OEP^

NISTIR 88-3906

Report No. 18

ULTRASONIC RAILROAD WHEEL

INSPECTION USING EMATS

National Institute of Standards and Technology

(formerly National Bureau of Standards)

U.S. Department of Commerce

Boulder, Colorado 80303-3328

December 1988

Raymond E. Schramm

A. Van Clark, Jr.

Stimulating America's Progress 1913-1988

NISTIR 88-3906

Report No. 18

ULTRASONIC RAILROAD WHEEL

INSPECTION USING EMATS

Raymond E. Schramm

A. Van Clark, Jr.

Fracture and Deformation Division

Institute for Materials Science and Engineering

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

December 1988

Prepared for

U.S. Department of Transportation

Federal Railroad Administration

Washington, DC 20590

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

Ernest Ambler, Acting Under Secretary for Technology

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, Raymond G. Kammer, Acting Director

CONTENTS

Page

ABSTRACT 1 INTRODUCTION 1 EMAT Program 1 Residual Stress in Railroad Wheel Rims 2

Cracks in Railroad Wheel Treads 3

Publications 5

Publication Reprints and Preprints:

1.

"Ultrasonic Characterization of Residual Stress and 7

Texture in Cast Steel Railroad Wheels," A. V. Clark,

H. Fukuoka, D. V. Mitrakovic, and J. C. Moulder in

Review of Progress in Quantitative Nondestructive

Evaluation , Vol. 6B, D. 0. Thompson and D. E. Chimenti,

eds . , Plenum Press, New York (1987), 1567-1575.

2. "Characterization of Residual Stress and Texture in 17

Cast Steel Railroad Wheels," A. V. Clark, Jr., H.

Fukuoka, D. V. Mitrakovic, and J. C. Moulder,

Ultrasonics 2_4, 281-288 (1986).

3. "Flaw Detection in Railroad Wheels Using Rayleigh-Wave 25

EMATs," R. E. Schramm, A. V. Clark, Jr., D. V. Mitrakovic,

and P. J. Shull in Review of Progress in Quantitative

Nondestructive Evaluation . Vol. 7B, D. 0. Thompson and D. E.

Chimenti, eds., Plenum Press, New York (1988), 1661-1668.

4. "Ultrasonic Characterization of Residual Stress and 33

Flaws in Cast Steel Railroad Wheels," A. V. Clark, R. E.

Schramm, H. Fukuoka, D. V. Mitrakovic in Proceedings

:

IEEE 1987 Ultrasonics Symposium . B. R. McAvoy, ed.

,

Institute of Electrical and Electronic Engineers, New York

(1988), 1079-1082.

5. "EMAT Examination for Cracks in Railroad Wheel Treads," .... 37

R. E. Schramm, P. J. Shull, A. V. Clark, Jr., and D. V.

Mitrakovic, submitted to Proceedings: Nondestructive

Testing and Evaluation for Manufacturing and Construction , Urbana, Illinois, Aug. 9-12, 1988.

6. "EMATs for Roll-By Crack Inspection of Railroad Wheels," ... 45

R. E. Schramm, P. J. Shull, A. V. Clark, Jr., and D. V.

Mitrakovic, submitted to Review of Progress in

Quantitative Nondestructive Evaluation . Vol. 8, La Jolla,

California, Aug. 1988.

7. "Ultrasonic Characterization of Residual Stress and 53

Texture in a Heat-Treated Steel Railroad Wheel," A. V.

Clark, H. Fukuoka, D. V. Mitrakovic, and J. C. Moulder,

submitted to Materials Evaluation.

i ii
'
ULTRASONIC RAILROAD WHEEL INSPECTION USING EMATS

Raymond E. Schramm and A. Van Clark, Jr.

Fracture and Deformation Division

National Institute of Standards and Technology

Boulder, Colorado 80303

This is report number 18 in a series covering the

research performed by the National Institute of

Standards and Technology (formerly National Bureau of

Standards) for the Federal Railroad Administration.

This issue collects seven reprints and preprints of

papers written by the Fracture and Deformation Division

over the last two years on the ultrasonic nondestructive

evaluation of railroad wheels for the presence of

residual stresses and cracks. All this work

concentrated on the use of electromagnetic-acoustic

transducers (EMATs)

.

Tensile residual stresses and tread cracks are major

factors in wheel failure. Catastrophic breakdowns may

lead to injuries, repair costs, and lost time. Current

inspection methods are largely visual, and thus

inefficient. Two ultrasonic techniques are applicable

to these wear defects:

1. Birefringence. A stress field effects the

velocity of a shear horizontal wave depending on its

polarization. Precise velocity measurements in a wheel

rim may allow calculation of the amount and direction of

stresses

.

2. Pulse-echo. A Rayleigh (surface) wave

transducer mounted inside the rail can introduce a signal to interrogate the circumference of a wheel as it

rolls by. An echo indicates a flaw's presence and size.

For both systems, we hope to produce instruments for

testing at a field site.

Key words: EMAT ; nondestructive evaluation; railroad

wheel; residual stress; roll-by inspection; ultrasonic.

INTRODUCTION

EMAT Program

Railroad safety is the focus of two current programs in the Fracture and

Deformation Division of the U.S. National Institute of Standards and

Technology (NIST, formerly National Bureau of Standards) in Boulder,

Colorado. These areas of research are:

I
1. Detection and quantification of residual stresses present in the

rim of railroad wheels.

2. Detection and depth sizing of cracks in the tread of railroad

wheels

The results of these nondestructive evaluation (NDE) problems come together

since a tread crack in a wheel with a tensile hoop stress in the rim can

lead to slow or explosive wheel failure. The consequences to personal

safety, schedule delays, track, and rolling stock can be both catastrophic

and expensive. For each problem, we apply a special type of ultrasonic

transducer to the cast wheels. The Federal Railroad Administration is

sponsoring these projects.

NIST researchers have been active in applying the unique properties of

electromagnetic-acoustic transducers (EMATs) to ultrasonic nondestructive

evaluation since 1980. During this time we have developed considerable

experience in transducer design and have built state-of-the-art electronics.

Our prior work has involved quantitative measurement of such parameters as

weld flaw size, residual stress, and sheet metal texture.

An EMAT contains two basic elements: a coil which is excited by highcurrent radio -frequency power to produce an eddy current in the specimen,

and a magnet to generate a static field. The operational principle is that

the Lorentz force generates a mechanical wave inside the wheel itself. The

Lorentz force F is an electromotive force caused by the interaction of

moving charge (eddy current J) with the external magnetic field (B) ; i.e.,

F = J x B where x is the vector cross-product. Industrial devices,

especially motors, use this force extensively. Application to the

generation of sound is relatively recent. Since the ultrasonic signal is

generated directly inside the specimen, a major EMAT advantage over the

standard piezoelectric transducers is elimination of any couplant - the

sound does not originate inside the probe and then traverse several coupling

media before encountering the wheel. With a bit of cleverness in orienting

the electric and magnetic fields, it is possible to generate any mode and

polarization of ultrasound.

Res idual Stress in Railroad Wheel Rims

Tensile residual stress in a wheel rim from causes such as heating during

heavy drag braking can contribute to catastrophic failure. Current

procedures call for an inspector to measure the extent of a certain color

rust between the rim and the hub to determine whether to pull a wheel from

service. Destructive tests at the Transportation Test Center (TTC) in

Pueblo, Colorado, have indicated that about half the wheels removed were

still safe to use (Britto Rajkumar, TTC, personal communication). To

quantify these measurements, we are examining the possibility of using

acoustic birefringence. The speed of shear-wave sound varies slightly

depending on its polarization and propagation direction with respect to the

stress field. Measuring the traverse time of an echo through the wheel rim

to within a few nanoseconds for two transducer orientations (radial and hoop

directions) should determine whether there is any stress present, its

direction, and magnitude. The goal is to develop a simple EMAT probe and

meter system usable in a railroad wheel shop.

2
We have examined the feasibility of developing such a field instrument.

Some progress made to date includes:

1. Development in theory and experimental procedures.

2. Design and construction of a digital delay gate to permit timing

measurements with the required accuracy.

3. Comparison between birefringence measurements made with

piezoelectric transducers and EMATs

.

4. Qualitative comparison between ultrasonic and destructive

measurements

.

5. Preliminary tests on a series of drag-braked wheels.

Besides stress, another contributor to the acoustic birefringence, and often

the major one, is the texture or preferred orientation in the specimen. For

cast steel wheels, preferred orientation occurs as the grains orient along

thermal gradients during cooling. The ability to separate the effects of

these two elements remains the major question in this part of the program.

Since the texture is largely a function of the metal alloy and the heat

treatment it has received, it is our hope that it will be possible to

determine a base birefringence for each type or class of wheel and then

subtract this from the value measured on a wheel under test. To make an

initial determination of the feasibility of such a procedure, we plan to

make measurements on a series of rim blocks cut from the drag-braked wheels

tested in item 5, above. When these blocks are removed during destructive

tests at TTC , the residual stress will be relieved so that our subsequent

EMAT examinations will show only the effects of texture. With this

information, we can quantify our initial measurements, and determine the

consistency of the baseline on at least this small sample. While awaiting

the preparation of the rim blocks, we have shifted our emphasis to a second,

and separate, program to detect cracks in the tread.

Cracks in Railroad Wheel Treads

Currently, cracks in the treads of railroad wheels are found mainly by

visual inspection during maintenance. Since each major rail line suffers

about 25-30 derailments every year as a result of cracked wheels, this

approach does not appear to be satisfactory.

In the early 1970' s, a company introduced an automatic ultrasonic method to

examine each wheel as it rolled by a checkpoint in a railyard. The

ultrasonic probe, situated inside the rail, is a pair of piezoelectric

transducers within a liquid- filled rubber boot. To aid the transmission of

the probing signal into and back out of the wheel, there are two nozzles

spraying fluid onto the tread as it passes. The basic idea, as in many

ultrasonic approaches, is to inject high frequency sound and then

electronically analyze echoes generated by any flaws present. The concept

and engineering of this system are very good, but the device has achieved

only very limited acceptance (there are only two currently active

installations in this country) . Two problems seem to have prevented large

scale implementation: (1) a very high rate of false positive indications,

and (2) fragility, since the boot is very susceptible to small cuts which

release the pressurized liquid couplant and disrupt operation.

Unfortunately, as wheel treads abrade, wear does develop occasional slivers

which are short-lived but very sharp and hazardous.

3
For more than ten years, the Fraunhofer Institute for Nondestructive Testing

(IzfP, Saarbrucken, Federal Republic of Germany) has been working on an

ultrasonic inspection system based on the same concept and physics, but

taking a different approach to the transducers. The German approach employs

noncontacting electromagnetic-acoustic transducers (EMATs) that do not

require an acoustic couplant to the wheel as do the conventional

transducers. EMATs simplify the mechanics of the rail mounting (since they

are generally smaller and more compact) , cut the number of false positives

(since they have a purer mode of operation and eliminate many extraneous

interfaces in the signal path) , and should be more durable (since there is

no pressurized rubber boot) . About two years ago IzfP installed a prototype

system in Wuerzburg for use on the high-speed German Experimental Intercity

Train (ICE)

This new system inspects the forged wheels of passenger cars and locomotives

of German design, all of which have tightly controlled wear profiles. In

our current project, we seek to use EMATs to inspect cast wheels on freight

cars of U.S. designs, generally with a variety of tread wear patterns and

damage states. Thus there are major differences between German and U.S.

applications of roll-by inspection systems.

While EMATs cannot operate efficiently at large liftoffs (distance to the

workpiece) , they can generally tolerate a separation on the order of

millimeters, and this is sufficient for roll-by inspection. Another EMAT

advantage is its high degree of selectivity, an ability to reject most

signals with an undesired polarization. This means a simpler echo pattern

and less confusion in data interpretation. In all the work of interest

here, the signal is a Rayleigh (surface) wave which travels around the outer

tread of the wheel and does not penetrate into the surface very far (<10-15

mm, in this case)

Over the past two years, we have worked to develop an EMAT system tailored

to the environment of the U.S. rail system. Some of the progress made to

date includes:

1. Design of Rayleigh wave EMATs using the new NdFeB permanent

magnets and a meanderline coil on a flexible substrate.

2. Development of a MOSFET power amplifier for the transmitter that

is capable delivering as much as 140 A.

3. Construction of a very low noise preamplifier for the receiver.

4. Assembling a short (4 m) length of railroad track to allow us to

conduct low-speed rolling tests on actual wheel sets.

5. Packaging of the analog electronics (items 2 and 3) into a single,

rugged unit suitable for use in field tests.

6. A simple trigger system to indicate wheel presence.

7. Start on the construction of digital electronics that will process

the analog signals automatically and generate any desired alarms

to indicate flawed wheels.

8. Contact with railroads and other research facilities to obtain

materials and information.

4
Tests made with the system described above indicate:

1. Good signal- to-noise ratio which should simplify the signal

processing that will be necessary,

2. Some tolerance to transducer liftoff so a protective cover for the

meanderline should be possible.

3. Differences in echo amplitudes based on flaw depths, indicating an

ability to distinguish artificial flaws (sawcuts) with a depth <>

the critical 6 mm.

4. Good operation with the transducer mounted inside the rail.

5. An ability to trigger the electronics with the presence of the

wheel and the capture of a digitized signal from a moving wheel.

Publications

Over the past two years, we presented the results of the work done under

this sponsorship at six meetings:

1. Review of Progress in Quantitative Nondestructive Evaluation, La

Jolla, California, August 1986.

2. Review of Progress in Quantitative Nondestructive Evaluation,

Williamsburg, Virginia, June 1987.

3. 1987 IEEE Ultrasonics Symposium, Denver, Colorado, October 1987.

4. Review of Progress in Quantitative Nondestructive Evaluation, La

Jolla, California, August 1988.

5. Nondestructive Testing and Evaluation for Manufacturing and

Construction, Urbana, Illinois, August 1988.

6. Third International Symposium on Nondestructive Characterization

of Materials, Saarbrucken, Federal Republic of Germany, October

1988.

Papers were written for the proceedings of each of these meetings. In

addition, a paper was published in the international journal Ultrasonics . and another has been accepted for publication in Materials Evaluation , the

journal of the American Society for Nondestructive Testing. This NISTIR is

a collection of the reprints and preprints of these papers (the papers for

meetings 4 and 6 were prepared at the same time and are virtually identical

so paper 6 is not included here)

.

5
.

.

'

6
Review of Progress in Quantitative NDE , eds. D. 0. Thompson and D. E. Chimenti

(Plenum Press, NY, 1987), Vol. 6B,

1567-1575.

ULTRASONIC CHARACTERIZATION OF RESIDUAL STRESS AND TEXTURE

IN CAST STEEL RAILROAD WHEELS

A. V. Clark, H. Fukuoka*, D. V. Mitrakovic** and

J. C. Moulder

Fracture and Deformation Division

National Bureau of Standards

Boulder, Colorado 80303

ABSTRACT

An ultrasonic technique has been used to characterize the state of

residual stress and texture in the rims of cast steel railroad wheels.

Orthogonally polarized shear-horizontal (SH) waves are propagated through the

thickness of the rim in pulse-echo mode. The (normalized) difference of

arrival times of these waves (acoustic birefringence) depends upon both texture and stress. The birefringence, B, was measured with two transducers:

an electromagnetic-acoustic transducer (EMAT) and a piezoelectric transducer

made of PZT.

Two wheels were tested. The first wheel had a sawcut, which locally

relieved the residual (hoop) stress. Measurement of the birefringence at

the sawcut allowed us to estimate the contribution of texture, which we subtracted from values of B at stressed locations. Values of hoop stress

obtained with the EMAT and PZT transducer agreed to within 10 MPa, for transducers placed on the center of the back face of the rim.

The second (uncut) wheel had been heat treated and air quenched, giving

a different microstructure. Measurements of B were made with both transducers at different radial and circumferential locations. We estimate that

the values of hoop stress obtained with the two transducers will differ by

less than 25 MPa for this wheel. Furthermore, the EMAT required less surface

preparation of the rim than the PZT transducer, indicating the potential for use of EMATs for residual stress measurements in the field.

INTRODUCTION

Residual stresses can be a significant factor in railroad wheel failure.

These residual stresses have two origins: (1) stress due to fabrication of

the wheel and (2) in-service stress induced by drag braking.

* NBS Guest Worker; on leave from Osaka University, Osaka, Japan.

** NBS Guest Worker; on leave from University of Belgrade, Belgrade,

Yugoslavia.

Contribution of the National Bureau of Standards: not subject to copyright

in the United States.

1567

7
_

ca

f^

steel wheels, fabrication stresses occur during cooling due to differential shrinkage. If these stresses are above the yield stress inhomogeneous plastic deformation results and residual stresses are then' necessary so that compatibility is satisfied.

The same process that creates residual stresses during cooling occurs (in reverse) during drag braking. The heat input Into the tread of the wheel

element

b

rf

k

fh

8 causas ex Panslon ° f thia region of the wheel. Each volume element of the rim tries to expand but is constrained by its neighbors. If

*2® 8 ool

TM

"P resslve stresses exceed yield stress, inhomogeneous plastic results; as the wheel cools after braking, a tensile hoop stress will result to make all the regions of the rim fit together.

Residual stresses due to fabrication are compressive, while those due

former

8

the ?

8 ar® (usually) tensile. When the latter stresses exceed the fo mer, the rim will be in a state of net tensile stress. If a crack occurs in the rim, those tensile stresses will act as crack-driving forces and that

T ?

fallure - Therefore it is highly desirable to have a non- destructive testing method for measuring such residual stresses.

In this paper, we present results of a study on the feasibility of using ultrasonic methods to characterize residual stress state in cast steel railroad wheels.

THEORY

It can be shown, both analytically and experimentally, that the presence of stress m metals induces a small sound velocity change. For isotropic materials, the normalized difference in velocity of orthogonally polarized shear-horizontal (SH) waves is proportional to the difference of principal stresses. This normalized difference in velocities is called the acoustic birefringence, in analogy to the birefringence effect in photoelasticity, so

V

-

e 1/2(v

e

+ v ) r

( 1 )

where V

g and V

p are velocities of SH-waves polarized in the hoop and radial directions respectively.

For anisotropic (textured) materials, birefr ingence depends on texture and principal stresses [1,2]:

B “ B

o

+ C A

(o

e ' °r } » (2)

where B

Q

is unstressed birefringence due to texture, C. is stress-acoustic

constant and o

Q

and o p are principal stresses in hoop and radial directions.

Combining equations (1) and (2) we have

v

e - 1

1/ 2( y

e + V

C fl ( o. °r>- ( 3 )

From equation (3) it is possible to calculate the difference in princi- pal stresses if the unstressed birefringence and velocities of orthogonally polarized shear waves are known.

1568

8
Instead of measuring velocities V

Q

and V

p we actually measure arrival

times T

0 and T

p of shear waves polarized in hoop and radial directions.

Since the SH-waves propagate through the same thickness,

V - V T - T n 6 r

_ r 6 , (4)

1/2(V

q

+ V

p

) 1/2(T

r

- T

0

)

so that the equation for calculating the difference in principal stresses

is given by

T - T

r 6 1/2(T

r TQ

) B

o

* C

A

(o

e • ar

) - (5)

Problems arising in using equations (5) for obtaining the difference in

principal stresses are: (1) the influence of texture and (2) the small \/alue

of stress-acoustic constant ( - 10 /MPa). The value of B

Q can be obtained

by measurements on unstressed reference samples (provided that the wheels are sufficiently homogeneous) . Because of the small value of the stress-acoustic

constant, we use electronics capable of measuring arrival times within ±1 ns.

experiments

Fukuoka et al. used conventional piezoelectric SH-wave transducer to

measure the residual stress state in rolled steel wheels [3t*0. They measured birefringence in the as-received state (residual stress due to rolling),

after drag braking, and after cutting into blocks (stress relieved). The

total residual stress measured ultrasonically was compared with destructive

measurements and agreed within 40 MPa [3.^3

In our work, we investigated the feasibility of using electromagneticacoustic transducers for measurements in the field. The EMAT has the advantage that it requires no acoustic couplant to generate sound in a metal.

Consequently, an EMAT can be scanned and rotated over the rim of the wheel

with ease. Being noncontacting, an EMAT may require less preparation of the

surface (where the sound is generated) than a piezoelectric device.

To determine whether these potential advantages could be realized in

practice, we performed a series of experiments on two cast steel wheels

(one sawcut and one uncut) which had been removed from service. We measured

the birefringence with an SH-wave EMAT in pulse-echo setup using a simple

velocity measurement system (described elsewhere [5]). We also measured the

birefringence using the same piezoelectric transducer used in Refs. 3 and 4.

The birefringence method using this piezoelectric transducer has been

successful in determining residual stresses in rolled wheels [3,43. Consequently, we used results obtained with this transducer as a benchmark against

which our EMAT measurements were compared.

The first set of measurements was made on a sawcut wheel. The measurements were made from the back face of tne rim, 19 mm from the inner edge of

the rim. This corresponds to the centerline of the front face (see Fig. 1).

Six different regions of the back face of the rim were milled in preparation for using both PZT and EMAT transducers (see Fig. 2). We measured the

birefringence near the center of each milled region at least three times with

both transducers. The values of birefringence thus obtained are plotted in

Fig. 3. The error bars represent the standard deviation of the measurements

of birefringence at each of the milled regions.

1569

9
Fig. 1. Cross-section of rim, of cast steel railroad wheel. Transducers

placed on back face; center of ultrasonic beam shown as broken

line.

180 °

Fig. 2. Top view of sawcut wheel. Shaded regions correspond to areas where

back face of rim was milled.

We were able to measure B

Q for the wheel as the mean value of the birefringence measured on both sides of the sawcut. Knowing B 0 we could convert

the birefr ingence measurements to stress. We used the same value for the

stress-acoustic constant C

A (-7.6 x lO'^/MPa) that was used in Refs. St 1* and

subtracted B

Q from B to obtain Cq - o p (see equation 2). Results obtained

are shown in Fig. <4 for both transducers.

1570

10
360*

Fig. 3 . Birefringence as function of circumferential position.

Fig. *4. Principal stress difference as function of circumferential position.

There are several interesting features in this figure. First, there is

good agreement between EMAT and PZT results. In fact, the difference of peak

stress measured with the different transducers is less than 10 MPa.

Second, the peak stress occurs at 180° circumferentially around the

wheel from the sawcut. This is in qualitative agreement with results obtained by researchers at Transportation Test Center [ 6 ]. They found that

sawcutting relieved the stress near the cut but left the stress 180 ° from the

cut almost unchanged.

1571

11
Third, the EMAT results are almost symmetric about 180° (the line opposite the sawcuts) . In fact, if the residual stresses and the degree of texture were homogeneous, one would expect Og - o p to display this symmetry

after sawcutting. The symmetry of the EMAT results is encouraging, since it

does indicate that the residual stresses and the texture were axisymmetric

prior to sawcut. This reduces the number of required measurements around the

rim of the wheel.

With the EMAT we made measurements not only on the milled regions, but

also at other locations where we used various surface preparation treatments.

The results obtained using the EMAT on these variously prepared surfaces are indicated by different symbols in Fig. 4. Qualitatively, it appears that the

degree of surface preparation is not particularly significant for EMAT measurements.

A second set of birefringence measurements was made with both an EMAT

and PZT transducer on an uncut cast steel wheel which had been heat treated

and air quenched. This process results in a grain structure which is finer

than in cast steel wheels and consequently shows a better signal-to-noise

ratio in ultrasonic measurements. Measurements were performed on the front

face of the wheel, which is the side accessible in the field.

To characterize the effect of surface preparation we made measurements

at the center of the front face of the rim and for varying degrees of surface

preparation. Results of these measurements at different circumferential

locations are shown in Fig. 5. For comparison we also have shown results

obtained by the PZT transducer on a milled surface at the same locations.

There is a good agreement between results obtained by EMAT and the PZT transducer. The difference in EMAT birefringence measurements for different sur- face preparations is usually less than 2 x 10~ . This is equivalent to a stress uncertainty of about 25 MPa, assuming a value of -7.6 x 10 °/MPa for

the stress-acoustic constant, C^.

The good agreement between EMAT measurements on surfaces with varying

degrees of surface preparation is quite encouraging. From a practical point

of view, it minimizes the amount of time or labor necessary to prepare the

surface.

?

4 0

W 2 ^ 0

1 -21-

1(0

t r Handpollshed

•-Grinding A Handpollshed

o- Milled

o- MiOed 4 Handpoliehed, PZT

EMAT ~

o

oSo

0

180° e270* 360°

Fig. 5. Effect of surface preparation on birefringence measurements.

1572

12
We were also interested in determining whether there is a radial variation of birefringence. (In fact, a radial dependence of birefringence was

found by Fukuoka, et al. [3,^0 on rolled steel wheels, using the PZT transducer). We measured the birefringence at 5 different radii, at 4 different

circumferential positions (marked as 0°, 90°, 180° and 270°). Both EMAT and

PZT measurements were made after the wheel was milled.

The results of the birefringence measurement at different radii are shown in Fig. 6 . The data points are the mean value of measurements at

circumferential positions, and the error bars represent the standard deviation. The birefringence has a gradient, becoming increasingly negative as the outer edge of the rim is approached. The gradient as measured by the

EMAT is almost linear and, assuming a constant texture, it has value of about

6 MPa/mm.

In the ideal case, both the texture and residual stress would be

axisymraetric. If this is the case, then the standard deviations of B shown

in Fig. 6 would vanish. The presence of the standard deviations (which are larger than experimental errors) implies that there is some asymmetry in the

birefringence. To show this asymmetry better, we have plotted the birefringence as a function of circumferential position in Fig. 7, with the radial

distance (from center of the rim) as a parameter.

i

RADIAL POSITION, mm

Fig. 6 . Radial variation of birefringence.

Fig. 7. Birefringence as function of circumferential position, for

radii of + 6 mm, -3 mm. Radial position measured outward from

center of front face of rim.

1573

13
For axial symmetry, the birefringence would be a straight line with zero slope (constant as function of circumferential position). The data in Fig. 7 for the radius -3 mm from center of the rim show a departure from axial symmetry with a minimum algebraic value of B in the region about 180° (with good

agreement between an EMAT and a PZT measurements). On the other hand, the

values of B for +6 mm from the center of the rim show not only good agreement

for both transducers measurements but also a behavior which is very close to

axial symmetry.

CONCLUSIONS

We have performed proof-of-concept experiments which show that EMATs can be used to characterize residual stress in cast steel railroad wheels. We

have developed a system which measures arrival times with an electronic precision of ±1 ns, for pulse-echo operation.

The birefringence was measured on two railroad wheels with different

microstructure. The cast steel wheel had a sawcut, which enabled us to mea- sure B

0 and hence convert our birefringence measurements to stress. We found

good agreement between piezoelectric and EMAT results (stresses agreed to

within 10 MPa). We also found that the EMAT was somewhat insensitive to

surface preparation.

On the heat-treated wheel, we characterized this sensitivity and found

that the scatter in EMAT results due to surface preparation was equivalent tc

about 25 MPa. This indicates that for practical use (e.g. in a railroad

yard) only minimum surface preparation may be necessary.

We measured the birefringence on this wheel as a function of both radial

and circumferential position. A steep gradient of birefringence in the

radial direction was found; if the texture is homogeneous, the gradient was about 6 MPa/mm. Values of birefringence measured with EMAT and piezoelectric

transducers were in good agreement, showing that the EMAT could be used to

resolve this steep gradient.

The birefringence in the rim was found to be nearly axisymmetric close

to the tread (where the wheel sits on the rail) but more erratic as the

inner edge is approached.

To determine the accuracy of the method, we plan to perform destructive

tests to measure the residual stress. Blocks cut from the rim will be used

to measure B . An average value will then be used in equation (5) for ultrasonic predictions of residual stress and comparison made with destructive

results.

ACKNOWLEDGMENTS

The work was supported by the Federal Railroad Administration, Department of Transportation, under the supervision of Claire Orth, Research

Manager. We were greatly assisted by the generous cooperation and encouragement of Mr. Britto Rajkumar of the American Association of Railroads, Pueblo,

CO who provided the wheels.

REFERENCES

1 . Y. Iwashimizu, Y. , and K. Kubomura, Int. J. Solids Structures 9, 99-1 1 At (1973).

1574

14
2. K. Okada, J. Acoust. Soc. Japan (E) _1_, 193 _ 200 (1980).

3 . H. Fukuoka, H. Toda, K. Hirakawa, H. Sakamoto, and Y. Toya, "Nondestructive Assessments of Residual Stresses in Railroad Wheel Rim by Acoustoelasticity, " to be published in J. of Eng. for Industry, ASME,

New York City.

4. H. Fukuoka, H. Toda, K. Hirakawa, H. Sakamoto, and Y. Toya, Acoustoelastic Measurements of Residual Stresses in the Rim of Railroad

Wheels, in: "Wave Propagation in Inhomogenous Media and Ultrasonic

Nondestructive Evaluation," AMD-Vol. 6 , G. C. Johnson, ed., ASME, New

York City (1984).

5 . A. V. Clark, H. Fukuoka, D. V. Mitrakovic, and J. C. Moulder, "Characterization of Residual Stress and Texture in Cast Steel Railroad

Wheels," to be published in Ultrasonics.

6 . B. Rajkumar, Transportation Test Center, American Assoc, of Railroads,

Pueblo, CO, private communication.

1575

15
1

:

-

> -s '

.

. > \h

- A

16
Characterization of residual stress and

texture in cast steel railroad wheels

Fukuoka* A.V. Clark, Jr, H. *, D.V. Mitrakovie* and J.C. Moulder

Fracture and Deformation Division, National Bureau of Standards, Boulder, CO 80303,

Received 20 May 1986

Residual stress and texture were characterized in the rim of a cast steel railroad wheel, using both an electromagnetic acoustic transducer (EMAT) and a piezoelectric transducer. Orthogonally

polarized shear horizontal waves were propagated through the thickness of the rim, and arrival times measured (by the pulse-echo method) with a precision of % i Q~5

. The difference in arrival times (birefringence) is related to the difference in principal stresses and also to texture. The wheel

had been sawcut in a previous experiment and the residual stress had been relieved at the sawcut.

The birefringence was measured at the sawcut and subtracted from the birefringence measured at stressed regions. This allowed us to map variations in stress around the circumference of the

wheel. Stresses measured with the EMAT and piezoelectric transducer aqreed to within

10 MPa.

Keywords: stress; texture; steel; EMATs; transducers

The presence of residual stress in railroad wheels can be a significant factor in wheel failure. These residual stresses have two origins: 1. stresses due to fabrication of the wheel

and 2, in-service stresses induced by drag braking. For cast

steel wheels (the most common type in the US) fabrication

stresses occur as the wheel cools in the mould; differential

shrinkage gives rise to thermal strains (and stresses). If these stresses are above yield stress (for the corresponding

temperature), then inhomogeneous plastic deformation

results. Residual stresses are then necessary so that

compatibility is satisfied; e.g. so all regions of the wheel fit together without voids.

The same process that creates residual stresses on

cooling also occurs (in reverse) during braking. The heat

input into the tread of the wheel (region in contact with the

rail) causes expansion of this region of the wheel,

especially the rim. Each volume element of the rim tries to expand but is constrained by its neighbours. Consequently, thermal expansion during braking causes a compression; if these compressive stresses exceed yield

stress, inhomogeneous plastic deformation results. As the

wheel cools after braking, a tensile hoop stress may result

to make all the regions of the rim Tit together'.

Residual stresses due to casting will probably be

compressive, whereas those due to excess braking will probably be tensile. If the latter stresses exceed the former,

then the rim will be in a state of net tensile stress. Consequently, if a crack occurs in the rim. these tensile stresses can act as crack-driving forces and lead to wheel

failure. *NBS guest worker, on leave from Osaka University, Osaka,

Japan

f NBS guest worker, on leave from University of Belgrade, Belgrade,

Yugoslavia

It is highly desirable to have a non-destructive testing

method for measuring such residual stresses. One promising technique which has been investigated is the acoustic

birefringence technique. It can be shown both analytically

and experimentally that the presence of stress in metals

induces a small velocity change (acoustoelastic effect).

Subject to certain restrictions, the difference in velocity of

orthogonally polarized shear horizontal (SH) waves will be proportional to the difference ofprincipal stresses. This

normalized difference in velocities is called the acoustic

birelringence. in analogy to the birefringence effect in photoelasticity.

Piezoelectric SH wave transducers have been used to measure^ the residual stress state in Japanese rolled steel wheels1 ' 2

. The birefringence was measured in the as- received state (residual stresses due to rolling), after drag

braking (additional stresses due to thermal expansion),

and after cutting into blocks (stress relieved). The total residual stress (measured ultrasonically) was compared

with destructive measurements; the two agreed to within

40 MPa1 - 2

. In this Paper, we present results of a study on the

feasibility of using ultrasonics to characterize residual

stress states in cast steel wheels of US manufacture. We

were interested in using electromagnetic acoustic trans- ducers (EMATs) for these measurements.

The EMAT has the advantage that it requires no

acoustic couplant to generate sound in a metal (or other

conductor). Consequently, an EMAT can be scanned,

rotated, etc. over the rim of the wheel with ease. Being noncontacting, it may require less preparation of the surface

(where sound is generated) than piezoelectric devices. To

determine whether these potential advantages could be

realized in practice, we performed a series of experiments

Ultrasonics 1 986 Vol 24 September 281

17
Residua! stress and texture in railroad wheels: A V. Clark et al. on a cast steel wheel which had been removed from

service. We measured the birefringence with an SH wave EMAT using a simple velocity measurement system

(described later). Since the birefringence method using

piezoelectric transducers had been successful in determining residual stresses1 ’ 2

, we measured the birefringence

using both an EMAT and a piezoelectric transducer. (In

fact we used the same transducer as was used in References 1 and 2.) The residual stress, as measured by

both devices, was found to be in excellent agreement.

Theory

Stress causes a small change in phase velocity in metals.

Consider the case of a homogeneous, isotropic metallic

plate in a state of plane stress. Let a pure mode shear

horizontal (SH) wave be propagated through the thick- ness of the plate; the wave is polarized first along the tr, (principal stress) axis, and then along the <r

2 axis, with

corresponding transit times 7", and T2

. The acoustic

birefringence, B, is defined as the normalized difference in arrival times and is proportional to the difference in principal stresses

B =

t2 -t1 (1/2) (T2 +7’, ) = CA(oj ~°2 ) (la)

Since both SH waves travel through the same material

thickness

B =

Ei - V2 (1/2) (E, + V2 ) (lb)

where E, is the phase velocity of an SH wave polarized

along cr, and E2 is the phase velocity of an SH wave

polarized along o2. The magnitude of the stress-acoustic constant. CA . is rather small. 7.6 X 10~6 MPa for railroad wheels 1,2

. Consequently, the stress-induced birefringence is also

small. In fact, for many common structural elements,

there will be unstressed birefringence. B0. due to fabrication processes (rolling, casting, etc.) which is the same

magnitude as the stress-induced birefringence.

The theoretical problem of an SH wave propagating

through a slightly anisotropic body in a state of plane

stress has been solved by iwashimizu and Kubomura3 and also by Okada4

. For brevity, we repeat only the results oftheir calculations here. It was found that pure mode SH

waves could be propagated only when the waves were

polarized along certain directions (also called acoustic

axes). These axes were oriented at angle 0 relative to the axes of material symmetry (to be defined later). 0 is given

by3

tan 20 =

2 CA o12 B0 + CA (on - o22 ) ( 2)

Here the normal stresses, cr,,. a22 . and the shear stress. a12 . are referenced to the material symmetry axes. In the

absence of shear stress, the acoustic axes coincide with the

material symmetry axes. Plane stresses are assumed in the

above.

The birefringence (difference in velocity of SH waves

polarized along the acoustic axes) is given by3

3

2 = I B0 +CA(on - Oj2)]

2 +(2CA onr (3)

180*

Figure 1 Top view of the sawcut wheel used in the experiments.

Shading indicates milled regions 1-6. Angle 6 is measured

circumferentially around the rim from the midpoint of the sawcuts,

in an anticlockwise direction

Note that Equation (3) reduces to Equation ( 1) when there

is no shear stress and no unstressed birefringence.

A discussion of material symmetry axes is in order

here. Normally one treats common alloys such as steel and aluminium as isotropic. However, as stated

previously, processes such as rolling and casting induce a preferential orientation of the crystallographic axes of the

grains (single crystals) which make up the structural

elements. It is this anisotropy that gives rise to the

unstressed birefringence. B0

. The directions of the preferential orientation are the material symmetry axes. The theory which leads to Equations (2) and (3) assumes orthotropic symmetry, i.e. any small volume

element of the material has three axes of two-fold

symmetry. This assumption works quite well for rolled

plates of aluminium and steel alloys. One of the purposes

of our experiments was to test its validity for cast steel wheels.

Experimental procedure

Railroad wheel

A cast steel wheel, shown in Figure 1 . was obtained from

the Department of Transportation (DOT) Test Center in Pueblo, Colorado, USA. This wheel has been removed

from service because of the ‘four inch rule'5 to prevent

possible failure due to build-up of residual stresses induced by braking* (If such stresses are tensile, they

become crack-driving forces for any cracks existing in the

wheels.) The wheel had been sawcut as part of a test programme being conducted by the American Association of Railroads for the DOT. Consequently, the

circumferential (hoop) stress had been relieved at the sawcut. The wheel was milled at several locations on the

front and back faces of the rim. The milled locations are

‘Simply put, the four inch rule prescribes that if a band of discoloration > 4in wide forms near the rim, the wheel has to be

removed

282 Ultrasonics 1986 Vol 24 September

18
Residua! stress and texture in railroad wheels: AV. Clark et al.

—^1818 mm p-p~ Back Face

Centre of / N Front Face

Ultrasonic Baam /

Figure 2 Cross-section of the wheel in the vicinity of the rim. Transducers were placed on the back face of the rim, 1 9 mm radially

outward from the (inner) edge of the rim

indicated in Figure L and a cross-section of the rim is shown in Figure 2. The milling was done to give a smooth

surface for coupling acoustic energy from a piezoelectric

transducer into the wheel (as described later). No

particular effort was made to make the milled surfaces

plane parallel. In addition, other locations around the circumference

of the wheel were cleaned by a variety of techniques. Some

locations were merely hand-sanded to remove surface

rust, leaving large pitted areas on the surface. Others were ground with a hand grinder, and then sanded. This was done to characterize the effect of surface preparation on

the velocity measurements made with electromagnetic

acoustic transducers (EMATs). All measurements were made with the centre of the transducer aperture 19 mm

from the inner edge of the rim. as shown in Figure 2.

Velocity measurement system

A simple time-interval averaging system was used for birefringence measurements. The set-up used with the

EMAT is shown in Figure 3. The essentials of the

operation are as follows. When the pulser excites the

transducer, it also excites a time-interval probe which

generates a TTL pulse which sends a start signal to a counter. The counter runs at 100 MHz clock rate until it receives a stop signal. To suppress the effect of noise (a

significant problem with EMAT operation), the counter can be set to average many such time intervals. We

typically used an average of 103 such intervals.

We wished to avoid dependence of arrival time

measurements on signal amplitude. This was accomplished as follows. The received signal was amplified by a broadband receiver, followed by a high-pass filter to remove any variations in d.c. level with amplifier gain.

The signal was then input to the stop probe, which

generates a TTL pulse when the signal reaches a selected

threshold. In order to make arrival time measurements

insensitive to amplitude variations, the threshold level was set to 0 V. The output of the probe was input to a specially constructed digital delayed gate (DDG),

described in more detail in the Appendix. In essence the

DDG accomplishes the same purpose as a distortion-free

analog gate, but does so by using logic circuit

electronics.

The stop probe will, of course, generate a signal each

time the voltage level reaches zero. Since the EMAT

system has a low signal-to-noise ratio, it is necessary to eliminate the stop probe output pulses which corres- pond to those zero crossings generated by noise (rather

than the echo received by the transducer). Typically, we

set the delay on the DDG so that its output corresponded

to a selected zero crossing in the RF echo waveform.

Velocity measurements with EMATs

The wheel was placed in a horizontal position. Since the

EMAT contained a permanent magnet, no fixture was

necessary to keep it in place. Measurements were made in the pulse-echo mode, with the transducers (both EMAT

and piezoelectric) placed on the back face of the rim. The

EMAT has an aperture 1 x 1 cm and was excited with a quasimonochromatic tone burst having a frequency of

2.8 MHz. Arrival times were measured with the EMAT

polarized in the radial (r) and circumferential (6)

directions. For the cast steel wheel used here, only the first echo was useable for these measurements; the signal-tonoise ratio was too small for the second echo and further

echoes could not be distinguished from noise on the scope

trace. Our measurements were conducted in close proximity

to a welding laboratory and a laboratory equipped with

mechanical testing machines (a noisy RF environment).

Nevertheless, the precision in arrival time measurements

made with our time-interval averaging system was ± 1 ns

(or better). Since the arrival time of the first echo was % 90 jus, our (electronic) precision is of the order of

10 ppm. This was achieved with a signal whose amplitude

(after amplification) ranged from « 200 to 800 mV (zero

to peak).

We observed that the signal amplitude varied with

orientation. In fact, the peak amplitude was about twice as

large for an SH wave polarized in the radial direction as in the circumferential direction. Consequently, it is essential

that the stop signal to the counter be triggered by a TTE

pulse (from the stop probe) which corresponds to a zero

Time-Interval Averaging Counter

Preamplifier

|em at]

Diode Expander Tuned Amplifier

Figure 3 Time-interval averaging system used for EMAT measure- ments

Ultrasonics 1986 Vol 24 September 283

19
Residua! stress and texture in railroad wheels: AV. Clark et al.

crossing (phase = 2mr) in the received RF waveform

(first echo).

The permanent magnet in our EMAT has a rotationally symmetric magnetic field. This is important, since it avoids the possibility that magnetic hysteresis effects could cause variations in the efficiency of converting

electromagnetic energy into acoustic energy as the EMAT

is rotated. We also observed an increase in signal strength

when the EMAT was left >n a given location for several

hours. This is probably due to an increase in magnetic

induction caused by the permanent magnet. (The amplitude of the acoustic signal is proportional to the magnitude of the applied magnetic induction in the skin depth

of an electromagnetic wave incident on the wheel.)

Relative velocity (birefringence) measurements were made with the EMAT polarized first in one principal

stress direction (e.g. radial direction) and then in the

orthogonal direction. This operation was repeated at least three times, since it was observed that arrival time

measurements at the same location with the same orientation exhibited a scatter typically of the order of ^5 ns. The arrival times in each orientation were averaged, and

the differences (of the two averages) were taken to give the

birefringence

<rr > ~(Te ) l/2«7’r

> +<70» (4)

where (Tq) is the average arrival time of the SH wave

polarized in the circumferential direction and (T,) is the

average arrival time of the SH wave in the radial

direction.

We attribute the scatter in arrival times to variations in the coupling between the EMAT and wheel surface. Even

though EMATs are non-contacting devices, there are

effects of lift-off (distance between EMAT and surface).

One effect is, of course, a decrease in the amplitude of the

acoustic signal generated by the EMAT. A second (usually

smaller) effect is a phase shift (in frequency space) or an

equivalent delay in the time domain. One explanation for

this phenomenon is as follows. Consider the equivalent

circuit for the EMAT. This consists of the inductance of

the EMAT coil plus a complex impedance due to the

electromagnetic field set up by the EMAT in the skin

depth of the conductor beneath it6

. This impedance

consists of a resistance (eddy current resistance) plus an

inductance. The impedance (in frequency space) has been

Figure 4 Time-interval averaging system used for PZT transducer

measurements, (a) Upper transducer; (b) lower transducer

shown to be a function of lift-off6

, so variations in lift-off will give variations in arrival times.

Variations in lift-off come from two sources in our

experiments. At locations which have not been milled,

there are pits remaining on the surface. Since the EMAT is not precisely registered each time it is placed on the

surface, the effective lift-off varies from one placement to another.

The other source of lift-off is caused by the construction

of the EMAT itself. A copper shield is placed over the front

of the EMAT. The shield has an aperture 1 x 1 cm cut in it. (This is done to obtain a linearly polarized SH wave.) The

copper shield is not flat but has small bulges which must

be flattened out by a (small) force applied to the case which houses the EMAT. To apply a constant force, we

usually placed a weight on top of the EMAT case. This

partially succeeded in reducing lift-off variations, but still left the previously mentioned spread of as 5 ns in arrival

times.

Velocity measurements with piezoelectric

transducers

We wished to compare our EMAT measurements with

those performed using conventional piezoelectric

transducers. We used a specially constructed device

(fabricated at Osaka University, Japan) which consisted of

two transducers of PZT. The transducers were orientated

so that they generated orthogonally polarized SH waves,

and then one transducer was bonded on top of the other.

Leads were arranged so that either the top or bottom

transducer could be separately excited by means of a switch.

A powder backing made of copper and epoxy was bonded to the upper transducer to act as an acoustic

absorber and increase the bandwidth of the device. A

small piece of rubber was bonded to the backing material.

Even if the axis of the device is not normal to the wheel

surface, the transducers will have enough freedom to rotate so that they are at a tangent to the surface.

The piezoelectric transducer was spring loaded to ensure a constant pressure on the viscous couplant which was used to couple a shear wave into the wheel. The device

which housed the transducer and the spring was mounted

on the wheel with a stand which had a magnetic base.

The electronics used to drive this stacked transducer

device were different from those used to drive the EMATs;

compare Figures 3 (EMAT) and 4 (PZT). The reason for using two different arrangements is as follows. With the

PZT device, the waveforms generated by the upper and

lower transducer are different. The upper transducer is 'loaded' on its front face by the lower transducer, whereas

the lower transducer is 'loaded' on its front face by the

wheel. When usingthegated amplifier (shown in Figure 3). we could not identify corresponding zero crossings in the

waveforms generated by the lower and upper transducer.

Consequently, we used a single pulser which shock

excited the PZT transducers. By properly tuning an impedance matching device connected between pulser

and transducers it was possible to obtain clearly

identifiable features in the waveforms generated by

each transducer and identify the corresponding zero

crossings.

We found that (for the same receiver gain) the signal

received by the upper transducer was always smaller than

that of the lower transducer. Furthermore, the waveforms

generated by the transducers were not purely sinusoidal.

We attempted to compensate for this by changing the

receiver gain so that the peak signal amplitude was the same in both waveforms.

284 Ultrasonics 1 986 Vol 24 September

20
Residua/ stress and texture in railroad wheels: AV. Clark et al. We used the following procedure. The signal received

by the lower transducer usually had a peak amplitude of

^400 mV with the (variable) receiver gain set to zero

+

. Consequently, the vertical amplifier on the oscilloscope was first set on 200 mV per division and then taken out of

calibration and adjusted until the peak of the waveform

was two (large) units above the ground level (with the

scope operating in the d.c. mode, rather than a.c.). The

switch (shown in Figure 4) was then operated so that the

signal from the upper transducer was received and

displayed on the scope with the (uncalibrated) gain on the

vertical scope amplifier unchanged, and the variable gain on the receiver was changed until the peak waveform for

the upper transducer reached the same level on the

oscilloscope (two large units) as that of the lower

transducer. We found that this procedure seemed to result in the minimum shift in apparent arrival time with gain

( **» 20 ns).

Using this procedure is a somewhat arbitrary’ choice,

since we noted a change in apparent arrival time with

receiver gain. In fact, we would expect some differences in

birefringence as measured with the EMAT and the PZT

transducer. With the EMATs excited by a toneburst. zero

crossings correspond to zero phase in the echo. For shock

excitation of the PZT transducers, which do not have a purely sinusoidal response, zero crossings do not necessarily correspond to zero phase, since a change in gain

caused a change in time of zero crossing. In fact, we expect

some systematic errors, 8te and St T

. in PZT arrival time

measurements. However, if we use a consistent arrival

time measurement procedure, we expect that the dif- ference between birefringence measured by EMAT and

PZT will be constant. The results of our measurements

seemed to bear this out. as will be shown subsequently.

One advantage of the design of this stacked transducer

is that couplant thickness variations are eliminated. Both

transducers generate SH waves which propagate through

the same couplant thickness. The arrival time of an SH

wave in the upper transducer will, of course, be increased

by twice the time an SH wave requires to propagate

through the lower transducer. To compensate for this, the

arrival times for both transducers were first measured with

the lower transducer polarized in the radial direction,

then the (stacked) transducer was rotated 90° and the new

arrival times measured. The birefringence was calculated

from

b = i /2 [ov' + r,

u

) - (r„

5 + r„")] /rQ (5)

where superscripts £ and u refer to the lower and upper

transducers, respectively, e.g.

7“

r£

is the arrival time

measured with the lower transducer polarized in the

radial direction. T0

is the average of all arrival times.

Results

Texture study

The theory of acoustoelasticity', which led to the relations

of Equations (2) and (3), was based on the assumption of

orthotropic symmetry3,4

, which usually occurs in rolled

plates of aluminium and steel alloys. Since we are dealing

with a cast steel wheel, it w

T as necessary to characterize the

texture (preferred orientation) to see whether the theories

of References 3 and 4 could be applied to our wheel. It has

been observed that for a slightly orthotropic rolled plate.

The receiver has a fixed gain of s» 30 dB, plus a variable gain

*

Figure 5 Angular variation of arrival times for unstressed region

( ) and stressed region ( ) of sawcut wheel. Angle ip is measured anticlockwise; ip = 0°, 1 80° when measuring T,; tp = 90°,

270° when measuring Te

the measured arrival time of an SH wave (propagating

through the plate at normal incidence) varies in a sinusoidal manner7

. The maximum and minimum arrival

times are obtained w'hen the transducer is polarized along

the acoustic axes. (For a treatment of the theory behind

this effect, see Reference 7). We measured the angular variations of arrival times in

the sawcut wheel. Locations 1 and 2 on the wheel

(adjacent to the sawcuts) experienced a relief of hoop

stress. We used our EMAT to measure the arrival time

variation at location 1 as the transducer was rotated. The

result is shown by the solid line in Figure 5. The velocity is fastest for a wave polarized in the radial (r) direction and

slowest for the wave polarized in the circumferential ( 6

)

direction.

To be sure that the acoustic axes are oriented along

radial and circumferential directions at stressed locations

in the sawcut wheel, we also measured the angular

variation of arrival times at location 4 (s ze Figure 1). This

location was 90

c around the w-heel circumference from the

sawcuts; hence, the residual stress there should not be

completely relieved by the saw'cuts. The angular variation

of arrival times at location 4 is shown as the broken line in Figure5. The data show that the radial and circumferential

directions are still acoustic axes for stressed locations.

Consequently, a necessary7 condition for the application

of the theory of References 2 and 3 has been satisfied. Furthermore, we see that the radial and circumferential

directions still appear to correspond to the fast and slow’ acoustic axes, respectively. The material symmetry' axes

still coincide with the principal stress axes, and from

Equation (2) the shear stress, vanishes. The birefringence Equation (3) becomes

B -B0 = CA(oe - ffr) (6)

for the cast wheel.

The unstressed birefringence. £0 , was measured at locations 1 and 2 (on either side of sawcuts). The values were slightly different at each of the two locations, and

Ultrasonics 1 986 Vol 24 September 285

21
Residua! stress and texture in railroad wheels: A V. Clark et al.

Table 1 Values of unstressed birefringence

Location Transducer B0 x 1 cr

4

1 PZT 7.6 2 PZT 6.2

1 EMAT 8.1 2 EMAT 7.4

were different for EMATs and the PZT transducer (see

Table 1). We took the average of the unstressed hire- <o fringence measured at locations 1 and 2 as the value of B0 to be used in Equation (6). For the PZT measurements, we have50 = 6.9 x 10~\ for the EMAT, B0 = 7.8 x 10“4

. The

difference between these values may be due to the way

arrival times are measured with the EMAT and PZT

transducers, as described previously.

Birefringence measurements

The birefringence was measured at the milled regions 1 -6. usingthe PZT transducer. The milled surface was ss 18 cm

long (arc length) at each region (see Figure 1). We

measured B at least three times at each milled region. The

birefringence was measured near the centre of each milled

region. The values of birefringence thus obtained are plotted in Figure 6. The vertical axis actually equals —B

since we defined B by B = (Tt — Tg)/T0

. The error bars

represent the standard deviation of the (three or more)

measurements of B at each of the six milled regions.

Also shown in Figure 6 are the results of EMAT

measurements on the milled surfaces. With the EMATs,

we measured B at three separate locations on each milled

region and at locations ±25 mm from the centre. Consequently. B was measured at a total of 18 locations.

At each location, we measured B at least three times. We

took the average of all measurements at a given milled

region (nine or more) as the value of B; we measured at the centre of the milled region. This tends to minimize any

(local) variation in B0 . remembering that B = B0 +

Ca(<T0 - or

). Values of B thus obtained are in good agreement with

PZT values. Note that the PZT values are usually lower

than EMAT values; this 'd.c. shift' is due to the difference

in the way arrival times are measured with the two

different transducers. Furthermore, the shift is about the same as the difference in B0 as measured with the EMAT

and the PZT transducer.

RADIAL POSITION, mm

Figure 6 Birefringence measurements made on rim of sawcut

wheel, using both the EMAT (O) and PZT (•) transducer

Figure 7 Differences in principal stresses measured on the rim of the sawcut wheel, using both the EMAT and PZT transducer. The

EMAT measurements were made using a variety of surface

preparations: •.milled; O, rust removed; A, hand sanded; D.hand

ground and hand sanded. 0- PZT, milled and hand sanded

Stress measurement

Since we were able to measure BQ for the wheel (due to the

sawcut). we can convert the birefringence measurements

to stress. We used the same value of the stress-acoustic constant. CA = -7.6 X 10~6 MPa. that was used in References 1 and 2 and subtracted B0 from B to obtain

cfq —<x

r [see Equation (6)]. The results obtained are shown

in Figure 7 for the EMAT and PZT.

There are several interesting features about this figure.

First, notice that the peak stress occurs at 180° circumferentially around the wheel from the sawcuts. This is in qualitative agreement with results obtained by researchers

at the Transportation Test Centre, USA8

. There, they

found that sawcutting relieved the stress near the cut. but

left the stress 180° from the cut almost unchanged. The

second interesting feature about Figure 7 is the good

agreement between the EMAT and PZT results. In fact, the difference of peak stress as measured with the different

transducers is < 10 MPa. This agreement shows that there was indeed a constant difference in birefringence

measurements between the PZT and EMAT; since we

substract B~B0 to obtain <Jq—<jx

, the differences cancel

out.A third feature of note is that the EMAT results are almost symmetric about 180° (the line opposite sawcuts). In fact, if the residual stresses (due to casting and braking),

and the degree of texture were homogeneous, then one would expect oe — crr to display this symmetry (about 180°)

after sawcutting. The symmetry of the EMAT results is pleasing, since it does indicate that the residual stresses and the texture were axisymmetric prior to sawcutting.

We hope to check this hypothesis of axial symmetry by

measurements on an uncut wheel.

286 Ultrasonics 1986 Vol 24 September

22
Residua! stress and texture in railroad wheels: A V. Clark et al. For the EMAT, we made measurements not only on the

milled regions but also at other locations where we used

various surface preparation treatments. At some locations, we merely removed a layer of rust with abrasive

paper leaving a surface consisting of both bright metal

and pits. At other locations, we used a hand grinder

followed by hand sanding with abrasive paper. The

resulting surface was smoother, but still contained pits. At

other locations, we used only abrasive paper, but sanded

for a longer time than when merely removing rust. The

surface finish was intermediate between the rust-removed

surfaces and surfaces cleaned with a hand grinder.

The results obtained using the EMAT on these various

surfaces are indicated by different symbols in Figure 7. Qualitatively, it appears that the degree of surface

preparations is not particularly significant as far as EMAT measurements are concerned. For instance, the

EMAT data from 0 to 90° agrees quite well with PZT data,

except for two points which are off the PZT curve. However, even these points are only out by 25 MPa. For

180-360°, the EMAT data is, as mentioned above,

approximately the mirror image of the data from 0 to 1 80°

(symmetry about the 180° line).

We expected systematic errors, Sts• in PZT

measurements of arrival times. The effect of these errors appears to have cancelled out in taking the difference

B—B0 to obtain stresses, since the EMAT and PZT

measurements of a#—ff

r are in good agreement.

Conclusions

We have developed a simple time-interval averaging

system which allows us to measure arrival times with a precision of ±1 ns or better, using SH waves propagating

through the rim of a cast steel railroad wheel. This

precision was obtained with a PZT transducer and also

with an EMAT, both operated in the pulse-echo mode.

We used our EMAT to measure the variation in arrival

times on regions next to the sawcut in our wheel. For these

cases, the hoop stress (assumed much larger than the

radial stress) vanished, so differences in arrival times are due to texture induced by casting the wheel. We found that

when the SH wave was polarized in the radial direction, it had the fastest arrival time: the slowest arrival time

occurred for polarization in the circumferential direction.

Furthermore, the arrival time displayed an approximately

sinusoidal variation with polarization angle. This satisfies a necessary condition for the wheel texture (averaged

through the rim thickness) to be orthotropic. The material

symmetry axes for the rim appear to be oriented in the

thickness, radial and circumferential direction.

This allows us to use the current theories of acoustoelasticity. which assume orthotropic symmetry3,4

. The

birefringence is related to stress through Equation (3). The

angle 0 between pure mode polarization directions

(acoustic axes) and the material symmetry axes is related

to stress through Equation (2). We measured the angular variation of arrival time at a stressed location and again found that the fastest and

slowest arrival times occurred for SH waves polarized

along the radial and circumferential directions, respectively. Since the acoustic axes still correspond to the

material symmetry directions (0 = 0), Equation (2) shows

that there is no shear stress ( o^ = 0). The relation between

birefringence and stress can be simplified to

B - B0 = CA(oe - ot ) (6)

To obtain the hoop stress, (assuming neligible or ) we

must first obtain the unstressed birefringence (which is due to casting induced texture). We were able to do this for

the sawcut wheel by measuring the birefringence on either

side of the sawcuts (locations 1 and 2). We did this with

both the EMAT and PZT transducer and found a small

difference ( % 1.0 x 10

-4

) in B0 as measured by the two

transducers.

We measured the birefringence at different locations

around the circumference of the wheel. To avoid damage

to the PZT transducer, we had certain regions of the rim

milled and then hand polished. We measured B at these

regions (1-6) with both types of transducers and found

that the EMAT values were usually slightly larger than the

PZT values ( % 1.0 x 10~4

). We also measured B at other

locations with just the EMAT. The rim was subjected to various kinds of surface preparation at these locations.

We did this so that we might characterize the relation

between quality of EMAT measurements and degree of

surface preparation.

Finally, we converted our values of B—

0

to hoop

stress, and compared the EMAT and PZT results. We

found that the peak stress occurred at the 180° position

(opposite the sawcuts), as expected. The peak stress as obtained by the EMAT was within 10 MPa of the stress obtained by the PZT transducer. We also obtained good

agreement at the other milled locations as well. The stress displayed a high degree of symmetry about the 180°

position, indicating that the texture is axisymmetric, and

that the hoop stress was axisymmetric before sawcutting.

The EMAT results also indicated that reliable measure- ments of B could be obtained with minimal amounts of

surface preparation, e.g. using a hand grinder.

The good agreement between the EMAT and PZT

results indicates that in field applications, the former can

be used instead of the latter. This is important, since the

EMAT requires less surface preparation, operates with a simpler mounting fixture, needs no couplant. and can be

easily scanned and rotated. Some of the disadvantages of

the EMAT (such as poor signal-to-noise ratio) have been overcome with our system, which uses carefully matched

electronics to minimize noise. Furthermore, the timeinterval averaging feature of our velocity measurement

system allows us to operate in an RF environment which

approximates conditions in the field. Clearly, much remains to be done in the future. We

hope to make additional measurements on uncut wheels

to determine, for example, whether the stress and texture

are axisymmetric. We also will measure the birefringence

as a function of radial position. We hope to develop a method of determiningfi0 on uncut wheels and. finally, to determine Os by ultrasonic results. Acknowledgements

The work was supported by the Federal Railroad

Administration, Department ofTransportation. under the

supervision of C. Orth. Research Manager. We were

greatly assisted by the generous co-operation of Mr Britto

Rajkumar of the American Association of Railroads.

Pueblo. CO. who provided both the wheel and encouragement. This Paper is a contribution of the National Bureau

of Standards and is not subject to copyright.

References

1 Fukuoka, H., Toda, H., Hirakawa, K., Sakamoto, H. and Toya, Y. Non-destructive assessments of residual stresses in railroad wheel rim by acoustoelasticity JEng Industry (ASME) submitted

for publication

2 Fukuoka, H., Toda, H., Hirakowa, K., Sakamoto, H. and Toya, Y.

Ultrasonics 1986 Vol 24 September 287

23
Residual stress and texture in railroad wheels: AV. Clark et a!.

Acoustoelastic measurements of residual stresses in the rim of

railroad wheels, in: Wave Propagation in Inhomogeneous Media

and Ultrasonic Nondestructive Evaluation Vol 6 (Ed. Johnson, G.C.)

ASME (1984)

3 Iwashimizu, Y. and kubomura, K. Stress-induced rotation of

polarization directions in slightly anisotropic materials Ini J

Solids Structures (1973) 9 99-1 14 4 Okada, K. Stress-acoustic relations for stress measurements by

ultrasonic techniques J Acoust Soc Jpn (E) (1980) 1 (3) 1 93— 200

5 Association of American Railroad Specification, Interchange

Rule 41 6 Frost, H.M. Electromagnetic-ultrasound transducer: principles. practice and applications" in: Physical Acoustics Vo! XIV

(Eds Mason. W.P. and Thurston, R.N.), Academic Press, New

York. USA (1979)

7 Jackson, C.W. Characterization of stressed metals using

acoustic shear waves PhD Dissertation Stanford University, USA

(1984)

8 Sajkumar, B. American Association of Railroads. Pueblo. CO,

USA. personal communication (1985) 9 Clark, A.V. and Moulder, J.C. Residual stress determination in aluminium using electromagnetic acoustic transducers

Ultrasonics (1985) 23 253-259

Appendix

EMATs have a relatively low signal-to-noise ratio. Consequently, our time-interval probes will often trigger

(generate a square wave pulse) on noise as well as on the zero crossings in the echo. It is necessary to reject these

spurious pulses when measuring arrival times. On thin

aluminium plates, we had found that using an analog gate was a satisfactory solution9

. However, this solution was

found to be unsatisfactory for thick steel sections such as the rim. We therefore adopted the following approach.

Rather than gate out the echo and send the gated signal to the probe, we simply gated out the desired probe pulses

from the undesired pulses (The former pulses, of course, correspond to zero crossings in the echo, and the latter to noise.) The gating was accomplished using digital logic

circuits rather than analog circuits. This resulted in less

To

Counter

Figure 8 Block diagram of digital delay gate

signal distortion (to avoid changing the apparent arrival

time).

The circuits (shown in Figure 8) work as follows.

Voltages from the probes were converted from the range

-0.5 to +0.5 V to digital logic circuit levels (TTL) by using a

fast preamplifier level shifter. The preamplifier had a

typical rise time of ^ 6 ns so that it could follow the rapid

rise times of the probe pulses. After a time delay (which we

can vary), a ‘one-shof device is triggered. This causes the

voltage on one terminal of a logic gate (AN D gate) to be at logical level T (+5 V). The signal from the preamplifier is input to the other gate terminal. When the probe generates

the leading edge of a square wave pulse (and the pre- amplifier correspondingly generates the leading edge of a TTL pulse), both terminals of the AND gate will be at logical level T and the gate output will also be at logical

level " 3

' (5 V). When the probe generates the falling edge of

the square wave pulse, the gate output returns to logical

level ‘0 (0 V). Consequently, the AND gate output

essentially replicates the probe pulses. The gate output

goes to the STOP channel of the counter. The trigger level of the counter can be set to stop the counter when it receives a voltage anywhere in the level -5 to +5 V. We

adjusted the trigger level to obtain the optimum precision

(better than ±1 ns) in arrival times.

288 Ultrasonics 1986 Vol 24 September

24
Review of Progress in Quantitative NDE , eds. D. 0. Thompson and D. E. Chimenti

(Plenum Press, NY, 1988), Vol. 7B,

1661-1668.

FLAW DETECTION IN RAILROAD WHEELS USING RAYLEIGH -WAVE EMATS

R. E. Schramm, A. V. Clark, Jr., D. V. Mitrakovic , ** and P. J. Shull

Fracture and Deformation Division

National Bureau of Standards

325 Broadway

Boulder, Colorado 80303

INTRODUCTION

Early work at the University of Houston [1] showed the efficacy of

Rayleigh waves for detecting cracks in the tread of railroad wheels.

Development work has been going on at the Fraunhofer Institute in Saarbrucken

(IzfP) on an automated system using EMATs that could be installed in a railyard and used in a roll-by mode [2]

A prototype of their system is now

operational in Luxembourg.

European railroad wheels are generally forged while U.S. wheels are cast.

Also, the IzfP system characterizes flaws in locomotive wheels; our system

inspects rolling stock (freight cars). Because the tread and track shapes

differ between U.S. and European equipment, the U.S. Department of

Transportation Federal Railroad Administration started a program to develop

an on-line system for use on American equipment. Our previous work with

EMATs has demonstrated their usefulness in generating long-wavelength SH waves to detect and size planar flaws in weldments [3,4].

EXPERIMENTAL

Our EMAT transducer was actually a separate transmitter and receiver for

pitch-catch operation. As noted in Fig. 1, the transmitter meanderline was a printed circuit on polymer film. The conductor was 1-mm wide and the 6 -mm period corresponds to the desired wavelength at 500 kHz. The choice of a five cycle length was based on the curvature of the wheel and the space

required to mount the EMAT in the track. The geometry of the receiver coil

was identical. This meanderline, however, was laid-up with wire on acetate

based adhesive tape. The enamel coated wire was 0.14 mm in diameter and

series-wound through the pattern six times. This multiplicity added to the

receiver sensitivity. Both coils were flexible to conform to the wheel shape

and achieve the best electromagnetic coupling. We laid these two units atop

one another and then shifted them along their length by a quarter period to

keep the elements of one from shielding the other.

** NBS guest worker, on leave from the University of Belgrade, Belgrade,

Yugoslavia

1661

25
6 MM 1 MM

TRANSMITTER

Printed circuit

ON POLYMER FILM

RECEIVER

Wire-wound (6X)

ENAMEL COATED WIRE, 0.14mm,

ON ACETATE TAPE

Fig. 1. Configuration of transmitter and receiver meanderlines on flexible

substrates. They were placed on top of each other with a 1/4

wavelength lateral shift to prevent mutual shielding.

Figure 2 is an exploded sideview of the entire EMAT package. The two

meanderlines were closest to the wheel (generally with a thin plastic sheet

between for mechanical protection of the coils). Next was a thin (about 4 mm uncompressed) flexible polymer foam to allow good contact pressure against

the curved tread. The compliance of this foam should also help with the

problem of wheel profile changing with wear. The 1-mm thick aluminum sheet

had several purposes. It was the pressure plate acting on the coils through

the foam layer. The aluminum served as an eddy current shield to prevent any

ultrasound generation in the magnet. This sheet was also part of a box which

served as mechanical support for the overall system as well as container and

handle for the magnet. The single Nd-Fe-B magnet was 52*mm long, 26-mm wide,

and 31 -mm high. Since the wheel is ferromagnetic, the magnet is strongly

attracted and provides considerable pressure to hold the coils tightly

against the tread. The field was normal to both the coils and the wheel rim.

This transducer is bidirectional, generating or receiving Rayleigh waves traveling in both directions around the tread circumference.

The block diagram in Fig. 3 shows the relatively simple system used for

these preliminary measurements. A function generator provides the rf signal

for the gated MOSFET power amplifier which drove the transmitter. For these

measurements, the input current to the transducer was ten cycles at 500 kHz

and limited to 120 A peak- to-peak. (Present system maximum is 140 A.)

Careful impedance matching of both coils on the wheel was necessary to ensure maximum efficiency. The preamplifier and amplifier were tuned devices with

very low noise capable of maintaining a good signal - to-noise ratio. In the

present system, only the function generator and the oscilloscope are commercial devices.

1662

26
Nd-Fe-B

Magnet

Aluminum sheet

EDDY CURRENT SHIELD

AND SUPPORT

Foam sheet

CURVATURE COMPLIANCE

Receiver \ shifted 1/a

Transmitter j wavelength

Fig. 2. Exploded sideview showing sequence of transducer parts.

RAYLEIGH-WAVE EMAT SYSTEM

Function

Generator

Gated

Power Amplifier

Impedance

Matching

Rayleigh Wave

Preamplifier

£ Tuned

Amplifier

H

Impedance

Matching

Receiver EMAT

X

L———

Transmitter Emat Rayleigh Wave

Fig. 3. Block diagram of laboratory system.

1663

27
RESULTS

To characterize the ultrasonic beam from our EMAT, we cut a shallow

circular slot into a flat carbon-steel plate and made several measurements

of the signal reflected from this flaw. These involved both translating the

EMAT parallel to the slot at 16- and 32-cm separations and also rotating it

at the 32-cm location. The results indicated that the beam is about 9 degrees wide (6-dB down points). On repeating the rotational sequence with

the transducer turned end-for-end, the results were virtually identical and

showed the truly bidirectional nature of this device.

With the EMAT on an unflawed railroad wheel, the signal traveled

unimpeded around the rim. It was possible to observe at least 14 round trips

before the signal decayed into the noise level. With a wheel circumference

of about 2.6 m, this meant we could watch the signal travel more than 36 m.

With a small slot in the rim only 1-mm deep, it was possible to see a distinct reflection. However, the critical cracks for U.S. wheels are about

6 -mm deep. Therefore, we are interested in distinguishing between cracks of

greater depth (no longer safe) and those of lesser depth (satisfactory for

continued use). Accordingly, we sawcut an initial circular flaw into the

center of the tread along a wheel radius with a maximum depth of 5.6 mm and a surface length of 38 mm.

While EMATs are non-contact devices, the signal strength does decrease

rapidly with any increase in separation from the specimen. Since an inspection system in a railyard would have to cope with a wide range of

conditions during actual use, we examined the decrease in the signal- to-noise

ratio (S/N) with increasing liftoff (Fig. 4). The signal reflected from the

5.6-mm deep slot does drop dramatically but is still useful even at the 1.5- mm liftoff; probably, 1 mm or less should be possible in practice.

A typical oscilloscope trace taken from the sawcut wheel is in Fig. 5. The first signal, A, is a reflection of the Rayleigh wave which has traveled

about 30% of the circumference to the slot and back. Signals B and D have

traveled once and twice, respectively, around the entire rim. Signal C is a reflection of the beam which has traveled 70% of the circumference to the

slot and back. Since the tread was fairly rusty from exposure while in

storage, the transducer coupling varied slightly with location. (Note: after

sanding to clean the rust from a small section of tread there was virtually

no change in the data.) In railyard usage the tread will likely be bright

metal from rolling wear, but other factors such as dirt may cause variations

in liftoff. As a consequence, the round-trip signal, B, was used as a normalizing factor, i.e., the measurement parameter was the ratio of the

amplitude of flaw signals and first round-trip signal (A/B and C/B)

.

With a slot depth of 5.6 mm and length of 38 aun, we measured the

amplitudes of signals A, B, and C as a function of position on the wheel,

moving the transducer in 5-cm steps from one side of the flaw all the way

around to the other. The arrival time of signals B and D remained constant;

as the distance from the slot increased, signals A and C moved closer to B

until they coincided with it at the half-way point. On advancing the EMAT,

the signals crossed in arrival time and then receded from B. Obviously, the

signal timing indicates the flaw position relative to the probe.

The plot of normalized signal versus EMAT position in Fig. 6a has three

very distinct regions. The first section shows a very large negative slope

up to about 70 cm. With the beam divergence of 9 degrees noted above, the

ultrasonic energy has not spread out to the full tread width of 10 cm until

it has traveled 70-80 cm. Therefore, inside this distance the flaw

intercepts a very large fraction of this concentrated energy and generates a

1664

28
Fig- 4. Signal -to -noise

wheel

ratio as a function of separation between EMAT and

Fig. 5. Typical oscilloscope pattern on wheel.

B and D are round trip signals.

A and C are slot echos.

1665
c

&> CO

fr

*Dc

=3O

£EN.c

.fcP CO2

c

S5P CO

TD

O) .W

O

z

Slot Signal Travel Distance (cm)

b

Fig. 6. Signal amplitude as a function of transducer position

around the vheel circumference. The slot was located

at 0 cm (262 cm)

.

a) Slot depth - 5.6 mm.

b) Slot depth - 12.1 mm.

1666

30
very large reflection. Once the beam has spread out to full width, the flaw

and round-trip signals remain fairly constant until about 225 cm. At this

point the flaw again intercepts a large fraction of the ultrasonic beam. The

flaw consequently decreases the size of the round- trip signal and causes an increase in the signal ratio.

The signals traveling in the opposite directions should generate the same data. While this trend appears, there does seem to be a slight

systematic shift. Possibly there is a slight difference in the smoothness of

the two sides of the sawcut slot.

As a next step, we used the same saw blade to enlarge the slot to a depth

of 12.1 mm and a surface length of 53 mm. The repeat measurements are plotted in Fig. 6b and the data follow the same basic pattern as in Fig 6a.

For the deeper flaw, the initial high-slope section extends to approximately

100 cm. While there seems to be some increase in scatter, the signal level

is somewhat higher than for the shallower flaw; i.e., size discrimination

does seem possible. In both Figs. 6a and b, there seems to be a local

maximum occur ing at about 150 cm; we are currently seeking an explanation

for it.

To explore the feasibility of mounting a transducer of this general

design in a rail, the magnet was placed in a brass box and inserted into a machined recess in a short section of track. With the foam and coils taped

onto the box, the track assembly was placed in a position relative to the

wheel approximating that expected in service. Even with this crude form of

the transducer, the ultrasonic signals approached those seen with the

aluminum box mount and without the surrounding ferritic iron. The steel rail

does not seem to cause any deterioration in performance due to distortion of

the magnetic field. Furthermore, the liftoff between the tread and the

track-mounted EMAT does not seem to degrade the signal appreciably.

DISCUSSION

While these results are preliminary, they have been achieved in only a few months. They do indicate that the electronics and this transducer design

will generate a strong Rayleigh wave in a cast iron railroad wheel. On the

basis of two artificial flaws it appears likely that depth discrimination is

possible with a rather simple approach.

Two locations on the circumference, each about 10 -cm long, cannot be

inspected with the current design. One location is centered right at the

transducer and is the result of dead time due to the recovery of the receiver

amplifier following the transmitter pulse. Another is exactly half way

around the circumference where any flaw signal will coincide with the roundtrip signal.

In addition to the signal amplitude, it will be necessary to determine

the signal time in order to locate the fault (if desired) and determine which

portion of the sizing curve to use. Another possible flaw sizing parameter

is the decay rate of the round-trip signals: the deeper the flaw the more rapid the decay.

Some steps remaining to produce a useful system are:

1. Measure additional artificial flaw sizes at various locations across

the tread.

2. Inspect real cracks.

3. Determine size discrimination capability.

4. Develop a sizing algorithm for automatic operation.

31

1667
5. Develop a reliable in-rail transducer counting system.

6. Determine variability due to wheel position and liftoff.

7. Perform a field test.

ACKNOWLEDGEMENTS

This work was sponsored by the U.S. Department of Transportation Federal

Railroad Administration, Washington, D.C. Conversations with H. J.

Salzburger of IzfP were an invaluable guide in this work. We also wish to

thank the Transportation Test Center, American Association of Railroads,

Pueblo, Colo., and, in particular Britto Rajkumar, for advice and also

providing test wheels.

REFERENCES

1. D. E. Bray, N. G. Dalvi, and R. D. Finch, Ultrasonics H, 66 (1973).

2. H. J. Salzburger and W. Repplinger, in Ultrasonics International 83 . conference organizers Z. Novak and S. L. Bailey (Butterworth & Co.

Ltd., Kent, Great Britain, 1983), pp. 8-12.

3. R. E. Schramm and T. A. Siewert, in Review of Progress in Quantitative

NDE . edited by D. 0. Thompson and D. E. Chimenti (Plenum Press, New

York, 1986), Vol. 5B, pp . 1705-1712.

4. R. E. Schramm and T. A. Siewert, Mat. Eval. 44, 1136 (1986).
Proceedings : IEEE 1987 Ultrasonics

Sympos ium , ed. B. R. McAvoy (Institute

of Electrical and Electronic Engineers,

NY, 1988) 1079-1082.

ULTRASONIC CHARACTERIZATION OF RESIDUAL STRESS AND FLAWS IN CAST STEEL RAILROAD WHEELS

A. V. Clark, R. E. Schramm, H. Fukuoka

1

, D. V. Mitrakovid^

Fracture and Deformation Division

National Bureau of Standards

Boulder, Colorado 80303

ABSTRACT

Two ongoing safety problems in cast steel railroad

wheels are the buildup of residual stress in the

rim and the growth of cracks in the tread.

Electromagnetic-acoustic transducer (EMAT)

technology shows promise for the nondestructive

evaluation of both. The acoustoelastic effect

generates a small but measurable acoustic

birefringence in the presence of stress fields,

while Rayleigh waves are sensitive to surface

disruptions from flaws. Experimental

investigations of both problems indicate that these

noncontact inspection methods lend themselves to

automated use in a railyard.

Residual Stress Characterization

Tensile residual stresses are a significant factor

affecting railroad wheel integrity. Should a crack

occur in the rim, tensile stresses will act as crack-driving forces and that can lead to wheel

failure. Therefore a non-destructive testing

method for measuring residual stresses is highly

desirable. We present results of a study using

ultrasonic methods to characterize the stress state

in cast steel railroad wheels.

Stress in metals induces a small sound velocity

change. For isotropic materials, the normalized

difference in velocity of orthogonally polarized

shear-horizontal (SH) waves is proportional to the

difference of principal stresses [1, 2]. This

normalized difference in velocities is called the

acoustic birefringence, B:

e r r e

l /2( Ve

+ V

r

) " l/2(T

r + T@

)

where V@ and Vr are velocities of SH-waves

polarized in the hoop and radial directions

^NBS Guest Scientist; on leave from Osaka University

2 NBS Guest Scientist; on leave from University of

Belgrade

Contribution of National Bureau of Standards;

not subject to copyright

respectively, and Tr and Tq are the respective

signal arrival times. For anisotropic (textured)

materials, birefringence depends on texture and

principal stresses [1,2]:

B " B

o + ca

( *e ” V- (2)

where B 0 is an unstressed birefringence due to

texture, is the stress - acoustic constant and oq

and a T are principal stresses in hoop and radial

directions

Problems arising in using Eq . 2 for obtaining the

difference in principal stresses are: (1) the

influence of texture and (2) the small value of

stress - acoustic constant (C^ ~ 10~ 5 /MPa) . The

value of B 0 can be obtained by measurements on unstressed reference samples (provided that the

wheels are sufficiently homogeneous). Because of

the small value of the stress - acoustic constant, we use electronics capable of measuring arrival times

within ±1 ns in a 90 ps interval.

We investigated the feasibility of using

electromagnetic-acoustic transducers (EMATs) for

measurements in the field. The EMAT has the

advantage that it requires no acoustic couplant to

generate sound in a metal. Consequently, an EMAT can be scanned and rotated over the rim of the

wheel with ease. Being noncontacting, an EMAT may

require less preparation of the surface (where it

generates the sound) than a piezoelectric device.

To determine whether these potential advantages

could be realized, we performed a series of

experiments on a cast steel wheel. We measured the

birefringence with a commercial SH-wave EMAT in a pulse-echo setup using a simple velocity

measurement system (described elsewhere [3]). We

also measured the birefringence using a pie zoelectric transducer. The birefringence method

using this piezoelectric transducer has been

successful in determining residual stresses in

rolled wheels [4, 5]. Consequently, we used

results obtained with this transducer as a benchmark against which our EMAT measurements were compared

To characterize the effect of surface preparation

we made measurements for varying degrees of surface

preparation. For comparison we also have results

1987 ULTRASONICS SYMPOSIUM — 1079

33
obtained by the PZT transducer on a milled surface

at the same locations. There is a good agreement

between results obtained by both EMAT and PZT

transducers. Also, the change in EMAT

birefringence measurements for different surface

preparations is usually less than 2 x 10“4

. This

is equivalent to a stress uncertainty of about 25

MPa, assuming a value of -7.6 x 10_6 /MPa for the

stress -acoustic constant, C^. The good agreement

between EMAT measurements on surfaces with varying

degrees of surface preparation is quite

encouraging. From a practical point of view, it

minimizes the amount of time and labor necessary to

prepare the surface.

We were interested in determining whether there is a radial variation of birefringence. We measured

the birefringence at five different radii in each

quadrant. The results are shown in Fig. 1. The

data points are the mean value of measurements at

four circumferential positions, and the error bars

represent the standard deviation. The PZT and EMAT

results are in good agreement. The birefringence

has a gradient, becoming increasingly negative as the outer edge of the rim is approached. The

gradient is almost linear and, assuming a constant

texture, it has value of about 6 MPa/mm.

To determine the residual stress, it is necessary

to know Bc (see Eq. 2). We cut four sections out

of the rim, thereby relieving the geometric

constraints and any stresses; measurements were

made on these unstressed specimens to obtain B 0 as

a function of radial position (Fig. 2). We found

that B0 was more negative than B, at every radial

location. Since is negative, we find that the

hoop stress (dominant stress) is compressive. This

is in qualitative agreement with sawcutting

measurements made on this wheel by the American

Association of Railroads [6]. Comparison of the

stressed and unstressed measurements indicates a high residual stress gradient test is likely due to

a high thermal stress gradient which occurred

during drag braking.

Fig. 1. Birefringence as a function of the radial

position on the wheel.

0 -

•oK

O -5 -

-10 -

-15

° Stressed

• Unstressed

_L _L -8 -4 0 4 8 RADIAL POSITION, mm

Fig. 2 Comparison of EMAT birefrigence

measurements on the stressed wheel and or. wheel sections cut out of the rim to relieve residual stress.

Flaw Characterization

Railroad wheel flaws include cracks originating at or near the tread surface or in the flange, as well as "shelling" (peeling of the tread surface). Ue examined the use of Rayleigh-wave EMATs in a pitchcatch mode for possible noncontact wheel

inspection

.

Development work on a similar system has been going on at the Fraunhofer Institute in Saarbrucken

(IzfP) to produce an automated system using EMATs that could be installed in a railyard and used in a roll-by mode [7]. European railroad wheels are generally forged while U.S. wheels are cast. Also,

the IzfP system characterizes flaws in locomotive

wheels; our system inspects rolling stock (freight

cars). Because the tread and track shapes differ

between U.S. and European equipment, the U.S.

Department of Transportation Federal Railroad

Administration started a program to develop an on- line system for use on American equipment.

Our EMAT transducer was a separate transmitter and receiver for pitch-catch operation. The

transmitter meanderline was a printed circuit on polymer film with a 6 mm period which corresponds

to the desired wavelength at 500 kHz. The receiver

coil saeanderline was laid-up with wire on acetatebased adhesive tape. The enamel coated wire was series -wound through the pattern six times for

greater receiver sensitivity. Both coils were flexible to conform to the wheel shape and achieve

the best electromagnetic coupling. We laid these

two units atop one another and then shifted them

along their length by a quarter period to keep the elements of one from shielding the other [8].

1080 — 1987 ULTRASONICS SYMPOSIUM
In the laboratory EMAT package, the two

oeanderlines were closest to the wheel. Next was a thin flexible polymer foam to allow good contact

pressure against the curved tread. A 1 mm thick

aluminum sheet was the pressure plate acting on the

coils through the foam layer. The aluminum also

served as an eddy current shield to prevent any

ultrasound generation in the magnet. This sheet

was also part of a box which served as mechanical

support for the overall system as well as container

for the single Nd-Fe-B magnet. The field was

normal to both the coils and the wheel treads.

This transducer is bidirectional, generating or

receiving Rayleigh waves traveling in both

directions around the tread circumference.

In the electronic system, a function generator

provides the rf signal for the gated MOSFET power

amplifier which drives the transmitter. The input

current to the transducer was ten cycles at 500 kHz

and limited to 140 A peak- to-peak. Careful

impedance matching of both coils on the wheel was

necessary to ensure maximum efficiency. The

preamplifier and amplifier were tuned devices with

very low noise and so were capable of maintaining a good signal- to-noise ratio.

With the EMAT on an unflawed railroad wheel, the

signal traveled unimpeded around the rim. It was

possible to observe at least 14 round trips before

the signal decayed into the noise level. With a wheel circumference of about 2.6 m, this meant we

could detect the Rayleigh wave after it traveled

more than 36 m.

A typical oscilloscope trace taken from a saw cut

wheel is in Fig. 3. The first signal, A, is a flaw

reflection of the Rayleigh wave which has traveled

about 30% of the circumference to the slot and

back. Signals B and D have travelled once and

twice, respectively, around the entire rim. Signal

C is a reflection of the wave after traveling 70%

of the circumference to the slot and back in the reverse direction from signal A. To account for

possible variations due to liftoff (Fig. 4), the

signal we used for flaw characterization was the

slot reflection (A and C) divided by the round trip

signal (B).

As a first test, we examined this normalized flaw

signal as a function of the EMAT-flaw distance

around the tread. Measurements were made as the

transducer was moved circumferentially in 5 cm increments from one side of the flaw to the other

(Fig. 5). Signals A and C moved closer to B as the

distance increased until they coincided with it

when the transducer was directly opposite the saw

cut. Signal timing indicates relative flaw

position.

The Rayleigh wave EMAT has a directivity pattern

with the 3 dB-down points at about ± 4°. This

factor leads to the three signal regions apparent

in Fig. 5. Up to about 70 cm, the ultrasonic beam

had not spread out to the full tread width of 10

cm; this means that the 4 cm long flaw intercepted

a large fraction of the energy and produced a large

reflection (A). Conversely, as the EMAT approached

the flaw from the other side, the slot shielded

Fig. 3. Typical oscilloscope pattern on wheel. A

and C are slot echos. B and D are round

trip signals.

much of the round trip energy and gave a small

normalizing signal (B) . Consequently, on either

side of our saw cut, the normalized signal was very

large when the transducer was within 70 cm.

The Rayleigh waves traveling in opposite directions

should generate the same data. This trend appeared

in Fig. 5, but a systematic shift did emerge.

Perhaps there was a slight difference in the

smoothness of the two sides of the saw cut slot.

LIFTOFF (mm)

Fig. 4. While the signal/noise ratio drops

rapidly as the liftoff increases, it is

still useful over a considerable range of

separation between the transducer and the

specimen surface.

1987 ULTRASONICS SYMPOSIUM — 1081

35
References

Fig. 5. Signal amplitude as a function of flaw

position around the wheel circumference.

The simulated flaw was a circular saw cut

in the center of the tread at 0 cm (262

cm)

.

To explore the feasibility of mounting the

transducer in a rail, the magnet was placed in a brass box and inserted into a machined recess in a short section of track. With the foam and coils

taped onto the box, the track assembly was placed

in a position relative to the wheel approximating

that expected in service. With this form of the

transducer, the ultrasonic signals approached those seen with the aluminum box mount and without the

surrounding ferritic iron. The steel rail does not seem to cause any deterioration in performance due

to distortion of the magnetic field. Furthermore,

the liftoff between the tread and the track-mounted

EMAT does not seem to degrade the signal

appreciably.

1. Y. Iwashimizu, Y. , and K. Kubomura, l nt. j Solids Structures £, 99-114 (1973)'

2. K. Okada, J. Acoust. Soc . Japan (E) 1 i<n w

(1980).

A * J * 20c

3. A . V . Clark, H. Fukuoka, D.V. Mitrakovic, and J.C. Moulder, Ultrasonics 2J±, 281-288 (1986)

4. H. Fukuoka, H. Toda , K. Hirakawa, H. Sakanoto and Y. Toya , "Nondestructive Assessments of

’ Residual Stresses in Railroad Wheel Rim by Acoustoelasticity," to be published in J . 0 f Eng. for Industry, ASME, New York City.

5. H. Fukuoka, H. Toda, K. Hirakawa, H. Sakamoto

and Y. Toya, Acoustoelastic Measurements of Residual Stresses in the Rim of Railroad

Wheels, in: "Wave Propagation in

Inhomogeneous Media and Ultrasonic

Nondestructive Evaluation," AMD-Vol. 6, G.C Johnson, ed. , ASME, New York City (1984).

6. B. Rajkumar, Transportation Test Center,

American Assoc, of Railroads, Pueblo, CO,

private communication.

7. H. J. Salzburger and W. Repplinger, in

Ultrasonics Inte rnational 83 . conference

organizers Z. Novak and S. L. Bailey

(Butterworth & Co. Ltd., Kent, Great Britain

1983), pp. 8-12.

8. R.E. Schramm, A.V. Clark, Jr., D.V. Mitrakovid

and P.J. Shull, to be published in Review of

Progress in Quantitative NDE . edited by D.O. Thompson and D.E. Chimenti (Plenum Press, New York, 1988), Vol. 7.

Conclusions

Ultrasonic methods are viable tools for the NDE of

important fracture mechanics parameters in railroad

wheels. Acoustic birefringence is sufficiently

sensitive to determine residual stresses while

Rayleigh-wave reflections detect the presence of

rim cracks. EMAT systems for field use appear

practical

.

Acknowledgments

We are grateful for conversations with H.J.

Salzburger of IzfP. This work was sponsored by the

U.S. Department of Transportation Test Center,

American Association of Railroads, Pueblo,

Colorado, and, in particular, Britto Rajkumar for

advice and test wheels.

1082 — 1987 ULTRASONICS SYMPOSIUM
EMAT EXAMINATION FOR CRACKS IN RAILROAD WHEEL TREADS*

R. E. SCHRAMM, P. J. SHULL, A. V. CLARK, JR., D. V. MITRAKOVIC**

National

Fracture

Boulder

Bureau of Standards

and Deformation Division

Colorado 80303

INTRODUCTION

Submitted to:

Proceedings, Nondestructive

Testing and Evaluation for

Manufacturing and Construction,

Urbana, 111.

Aug. 9-12 , 1988

Cracks in railroad wheels may result from high stresses due to dynamic or

static loads and residual stresses generated by such events as heating

during braking. These flaws generally originate in the tread surface or

flange and can lead to catastrophic wheel failure resulting in

considerable equipment damage and possible derailment. This threat to

personnel safety and the potential costs in time and money are

inducements in the search for an effective, automated method for

nondestructive examination that will identify damaged wheels needing

replacement

The current method of examination is visual observation. In the early

1970' s, an ultrasonic method was introduced (1-4). This basically

involves an in-rail system of piezoelectric transducers generating

Rayleigh waves, electronics for signal generation and processing, and a method for tagging suspect wheels. The goal is to examine each wheel of a train as it rolls by a checkpoint in a railyard. There are currently two

of these systems in operation in this country. A companion portable handheld system subsequently verifies any flaw indications. The Fraunhofer

Institute (IzfP) in Saarbrucken, Federal Republic of Germany, has been

developing another ultrasonic system along this line (5) but using

electromagnetic-acoustic transducers (EMATs). IzfP has placed a prototype

into operation.

Our current research in this area (6, 7) is the development of an

ultrasonic system using EMATs designed to work on American- style wheels

and rails. The objective is to check every wheel on a train, also in a roll-by mode. Like the earlier works, our system will utilize Rayleigh

waves that travel around the wheel tread. We are using EMATs for two

main reasons: 1. No ultrasonic path in the transducer itself and low

sensitivity to phase - transformed signals mean a simplified, low-noise

signal that is relatively easy to interpret with high reliability, even

in an automated system; 2. Their non-contact nature eliminates the need

for ultrasonic couplants, i.e., no water sprays or liquid- filled boots.

Two extensive reviews of EMAT designs and applications have recently been

published (8,9)

^Contribution of the National Bureau of Standards and not subject to

copyright

**NBS guest scientist on leave from the University of Belgrade,

Belgrade , Yugoslavia.

37
EQUIPMENT

In the electronic system, a function generator provides the rf signal for

the gated MOSFET power amplifier which drives the transmitter. The burst

consists of 5-10 cycles at 500 kHz with a pulse repetition frequency (PRF)

of 60 Hz. The receiver preamplifier is a very- low-noise design.

Impedance matching is very important for both transmitter and receiver to

ensure maximum efficiency.

Our EMAT device is designed for insertion into a recess cut out of the

rail. Therefore, one goal in the transducer design is to keep the

configuration as compact as possible to minimize both the amount of

required machining in the rail and the loss of weight-bearing surface.

Toward this end, we use a single permanent magnet (Nd-Fe-B for maximum

field strength) 32 mm X 26 mm X 52 mm with the magnetization direction

along the 32 mm direction and oriented normal to the tread. For pitchcatch operation, there are separate transmitter and receiver coils.

These are meanderlines with a periodicity of 6 mm to generate a Rayleigh

wave with this wavelength at 500 kHz (velocity — 3 km/s) . The two coils

are stacked with a slight shift to prevent their shielding each other,

and then laid on top of the magnet to contact the wheel tread (Fig. 1).

To prevent signal complications from ultrasound production in the magnet,

we place a thin foil of copper or aluminum between the coils and the

magnet to minimize eddy current generation in the Nd-Fe-B. The original

coil design contained five cycles since this length was likely to

correspond to the expected length of contact with the curved wheel.

The receiver coil is AWG 36 enamel -coated wire that has been wound

through the pattern six times on acetate -based adhesive tape. This

multiplicity adds to the sensitivity since the repetitions are seriesconnected. The flexible tape allows the coil to conform to the wheel's

curvature

.

Nd-Fe-b

Magnet

(a)

EDDY CURRENT SHIELD

Foam sheet

CURVATURE COMPLIANCE

Receiver \ shifted 1/a

Transmitter » wavelength

Wheel

FIGURE 1. Rayleigh-wave EMAT construction.

(a) Two meanderline coils, shifted to prevent shielding, placed atop a magnet with field H.

(b) Exploded sideview showing sequence of transducer parts.

38
The transmitter coil has taken two forms. The first is a printed circuit

on polymer film. The conductor is about 1 mm wide and 0.025 mm thick.

In this form, the impedance is very low (about 1.3 including a current

limiting resistor) . Our power amplifier can deliver about 140 A of rf

current into this coil. The second form is identical to the wire -wound

receiver, and the much higher impedance limits the drive current to only

about 30 A.

The greater the current flow through the transmitter, the greater is the

eddy current induced in the specimen and the greater is the ultrasound

generated. While the printed circuit coil permits the maximum current,

the wirewound coil passes the current through the system multiple times

and the current density (A/mm^) induced in the specimen is roughly the

same, i.e., the size of our ultrasonic signal is nearly identical. Using

the lower current coil will increase the longevity of the power

amplifier, and will allow an increase in the pulse repetition frequency

(PRF) should this prove desirable later. At present, the limit on the

PRF is due to two factors in the power amplifier: ohmic heating of the

MOSFET output and the time required to recharge the power supply

filtering capacitors.

While EMATs are noncontact devices, their efficiency decreases rapidly

with liftoff. To assure the greatest coupling with the curved wheel

tread, we have been inserting a few millimeters of compliant material

between the magnet and the coil pair. This material compresses under the

weight of the wheel and the flexible coil backing assumes the tread

shape

By its design, this transducer should be bidirectional and generate

identical sound pulses from both ends of the coil. Their travel

direction is normal to the legs of the meanderline. Experimental

measurements on a flat steel plate with a saw-cut notch confirmed that

this was the case. Translating and rotating the transducer on this same

plate indicated that the ultrasonic energy radiates in a beam spreading

at about 8-9° (using the 3-dB down points). Locating the sensor midway

in the tread means that the signal must travel approximately 70 cm before

it irradiates the entire width of the tread.

EXPERIMENTAL RESULTS

For the initial wheel measurements, the transducer was mounted in a small

aluminum box and moved manually around a stationary wheel. On a used but

unflawed wheel, this arrangement produces a signal which is still

detectable after 14 round trips or more than 36 m of travel.

A small cut only about 1 mm deep generated a distinct reflection. The

critical flaw depth, however, is about 6 mm so our interest is in

distinguishing between cracks of greater aepth (no longer safe) and those

more shallow (safe for continued use). Accordingly, we sawcut an initial

circular (half-penny) flaw into the center of the tread along a wheel

radius. This flaw depth into the wheel was 5.6 mm at the maximum; the

length was oriented along the tread width and 38 mm long at the surface.

In any field measurements, there will be many physical variables that may

change the transducer-wheel coupling; among them will be variations in

the tread profile due to different degrees and types of wear. Another

factor is the taper of the wheel tread, i.e., the tread and the top

39
surface of the rail are not parallel. The use of compliant material

under the coils to allow them to conform to each wheel will minimize any

separation. Figure 2 gives an initial indication of what the allowable

limits may be. In this case the echo amplitude from the 5 . 6 mm slot

yielded the signal- to-noise ratio (S/N) as a function of the liftoff from

the surface. An S/N of 2.5, while rather small, ought to be workable.

This lower limit on S/N corresponds to a liftoff of 1.5 mm which should

be quite possible to obtain in most circumstances. These preliminary

data show that the test system will be useful even before future

refinements

.

To determine how far a wheel can move and still return a useful signal,

we first built a wooden form to simulate the track surface and then

examined the signal strength as the transducer moved past the wheel

contact point. Again using a 3-dB down point as the outer limits, a

usable signal should be available during at least 2 cm of train travel.

This is the space window in which the wheel must be located to both

transmit and receive our test signal. While it would be desirable to

introduce multiple pulses into the wheel in order to improve statistics

for a higher signal- to-noise ratio, this may not be feasible since the

Rayleigh wave travels multiple times around the circumference. To avoid

processing confusion, it will be necessary to delay any new pulse until

the prior one has sufficiently decayed. Since each round trip takes

nearly a full millisecond, this means a delay interval of at least 10 ms.

To get even two test pulses into the wheel (one at the beginning and end

of the 2 cm window) means the train cannot travel faster than 2 cm/10 ms

or about 7 km/h. This would be unacceptably slow for a field system so

we shall have to settle for a single shot. A likely scenario is to use a

trigger device to start the pulse as the wheel enters the window.

Recording just the data arriving before the return of second round trip

signal (see below) takes nearly 2 ms so the maximum train speed could be

about 2 cm/2 ms, or 36 km/h. This is much better and likely improvement

in the window length could increase the allowable speed.

LIFTOFF (mm)

FIGURE 2. Signal- to-noise ratio as a function of separation between EMAT

and wheel.

40
As noted above, the transducers are bidirectional. Figure 3 indicates

schematically the signal paths to a flaw and the resulting scope trace.

Signals A and C are echos that have traveled about 30% and 70% of the way

around the wheel circumference to a flaw and then back. B and D are

first and second round trip signals. Frequently some splitting occurs in

these signals and this likely has at least two sources. Each round trip

signal is a combination of counter-rotating signals. The path length is

quite long (about 2.6 m) and some phase incoherence between these two

becomes likely over this distance before they again combine on arriving

back at the transducer. Also, the Rayleigh wave has some tendency to

spread out to the wheel flange where it has a somewhat longer path than

along the tread. The amount of ultrasonic energy in a wheel depends on

the closeness of EMAT coupling and the wheel condition, e.g., grain size

and residual stress state. Consequently, we are presently using the

amplitude of the first round trip signal as a normalizing factor; the

amplitude ratios A/B and C/B are our current flaw measurements.

To determine how the flaw position relative to the transducers affects

the ultrasonic data, we measured the amplitudes of signals A, B, and C at

5 cm intervals around the entire circumference of the wheel. To obtain

an idea of the sizing capability of this approach, the data were repeated

for two flaw sizes, both circular sawcuts in the center of the tread;

one was 5.6 mm deep and 38 mm long at the surface, and the other was the

same cut enlarged to 12.1 mm deep and 53 nun long.

Data for the small flaw are in Fig. 4 and show three distinct regions.

The first has a very large negative slope up to about 70 cm. With the

beam divergence noted above, the ultrasonic energy needs this distance to

spread out to the full tread width of 10 cm. Inside this distance, the

flaw intercepts a very large fraction of this concentrated energy and

generates a very large reflection. Once the beam has spread out to full

width, in the second region, the flaw and round-trip signals remain

fairly constant. In the third region, past about 225 cm, the flaw

intercepts a large fraction of the counter-rotating ultrasonic beam;

from this side, the flaw blocks a large portion of the round-trip signal

and so increases the signal ratio.

EMAT

(a)

FIGURE 3. Flaw echoes.

(a) Schematic of bidirectional Rayleigh-wave travel to a flaw. A is the

short path and C is the long path.

(b) Typical oscilloscope signal. A and C are short and long path flaw

echoes. B and D are first and second round trip signals.

41
TOC

DO

CO

CL

£

CO

TOC

DC

in

TD

&>

TO

Slot Signal Travel Distance (cm)

FIGURE 4. Signal amplitude as a function of flaw position around the

wheel circumference. The simulated flaw was a saw cut in the center of

the tread at 0 cm (262 cm). The two curves indicate data from signals traveling in opposite directions.

The data from the larger flaw show the same general trend (7) although

the center region had somewhat higher values. This encourages our

expectations that we will be able to achieve meaningful size

discrimination.

The signals traveling in the opposite directions should generate the same

data. While this appears basically true (the solid and dotted lines in

Fig. 4 are close to each other), there does seem to be a slight

systematic shift. Possibly there is some difference in the smoothness of

the two sides of the sawcut.

Signal B's arrival time remains constant, of course, while A and C move

closer to B as the EMAT-flaw distance increases until they all coincide

when the flaw is exactly opposite the transducer. Thus, there are two

zones on the circumference where meaningful measurements are not

possible. Both of these are about 24 cm long; one is centered at the

transducer and the other is exactly opposite. The first is due to

recovery of the receiver amplifier following the transmitter pulse; the

second is due to the merger of the round-trip and flaw signals when their

acoustic path lengths become identical. As a result, approximately 80%

of the tread can be inspected with each pass.

42
FUTURE WORK

We have recently constructed a short (4 m) section of track in which to

mount the transducer (Fig. 5). Rolling an actual wheel set over the

device will much more closely simulate field conditions and permit a

reasonable debugging of several parts of the system: mechanical

mounting, compliant coil backings, geometric variations in the size and

position of magnet and coils, triggering (if necessary), and signal

collection and storage. A new meanderline with 8 cycles (69% longer)

will take advantage of the full length of our largest magnet. Testing

will show if this will increase the space window for injecting the

ultrasonic pulse and so increase the allowable train speed. With

multiple wheels now available, it will be possible to generate a variety

of artificial flaw sizes in the pursuit of a workable sizing algorithm.

Since the wheels have a few centimeters of side-to-side play in normal

operation, this rolling test will also show us the importance of the

wheel position with respect to the transducer.

Automatic signal processing will be necessary before moving the system to

a railyard to look for actual cracks in the field.

ACKNOWLEDGMENTS

We are grateful for discussions with H. J. Salzburger of IzfP as well as

John Cowan and Richard Conway of The Fax Corporation. This work was

sponsored by the U.S. Department of Transportation, Federal Railroad

Administration, Washington, D.C. We also wish to thank the

Transportation Test Center, American Association of Railroads, Pueblo

Colorado, and, in particular, Britto Rajkumar and Dominic DiBritto for

providing test wheels.

FIGURE 5. Current test configuration.

(a) Standard wheel set rolled along a short length of track.

(b) EMATs mounted in rail. A protective film covers the coils and they

are represented schematically in this photo.

43
REFERENCES

1 .

2 .

3.

4.

3 .

6 .

7.

8 ,

9.

Inspect Wheels "In Motion," Progressive Railroading . Jan., 1974.

Spotting the Defects in Railroad Wheels, Business Week , p. 44, Sept.

28, 1974.

Cracked-Wheel Detector "Could Save Millions," Railway Systems

Control . May, 1975.

U.S. Patent 3,978,712: Sept. 7, 1976, John Vincent Cowan, Gerald De

G. Cowan, and John Gerald Cowan.

Salzburger, H. J., and Repplinger, W. , Ultrasonics International 83 . conference organizers Z. Novak and S. L. Bailey (Butterworth & Co.

Ltd., Kent, Great Britain), pp . 8-12, 1983.

Clark, A. V., Schramm, R. E. , Fukuoka, H. , and Mitrakovic, D.V.

,

Ultrasonic Characterization of Residual Stress and Flaws in Cast

Steel Railroad Wheels, Proceedings. IEEE 1987 Ultrasonics Symposium.

Vol . 2 . B. R. McAvoy, editor (Institute of Electrical and Electronic

Engineers, New York), pp . 1079-1082, 1987.

Schramm, R. E. , Clark, Jr., A. V., Mitrakovic, D. V., and Shull, P.

J., Flaw Detection in Railroad Wheels Using Rayleigh-Wave EMATs

,

Review of Progress in Quantitative Nondestructive Evaluation. Vol.

7B . D. 0. Thompson and D. E. Chimenti, editors (Plenum Press, New

York), pp. 1661-1668, 1988.

Maxfield, B. W. , Kuramoto, A., and Hulbert, J. K. , Evaluating EMAT

Designs for Selected Applications, Materials Evaluation , vol. 45, no.

10, pp. 1166-1183, 1987.

Alers , G. A., and Burns, L. R. , EMAT Designs for Special

Applications, Materials Evaluation , vol. 45, no. 10, pp . 1184-1194,

1987

44
A nearly identical paper entitled "Crack

Inspection of Railroad wheel Treads by

EMATs" was submitted to Proceedings

Third International Symposium on Nondestructive Characterization of Materials , 3-6 Oct., 1988, Saarbriicken, Federal

Republic of Germany.

Submitted to: Review of Progress in Quantitative NDE , eds. D. 0. Thompson and D. E. Chimenti

(Plenum Press, NY, 1989), Vol. 8.

EMATS FOR ROLL- BY CRACK INSPECTION OF RAILROAD WHEELS*

R. E. SCHRAMM, P. J. SHULL,

A. V. CLARK, JR., and D. V. MITRAKQVIC**

National Institute of Standards and Technology

Fracture and Deformation Division

Boulder, Colorado 80303

INTRODUCTION

Railroad safety depends on many factors. The integrity of the

wheels on rolling stock is one that is subject to nondestructive

evaluation. For some years, ultrasonic testing has been applied to the

detection of cracks in wheel treads, with particular attention to

automatic, in-rail, roll -by methods. We have begun constructing a

system aimed at using relatively low frequency Rayleigh waves generated

by electromagnetic-acoustic transducers (EMATs) . The current design

uses a permanent magnet to maintain a compact structure and minimize

the size of the pocket machined into the rail. Measurements thus far

indicate a responsiveness, even to small flaws. With the development

of a signal processing and analysis system, field tests should soon be

possible

Cracks in railroad wheels may result from high stresses due to

dynamic or static loads and residual stresses generated by such events

as heating during braking. These flaws generally originate in the

tread surface or flange and can lead to catastrophic wheel failure

resulting in considerable equipment damage and possible derailment.

This threat to personnel safety and the potential costs in time and

money are inducements to search for an effective, automated method for

nondestructive examination that will identify damaged wheels needing

replacement

The current method of examination is visual observation. In the

early 1970's, an ultrasonic method was introduced [1-4]. This involves

an in- rail system of piezoelectric transducers that generate Rayleigh

waves, electronics for signal generation and processing, and a method

for tagging suspect wheels . The goal is to examine each wheel of a

train as it rolls by a checkpoint in a railyard. Two of these systems

* Contribution of the National Institute of Standards and Technology

(formerly National Bureau of Standards), not subject to copyright.

** Guest scientist on leave from the University of Belgrade, Belgrade,

Yugoslavia

45
are currently in operation in this country. A companion portable handheld system subsequently verifies any flaw indications. The Fraunhofer

Institute (IzfP) in Saarbrucken, Federal Republic of Germany, has been

developing another ultrasonic system along this line [5], but using

electromagnetic-acoustic transducers (EMATs). IzfP has placed a

prototype into operation.

Our current research in this area [6-8] is the development of an

ultrasonic system using EMATs designed to work on American- style wheels

and rails. The objective is to check every wheel on a train, also in a

roll -by mode. Like the earlier systems, ours uses Rayleigh waves that

travel around the wheel tread. We are using EMATs for two main

reasons: 1. No ultrasonic path in the transducer itself and low

sensitivity to mode -converted signals mean a simplified, low-noise

signal that is relatively easy to interpret with high reliability, even

in an automated system; 2 . Their noncontact nature eliminates the need

for ultrasonic couplants, i.e., no water sprays or liquid- filled boots.

Two extensive reviews of EMAT designs and applications have recently

been published [9, 10].

EQUIPMENT

In the electronic system, a function generator provides the rf

signal for a gated MOSFET power amplifier which drives the transmitter.

The toneburst consists of 5-10 cycles at 500 kHz with a pulse

repetition frequency (PRF) of 60 Hz. The receiver preamplifier is a

very low noise design. Electronic impedance matching is very important

for both transmitter and receiver to ensure maximum efficiency.

Our EMAT device (Fig. la) is meant for insertion into a recess cut

out of the rail. Therefore, one goal in the transducer design is to

keep the configuration as compact as possible to minimize both the

Fig. 1. Rayleigh-wave EMAT construction.

(a) Two meanderline coils, shifted to prevent shielding,

placed atop a magnet with field H, A foil eddy current

shield lays over the magnet and a thin, compliant foam

sheet is under the coils.

(b) Details of the wirewound meanderline.

46
amount of required machining in the rail and the loss of weight-bearing

surface. Toward this end, we use a single permanent magnet (Nd-Fe-B

for maximum field strength) 32 mm X 26 mm X 52 mm with the

magnetization direction along the 32 mm direction and oriented normal

to the tread. For pitch-catch operation, there are separate

transmitter and receiver coils. These are meanderlines with a

periodicity of 6 mm to generate a Rayleigh wave with this wavelength at

500 kHz (velocity - 3 km/s). This design generates and receives

bidirectional Rayleigh waves traveling normal to the coil legs.

Rather than stacking the transmitter and receiver coils directly

atop one another, we shifted them by a quarter period so they do not

shield each other. To prevent signal complications from ultrasound

production in the magnet, we cover it with a thin foil of copper or

aluminum to minimize eddy current generation in the Nd-Fe-B. Between

this shield and the coils are a few millimeters of compliant material

(currently, polymer foam); this layer compresses under the wheel's

weight and their flexible substrate allows the meanderlines to conform

to the curvature and taper of the wheel tread. This minimizes liftoff

for maximum efficiency and signal/noise.

The present coil design contains eight cycles or loops in the

meanderline. The receiver coil is AWG 36 enamel-coated wire that has

been wound through the pattern six times on acetate -based adhesive tape

(Fig. lb). This multiplicity adds to the sensitivity since the

repetitions are series-connected. The flexible tape allows the coil to

conform to the wheel's curvature.

We've constructed the transmitter coil in two forms. The first is

a printed circuit on polymer film. The conductor is about 1 mm wide

and 0.025 mm thick. In this form, the impedance is very low (about 1.3

0 including a current limiting resistor) . Our power amplifier can

deliver about 140 A of rf current into this coil. The second form is

identical to the wire -wound receiver, and the much higher impedance

limits the drive current to about 30 A.

The greater the current flow through the transmitter, the greater

is the eddy current induced in the specimen. While the printed circuit

coil permits the maximum current, the wirewound coil passes the current

through the EMAT aperture multiple times and the current density

(A/mm2

) induced in the specimen is roughly the same, i.e., the size of

our ultrasonic signal is nearly identical with either form of

transmitter. Using the lower current coil will increase the longevity

of the power amplifier, and will allow an increase In the pulse

repetition frequency (PRF) to as much as 250 Hz, should this prove

desirable later. At present, the limit on the PRF is due to two

factors in the power amplifier: ohmic heating of the MOSFET output

drivers and the time required to recharge the power supply filtering

capacitors

EXPERIMENTAL RESULTS

We have recently constructed a short (4 m) section of track in

which we mounted the transducer (Fig. 2). Rolling an actual wheel set

(two wheels mounted on an axle) over the device much more closely

simulates field conditions than our initial static measurements [7],

The goal is to debug several parts of the system: mechanical mounting,

compliant coil backings, geometric variations in the size and position

47
of magnet and coils, triggering (if necessary), and signal collection

and storage. Since the wheels have a few centimeters of side-to-side

play in normal operation, this rolling test will also show us the

importance of the wheel's transverse position with respect to the

transducer

.

For these initial measurements, we had two wheel sets, well used

but nominally uncracked. The wheels were rolled onto the transducer

and held stationary or rolled over the EMAT site at a few kilometers

per hour. In both wheels of one set, this arrangement produces a

signal which was still detectable after 16 round trips or about 42 m of

travel. In both wheels of the other set, the signal traveled around

the tread only six times before being attenuated into the noise level.

Metallurgical variables due to such factors as manufacture, wear, etc.,

have a very large influence on the wheel's acoustic response. Any

signal analysis will have to account for this effect.

The critical flaw depth is about 6 mm, so our interest is in

distinguishing between cracks of greater depth (no longer safe) and

those more shallow (safe for continued use). Initially, however, we

saw-cut a very shallow circular flaw into the center of the tread along

a wheel radius. This flaw depth into the wheel was almost 2 mm at the

maximum; the length was oriented along the tread width and 12 mm long

at the surface.

To determine how far a wheel can move across the transducers and

still return a useful signal, we rolled the wheel in small steps across

the EMATs and measured the signal strength as the initial contact point

moved across the coils. This indicated that the effective EMAT

aperture within which the wheel must be located to both transmit and

receive our test signal is about 5-8 cm long. This aperture is

actually slightly longer than the coils because the wheel radius is

relatively so much larger, and the flexible coils conform to the tread

curvature. While it would be desirable to introduce multiple pulses

into the wheel in order to improve statistics for a higher signal- to

noise ratio, this may not be feasible since the Rayleigh wave travels

several times around the circumference. To avoid processing confusion,

it will be necessary to delay any new pulse until the prior one has

sufficiently decayed. Since each round trip takes nearly one full

millisecond, this means a delay interval of about 10-16 ms. To get

even two test pulses into the wheel (one at the beginning and end of a

5 cm window) means the train cannot travel faster than 5 cm/10 ms or

about 18 km/h. This would likely be unacceptably slow for a field

system, so we shall probably have to settle for a single transmitter

pulse. A likely scenario is to use a trigger device to start the pulse

as the wheel enters the window. Recording just the data arriving

before the return of second round trip signal (see below) takes nearly

2 ms , so the maximum train speed could be about 5 cm/2 ms, or 90 km/h.

There are other factors (signal processing time, timing accuracy of

pulse trigger, etc.) that will likely limit the allowable speed to

somewhere under this value.

There are possibilities for triggering mechanisms to signal the

proper moment during a wheel's transit to pulse the EMAT . For these

initial tests, we placed a simple membrane switch between the coils and

the magnet. The compliant foam transmitted the wheel pressure and

switch closure triggered both the high current pulser and a digital

oscilloscope to capture the transient ultrasonic signal from the single

pulse. At the low speeds possible on our short test track, this system

has been both reliable Wand consistent with our current spatial window.

48
The transducers are bidirectional and generate Rayleigh waves

traveling clockwise and counterclockwise around the railroad wheel

tread. Figure 3 indicates schematically the signal paths to a flaw and

the resulting scope trace. Signals A and C are echos that have

traveled about 22% and 78% of the way around the wheel circumference to

the 2 mm deep flaw and then back. Signals B and D are first and second

round trip signals. Signal A' is the short path echo after the initial

pulse has already traveled once around the entire circumference.

Frequently some splitting occurs in these signals and this likely has

at least two sources. Each round trip signal is a combination of

counter-rotating signals. The path length is quite long (about 2.6 m)

and some phase incoherence between these two becomes likely over this

distance before they again combine on arriving back at the transducer.

Also, the Rayleigh wave has some tendency to spread out to the wheel

flange where it has a somewhat longer path than along the tread. The

amount of ultrasonic energy in a wheel depends on the closeness of EMAT

coupling and the wheel condition, e.g., grain size and residual stress

state. Consequently, we are presently using the amplitude of the first

round trip signal as a normalizing factor; the amplitude ratios A/B

and C/B are our current flaw depth indicators.

The arrival time of signal B remains constant, of course, while A

and C move closer to B as the EMAT- flaw distance increases until they

all coincide when the flaw is exactly opposite the transducer. Thus,

there are two zones on the circumference where meaningful measurements

are not possible. Both of these are about 24 cm long; one is centered

at the transducer and the other is exactly opposite. The first is due

to recovery of the receiver amplifier following the transmitter pulse;

the second is due to the merging of the round-trip and flaw signals

when their acoustic path lengths become identical. As a result,

approximately 80% of the tread can be inspected with each pass. A

second pair of EMATs located 0.5-1 m down track would assure 100%

inspection.

FUTURE WORK

The electronics for this system fall in two basic categories: 1.

transducer (high current pulser for the transmitter and amplifier for

the receiver) and 2. signal processing (digitizing and logic). The

designs for the first category are working very well, and we are

packaging them in a form suitable for routine use. Work on the second

instrumentation system has now begun.

While our present EMAT design is producing excellent signals,

there are several possible improvements we will be investigating.

Among these are the use of a physically smaller magnet to reduce the

size of the pocket machined into the rail, alternate materials for the

compliant layer under the coils, and variations in the design and

physical structure of the coils. Other wheel “detection mechanisms to

trigger the current pulse are possible.

Further investigation into the size discrimination capabilities of

this system is also necessary. This will entail a series of

measurements on artificial flaws, initially, and then actual cracks.

Our goal is to deliver a working system to the Transportation Test

Center in Pueblo, Colorado, and conduct full-scale field tests.

49
Fig. 2. Current test configuration.

(a) Standard wheel set rolled along a short length of

track.

(b) EMATs mounted in rail. A protective film covers the

coils and they are represented schematically in this photo.

Fig. 3. Flaw echoes.

(a) Typical oscilloscope signal for 2 mm deep cut. A and

C are short and long path flaw echoes. B and D are first

and second round trip signals. A' is the short path echo

after a complete round trip (A + B)

.

(b) Schematic of bidirectional Rayleigh-wave travel to a

flaw. A is the short path and C is the long path.

50
ACKNOWLEDGMENTS

We are grateful for discussions with H. J. Salzburger of IzfP as

well as John Cowan and Richard Conway of The Fax Corporation. This

work was sponsored by the U.S. Department of Transportation, Federal

Railroad Administration, Washington, D.C. We also thank the

Transportation Test Center, American Association of Railroads, Pueblo

Colorado, and, in particular, Britto Rajkumar and Dominic DiBrito for

providing test wheels.

REFERENCES

1. Inspect Wheels "In Motion," Progressive Railroading . Jan., 1974.

2. Spotting the Defects in Railroad Wheels, Business Week , p. 44 „ Sept. 28, 1974.

3. Cracked-Wheel Detector "Could Save Millions," Railway Systems

Control . May, 1975.

4. U.S. Patent 3,978,712: Sept. 7, 1976, John Vincent Cowan,

Gerald De G. Cowan, and John Gerald Cowan.

5. H. J. Salzburger and W. Repplinger, in Ultrasonics International

83, conference organizers Z. Novak and S. L. Bailey (Rutterworth

& Co. Ltd., Kent, Great Britain, 1983), pp. 8-12.

6. A. V. Clark, R. E. Schramm, H. Fukuoka, and D. V. Mitrakovic,

Ultrasonic Characterization ©£ Residual Stress and Flaws in Cast

Steel Railroad Wheels, in Proceedings, IEEE 1987 Ultrasonics

Symposium, Vol. 2 . edited by B. R. McAvoy (Institute of

Electrical and Electronic Engineers, New York, 1987), pp. 1079-

1082.

7. R. E. Schramm, A. V. Clark, Jr., D. V. Mitrakovic, and P. J.

Shull, Flaw Detection in Railroad Wheels Using Rayleigh-Wave

EMATs , in Review of Progress in Quantitative Nondestructive

Evaluation . edited by D. 0. Thompson and D. E. Chimenti (Plenum

Press, New York, 1988), Vol. 7B, pp . 1661-1668.

8. R. E. Schramm, P. J. Shull, A. V. Clark, Jr., and D. V.

Mitrakovic, EMAT Examination for Cracks in Railroad Wheel

Treads, submitted to Proceedings: Nondestructive Testing and

Evaluation for Manufacturing and Construction , Urbana , 111.,

Aug. 9-12, 1988 (Hemisphere Publishing Corp., Washington, D.

C.).

9. B. W. Maxfield, A. Kuramoto, and J. K. Hulbert, Evaluating EMAT

Designs for Selected Applications, Materials Evaluation , vol.

45, no. 10, pp. 1166-1183 (1987).

10. G. A. Alers, and L. R. Burns, EMAT Designs for Special

Applications, Materials Evaluation , vol. 45, no. 10, pp. 1184-

1194 (1987).

r

51

Ultrasonic Characterization of Residual Stress and Texture

in a Heat-Treated Steel Railroad Wheel*

A. V. Clark, H. Fukuoka**, D. V. Mi trakov ic*** and J. C. Moulder

National Bureau of Standards

Fracture and Deformation Division

Boulder, Colorado 80303

Submitted to Materials Evaluation

Some minor revisions have been made in this manuscript in

accordance with a reviewer's remarks.

** NBS Guest Worker; on leave from Osaka University, Osaka University,

Japan.

*** NBS Guest Worker; on leave from University of Belgrade, Belgrade,

Yugoslavia.

*Contr ibution of the National Bureau of Standards; not subject to copyright.

53
ABSTRACT

The state of residual stress in the rim of a heat-treated steel railroad

wheel has been characterized using the acoustoelastic effect. Shearhorizontal (SH) waves were propagated through the rim thickness by transducers operating in pulse-echo made. The difference in arrival times of

orthogonally polarized SH-waves (acoustic birefringence) was measured with a

precision of ±1 ns or better. This difference is related to the state of

stress and texture in the rim.

Measurements were made using two transducers; a piezoelectric device

made of PZT, and an electromagnetic-acoustic transducer (EMAT). The PZT

device had been used to measure residual stresses in rolled steel wheels [1]

and was used as a reference against which EMAT measurements were compared.

Measurements made as a function of radial position on the rim indicated

a gradient of stress (assuming constant texture). The stress gradient

measured by the EMAT was in good agreement with that measured by the PZT

transducer

Measurements made around the circumference of the rim indicated an

axisymmetric state of stress and texture for regions closest to the tread,

with increasing departure from axial symmetry as the inner edge of the rim is

approached. The EMAT and PZT measurements both indicated this trend.

The effect of surface preparation on EMAT measurements was

characterized. The difference in birefringence as measured on rough surfaces

was usually less than 2 x 1 0

24

, which gives an (approximate) difference in

stress of 25 MPa.

The good agreement between EMAT and PZT values of birefringence, plus

the relative insensitivity of the EMAT to surface preparation, indicate that

EMATs are quite suitable for field measurements of residual stress.

54
INTRODUCTION

The fact that stress induces a (small) change in phase velocity in

metals has been known for decades. Recently, there have been numerous

experiments which use this acoustoelastic effect to map out states of applied

stress; many of these results are summarized in a review article [2],

Researchers have also attempted to use the acoustoelastic effect to

measure residual stress states. This problem is generally more difficult

than the applied stress problem, since the effect of texture must be accounted

for. (Presumably, the effect of texture can be eliminated in applied stress

problems by making velocity measurements before and after application of

stress. Then the change in velocity is related solely to stress.)

Measurements of residual stress due to welding have been conducted by

several researchers [3, *1,5,6] with good agreement with other techniques.

Likewise, measurement of the residual stress state in a shrink-fit specimen

have been performed [7,8,9,10] with good agreement with the theoretical

values.

The acoustoelastic effect was also used to measure the state of residual

stress induced by drag braking in the rim of rolled steel railroad wheels of

Japanese manufacture [1]. The values of stress so determined were compared

with two destructive residual stress measurement techniques; the stresses

measured using acoustoelasticity agreed within 140 MPa with the destructively

measured stresses.

In Ref.1, a novel piezoelectric transducer was used to make the acoustoelastic measurements. In order to protect the transducer from excessive

wear, the (rough) surface of the rim was machined to a smooth state.

Clearly, this machining is admissible in a laboratory setting, but is a

potential limitation on the use of acoustoelasticity in the field.

55
In a previous paper we demonstrated the feasibility of using

electromagnetic-acoustic transducers (EMATs) for characterization of the

residual stress state in the rim of a cast steel railroad wheel [11]. In

that work, we propagated shear-horizontal (SH) waves at normal incidence

through the thickness of the rim, using both an EMAT and a piezoelectric

transducer. The transducers were placed on the back face of the rim and

SH-waves were reflected from the front face. (The crosssection of a typical

rim is shown in Fig. 1.) Measurements of arrival times were made in pulseecho mode, so the wave had propagated through about 290 mm of cast steel when

it reached the transducer ( s)

The cast steel wheel (obtained from the Department of Transportation Test

Center, Pueblo, Colorado) had a sawcut. We assumed the hoop stress (induced

by braking the wheel, and also by the casting process) to be relieved near

the sawcuts and assumed that radial stress was negligible. We found that

the acoustic axes (polarization direction for maximum and minimum arrival

times of SH-waves) were oriented along the radial and circumferential directions. This was true both in the stress-relieved state (adjacent to sawcut)

and in the stressed state.

Consequently, we assumed that the residual stress could be obtained from

the birefringence equation [12]:

B - B

o

+ cA<°e - <Y> (1)

where o

Q

and o r are hoop and radial stresses, respectively, and CA

is the

stress-acoustic constant. 1 The acoustic birefr ingence , B, is defined as the

^It has been shown that the stresses vary with depth in the rim [13*1*0*

Consequently, the birefringence method will measure stresses averaged through

the rim thickness. When referring to stresses, we mean thickness-averaged

values unless stated otherwise.

56
difference of velocities of orthogonally polarized SH-waves, propagating

through the same material thickness:

B

V

o (2)

Here V

Q is the velocity of an SH-wave polarized in the 9 (hoop) direction,

and T

0 is the corresponding time of flight. B 0 is the birefringence in the

unstressed state; V

Q is the average of Vg and Vr

.

We measured B

Q on both sides of the sawcut, and took the average of

these two measurements. This value was subtracted from the birefringence

measured at stressed locations and equation (1) was used to obtain a

0

. The

values of hoop stress thus obtained with EMAT and piezoelectric transducer

were compared; we found that the peak values of hoop stress agreed within

10 MPa [11].

The present work concerns birefringence measurements made with both an

EMAT and piezoelectric transducer on a cast steel wheel which has been heat

treated and air quenched. This process results in a grain structure which is

finer than in cast wheels. (In fact, we found that we were able to detect up

to five echoes, using the EMAT, for the heat-treated wheel; for the cast

wheel, we obtained only two echoes at most.) Consequently, we expect a

different microstructure for the heat-treated wheel.

In Ref. 11, we measured B at the center of the back face of the rim.

We wished to determine if there was a gradient of stress in the radial direction, and, if so, whether we could measure the gradient with an EMAT.

Consequently, in the present study we measured B at 5 radii with our EMAT and

also with a piezoelectric transducer. In general, the values of B were in

good agreement.

57
In addition, we wished to determine whether the state of stress is

axisymmetric . Obviously, it is desirable that this be the case, since axial

symmetry greatly reduces the number of measurements necessary to determine

the stresses in the rim of the wheel.

Furthermore, we wished to make our measurements with the transducers

situated on the front face of the rim. This is the side that is accessible

in the field; to reach the back face necessitates reaching around the wheel,

which is considered a hazardous practice in a rail yard. Since the front

face is narrower than the back face, we anticipated that problems due to

acoustic beam spreading, sidewall reflections, etc. might arise. There is

also a potential problem with fringing of the eddy current field induced in

the rim by the EMATs; more will be said about this later.

EXPERIMENT

Method of Birefr ingence Measurement with EMAT

We measured arrival times (for both the EMAT and piezoelectric transducer) with a simple time-interval averaging system. The block diagram for

operation with the EMAT is shown in Fig. 2.

For the EMAT measurements, we simply oriented the transducer to generate

SH-waves polarized in the radial (or hoop) direction. The EMAT, which has an

aperture 1 cm x 1 cm, was excited with a tone burst having center frequency

of 2.4 MHz. The START time- interval probe (see Fig. 2) sends a square-wave

pulse to the counter when the voltage in the exciting pulse reaches a predetermined level. The counter runs at a 100 MHz rate until it receives a STOP

signal

58
The echoes are received by the EMAT , which is followed by a preamplifiertuned amplifier combination. The final stage of amplification is accomplished by a broadband receiver. The broadband receiver displays little

change of d.c. level with gain; this dc shift is further suppressed by the

high-pass filter. This is crucial, since we are using a zero crossing

(voltage - 0) as an arrival-time marker; any change in dc level with gain

could cause a change in apparent arrival time.

We chose a certain zero crossing (phase = 2nu) in the first echo as a

marker to determine arrival time. When this zero crossing occurs, the digital delayed gate (DDG) sends a TTL pulse to the counter.

Note that the STOP time-interval probe generates a pulse every time a

zero crossing occurs. Because the signal/noise ratio using an EMAT is so

low, there will be many such zero crossings unrelated to arrival of the first

echo. The purpose of the DDG is to (a) effectively gate out unwanted pulses

and (b) use logic circuitry to avoid distortion of the waveform (which occurs

with many analog gates).

Basically, the DDG operates as follows. A one-shot device is triggered

after a selected time delay. The time delay is selected to correspond (approximately) to the arrival of the selected zero crossing in the first echo.

The output of the "one-shot" device (a TTL signal at logical level "1") is

input to one port of a logical AND gate. The AND gate generates a TTL signal

when the other port is at logical level "1". This occurs when the next

square wave pulse is generated by the STOP time-interval probe. (We used a

preamplifier-level shifter circuit to change the output of the probe to TTL

level. The operation of the DDG is described in more detail in Ref. 11.)

59
With the EMAT in place, the precision of the arrival time measurement

was better than ±1 ns, for an arrival time of about 90 ps. Consequently, the

electronic precision is of the order of 10 p.p.m. To obtain this precision,

we set the counter to average 103 arrival times per reading. This averaging

allowed us to obtain good precision while operating in a noisy rf environment (proximity to a welding lab and a fatigue testing lab).

After an arrival time was measured in a given orientation, we rotated the

EMAT 90° and made another arrival time measurement, taking care to use

the same zero crossing as our marker.

We then rotated the EMAT to its original orientation, and repeated

the arrival time measurement. Typically, we found that this (new) arrival

time could be as much as ±5 ns different from the first measurement. We

attributed this to variations in the electromagnetic coupling between the

EMAT and the wheel.

We measured B by making at least three measurements of T

p and T

0 at each

location. This tended to reduce the uncertainty of arrival times; typically,

the standard deviation of the three (or more) arrival times was about ±3 ns.

For a typical arrival time of about 90 ys, this gives an uncertainty of about

3 x 10

-5

. The birefringence was obtained from

< T > - < T > B '

< T > (3)

0

where < > denotes an average.

Birefringence Measurements with Piezoelectric Transducer

The velocity (arrival time) measurement system was slightly modified

when using the piezoelectric transducer; the block diagram is shown in

60
Fig. 3. A thin layer of couplant was used to acoustically couple the

transducer and the wheel.

The transducer actually consists of two stacked PZT SH-wave transducers

which have an aperture of 7 mm x 7 mm. The transducers are orthogonally

polarized and bonded together so that each will propagate an SH-wave through

the same couplant thickness. This is an advantage for ultrasonic stress

measurement, where small changes in couplant thickness can cause errors in

arrival time measurements.

The lower transducer is, of course, acoustically coupled to the wheel

through the couplant. The upper transducer is coupled to the lower, so the

waveforms generated by the transducers will be different. To minimize this

difference, we used a shock excitation of the PZT transducers (as opposed to

a toneburst for the EMAT ) . We also adjusted the gain on the receiver so the

peak received signal is the same for both the upper and lower transducers.

(More details can be found in Ref. 11.) With shock excitation the waveform

is essentially the ring-down of the transducer (plus backing, etc.). We used

the second zero crossing as a marker for arrival time measurements; i.e., we

set the DDG to stop the counter when the second zero crossing occurred.

The stacked PZT transducer was mounted in a housing which allowed it to

be pressed down on the couplant with a constant force (supplied by a spring).

The housing in turn was connected to the wheel by a stand with a magnetic

base:

To avoid damage to the plating on the lower transducer, we had the

surface of the front face of the rim milled (in a machine shop), and then we

lightly hand polished the surface with abrasive paper (up to 320 grit). A

small amount of couplant was applied to the surface, the transducer was

locked in place with the magnetic stand, and the spring force applied.

61
We measured the arrival time by exciting the lower transducer when it

was polarized in, e.g., the radial direction. We then threw a switch (which

allowed the pulser to shock excite the upper transducer) and measured the

arrival time of the SH-wave polarized in the hoop direction. The stacked

transducer was then rotated 90° (so that the lower transducer was polarized

in the hoop direction) and the arrival times for both transducers measured.

To compensate for the additional acoustic path length for the upper

transducer, we calculated the birefringence from

B = [(T<u) + T^) - (T^u) + T^ } )]/2T0 (A)

where the superscripts (u) and (£) refer to the upper and lower transducers,

respectively and T

Q is the average arrival time. The additional path length

cancels out in the formula above.

For both transducers (EMAT and PZT) we measured B several times at the

same location (usually on separate days) to check repeatability. We took the

average of these measurements to give the final values of B.

RESULTS

Effect of Surface Preparation on EMAT Measurements

In our previous work (on the back face of the rim) we found that the

EMAT measurements were relatively insensitive to the degree of surface preparation [11]. To see if a similar result could be obtained on the front face of

the (heat-treated) wheel rim, we first measured the birefringence around the

rim circumf erence after only hand polishing with 60 grit abrasive paper.

Measurements were made at the center of the front face of the rim. The

results are shown as open circles in Fig. A, where the angle 6 is measured

counterclockwise around the rim from an arbitrarily chosen location. Each

62
circle represents one birefringence measurement, done as described above.

We measured B every 45 c' (8 locations) around the rim twice.

Next, we cleaned the front face surface more thoroughly (at the 8 locations) with a hand grinder (using 60 grit paper) and followed this with hand

sanding. We remeasured B, with results that are shown as solid circles in

Fig. 4. The difference in B between the two surface treatments is usually

less than 2 x 10”^. Assuming a value of -7.6 x 10”^ MPaVor the stressacoustic constant, CA> this difference of 2 x 10~^ is equivalent to a stress

difference of about 25 MPa (less than 4 ksi).^

The front face was then milled in a machine shop to obtain a smooth

surface. The EMAT measurements of B made on the smooth surface are shown as

diamonds in Fig. 4; in general, these measurements are within 2 x 10~^ of

previous measurements of B.

The fact that there is good agreement between EMAT measurements made on

surfaces of varying degrees of surface preparation is quite encouraging.

From a practical point of view, it is convenient to minimize the amount of

time and labor necessary to prepare the surface.

The first two methods (hand polish only and hand grinding followed by

hand polish) both left large pitted regions on the front face surface. Only

milling removed these pits.

We initially expected that the presence of pits could be a problem for

birefringence measurements with the EMAT; the pits could conceivably: (a)

cause local variations in liftoff, and (b) distort the eddy current field

induced in the rim. These effects might cause degradation in the acoustic

2 The value of C A was obtained in Ref. 1 for rolled steel wheels; here we

assume that C

A for heat-treated wheels will be approximately the same as for

rolled wheels.

63
signal amplitude and the purity of polarization. In addition, local variation of liftoff might cause a phase shift (in frequency space) or an equivalent time delay. However, it appears that to a first approximation that these

effects average over the transducer aperature.

After the front face of the rim was milled and hand polished, measurements were made with the PZT transducer. The results are shown by the

squares in Fig. 4. There is good agreement between the birefringence

measured by the EMAT and by the PZT transducer; in general, the difference

between these birefringence measurements is less than 2 x 10“^.

Radial Variations of Birefringence

The data shown in Fig. 4 were taken at locations in the middle of the

front face (13 mm from inner edge of rim). We were also interested in

determining whether there is a radial variation of birefringence. In fact,

the thickness-averaged residual stress due to braking should be largest at

the outer edge of the rim [1], since it is nearest the tread (where the

braking force is applied). A radial dependence of birefringence was found by

Fukuoka, et al. [1] on rolled steel wheels (using the PZT transducer).

We measured the birefringence at different radii; for each radius, we

measured B at 4 different circumf erent ial positions (e = 0°, 90°, 180°,

270°). The radii were at ±3 mm and ±6 mm from the middle of the front face of

the rim; radial position increases outward, as shown in Fig. 1. Both EMAT

and PZT measurements were made after the wheel was milled.

64
The results of the measurements at different radii are shown in Fig, 5;

the error bars represent the standard deviation of measurements of B at each

of the four circumferential positions (0 C

, 90°,...). The birefringence has a

gradient, becoming increasingly negative as the outer edge of the rim is

approached. The gradient as measured by the EMAT is almost linear.

If we assume that the texture is constant in the radial direction, then

the data show that the hoop stress is becoming larger (in the positive, or

tensile, sense) as the tread is approached. This is in agreement with the

physics of the process whereby the residual stress in the wheel is created.

The effect of braking is to cause compressive hoop stresses as the rim tries

to expand during heating. These thermal stresses are, of course, largest at

the tread where heat input (from braking) is applied. On cooling, a state of

tensile hoop stresses results, with the largest values at the tread.

Assuming constant texture, the EMAT values show a difference of about

75 MPa between -6 and +6 mm (a gradient of about 6 MPa/mm). The PZT values

indicate a difference of about 60 MPa between -6 mm and +6 mm. Note that

these are differences of stresses, and not the actual stress magnitudes,

since we do not as yet know the value of B

Q

.

For all five radii where measurements were made, the (average) EMAT values

of B are smaller than the PZT values. However, any systematic difference

should not cause the EMAT value of o

0 to differ from the PZT value since we

need to subtract B

Q from B to obtain o 0 (see equation 1). In fact, the data

of Ref. 11, taken on a sawcut wheel (where we could obtain B

Q

), showed good

agreement between o

0

as measured by EMAT and PZT transducer.

The largest discrepancy in Fig. 5 (about 2 x 10"^) between EMAT and PZT

measurements occurs at 6 mm, the position closest to the tread. At this

65
radius, we found that the signal from the upper PZT crystal was different

when it was polarized in the radial and hoop directions. Since this phenomenon was not noted at other radii, it may explain the difference between EMAT

and PZT results at 6 mm.

Circumferential Variation of Birefringence

In the ideal case, both the texture and residual stress would be

axisymmetric. If such were the case, then the standard deviations of B shown

in Fig. 5 would vanish (assuming no experimental error). In fact, we see

that the standard deviation is typically about 3 x 1 0“ 24

, implying that there

is some asymmetry in the birefringence. (The uncertainty in arrival time

measurements is about 3 x 10"^.)

To show this asymmetry better, we have plotted the birefringence as a

function of circumferential position in Fig. 6, with the radial distance

(from center of the rim) as a parameter. For axial symmetry, the birefringence would be a straight line with zero slope (constant as function of

circumferential position). The data in Fig. 6 (radii ±3 mm from center of

rim) show that there appears to be a departure from axial symmetry with the

minimum algebraic value of B in the region l80°-270°.

We see that the values of B as measured by the two transducers agree

reasonably well for radial positions ±3 mm from the center of the front face

(to within less than about 2 x 10

-it

). The agreement is not as good for the

center ( see Fig . 4 )

The values of B (as function of cir cumferent ial position) for ±6 mm from

the center of the rim are shown in Fig. 7. Now we see a behavior which is

very close to axial symmetry for +6 mm, for both EMAT and PZT measurements.

Furthermore, there is (approximately) a constant difference between the two

66
sets of data at 6 mm. For -6 mm, the values of B are such as to suggest that

the texture and/or the stress is rather inhomogeneous.

Effect of Edges on EMAT Measurements

As mentioned previously, the process of drag-braking the wheel usually

induces the largest stresses at the largest radius in the rim. (This is

because the brakes are applied on the tread.) Consequently, it is important

to be able to measure the birefringence at locations as close as possible to

the outer edge of the rim.

At the- +6 mm position, the edge of the EMAT aperature is about 2 mm from

the outer edge of the front face of the rim. We found that the signal amplitude was markedly less f or r > 6 mm (r measured from rim center), as compared

with the other radial locations. Consequently, we made no measurements for

r > 6 mm.

We think that the drop in signal amplitude near the rim edge is due to

the effect of fringing field on the EMAT. 3 The EMAT generates eddy currents

in the surface of the rim (within the skin depth of an electromagnetic wave).

The eddy currents must form closed conduction paths. Our EMAf was designed

so that the eddy currents in the surface were unidirectional, under the EMAT

aperture. The effect of the edge of the conductor (rim) is not the same for

different orientations of the EMAT (see Fig. 8). We conjecture that the

effect of the edge is to change the effective impedance offered by the

the surface to the eddy currents and that this impedance change depends upon

^We note that the EMAT magnet is a single pole piece. Consequently, the

magnetic induction in the rim is invariant under rotation of 90°, i.e., the

magnetic induction is the same when the EMAT generates SH-waves polarized in

the r- and 6- directions.

67
the orientation of the EMAT. These effects precluded measurement of B when

the edge of our EMAT aperture was within 2 mm of the edge of the rim.

We also performed an experiment on a (cast steel) rim block whose front

and back surfaces were polished so that they were plane-parallel. The edges

of the flat regions are denoted as location 1 and 2 for the front face, and

as 3 and 5 for the back face; see Fig. 9. Note that location 2 is directly

above location 3, but locations 1 and 5 are offset.

We measured arrival times with our EMAT placed first on the front face,

and then on the back face. When the center of the EMAT aperture was within

6 mm of location 1 (front face) or 3 (back face), the signal amplitude

dropped and erroneous arrival times were obtained. When the EMAT was farther

than 6 mm from these locations, the same value of arrival times (within ±1 ns

on the average) was obtained, regardless of which face the EMAT was on. This

gave us confidence that correct arrival time could be measured when the EMAT

was farther than 6 mm from an edge.

We also checked this by measuring arrival times with the EMAT on the

front face, 7 mm from location 2 (7 mm from an edge). The average value was

within ±1 ns of the corresponding value obtained with the EMAT 7mm from location (27 mm from an edge). Consequently, we conclude that the effect of

the edge is negligible when the EMAT is centered 7 mm or more from an edge.

These results may pertain only to this design of EMAT, operated of

2.H MHz on steel. For other applications, frequencies, and EMATs, a similar

experiment should be carried out to determine the effect of edges on EMAT

operation.
DISCUSSION

We used the stacked PZT transducer (which is, of course, insensitive to

the edge effect) as a check on our EMAT results. As shown in Fig. 7, the

EMAT and PZT values of B agree within about 2 x 10 -i4

at +6 mm from the

center of the rim. In contrast, we see better agreement between EMAT and PZT

measurements at +3 mm (Fig. 6).

There is a possibility of a systematic difference between EMAT and PZT

measurements of B, since a toneburst excitation was used for the former and

shock excitation was used for the latter. In fact, a small systematic

difference was noted in Ref. 11 for measurements made at the center of the

back face of the rim of a cast steel wheel. However, this difference was

only about 1 x 10"^ (or less). In fact, this systematic difference essentially cancelled out since the stresses are calculated from the difference

(B - B

0 ) in equation (1). In Ref. 11, we found that EMAT and PZT measurements of o

0

were almost identical for all measurement locations on the wheel;

the maximum difference was less than 10 MPa.

For the heat-treated wheel, there appears to be a systematic difference

between EMAT and PZT measurements of B at radial distances of +6 and ~3 mm

(from center of the rim). For -6 mm, the data of Fig. 7 suggest a rather

inhomogeneous texture. At the other locations, there is no clear evidence of

a systematic difference; i.e., at some circumferential locations, the EMAT

measurements of B are larger, while the reverse is true at other locations.

There are several issues that remain to be resolved. First, how

homogeneous is the texture? This may account for the rather erratic

behavior of B at -6 mm, as shown in Fig. 7. Second, is the difference in

EMAT and PZT values of B at +6 mm systematic? (We found a different

69
waveform for the upper PZT transducers at this radius.) If so, will the

difference cancel if we measure B - B

Q

?

The obvious way to resolve these issues is to cut the rim into small

blocks, so that the stresses are relieved. Then one can measure B

Q with

both EMAT and PZT transducers and determine whether both measurements give

the same values of stress (cancellation of systematic errors).

In addition, we need to perform destructive tests on the wheel to

measure the actual state of residual stress and compare within the ultrasonic

results.

We hope to perform all of these tests in the near future.

CONCLUSIONS

The radial variation of <B> as measured by EMAT and PZT transducers are

in good agreement for the range -6 mm to +3 mm; here <B> is the average of

4 circumf erential locations. Over this range, EMAT and PZT values of <B>

differ by 2 x 10

-

^ or less. For +6 mm the difference is less than 3 x 10~^.

Assuming no systematic differences between .EMAT and PZT measurements (no

cancellation of errors when taking the difference B - B

Q ), this gives a

stress differences of about 27 MPa (for radii ranging from -6 mm to +3 mm

from the rim center) and 40 MPa (+6 mm from rim center). If there are

systematic errors which can be (removed by subtracting B

Q from B), the

differences will be less.

We found that, as a function of circumf erent ial position, EMAT and PZT

values of B were in good agreement (difference less than 2 x 10~^) for radial

positions -3 mm to +3mm. There appears to be a systematic difference (EMAT

values of B less than PZT values by about 3 x 10”^ or less) at +6 mm. At

70
-6 mm, both sets of data are erratic, perhaps indicating inhomogeneity in

texture.

It is fortunate that the birefringence displays the axisymmetric

behavior at +6 mm, since this will be the radius of maximum (thicknessaveraged) hoop stress [1]. The axisymmetric behavior suggests that the

texture and stress may be homogeneous at this radius for the heat-treated

wheel. If such is the case, then the following procedure may be used to

determine value of o

0 at +6 mm on in-service wheels.

Several heat-treated wheels would be cut into rim blocks (as in Ref. 1)

to relieve the residual stress. The value of B

Q would then be measured on a

statistically significant number of rim blocks. The mean value, <BQ

> , would

then be used when determining the value of Oq for an in-service wheel. To do

this, the birefringence would be measured for the in-service wheel, and <B Q

>

subtracted to obtain a

0

:

o

0 = (B - <B 0>)/Ca

. (5)

This is the method used in Ref. 1 to obtain o

0

for rolled steel wheels.

The uncertainty 6o

0

in o

0

can be estimated from the standard deviation

of B

0 measured on the population of rim blocks:

6a

0 = 6 B

q /Ca

. (6)

We hope to perform such experiments in the future.

The EMAT measurements of B at the center of the rim are relatively

insensitive to the degree of surface preparation. The degree of surface

preparation was varied between the extremes of (a) handsanding to remove rust

(leaving a pitted surface) and (b) milling in a machine shop, followed by

handpolishing with fine abrasive paper (leaving a very smooth surface). The

71
EMAT measurements of B for these surface treatments were usually within

2 x 10

-1

^ of each other. For practical applications, this indicates that a

minimal amount of surface preparation may be necessary for EMAT measurements

in the field.

ACKNOWLEDGMENTS

This work was supported by the Federal Railroad Administration, Department of

Transportation, under the supervision of Mr. Claire Orth, Research Manager. We

were greatly assisted by the generous cooperation of Britto Rajkumar of

the American Association of Railroads, Pueblo, CO, who provided both the wheel

and encouragement.

72
REFERENCES

1. Fukuoka, H., Toda, H., Hirakawa, K. , Sakamoto, H., and Toya, Y.,

’’Acoustoelastic Measurements of Residual Stresses in the Rim of Railroad

Wheels,” Wave Propagation in Inhomogeneous Media and Ultrasonic Nondestructive Evaluation, G. C. Johnson, ed., AMD-Vol. 6 (published by

ASME, 1984).

2. Pao, Y. H., Sachse, W. , and Fukuoka, H., "Acoustoelasticity and Ultrasonic Measurements of Residual Stress,” Physical Acoustics , Vol. XVII,

Academic Press, New York, 1984, pp. 61-143.

3. Fukuoka, H., Toda, H., and Yamane, T., "Acoustoelastic Stress Analysis

of Residual Stress in a Patch-Welded Disc,” Exp. Mech. , 1

, 1978,

pp . 277-280

4. Schneider, E., "Determination of Residual Stresses by Time of Flight

Measurements with Linearly-Polarized Shear Waves" Proc. IEEE Symp. on

Sonics and Ultrasonics , 1981 , pp. 956.

5. Toda, H., Fukuoka, H., and Aoki, Y., "R-value Acoustoelastic Analysis of

Residual Stress in a Seam Welded Plate," Proc. 4th Symp. on Ultrasonic

Electronics, Tokyo, 1983- Jap. Jour. Appl . Phys. , 23 , 1984, pp. 86-88.

6. Clark, A. V., Moulder, J. C., Trevisan, R. E., Siewert, T. A., and

Mignogna, R. B., "Ultrasonic Techniques for Residual Stress Measurement

in Thin Welded Aluminum Alloy Plates," to be published in Rev. Prog, in

Quant. NDE , D. 0. Thompson and D. E. Chimenti, eds., 1985.

7. King, R. B., and Fortunko, C. M., "Determination of In-Plane Residual

Stress States in Plates Using Horizontally Polarized Shear Waves,"

J. Appl. Phys. , 54, 1983, pp. 3027-3035.

8. Blessing, G. V., Hsu, N. N., and Proctor, T. M., "Ultrasonic-Shear-Wave

Measurement of Known Residual Stress in Aluminum, Exp. Mech , 24 , 1984,

pp. 218-222.

9 . Clark, A. V., and Moulder, J. C., "Measurement of Residual Stresses in

Slightly Anisotropic Aluminum Alloy Specimens by the Method of Acoustic

Birefringence," Ultrasonics , 23 , 1985, pp. 253 ” 259 .

10. Jackson, C. W., "Characterization of Stressed Metals Using Acoustic

Shear Waves"; Ph.D. Thesis, Dept, of Mechanical Eng., Stanford Univ.,

1984.

11. Clark, A. V., Fukuoka, H., Mitrakovic, D. V., and Moulder, J. C.,

"Characterization of Residual Stress and Texture in Cast Steel Railroad

Wheels," to be published in Ultrasonics .

12. Iwashimizu, Y., and Kubomuro, K., "Stress-Induced Rotation of Elastic

Waves in Slightly Anisotropic Materials," Int. J. Solids Structures , 9 , 1973, PP. 99-114.

73
13. Nishimura, S., and Tokimasa, K., "Study on the Residual Stresses in

Railroad Solid Wheels and Their Effect on Wheel Fracture," Bulletin of

JSME, J_9 , 1976, pp. 1159-^68.

14. Johnson, M. D., Robinson, R. R., Opinski, A. J., and Stone, D. H.,

"Railroad Wheel Back Rim Face Failures III. Residual Stress

Calculations on 33" D One-Wear Freight Car Wheels," Am. Assoc, of

Railroads Rep. No. R-560, AAR Tech. Center, Chicago, IL, 1983.

74
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76

Fig.

2.

Block

diagram

of

system

used

to

measure

birefr

ingence

with

EMAT
E-M

a.

jC•H

<DOC

<1) bDC

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4.

Birefringence

measured

at

center

of

rim,

for

various

degrees

of

surface

preparation.
E-1 E N •>—i

a. t- •o c,_ c o

ns

i.

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79

transducer.

Radial

position
(fr-(H) °l/( @1 - 1)

80

Fig.

6.

Circumferential

(e)

variation

of

birefringence,

as

measured

with

EMAT

and

PZT

transducer.

Birefringence

measured

at

±3

mm

from

rim
oO

CO

<0oo

CVJ eO

00oo

o>

81

Fig.

7.

Circumferential

variation

of

birefr

ingence

at

±6

mm

from

center

of
EMAT Aperture

Fig. 8. Interaction of eddy current field with rim edge. EMAT oriented

to generate SH-wave polarized in circumferential direction (top

figure), and in radial direction (bottom figure).

82
Fig. 9. Cross-section of rim block use to test effect of edges on arrival

time measurements made with EMAT. Locations 1 and 3, 2 and are

directly opposite each other. Location 5 is where flat portion of

back rim face ends.

83
NBS-1 14A (REV. 2-8C) U.S. DEPT. OF COMM. 1. PUBLICATION OR 2. Performing Organ. Report No.

BIBLIOGRAPHIC DATA

REPORT NO.

SHEET (See instructions) NISTIR 88-3906

3. Publication Date

December 1988

4. TITLE AND SUBTITLE

Ultrasonic Railroad Wheel Inspection Using EMATs

Report No. 18

5. AUTHOR(S)

Raymond E. Schramm and A. Van Clark, Jr.

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s) Institute , m national BDKBAtf OF STANDARDS and Technology

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

7. Contract/Grant No.

8 . Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

U.S. Department of Transportation

Federal Railroad Administration

400 7th Street, S.W.

Washington, DC 20590

10.

SUPPLEMENTARY NOTES

] Document describes a computer program; SF-185, FlPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most significant information. I f document includes a si gn i fi cant

bi bl iography or literature survey, mention it here)

This is report number 18 in a series covering the research performed by the National

Institute of Standards and Technology (formerly National Bureau of Standards) for

the Federal Railroad Administration. This issue collects seven reprints and preprints

of papers written by the Fracture and Deformation Division over the last two years

on the ultrasonic nondestructive evaluation of railroad wheels for the presence of

residual stress and cracks. All this work concentrated on the use of electromagneticacoustic transducers (EMATs).

Tensile residual stresses and tread cracks are major factors in wheel failure.

Catastrophic breakdowns may lead to injuries, repair costs, and lost time. Current

inspection methods are largely visual, and thus inefficient. Two ultrasonic techniques

are applicable to these wear defects:

1) Birefringence. A stress field effects the velocity of a shear horizontal

wave depending on its polarization. Precise velocity measurements in a wheel rim

may allow calculation of the amount and direction of stresses.

2) Pulse-echo. A Rayleigh (surface) wave transducer mounted inside the rail

can introduce a signal to interrogate the circumference of a wheel as it rolls by.

An echo indicates a flaw's presence and size.

For both systems, we hope to produce instruments for testing at a field site.

12.

KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semi colon s) EMAT; nondestructive evaluation; railroad wheel; residual stress; roll-by inspection;

ultrasonic

.

13.

AVAILABILITY

Uni imited

j

i For Official Distribution. Do Not Release to NTIS ^ Order From Superintendent of Documents, U.S. Government Printing Office, Washington, D.C.

" 20402 .

[X] Order From National Technical Information Service (NTIS), Springfield, VA. 22161

14. NO. OF

PRINTED PAGES

88

15. Price

W U.S. GOVERNMENT PRINTING OFFICE: 1989 - 674-393/5025

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