I IV 1 1

United States Department of Commerce

I I Ai1^31 National Institute of Standards and Technology

NISTIR 3968

Report No. 23

RESIDUAL STRESS DETECTION IN

RAILROAD WHEELS:

AN ULTRASONIC SYSTEM USING EMATS

Raymond E. Schramm

Alfred V. Clark, Jr.

Dragan V. Mitrakovic

Stephen R. Schaps

Todd J. McGuire

.U56

3968

1991

C.2
7,

i

pi

V.

Ti^

'rtf~
NISTIR 3968

Report No. 23

RESIDUAL STRESS DETECTION IN

RAILROAD WHEELS:

AN ULTRASONIC SYSTEM USING EMATS

Raymond E. Schramm

Alfred V. Clark, Jr.

Dragan V. Mitrakovic*

Stephen R. Schaps

Todd J. McGuire

Materials Reliability Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

*NIST Guest Researcher, on leave from the University of Belgrade

May 1991

Prepared for

U.S. Department of Transportation

Federal Railroad Administration

Office of Research and Development

Washington, DC 20590

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director

CONTENTS

Page

ABSTRACT 1

INTRODUCTION 2

CONTRACT HISTORY 3

STRESS MEASUREMENT METHODS

Saw Cutting 4

X-Ray Diffraction 4

Neutron Diffraction 4

Hole Drilling 5

Visual Appearance 5

Acoustic 5

ACOUSTIC BIREFRINGENCE AND STRESS 6

EMAT OPERATION ' 7

Principles 7

Configuration 8

OPERATIONAL PRINCIPLES 12

Transducer Package and Fixture 12

EMAT Electronics 14

Digital Gate 17

Interval Timer 18

SYSTEM ASSEMBLY AND USAGE

Wheel Mount System 19

Connections and Cables 19

Data Collection 20

EXPERIMENTAL RESULTS

EMAT/PZT Comparison 21

Stress Sensitivity 22

Saw- Cut Comparison 24

Texture Measurement 24

ELECTRONIC CIRCUITS 25

CHECKLIST 39

TROUBLE SHOOTING 39

FUTURE CONSIDERATIONS 40

ACKNOWLEDGMENTS 40

REFERENCES 41

iii
-t

-

4 i

' :0i-

* • ti » j •*•.•</••' * * •'• f.- K ''

'

' ''4f- '-*<'’
RESIDUAL STRESS DETECTION IN RAILROAD WHEELS:

AN ULTRASONIC SYSTEM USING EMATS

Ra)nnond E. Schramm, Alfred V. Clark, Jr. , Dragan V. Mitrakovic'',* Stephen R. Schaps, and Todd J. McGuire

Materials Reliability Division

Materials Science and Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

* NIST Guest Researcher, on leave from the University of Belgrade

This is report number 23 in a series covering research performed

by the National Institute of Standards and Technology for the

Federal Railroad Administration. This report covers a project by

the Materials Reliability Division (formerly the Fracture and

Deformation Division) to develop and build a system to detect and

measure residual stress in the rims of railroad wheels.

Acoustic birefringence is the underlying principle of operation.

This is a measure of the relative difference in the propagation

times of two shear waves polarized in radial and circumferential

directions. Measurements employ an EMAT (electromagnetic-acoustic

transducer) as an ultrasonic probe. This type of transducer

requires little or no surface preparation and no acoustic

couplant. The system operates in a pulse -echo mode. A short

burst of shear horizontal waves travels through the rim thickness.

The rotation of the EMAT determines the orientation of the

polarization vector, radial or circumferential. Precise timing of

echoes in both directions reveals the degree of birefringence.

Changes are due to both stress state and metallurgical texture.

Initial tests indicate it may be possible to separate these two.

This report documents the design, construction, and testing of the

system. It is also to serve as an operational guide for the equipment

being delivered to the FRA.

Key words: birefringence; EMAT; nondestructive testing; railroad

wheel; residual stress; ultrasonic.

1
INTRODUCTION

In this country, most railroad operations now deal with the transportation

of freight in its many forms. Modern needs place ever greater demands on

the elaborate rail network that began developing early last century. Just

one of the important elements in this complex system is the integrity of the

many wheels that bear the loads. As massive as the cast-steel wheels used

in the U.S. are, they might not seem to be a problem. Accident statistics,

however, indicate otherwise. According to the Accident /Incident Bulletin of

the FRA Office of Safety, broken wheels caused 134 accidents in the four

year period of 1985-1988, resulting in losses totaling $27. 5M. Besides the

direct and indirect cost of such problems, there is the threat to human life

and health due to the accidents themselves or the potential environmental

impact from the release of dangerous materials. Disruption to commerce at

many levels is almost inevitable.

During their lifetime, wheels see a wide variety of sometimes extreme

conditions. Among these are large static and dynamic mechanical loads,

large-scale frictional wear, high temperatures from braking operations,

environmental extremes, and exposure to potentially corrosive materials.

All these factors operate to initiate tread cracks or aggravate any existing

faults. Depending on the stress conditions (applied and residual), crack

growth can be very slow and gradual or rapid, even explosive. The relative

rarity of failures under all these extreme conditions speaks well for the

progress made in wheel design and manufacture.

Wheel improvements will continue, but it will not be possible to eliminate

completely the tensile stresses that drive a crack to failure. For

economic, environmental, and safety reasons, replacement before failure is

the preferred approach. Because of the costs and uncertainties involved,

retirement for cause is favored, whenever possible, over replacement at

predetermined intervals. Effective and efficient nondestructive evaluation

makes it possible to evaluate fitness for service. For railroad wheels, as

in most other applications, the first NDE test is visual examination by a

skilled inspector. In practice, however, this simple approach has severe

limitations. The stress state itself is not directly observable. The ideal

test method would be simple, reliable, unintrusive, inexpensive, and

operator independent. No available techniques fulfill all these conditions

perfectly, but ultrasonic testing offers a possible major step in this

direction.

The ultrasonic system examined here follows upon prior efforts at NIST and

elsewhere, and upon the experience and expertise with electromagneticacoustic transducers in this laboratory. After laboratory testing and

development, we built a prototype for delivery to the FRA. This will

receive extensive field testing and exposure to potential users. The

desired goal is to develop an instrument of value to the railroad industry

that private industry would license for commercial production.

The intention here is to develop an instrument for use in a railcar shop

during maintenance operations. Subsequent developments may allow use at

trackside should this prove desirable.

2
CONTRACT HISTORY

As the Fracture and Deformation Division of the National Bureau of

Standards, we first received a Federal Railroad Administration research

contract to develop nondestructive evaluation (NDE) procedures for railroad

wheels in September 1985. During the period of this work, we have been

renamed the Materials Reliability Division of the National Institute of

Standards and Technology. Our contract monitors have been M. Clifford

Gannett, Claire Orth, and Donald E. Gray. The current contract is

Reimbursable Agreement No. DTFR53-89-X-00018 with an effective date of

4/21/89.

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

this sponsorship at two meetings:

1. Review of Progress in Quantitative Nondestructive Evaluation, La

Jolla, California, August 1986.

2. 1987 IEEE Ultrasonics Symposium, Denver, Colorado, October 1987.

There have been four publications on this part of the work:

1. "Ultrasonic Characterization of Residual Stress and 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 Cast Steel

Railroad Wheels," A. V. Clark, Jr., H. Fukuoka, D. V. Mitrakovic

and J. C. Moulder, Ultrasonics 24, 281-288 (1986).

3. "Ultrasonic Characterization of Residual Stress and Flaws in Cast

Steel Railroad Wheels," A. V. Clark, R. E. Schramm, H. Fukuoka,

and D. V. Mitrakovic in Proceedings: IEEE 1987 Ultrasonics

Symposium . B. R. McAvoy, ed. , Institute of Electrical and

Electronic Engineers, New York (1988), 1079-1082.

4.

"Acoustoelastic Measurements Pertaining to the Nondestructive

Characterization of Residual Stress in a Heat-Treated Steel

Railroad Wheel," A. V. Clark, H. Fukuoka, D. V. Mitrakovic, and J.

C. Moulder, Materials Evaluation 835-841 (1989).

Most of this material was collected together and published as:

Report No. 18: Ultrasonic Railroad Wheel Inspection Using EMATs . Raymond E. Schramm and A. Van Clark, Jr., NISTIR 88-3906, Dec. 1988.

Other work done under this contract focused on the detection of cracks in

the tread of railroad wheels. The approach has been to use an in-rail EMAT

to generate Rayleigh waves and listen for flaw echoes. Operation will be

roll-by and fully automatic. The results of this parallel study are in a

prior NISTIR 91-3967:

Report No. 22: Tread Crack Detection in Railroad Wheels: An

Ultrasonic System Using EMATs .

3
STRESS MEASUREMENT METHODS

Following are a few observations on some of the usual techniques.

Saw Cutting

While obviously not usable for in-service inspection, destructive methods do

usually provide definitive measurements and can provide verification of

nondestructive procedures. In the case of railroad wheels, one approach is

to make saw cuts along the radius. The presence of compressive or tensile

stresses will cause the wheel to close on the blade or open further.

Quantitative measurements of stress and its distribution are possible by

careful instrumentation with strain gages. Because of the complex shape of

a wheel, it is generally necessary to enter these data into some model (such

as finite element) for considerable calculations to reveal the stress state.

X-Ray Diffraction

The strains caused by residual stresses are evident even at the level of

interplanar spacing within the crystal lattice. The exact angular position

of X-ray powder diffraction lines is a very sensitive indicator of such

changes. Commercial devices designed for this specific task are on the

market. The main limitations of this approach fall into two categories:

1. All measurements must be referred to a stress-free specimen. At

the interatomic level, such a state is difficult to achieve and

also tends to be very dependent on exact chemistry and metallurgy.

2. X-rays penetrate only several atomic layers deep. The resulting

calculations reflect the status at the immediate surface, while

the stress distribution likely has a very sharp gradient there.

The stresses internal to thick materials can only be inferred.

Neutron Diffraction

Another way of measuring the interatomic strains is by looking at the

diffraction of neutrons caused by the periodicity of the nuclear magnetic

moments. Because of the great penetrating power of neutrons, it is possible

to obtain the stresses deep within very large specimens. Unfortunately, the

usual source of radiation is a nuclear reactor. The measurement

instrtunentation also is usually bulky and complex. While this is a valuable

research tool, it rarely sees service in an industrial setting.

4
Hole Drllllnfg

A mature technique that is "mostly" nondestructive is the measurement of

strain relief while drilling a very precise, shallow hole into the surface

of the specimen. Special strain gage rosettes and milling fixtures

specially designed for this purpose are commercially available. Assuming

that the damage from the small hole is acceptable, there are two problems:

1. The procedure is very time consuming and requires considerable

skill and experience.

2. Measurements represent the stress state within the top few

millimeters of the specimen. Again, conditions deep inside must

be inferred.

Visual Appearance

If a stress state is due to transient high- temperature conditions, there may

be evidence in the form of surface discolorations from oxidation reactions

.

In fact, the Federal Railroad Administration, DOT, Code of Federal

Regulations 49 CFR states:

^215.103 Defective wheel

A railroad may not place or continue in service a car if -

• • • (h) A wheel on the car shows signs of having been overheated as

evidenced by a reddish brown discoloration, to a substantially equal

extent on both the front and the back face of the rim, that extends on

either face more than four inches into the plate area measured from the

inner edge of the front or back face of the rim . .

.

The obvious problem here is the need for a color- calibrated eye and

excellent lighting conditions. In addition, destructive tests done by the

Transportation Test Center (TTC) on wheels condemned by this rule showed

that approximately half (more for Class C) of them contained compressive

hoop stresses and were still safe for service.

Acoustic

The theoretical effects of stress on the propagation of sound and a

considerable body of experimental work have been known for some time, much

of it done in this laboratory [1,2]. While most of this has involved the

relations between stress, polarization direction, and velocity, the magnetoacoustic effect has been under study by the Association of American

Railroads (AAR) [3j. A commercial device with special reference to railroad

applications is now available [4]

.

5
Acoustic methods have two possible problem areas:

1. Very high precision in signal timing is necessary to determine

velocity with sufficient precision.

2. It may be necessary to have a reference material representing the

metallurgical texture of the unstressed specimen.

With the expectation of overcoming these difficulties, we have chosen to

pursue the application of EMATs to the measurement of acoustic birefringence

for the calculation of residual stress.

ACOUSTIC BIREFRINGENCE AND STRESS

Birefringence (or double refraction) is the variation of the propagation

velocity of a wave with the direction of the polarization vector. This

property is probably most familiar as an optical phenomenon in some natural

crystals, such as calcite. Some polymers exhibit a photoelastic effect;

i.e., their birefringence is a function of mechanical stress. Such

materials are available commercially, and they permit a visual

representation of the development of an applied surface stress field.

Acoustic waves in anisotropic materials exhibit birefringence. There are

two principal causes for this: stress fields and metallurgical texture or

preferred orientation of grains. While there are several ways to probe this

anisotropy [1,2], Fig. 1 illustrates the approach pursued in this

Figure 1. Schematic of a section of wheel rim indicating the travel and

polarization directions of the ultrasonic pulse.*

6
work. We place a transducer on the front face of the wheel rim and launch

an ultrasonic pulse through the thickness of the rim. This pulse will

reflect back toward the transducer from the back face, and its transit time

is indicative of the velocity. The ultrasound is a horizontally polarized

shear wave. There are two timing measurements necessary, with the

polarization vector oriented along the radial (R) direction and tangential

to the wheel circumference (0). To achieve this, the transducer remains in

the same location and simply rotates 90®

.

The distance traveled by the two

pulses is exactly the same, so the experimental parameters are their arrival

times. We define the birefringence as the fractional change in the velocity

of the two polarizations:

B - (Vq - Vr) / (Ve + Vr)/2 - (tR - te) / (tR + te)/2. (i)

B is the birefringence; V is the velocity, and t is the transit time of

waves polarized in the R and 0 directions.

The functional relationship between birefringence and stress is:

B “ Bq + •

Bq is the birefringence due to the metallurgical texture. is the stress

acoustic constant. The a's are the stresses in the principal directions.

The value of for shear waves is small [6], about -7.0 x 10"^ (MPa)"-*- or

-4.8 X 10"^ (ksi)’^ for steel wheels. Effectively, this means that the

timing measurements must be very accurate (typically a few parts in 10^, or

<10 ns in a 100 /is interval). Bq is usually a significant or major portion

of the total birefringence, B. If we know Bq (e.g., from a measurement on

an unstressed specimen) , the stress difference is then

-1

^0 " ^R “ ®0) / ^A- ( 2 )

Usually, (7R is negligible.

EMAT OPERATION

EMATs (electromagnetic-acoustic transducers) are devices that generate and

detect sound energy directly in an electrically conductive specimen. A

major advantage of these devices is that they do not require the acoustic

couplants usually associated with piezoelectric transducers. This

simplifies operation and eliminates echoes and mode conversions that might

occur at the interfaces the signal would otherwise have to traverse.

Furthermore, they can operate effectively on rough and pitted surfaces.

Principles

To indicate basic EMAT operation. Fig. 2 shows a primitive element composed

of a wire conductor carrying a dynamic current 1^^ and a source of strong

7
magnetic field Hq. The current induces dynamic eddy currents in the

metal conductor surface. Hq causes the deflection of the moving electrons

in a direction defined by The resultant Lorentz body forces, T,

generate ultrasonic signals that propagate radially into the specimen.

The polarization and propagation direction of a signal from an EMAT are the

result of the orientation of the electric and magnetic fields and the

arrangement of the primitive elements. For the examination of the wheel

rim, we want a shear horizontal (SH) wave (polarization vector in the plane

of. the specimen surface) that travels normal to the surface. To generate

this ultrasonic signal, we used a planar coil (Fig. 3). This configuration

involves several design considerations generated here and elsewhere [5]

1. Because the two coils are counter-wound and connected as shown in

Fig. 3, ambient or stray magnetic fields will generate electrical

signals of opposite sign in the two halves. This gives noise

cancellation within the transducer.

2. To prevent the generation of confusing eddy currents within the

specimen, there is an electromagnetic window to define the desired

area of the coil. Here, the window (unshaded area in Fig. 3) is a

1 cm square etched in copper cladding on a Kapton substrate (0.03

mm of Cu on 0.15 mm of Kapton). The rf current direction in all

the exposed wires is in the same direction.

3. The coils are in the shape of an elongated spiral so that all the

wires within the window are parallel. Tliis geometry and the

window combine to insure that the eddy currents generated at the

specimen surface are well defined.

4. This balanced- coil geometry places the window at the center. A

large, single coil can generate the same acoustic signal, but the

window must then be offset to one side. This may cause some

difficulty in the physical construction of the transducer.

The ultrasonic pulses resulting from this careful design can be understood

and controlled. This coil is currently laid out by hand, but automatic

production is possible for large quantities.

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 simplifies field applications. 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.

Configuration

In the present design (Fig. 3), there are 35 turns of AWG 36 enameled wire

in each half of the coil. The turns are tightly packed as they are wound

into the mastic on a thick polymer tape. The aim is to get as many turns as

possible into the available space. The DC resistance of the connected pair

^ of coils is approximately 5.0 Q.

8
Figure 2. Primitive EMAT element.

Figure 3. Schematic of the EMAT coil used to generate and receive shear

horizontal (SH) waves. The 35 turns of AWG 36 enameled wire in

each of the two coils are tightly packed as they are wound into

the mastic on a thick polymer tape. This shows the manner of

connecting the two counter-wound halves. The shaded area is a

thin copper shield with a 1 cm square window over the active

section.

9
The permanent magnet supplying the field is a 52 mm x 26 mm x 31 mm block of

Nd-Fe-B with a nominal energy product of 0.28 MJ/m^ (35 MG-Oe). The field

direction is along the 31 mm dimension and must be normal to the test

surface. To confine and concentrate the field into the area of Interest,

there is a tapered pole tip made from a bar of cold-rolled, mild steel (Fig.

4).

A commercial aluminum case contains the magnetic pieces and the electrical

connections (Fig. 5). In this instance, the case is 105 mm high with a

specimen contact area of 70 mm x 42 mm. An opening machined to fit the tip

of the pole piece allows closer magnetic coupling to the workpiece and

mechanically fixes the tip location. The set screws hold the magnet

securely in place. A thin layer of copper tape over the pole tip shields it

from the electromagnetic field of the coil, and prevents sound generation in

the pole piece and the confusion this extraneous signal would generate. The

next layer is the coils on their polymer tape backing. The unshaded portion

in Fig. 3 must lie over the pole tip. The outer wrap in Fig. 5 is copperclad Kapton; the etched window must also line up with the pole tip. The

copper on the wrap is on the inside so the Kapton serves as a wear plate.

Figure 4. Configuration and dimensions of the cold- rolled, mild steel pole

tip. This confines and concentrates the field from a Nd-Fe-B

magnet. See Fig. 5.

10
Figure 5. Schematic assembly of the EMAT. The pole tip, coils, and etched

window must be carefully aligned. On the outer wrap, the copper

cladding is inside while the Kapton serves as a wear plate. The

twisted leads from the coils to the BNC connector are omitted for

simplicity.

11
OPERATIONAL PRINCIPLES

Figure 6 is a schematic of the basic units of this system. Figure 7 is a

photograph of the actual equipment. Some descriptive details follow.

Vfheel

Figure 6. Block diagram of ultrasonic stress measurement system.

Transducer Package and Fixture

The mechanical location of the EMAT on the front face of the wheel is very

important (see EXPERIMENTAL RESULTS). To facilitate precise, rapid

placement, we designed a fixture largely of polyester resin and fiberglass

mat (Figs. 8 and 9). With this apparatus in position and adjusted (see

SYSTEM ASSEMBLY AND USE) , the aluminum cylitider with the EMAT is eased onto

the wheel. Stops in the system allow rapid and accurate rotation between

the two arrival time measurements.

12
Figure 7. Photograph of the equipment represented in Fig. 6.

Figure 8. Ciose-up photographs of the wheel-mount fixture for the EMAT

A. The system attaches to the front face of the rim.

B. The cylinder with the EMAT is then inserted.

13
Aluminum Plate

for EMAT Cylinder

Radial

Adjustment

Screws

Leveling

Screw

Fiberglass /Res In Fixture

Figure 9. Schematic of wheel-mount fixture on a wheel rim (cf. Fig. 8).

After adjustments for position and leveling, the EMAT cylinder is

f inserted in the top plate.

EMAT Electronics

The first five modules of the electronics rack (Fig. 10) contain the power

amplifier to generate the electronic pulse. This produces a gated pulse of

power at 2 MHz. For all the results reported here, the number of cycles in

the pulse was set at 10 with the thumbwheel in module 4. Figure 11 shows

the voltage and current output as measured between the output of module 5

and the external matching circuit. The power amplifier operates at a pulse

repetition frequency of 78 Hz. It generates a trigger with each pulse as a

"start” signal for the interval timer and the oscilloscope.

Module 7 of the electronics rack is a high-gain, low-noise preamplifier for

the received signal. The output goes to the digital gate and the

oscilloscope. The gain is variable by screw adjustment through a hole in

the front panel. Figure 12 shows a typical signal. Upon the initial pulse

from the power amplifier, the preamplifier overloads and recovers after

about 0.03 ms. The signals occurring at just less that 0.1 ms intervals are

multiple acoustic echoes. On this time scale, the individual rf cycles are

not evident.

14
EMAT EMAT

Out In

Figure 10. Identification of the seven modules in the main electronic

unit. Coaxial cable connections go to modules 5-7. Input

power is U.S. standard 110 V, 60 Hz.

Figure 11. Voltage -current output of the power amplifier into the EMAT.

This was measured between module 6 and the external matching

network.

15
Figure 12. Oscilloscope picture of a typical signal from the pre-amp.

The box with the external matching electronics contains:

1 . A diode expander circuit to prevent received signals from

reflecting back into the power amplifier.

2. A low-pass filter to eliminate harmonics.

3. A series capacitor to tune the EMAT for resonance at 2 MHz.

Figure 13 indicates both zero phase angle and a minimum in

impedance to achieve maximum power transfer.

Frequency, MHz

Figure 13. Impedance analyzer plot indicating resonance achieved by the

tuning capacitor in series with the EMAT. This assures

greatest power transfer and signal amplitude.

16
Di gital Gate

The digital gate located in module 6 (Fig. 10) is a special circuit that

allows the high precision timing required by this procedure. It has coarse

and fine controls (accessible through front panel holes) to adjust the time

delay after the "start" trigger from the power amplifier. This roughly sets

the two-way transit time of the ultrasonic signal through the rim thickness.

After this delay, the gate looks for the next positive -going zero crossing

that exceeds 100 mV. When this occurs, it generates a TTL pulse fed into

the stop channel of the interval timer (see below) . This makes it possible

to time precisely the arrival of a selected cycle of the signal. Which

cycle is chosen is unimportant as long as it is the same cycle for both

polarizations. Usually, we select a cycle near the center of the signal

packet where it is strongest and most easily identifiable.

The top BNC in module 6 is identified as the GATE MONITOR. This signal

shows (on an oscilloscope) the allowed time window for the gate TTL pulses.

At the time of initial set-up, displaying the acoustic signal and this

monitor signal simultaneously allows adjusting the time delay so that the

leading edge of the square pulse is near the center of the first echo. The

final delay adjustment is easiest while looking at the individual TTL

signals from GATE OUT.

In practice, we use the oscilloscope to observe the outputs of both the preamp and the digital gate. Figure 14 shows a typical scope display. In this

case, the time base is greatly expanded around the arrival of the first

echo, the top trace. The bottom trace is the gate signal which has been

delayed to the arrival of the cycle at the center (cycle with the maximum

amplitude) . It is also possible to count the cycles from the beginning of

the packet. The first TTL pulse acts as the "stop" signal; the subsequent

pulses occur for each following cycle but are not used.

The gate delay is adjusted for the first of the time measurements and the

EMAT is then rotated for the second measurement. In the event the TTL pulse

then triggers at the preceding or following cycle, the arrival time will

jump by the cycle period. For a 2 MHz signal, this is 500 ns and/ is much

larger than what is usually due to birefringence. This large jump is a

warning that the gate delay needs a minor adjustment.

It is also possible that the gate delay is just at an edge where the TTL

pulse jximps back and forth between successive cycles of the acoustic signal.

This condition has two symptoms:

1. The first TTL pulse of the GATE OUT trace will visibly jitter in

time on the oscilloscope.

2. The readout from the interval timer will be very sporadic between

successive averages (see below)

.

The remedy is to adjust the fine time delay until the gate locks onto the

desired cycle.

17
Figure 14. Oscilloscope trace expanded around the arrival of the first

echo (top trace) . The bottom trace is the digital gate

output. The delay here places it at the center of the echo.

The first TTL pulse stops the interval timer.

Interval Timer

The instrument used to measure the echo arrival time is a commercial

instrument [7] set to read to the nearest nanosecond. The "start" signal

comes from the trigger of the power amplifier while the "stop" signal comes

from the digital gate. To achieve maximum accuracy, we usually set the

timer to average 100 readings; this renews the display every 1.3 s and the

scatter in the displayed arrival time is usually <±10 ns (assuming proper

adjustment of the time delay on the digital gate) . Averaging 1000 times

takes longer, obviously, but does yield very stable data.

The basic configuration (buttons pushed in) we have used is:

Left- to-right , bottom of panel - Power

TI, A-B

.001 /iS, RESOLUTION

Top -to bottom, right of panel - Channel A (start) - xlO

TRIG LEVEL, PRESET-IN

LPF

Channel B (stop) - xlO

18
SYSTEM ASSEMBLY AMD USE

Wheel Mount System

Since the railroad wheel tread wears and changes in profile, this fixture

(Fig. 8) indexes on the wheel's front face surface and the corner where it

turns into the wheel plate (Fig. 9). Spring-loaded plungers make contact

with the tread; these and three magnets support the apparatus in place. The

EMAT must maintain close coupling with the flat surface to produce a strong

signal; therefore, there is a screw for fine adjustment of the plane of the

transducer to assure it coincides with the rim face. The radial location of

the EMAT is very important (see RESULTS); therefore, a top plate, which

positions the transducer, has a limited range of travel in this direction

and locks in place. Pointers in the plate and rulers on the fixture

facilitate this adjustment. Experience with many wheels may (hopefully)

show that this position will very infrequently need change, and measurements

at one radial position will be sufficient.

An aluminiim cylinder holds the EMAT box (Fig. 5) fixed in place by set

screws. After positioning the fixture, the operator slips this cylinder

assembly into the hole in the top plate. There are mechanical stops 90**

apart to facilitate rotation to the two polarization directions.

On the basis of limited practice, it seems that this device allows very

repeatable time readings when it is removed from and replaced on a test

wheel. Moving between the two rotations while maintaining exact location is

very easy.

Connections and Cables

The block diagram in Fig. 6 shows three functional units. These electronic

components are in five discrete boxes:

1. EMAT

A. The EMAT, contained in a large aluminum cylinder.

2 . EMAT Electronics

B. The large rack with seven modules.

C. The matching electronics in a small box.

3. Signal Timing

D. Time interval counter (commercial).

E. Oscilloscope with dual channels and expandable time base.

(This is not part of the equipment delivered.)

Standard coaxial cables connect these five boxes. There are five major

lines (Fig. 15), and we identified each of them with a color to make it easy

to check or re-establish connections. There is a colored dot at each of the

BNC's and a colored band around each end of the coaxial cables.

19
Color Connections

White EMAT

Copper GATE

Red TRIG

Green GATE

Silver EMAT

OUT (module 5) - IN (external matching)

IN (Module 6) AMP OUT (module 7) - CH. 1 (scope)

OUT (module 6) -* CH. A (start, timer) -+ TRIGGER (scope)

OUT (module 6) -+ CH. B (stop, timer) CH. 2 (scope)

IN (module 7) OUT (external matching) - EMAT

Figure 15.

Time Interval Counter Oscilloscope

EMAT

Interconnection of the electronic units by coaxial cable.

Data Collection

To assure best accuracy, the interval timer should have a warm-up period of

at least 15-30 minutes. After completing the physical positioning of the

wheel fixture, placing the EMAT on the wheel rim, adjusting the gain of the

signal amplifier (if necessary), and setting the gate of the digital timer,

the operator can make the actual measurements quite rapidly.

1 . Rotate the EMAT cylinder to one orientation (note arrow marked on

transducer box to indicate polarization direction)

.

20
2. Observe interval timer readout. There will be some minor

variation (<±10 ns) between successive readings; record a t)rpical

value

.

3. Rotate the cylinder 90® to the second position and repeat step 2.

4. Repeat steps 1-3 several times (three to five) to increase

confidence in readings.

5. Average the sets of readings in the two orientations. Discard any

values that cannot be repeated.

6. Calculate the birefringence from eq. (1).

7. Subtract the proper value of the texture birefringence for the

radial position (see RESULTS), and calculate the stress from eq.

( 2 ).

The following is a typical data set:

I Arrival Times, Stress (erg - or)

Radial Circumferential B (10'^) MPa ksi

(Rim block measurement to determine Bq, or birefringence due to texture - generally, this value

such as this)

94.042

94.043

94.040

will be a

94.143

94.144

94.145

previously known average of measurements

Avg. 94.042 94.144 -10.8 ( - Bq)

(Wheel measurement to

90.920

90.917

90.919

determine

90.931

90.934

90.934

B, or total birefringence)

Avg. 90.919 90.933 -1.5 -130 -19

Since the stress in the radial direction is generally negligible, this

example indicates a small compressive hoop stress. The magnitude of the

arrival times depends on the exact thickness of the rim. It is the

fractional change between polarizations that is important.

EXPERIMENTAL RESULTS

EMAT/PZT Comparison

An early collaboration [8-11] with Prof. H. Fukuoka of Osaka University

compared measurements made with electromagnetic and more conventional

piezoelectric transducers

.

The measurements made with the two systems were the same within experimental

error. One important difference did emerge, however. To achieve good

21
acoustic coupling between the PZT device and the wheel rim, it was necessary

to mill a flat area on the rim, while the EMAT could operate over a rusted,

pitted surface. This has obvious advantages for field use where the EMAT

may require only modest wire -brushing to knock loose debris from the test

surface. Figure 16 indicates the comparability of a number of measurements.

The EMAT/PZT difference is smaller than the expected experimental error.

Figure 16 also confirms an earlier finding that the degree of birefringence

is strongly dependent on the radial position of the transducer [6]

.

Figure

17 shows the convention used here. The zero position was at the approximate

center of the width of the front rim face. Negative distances indicate

positions closer to the axle, while positive distances mean sites nearer the

tread. This position dependence is small, but, since it is comparable to

the size of the effects of interest, it is essential that the position be

carefully controlled. Some possible causes of this effect are signal

interaction with the tread surface, the strong variation of the

metallurgical texture (see Texture Measurements below) , or a significant

gradient in the stress state.

These small shifts between measurements meant exerting considerable care in

placing the transducers. The main reason for the eventual construction of

the wheel mounting fixture for the EMAT was to speed up this process while

making it easier and more reliable.

Stress Sensitivity

Previous work [6,12] had shown the sensitivity of acoustoelasticity to the

stress state. However, Dave Utrata at the Association of American Railroads

Technical Center in Chicago lent us a pair of test wheels to demonstrate

this effect in yet another way. While the wheel stress characterization was

not sufficient to quantify or calibrate our measurements, the data did

display the known and expected characteristics.

Both wheels had a saw cut along a radius, from tread to hub. At the outer

(tread) end, there was a larger cut-out to accommodate the actuator of a

small hydraulic press. With a hand-operated pump, it was possible to spread

two sides of the wheel rim apart and introduce a hoop compressive load. For

the measurements plotted in Fig. 18, the EMAT was positioned 180® from the

actuator. After pumping the hydraulic pressure to predetermined levels, the

ultrasonic wave transit time measurements were made at several radial

positions

.

The plots in Fig. 18 reveal several characteristics:

1 . The birefringence increases with higher pump pressure or greater

compressive stress. Using the sign conventions of eqs. (1) and

(2), this confirms that is negative.

2. If we assume that the rim hoop stress is linear with the pump

pressure. Fig. 4 shows the stress is also linear with the

birefringence, as per eq. (2). All the data sets have the same

slope or C/^.

22
I

Figure 16. Comparison of measurements using two types of transducers at

several radial positions on a wheel rim face. The error bars

indicate the standard deviation of the measurements

.

Figure 17. This is the convention for radial position used in the work.

The zero position is at about the mid- line of the front face.

Negative positions are closer to the axle, while positive

ones are closer to the tread.

23
4

Hydraulic Jack Pressure, ksi

Figure 18. Birefringence in two AAR test wheels as hydraulic pressure is

applied to generate compressive hoop stress. The ultrasonic

tests were made at three radial positions on each wheel.

3. The zero-pressure measurements from the two wheels are

considerably different. This is most likely due to different

residual stresses still remaining after the one radial saw cut.

They also likely have a different underlying metal texture since

they are different classes.

4. Again, there is a strong radial dependence, with the most negative

birefringence occurring nearest the tread.

Saw- Cut Comparison

Earlier reports of measurements on an intact wheel [7,10] indicated the

presence of compressive residual stress in the rim. Subsequent saw cutting

at TTC gave qualitative, if not quantitative, confirmation when the wheel

closed on the saw blade during the cutting.

Texture Measurements

As noted previously, the metallurgical texture of the specimen contributes a

significant fraction of the measured birefringence. This must be taken into

account when calculating the stress, eq. (2). If the specimen is available

before being stressed or there is always an unstrained reference specimen

available that contains the same texture, the problem is simply to make two

measurements and subtract. In practice, this is seldom possible- -never for

in-use tests of railroad wheels.

24
The photographs in Figs. 19 and 20 indicate the type of texture produced by

casting. These are the result of polishing and macroetching a thin slice

from the rim of a wheel taken from service. There is a very prominent

dendritic structure growing normally from each exterior surface. The

dendrites are about 20-25 mm long as the result of grain growth during cooldown after casting. This large structural gradient is a graphic indication

of the importance of establishing a fixed EMAT position before making the

timing measurements.

To help learn the magnitude of the texture problem as a prelude to working

on a solution, TTC sent us two rim blocks each from ten different wheels.

All of the wheels were manufactured by Griffin, but they were fabricated by

four different plants. The rim blocks were about 20 cm long and nominally

free of bulk residual stress. The measurements on these are in Fig. 21,

grouped by production plant. Figure 22 combines the four data sets.

Figure 22 gives a worst-case indication of the data scatter. A simple

average of the values is “ -5 x 10”^ with a spread (5 Bq) of ±5 x 10"^. This

translates to a stress uncertainty (^Bq/C^) of ±71 MPa (10 ksi) . Figure 21,

however, indicates two possible refinements of this process:

1. Take into account the plant of origin (known from wheel ID

markings)

.

2. Allow for the variation with radial position by curve fitting each

plant's data (probably with a simple line).

With these procedures and a larger data base, it is possible to cut the

texture uncertainty by a factor of almost 2.

While considerably more data are necessary, these initial measurements are a

positive indicator that this approach will have sufficient accuracy and

sensitivity to become a useful tool.

ELECTRONIC CIRCUITS

Figures 23-31 are the schematics for the stress measurement system,

comprised of five modules for the power amplifier, one module for the

digital gate, one module for the signal pre-amp, and an external box with

the matching electronics. The following list identifies all the components.

25
Figure 19. Cross section of a wheel rim after polishing and

macroetching. Note the dendritic structure that has grown

from each surface

.

Figure 20. Enlarged view of macrostructure in Fig. 19

A. Front face of rim.

B. Root of flange.

26
I

0 --

- 10-

0)o

(Dto

•HU

fH

(D •H

CQ - 10 ”

Colton Plant 1

:

*

1

•

• i

1

>

1

•

Bensenville Plant I

1

1

1

1 t i:

•

i

;

• .

1

1

\ : • . !

^

. i;

1

'

1 i 1

•

1

‘

: -|

1

1

1

1

1

Keokuk Plant !

^

1 ;

__i_ ^

* •..«!:

St. Hyacinthe Plant

•

1

:

. j1 i;

1

• 1

t

•

1

*

! I !

1

1 d

1 -

• Ill 1 turiij ^

0

-.-

-8 -4 -8 -4 Radial Position, mm

Figure 21. Birefringence measured on rim blocks from four production

plants. The effect here is due solely to the metallurgical

texture. There appear to be characteristic differences. An

expanded data base is necessary to refine these trends.

Wheels from Four Plants

Radial Position, mm

Figure 22. Collection of all the data in Fig. 21. Even in this worstcase scenario, with no knowledge of wheel's origin, the

uncertainty in the texture is still within acceptable limits.

27
DIGITAL

GftTE oo • UUJ zzZH

o_i o

(£. NU03

<ro

0. 0

zo

<i<i HU01

<ll- oz • o D Q Z O Z D

QC

DO o o o:

tt: o

(Ky

u in 3 _J H o <i in

0. z in o a O H I

<i in o uIIIu u

t- I- tin in in

Hi'* ^

28

Figure

23.

Power

and

signal

connections

in

the

main

electronic

unit.
inHz

Ul

_J3

O

o

z

<I

O'

ll.

(Ti

o

•Hc

o

4JO

Q) rH

0)

29

Figure

24.

Transformer

and

power

switches

in

module

1,

main

unit.
>oo oz >oo >

UJ

H

q:

>-

-1

Q.

a.D

U)

N

O'

u

llJ -J in X D in o o • 0. o q: X 2 u 0. o U1 >

<I in

L

X

tu 0) -I x X o h- £ > in 3Z

1-

X

U1 o H H

C Z X (U£3U0

0l

O

0*N <r

•Hin

IT*

Cta z >z> o ooo o o >X + X 1 o ??? 6 iio az UOO o <to<i > x>x X X

<II >X o

<IX

30

Figure

25.

High

voltage

power

supply

in

module

2,

main

electronic

unit.
NTE168

TTfV

UUO TTfi*

>.J utilbfbl

u> Kcas

OM

MNX9N

TT «b.

-<C V-<

•4N XUU

I^UU m

(i. 9N>

•Sfi UVN

k.9N>

n •u» UVN

^fXU >1

>03JL. >03K fHX

IHZ > IM

k.O V-l XU XU

"

1^

Ul 3 M O

Z

1 a

£ I

—a.

.js.

w

N

NNX

XU >1

>03»- hX

>U IMX

wu U-l

XU >1 >N MOJk. >03^ hX

IhX > UIA kU uu

XU XU

4J

•rHc

:3o

c

o

u

u

o

<u

r—{

(U

•rHCOs

CO

0)3

T3O

S

c

•H

—Ia

a

;3

(0

(Uo

a

0)bO

cO 4J

i-Ho

>

o

l-J

VO

CM

(U

:3bO

•H

pv,

31
32

Figure

27.

Burst

logic

in

module

4,

main

electronic

unit.
+1

33

High

power

driver

in

module

5,

main

electronic

unit.
34

Figure

29.

Digital

gate

in

module

6,

main

electronic

unit.
35

Figure

30.

Signal

preamplifier

in

module

7,

main

electronic

unit.
>u

q; 0

U1Hz Xo

CD

in

in

bJa:

hin

<rz

a:u

i-Xu c

Oi£3u0Q_mNH

in

•HL

Q. <r

36

Figure

31.

Matching

electronics

in

external

box.
1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

2223

2425

26

27

28

29

30

31

32

33

34

List of Electronic Components

Quantity Reference Part Item Quantity Reference Part

3 U1,U2,U4 m723 37 4 R30,R31,R32,R33 7.5k

4 C1,C2,C6,C11 1000 38 3 C36,C35,C37 .OOluF

1 R1 500 39 1 C38 680pF

1 R2 750 40 1 U8 AH0013

3 R3,R5,R10 5.6K 41 1 U9 TL071

12 C5,C8,C10,C13,C15,C21,

C31,C43,C46,C47,C59,C60

.1 42 1 C39 390

43 5 C41,C40,C49,C50,C56 .01

1 R4 820

44 2 C42,C44 1

2 U3,U5 Uf7805

45 2 R34.R35 POT

7 R6,R11,R14,R17,R49,R75,

R76

IK

46 1 JPl HEAOER 2X2

2 R7,R12 3K 47 1 UlO 759N

6 R8 , R13 , R27 , R28 , R29 , R67 3.3K 48 7 R37,R38,R41,R51,R52,R73,

R74

100

1 R9 2.2K

49 2 R39,R40 5.11K

3 D1,D2,D3 NTE168

50 6 R42 , R4 3 , R50 , R55 , R68 , R69 lOK

4 T1,T2,T1B1,T2B1 FP34-340

51 2 R44,R47 6.8

7 P1,P2,P4,P5,P6,U11,U12 CONN FRA8X2

52 2 R45,R46 30

3 O a a 2N2222

53 1 R48 7.5K

3 Q5,Q1,Q3 2N6122

54 1 02 0 NTE109

6 C16 , C17 , C18 , C61 , C63 , C64 InF

55 1 Q9 2N4904

14 C3 , C4 , C7 , C9 , C12 , C14 , C19

,

C20,C29,C30,C51,C57,C58,

4.7UF

56 1 QIO NTE190

C83

57 1 Qll 2N3906

2 U6,U7 TSC429

58 1 Q12 NTE189

4 D4 , D6 , D8 , D9 MUR1550

59 2 T3,T4 1:9

2 05, D7 1N5248A

60 1 SWl SW SPOT

2 R15.R16 3.3

61 1 J4 OOTPOT

1 J1 EMAT XMIT

62 1 J5 INPOT

2 Q7.Q8 IR9017

63 1 T5 XFMR

3 R18,R21,R54 220

64 2 018,019 1N4153

1 R24 150

65 1 U ISOnH

3 R25,R26,R36 10

66 1 L3 lOmH

4 16

R19,R20,R22,R23 4.7

67 1 C45 lOOpF

C23 , C24 , C25 , C26 , C27 , C28

,

C84 , C85 , C86 , C87 , C88 , C89

,

C90,C91,C92,C93

lOOuF

68 1 C48 SOOpF

69 1 ICl LM360

2 C33.C34 2uF

70 1 R53 680

2 C32,C22 luF

71 1 C52 27

4 D10,D12,D13,D16 1N3600

72 4 IC2,IC6,U6,U27 74S04

4 D11,D14,D15,D17 1N523S

73 8 IC3,IC4,U13,U14,U19,U20, 74121

2 J2,J3 BNC 021,024

2 L1,L2 2.5 uH 74 1 ICS 74S74

37
List of Electronic Components (cont.)

Item Quantity Referenc* Part

75 1 P3 204-lK 76 1 J6 GATE IN

77 1 J7 TRIG OUT

78 1 J8 GATE OUT

79 1 J9 GATE MONITOR

80 1 L5 7 BEAOS

81 4 D21, 022,023,024 1N4148

82 1 C54 3300

83 1 C55 1500pF

84 1 C53 270pF

85 2 U15,U23 74LS00

86 5 R56,R57,R58,R59,R60 470

87 1 U16 7474

88 2 U17,U18 74LS192

89 2 U22,U25 7404

90 2 C65,C66 47 91 2 R61,R62 10k

92 2 R63,R65 270

93 2 R64,R66 360

94 2 U26,U32 HCL2601

95 3 U28,U29,U31 74S140

96 1 U30 74HCT4046

97 1 R70 5. IK

98 1 R71 50K

99 1 JIO TO

100 1 SW2 BCO SW

101 15 C68 , C69 , C70 , C71 , C72 , C73 , C74 , C75 , C76 , C77 , C78 , C79

,

C80,C81,C82

0.1

102 1 C67 470pF

103 1 C62 150pF

104 1 T6 200KVA

105 1 SW3 POWER

106 1 SW5A1 HIGH VOLTAGE

107 2 SW5C1,SW5B1 3P2T

108 1 R72 15K

109 2 025,026 LN28RCP

110 4 027,028,029,030 1N4006

38
CHECKLIST

1. Check that all connecting cables are installed. Check color ID's.

2. Turn power ON: main electronic unit, time interval counter, and

oscilloscope. Check timer settings and allow 15-30 minutes warmup.

3. Locate wheel fixture, making sure the EMAT will seat flush on the rim

of the front face. Check radial position of top plate.

4. Insert EMAT cylinder in wheel fixture. Rotate to first position.

5. Check oscilloscope display for signal strength, and digital gate

position and stability.

6. Observe time display for reasonable stability (<±10 ns variation) and

record.

7. Rotate transducer 90* and repeat steps 5 and 6.

8. Rotate transducer to original position and repeat steps 5 to 7 about

two more times

.

9. Average the two sets of data and calculate the birefringence, eq. (1).

10.

Subtract the texture birefringence and calculate the stress, eq. (2).

TROUBLE SHOOTING

Some potential problems with possible solutions include:

1. Poor signal, low

amplitude 1st echo

Check to see that the EMAT is seated properly

on the rim face. This will likely mean

adjustment of the wheel fixture.

2. No signal The EMAT coil may be shorted or open. Its DC

resistance should be about 5 Q.

3 . Time j itter between Adjust the fine control of the digital gate

successive timer delay.

readings (>±10 ns)

4. Time differences The stress level may be very high or the

digital gate may be jumping a cycle between

the two readings. Check the time -expanded

oscilloscope display to be sure the gate TTL

signal triggers on the same cycle. Adjust the

delay if necessary.

between the radial

and circumferential

signals of about

400-500 ns

39
FUTURE CONSIDERATIONS

1. It may be possible to stack two sets of EMAT coils, one atop the other,

in orthogonal orientations . Connecting first one and then the other

coil to the EMAT electronics may permit timing the signals from the two

polarizations without physically rotating the transducer.

2. Computer controls are possible to fully automate this procedure:

A. Find the center of the acoustic pulse of the first echo and adjust

the window of the digital gate.

B. Switch between the two coils of #1.

C. Use a time interval counter with an IEEE-488 bus to allow direct

input

.

D. Perform all record keeping and calculations.

3. There may be advantages to using ultrasonic waves of other modes.

Rayleigh waves would yield specific information on surface conditions.

Longitudinal or P-waves can be directed along the surface or through

the bulk; in addition, their stress acoustic constant is larger so

they would be a more sensitive probe. It may be possible to combine

information from the different modes to minimize the texture effects.

4. The BNC coaxial connectors should probably be replaced with a more

durable type for long-time field operation.

5. Investigate a more durable wear plate for the contact surface of the

EMAT. Experience will indicate whether this is a problem.

6. An enlarged data base on the texture effect will increase the

reliability of eliminating this parameter.

7. Experience will dictate modifications to the wheel fixture to increase

its flexibility and reliability.

ACKNOWLEDGMENTS

A considerable number of people have been involved in this work in various

ways and at various time.

From the FRA, Clifford Gannett, Claire Orth, and Donald Gray have all

provided valuable guidance.

From TTC we received a great deal of help and cooperation in obtaining test

wheels. Of course, many tests would have been impossible without their

collaboration. Our main contact has been Robert Larson, Jr.

,

who put a

great deal of effort into this work. Some of the many others include Britto

Rajkumar, Robert Vandeberg, and Robert Florom.

40
Both before and after his stay as a guest researcher in our laboratory,

Professor Hidekazu Fukuoka of Osaka University was a valuable guide to the

stress effects on ultrasound, particularly in railroad wheels.

Within our own lab, there have been many helpful ideas and considerable

assistance at all stages. We are especially grateful to Dale Fitting, Jon

Fowler, Peter Shull (now at the Dept, of Materials Science, Johns Hopkins

University), Christopher Fortunko, Daniel Vigliotti, and many others. Tim

Waldorf's skills were responsible for producing the wheel fixture. Avinoam

Tomer (guest researcher from the Nuclear Research Center Negev, Beer Sheva,

Israel) polished and etched the wheel rim cross section.

REFERENCES

[1] "Residual-Stress Measurements Using Shear -Horizontal Waves from

Electromagnetic -Acoustic Transducers," R. B. King and C. M. Fortunko,

NBSIR 84-3002 (March 1984). (This is a collection of published

papers .

)

[2] "Acoustoelasticity and Ultrasonic Measurements of Residual Stresses,"

Y.-H. Pao, W. Sachse, and H. Fukuoka in Physical Acoustics . Vol. XVII,

Academic Press, New York (1984), 61-143.

[3] "Magneto -Acoustic Stress Responses of Various Rail Metallurgies," D.

Utrata and M. Namkung in Review of Progress in Quantitative

Nondestructive Evaluation . Vol. 9B, D. 0. Thompson and D. E. Chimenti,

eds.. Plenum Press, New York (1990), 1903-1910.

[4] DEBRO-30 Stress Meter,* developed by the Institute of Fundamental

Technological Research, Polish Academy of Sciences. Marketed by

ELTRONA GmbH, Hamburg, Federal Republic of Germany.

[5] "The Design and Use of Electromagnetic Acoustic Wave Transducers

(EMATs)," B. W. Maxfield and C. M. Fortunko, Materials Evaluation 41 . 1399-1408 (1983).

[6] "Acoustoelastic Measurements of Residual Stresses in the Rim of

Railroad Wheels," H. Fukuoka, H. Toda, K. Hirakowa, H. Sakamoto, and Y.

Tota in Wave Propagation in Inhomogeneous Media and Ultrasonic

Nondestructive Evaluation . Vol. 6, G. C. Johnson, ed. , ASME (1984),

185-193.

[7] B6tK model 1822, 175 MHz Universal Counter

The use of tradenames in no way implies endorsement or approval by NIST

and is included only for identification.

41
[8] "Ultrasonic Characterization of Residual Stress and 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.

[9] "Characterization of Residual Stress and Texture in Cast Steel Railroad

Wheels," A. V. Clark, Jr., H. Fukuoka, D. V. Mitrakovic and J. C.

Moulder, Ultrasonics 2^, 281-288 (1986).

[10] "Ultrasonic Characterization of Residual Stress and Flaws in Cast Steel

Railroad Wheels," A. V. Clark, R. E. Schramm, H. Fukuoka, and D. V.

Mitrakovic in Proceedings: IEEE 1987 Ultrasonics Symposium . B. R.

McAvoy, ed. , Institute of Electrical and Electronic Engineers, New York

(1988), 1079-1082.

[11] "Acoustoelastic Measurements Pertaining to the Nondestructive

Characterization of Residual Stress in a Heat-Treated Steel Railroad

Wheel," A. V. Clark, H. Fukuoka, D. V. Mitrakovic^, and J. C. Moulder,

Materials Evaluation 47. 835-841 (1989).

[12] "Nondestructive Evaluation of Material Properties with EMAT

(Formability in Cold Rolled Steel Sheets and Residual Stress in

Railroad Wheel)," R. Murayama, K. Fujisawa, H. Fukuoka, M. Hirao, and S.

Yonehara in Proceedings : IEEE 1989 Ultrasonics Symposium . B. R.

McAvoy, ed. , Institute of Electrical and Electronic Engineers, New York

(1990), 1159-1162.

42
BL-114A

(5-90)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

bibliographic data sheet

1. PUBUCATION OR REPORT NUMBER

f^ISTIR 3968

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBUCATION DATE

May 1991

. TITLE AND SUBTITLE

Report No. 23 - Residual Stress Detection in Railroad Wheels: An Ultrasonic System

Using EMATs

AUTHOR(S)

Ra3nnond E. Schramm, Alfred V. Clark, Jr., Dragan V. Mitrakovic^, Stephen R. Schaps, and

Todd J. McGuire

PERFORMING ORGANIZATION (IF JOINT OR OTHER THAN NIST. SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BOULDER, COLORADO 60303-3328

7. CONTRACT/GRANT NUMBER

DTFR 53-89-X-00018

8. TYPE OF REPORT AND PERIOD COVERED

Final 9/85 - 12/90

I. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET. CITY. STATE, ZIP)

U.S. Dept, of Transportation, Federal Railroad Admin.

Office of Research and Development, Washington, DC 20590

0. SUPPLEMENTARY NOTES

11. ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOGRAPHY OR

LITERATURE SURVEY, MENTION IT HERE.)

This is report number 23 in a series covering research performed by the National Institute

of Standards and Technology for the Federal Railroad Administration. This report

covers a project by the Materials Reliability Division (formerly the Fracture and

Deformation Division) to develop and build a system to detect and measure residual

stress in the rims of railroad wheels.

Acoustic birefringence is the underlying principle of operation. This is a measure

of the relative difference in the propagation times of two shear waves polarized in

radial and circumferential directions. Measurements employ an EMAT (electromagneticacoustic transducer) as an ultrasonic probe. This type of transducer requires little

or no surface preparation and no acoustic couplant. The system operates in a pulseecho mode. A short burst of shear horizontal waves travels through the rim thickness.

The rotation of the EMAT determines the orientation of the polarization vector, radial

or circumferential. Precise timing of echoes in both directions reveals the degree

of birefringence. Changes are due to both stress state and metallurgical texture.

Initial tests indicate it may be possible to separate these two.

This report documents the design, construction, and testing of the system. It is

also to serve as an operational guide for the equipment being delivered to the FRA.

12. KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITALIZE ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

birefringence; EMAT; nondestructive testing; railroad wheel; residual stress; ultrasonic

13. AVAILABILITY UNUMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS).

X

ORDER FROM SUPERINTENDENT OF DOCUMENTS. U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD, VA 22161.

14. NUMBER OF PRINTED PAGES

48

15. PRICE

ELECTRONIC FORM

'A’U.S. GOVERNMENT PRINTING OFFICE: IWl • 575-449/45109
v.’vT-^ :•

' .. . ^- - -f i. . -f. I. •-

. 'S«E ‘

' '

. ' 'i' iJWA't'jiL 'i - 4'»'_f”of

.

- .I'M

••; ^'^'- <»UV-V, ’^.'

^ ^-.r. . ,

•

: ii,v W?S^'

jV "i?

,,'\J':.

„ ;_

'S'

I '.' \

•( “ sf' ikA ' - .• - ’'.

, >

V- - ^ih, 4" E

' ^ ' : 1 . ,<«*

. =' .•^

•' h>% ''= *: ’'•'Ci "iV- '=?*ii.4, i': ' fit ,

- :'i

r' .•' J ,

'. r *•

. '-I - .'-, 4. .

...-.H ;' :

- -t «"':i

' ''.!

. .-ur- '^»V"=*

' • - . X.^.-

:

; '*,.St:«! . '^: t ” "" ' *

’ "

' • .• ' , ma»

• *':'-

;'ic. ; . j . .. i.'K*-,;-.tfi .;

-' ;<"' . .i K'.*S^K

. . '^ 'V

.' ' ..

' i-'i'^

' 4^4 'i f/CI;4iv 4i^:V"

•t; , : ^ ; ',: -J .

.':

, ./• .•i<*-;^r:.^-<15‘ -;iv.

tf T.

» :.*f ^ >>. <

':wv-: ^i4T

. , ,!.s'‘\'-

-4'/:

•' .»

ir.:o ;.V"i-

, V, r'V i

y

..

' i ^ '1 'd

.a^.t-: = !

’ A .V

"

.,..4

’.^ 'V ' 'kv ' M ' i.'ir 4-. ^-4- 4.-- . 4-v'

'

I

, /

•

' .. r,. ,

'' 4 "" '

'

f , '

. '4 \,

,,('*' ••v'^r '?4'‘ #

•••&•.

!, . 4 . * ..-> ' ...4' ;: . 4 . ;-v4, .'V , . V.44- =

i i--.;

Mi.

[<w-

-,v."

'kv'ik' ^4' ...T .:’'""^4''-'

,• , / .1 flWijp'k:-''',

I '** " " -"'4

' -'

. .A-' -lYv.^ :

'

’ 4 ;,,

’ .W: ;. . :i4IMliUi .>,. rYSfO .