A 1 1 1 0

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573153

WBSIR 33-

An Assessment of Ultrasonic

Reference Block Calibration

Methodology

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*

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Manufacturing Engineering

Mechanical Production Metrology Division

Ultrasonic Standards Group

Washington, DC 2C234

NBS

PUBLICATIONS

A 1 1 1 0 4 5043^1

2710

June 1383

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U S. DEPARTMENT OF COMMERCE

NAT!0NAL BUREAU CF STANDARDS

NBSIR 83-2710

A« « AN ASSESSMENT OF ULTRASONIC

REFERENCE BLOCK CALIBRATION

METHODOLOGY

NATIONAL EUREAU OF STANDARDS

LIBRARY JUL 5 1983

noY o^cjc. - ore.

<Tp c \ oO

G. V. Blessing

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Manufacturing Engineering

Mechanical Production Metrology Division

Ultrasonic Standards Group

Washington, DC 20234

June 1 983

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

TABLE OF CONTENTS

Page

List of Figures 1

List of Tables 2

Abstract 3

1. Introduction 4

2. Background 4

3. Approach 6

4. Results 6

4.1. Block Material 6

4.2. Transducer 9

4.3. Pulser/Receiver 11

4.4. Primary Reference Standard 12

4.5. Operator Judgment & Procedures 13

4.6. Block Geometry 14

5. Recommendations 14

6. Conclusions 16

7. Acknowledgements 16

8. References 17

Appendix 20

LIST OF FIGURES

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Illustration of an ultrasonic flat-bottom-hole reference block.

The principal components of an ultrasonic calibration system for

reference blocks: the transducer, the pulser/receiver , the primary

reference standards, the reference blocks and the immersion

tank and yoke system to suspend and manipulate the transducer relative to

the targets.

Echo amplitude data as a function of metal-path distance for an anomalous reference block set (diamonds), compared with typical

values (solid line).

A continuous profile scan of the back-surface echo amplitude from A)

the anomalous block material, and B) the normal block material.

Far-field center axis beam profiles of a ceramic (dashed line) and

a quartz transducer (solid line).

Reference-block-echo amplitude data taken with a ceramic transducer

(discrete points), relative to that taken with a quartz transducer (solid

line), as a function of metal path distance for a No. 5 hole set. Ceramic

values using two distinct targets before the application of correction

factors (diamonds and triangles), compared to the corrected values

(stars) using a block target.

1
LIST OF TABLES

Table 1

Table 2

Relative echo amplitude responses of a reference block (5-0050) and a steel

sphere (5/16-inch diameter) as a function of pulse width.

Steel sphere diameters used to set system sensitivity for the respective

block hole sizes, according to ASTM E127.

2
ABSTRACT

The state of the art in aluminum ultrasonic reference block calibration

practices is reviewed, especially as it has been guided by the recommended

practices of ASTM for aluminum blocks. The principal system variables in the

calibration procedure are identified, and recommendations for reducing their

associated measurement errors are presented. Quantitative evaluations of the

limitations to improving measurement precision are made in light of present

technology. Suggestions for improving present practices are given, and

extensive reference to the relevant technical literature is made.

Key words: ASTM E127; ASTM reference block calibrations; ultrasonic aluminum

reference blocks; ultrasonic system calibration; ultrasonic

transducer calibration; nondestructive evaluation.

3
1. INTRODUCTION

Flat-bottom-hole reference blocks have been used as defect artifact standards in

ultrasonic nondestructive testing for many years. They were first introduced into

an official document of recommended practice in 1958 by the American Society for

Testing and Materials (ASTM), where the application was specifically to aluminum

alloy blocks [1]1. Whereas the title of that document, "Fabricating and Checking

Aluminum Alloy Ultrasonic Standard Reference Blocks," has not changed, its

requirements for block acceptance have changed considerably. At its inception, the

practice called for a tolerance of + 1 dB relative to a set of standard tabulated

values for echo amplitudes from the flat-bottom-holes. While subsequent efforts

were made to improve the original practice, the tolerance has in fact been gradually

relaxed to its present level of +2 and -3 dB [2].

Recently, the United States Department of Defense, by way of an Army-sponsored

project, sought to reduce that tolerance to the original + 1 dB level in its own applications. For that purpose, the National Bureau of Standards was asked to

define the necessary practice and rewrite the appropriate documentation. This

report^ summarizes the feasibility of doing so, and the requirements involved.

This report is not intended to reflect on the intrinsic value of flat-bottom-holes

as artifact standards or as defect simulators. (Refer to the Appendix.) That is,

it is not the purpose of this work to develop a better standard with which to

enhance flaw assessment. Rather, it is our goal to achieve greater uniformity of

measurement results among different laboratories by means of a more precise practice

to fabricate and utilize the artifact standards prescribed in ASTM.

2. BACKGROUND

In this report, ultrasonic reference blocks refer to a set of aluminum cylinders two

inches in diameter, and lengths ranging nominally from one to seven inches. At one end of each cylinder, a flat-bottom-hole of a specific diameter (typically 3/, 5/, or 8/64 inch) is coaxially drilled to a depth of 0.75 inches. Figure 1 illustrates

this geometry. With system sensitivity for each hole size defined by the response

from a corresponding diameter metal sphere, a calibration is performed by measuring

the amplitude of the ultrasonic echoes from the flat-bottom-hole as a function of

metal path distance. The originally prescribed tolerance for these echoes, to which

we hope to return, was + 1 dB relative to a set of standard amplitude values.

Two distinct philosophies may be considered for achieving the + 1 dB objective.

With one, a procedure similar to ASTM E127 may be followed wherein each separate

laboratory facility would be self-sufficient for its own calibration work. With the

Numbers in brackets indicate the literature references at the end of this

report.

2

Presented, in part, at the 1982 Spring and Fall conferences

of the American Society for Nondestructive Testing [3,4].
Ultrasonic Wave Entry Surface

Figure 1

.

Illustration of an ultrasonic flat-bottom-hole reference block

Reference

Standards

Figure 2.

Reference

Blocks

The prinicpal components of an ultrasonic calibration system for reference

blocks: the transducer, the pul ser/receiver , the primary reference standards,

the reference blocks and the immersion tank and yoke system to suspend and

manipulate the transducer relative to the targets.

5
other, a central calibration facility would evaluate all blocks used in Defense

Department work. The principal advantage of the first approach is the independence

each field laboratory would have to perform its own calibrations. The principal

advantage of the second is to substantially reduce interlaboratory system variables

by applying the same instrumentation and detailed procedures to all calibrated

blocks. Correction factors could then be applied to correlate with the readings of

the central calibration facility. For a comparison of these approaches with other

national and international standard practices, see references [5] and [6]

.

For this work, the first approach was chosen, whereby each individual laboratory

facility would perform its own ultrasonic calibration using its own block set

purchased directly from the manufacturer. This is the more ambitious approach,

requiring greater block and test system uniformity amongst different facilities, but

also allows for a large degree of inter-facility flexibility and independence. In

either case, it is necessary to explore the detailed system variables to determine

the optimum feasibility.

3. APPROACH

A review of the literature, supplemented by experimentation in specific areas, was undertaken to define the major system variables. Consideration was limited to

liquid-immersion testing, as direct contact techniques were judged not feasible for

the desired measurement precision. In direct contact measurements, repeatability

error alone far exceeds the desired + 1 dB tolerance.

The ultrasonic system may be divided into two principal components: the reference

blocks to be measured, and the measurement system consisting of the. electronic

equipment and ultrasonic transducer. While our goal is to evaluate block variables,

to achieve that we must consider the variables of the entire system. For purposes of

analysis, the principal system variables were categorized as follows (see Fig. 2):

1. Block Material

2. Ultrasonic Transducer

3. Pulser/Receiver

4. Primary Reference Standards

5. Operator Judgment and Procedures

6. Block Geometry

These principal variables, listed in an approximate order of descending importance,

were in turn subdivided and quantitatively analyzed, where possible, for their

contribution to net system variability.

4. RESULTS

4.1. Block Material

The block material has proved itself to be a significant variable in the ultrasonic

calibration system. The material parameters which may contribute to variations in

ultrasonic scattering and, therefore, to variations in the received echo amplitude

include texturing, grain size, microporosity, and residual stress.

6
Although ASTM E127 recommends specific procedures for processing aluminum bar, the

subsequent evaluation of material ultrasonic properties is very limited. Prior to

sectioning, ultrasonic waves are propagated diametrically through the extruded or rolled bar, and the material is judged acceptable if the back-scattered waves from

within the bar are below a certain level. Prior to hole drilling, no evaluation is

made of scattering for wave propagation parallel to the bar axis, which is the wave

direction for echo amplitude calibrations.

While the present screening requirements for raw bar material may be inadequate, the

difficulty in establishing more precise quantitative criteria becomes apparent with

the following considerations. If we arbitrarily decide to limit the contribution of

material variations to one-half of the desired +1.0 dB net system tolerance, we can easily calculate the maximum allowable variation in ultrasonic attenuation for the

bar material. For the 0575 (i.e. 5.75 inch metal path distance) block the 11.5 inch

round trip path for the ultrasonic waves results in a + 0.04 dB/inch restriction in

material variation. This would probably be a difficult tolerance to meet in

aluminum and would, in any event, certainly be difficult to measure.

In practice, a reasonable approach would be to establish a reference standard for

evaluating relative material attenuation; , for example, a long (0575) block with a large (No. 8) hole (i.e. 8/64 inch diameter hole). Measuring relative material

attenuation (or scattering) by an echo from a flat-bottom-hole serves to evaluate

the wave propagation properties of the material in the center region of the block,

which is the region of most pertinence for block calibrations. Using a large hole

minimizes the effect of fabrication errors on echo response, and provides a greater

signal to noise ratio for evaluating the echo amplitude.

An extreme example of material variability was reported in a particular No. 5

hole block set [7]. Figure 3 compares the results on that anomalous set with

typical data for acceptable blocks (solid line). Two facts of note are that the

anomalous data (a) are greater in amplitude than the standard data, and (b) increase

in amplitude for metal path distances greater than two inches, dramatically

diverging from the standard data in the case of the longest blocks. The apparent

inference from this second observation of a negatively attenuating block set is very

difficult to explain, but may be conjectured to be the result of beam focusing due

to material texturing. Upon confirming hole size, shape, and depth integrity by

means of radiography and block sectioning, this hypothesis was tested by

ultrasonically scanning the 0375 block (3.75 inch metal path distance) material

itself with the hole removed. A continuous-amplitude profile of the back-surface

echo amplitude from this specimen is compared with that from a like-sized normal

cylinder^ in Fig. 4. The signal level of the peak observed in the anomalous

cylinder is 4.5 dB greater than the corresponding level in the normal cylinder,

representing an apparent attenuation difference of 0.6 dB/inch at the center of the

blocks. Metallographic studies revealed a finer dendritic cell structure (with

little or no cellular substructure) in an anomolous block than in a good block [8],

How this might be related to the observed ultrasonic phenomena is not clear,

however.

3

This normal cylinder was processed from the same bar material as a block set

which satisfied the specifications of ASTM E127.

7
100.0

Figure 3.

Echo amplitude data as a function of metal -path distance for an anomalous

reference block set (diamonds), compared with typical values (solid line).

Figure 4.

A continuous profile scan of the back-surface echo amplitude from A) the

anomalous block material, and B) the normal block material.

8
4.2. Transducer

A second significant contribution to the net tolerance of the ultrasonic calibration

system is the transducer or search unit. Transducers of supposedly similar or even identical construction have been observed to vary appreciably in their performance

characteristics. Substantial research has been undertaken to elucidate the causes

[9-13].

The transducer is itself a complex subsystem of the ultrasonic system. The

properties of its piezoelectric element vary widely, even for the same piezoelectric

material such as ceramic or quartz [14]. Furthermore, its physical structure or housing can have a significant effect on its performance via the damping effects on the piezoelectric crystal. The addition of tuning elements to the transducer

package, as is often done to increase sensitivity, causes additional performance

variations. Many researchers agree that the following minimum operating parameters

should be specified for transducer standardization: center frequency, bandwidth,

damping factor, effective area of the piezoelectric element, beam geometry, loop

sensitivity (conversion efficiency), and electrical impedance [15-18],

If we restrict ourselves to the use of a specific frequency and material type, e.g.

a 5 megahertz quartz crystal, for our pieozoelectric element, (in accordance with

the requirements of ASTM E127) , some variability still exists. Comparison tests

of a set of six quartz transducers, all meeting the transducer specifications of

ASTM E127, revealed variations of + 1 dB in their echo response from various block

sizes [11]. An encouraging aspect to that study, however, was the ability to com- pensate for the respective transducers’ beam pattern differences by means of a farfield echo normalization procedure. This procedure of correction factors was able

to reduce the variation in block response observed with the different transducers to

a few percent.

In a recent effort to extend this correction factor approach to include ceramic

transducers, an empirical study was made to compensate for the response of a given

ceramic transducer of a size and fundamental frequency equal to the ASTM El 27 quartz

standard [4]. The far-field center axis profiles, determined via the echo amplitude

from a 0.5-inch diameter spherical steel target, of the two transducers are shown in

Fig. 5. The transducers’ echo amplitudes were normalized relative to each other by

setting the response of each to 80% of full scale reading at their respective Y^

points. Correction factors were then calculated to raise the ceramic readings to

match those of the quartz transducer. Applying this approach to reference block

calibrations failed in one application, but proved successful in another. In the one application, initial system sensitivity was defined by the echo from a 5/16 inch

diameter steel sphere according to ASTM E127. Applying the correction factors

determined from the data of Fig. 5, however, resulted in severely overcompensating

for the ceramic transducers' lower echo values. In the other application, initial

system sensitivity was defined by the echo from a No. 5-0050 reference block. (The

sensitivity setting for the quartz transducer was the same for both targets.) In

this case, applying the same correction factors resulted in good agreement with the

quartz transducer data, as shown in Fig. 6.

9
Figure 5.

Far-field center axis beam profiles of a ceramic (dashed line) and a quartz

transducer (solid line).

20

Ui

Q 10

Q.

5

<

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X

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LULU>

-10

Ui

cc

-20

Quartz

Ceramic Bail Target

Block Target

* Bl. Tar. Corrected

*** * -

:

* *

X

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2 3 4

METAL PATH, in.

Figure 6.

Reference-block-echo amplitude data taken with a ceramic transducer (discrete

points), relative to that taken with a quartz transducer (solid line), as a function of metal path distance for a No. 5 hole set. Ceramic values using two

distinct targets before the application of correction factors (diamonds and

triangles), compared to the corrected values (stars) using a block target.

!'

10
A surprising feature to be noted in Fig. 6 is the improvement of the uncorrected

ceramic data taken with initial sensitivity determined by the block target

(diamonds), over that taken with sensitivity determined by the spherical target

(triangles). While the subsequent application of correction factors (stars)

enhances the agreement with the quartz data (horizontal line) , a very significant

factor for improving the agreement between the two transducers' data appears to be

the target used to set system sensitivity [19]. Apparently, the spherical target

interrogates features of the ultrasonic beam that are distinctive from those

interrogated by a flat-bottom-hole in metal and, therefore, the relative echo

difference between the two targets does not transfer proportionately when setting

system sensitivity with different transducers. This is attributed principally to the

following two factors: (1) target geometry differences—spherical versus flat, and

(2) refraction of the ultrasonic wave at the water-metal interface upon entering and

exiting the block.

In contrast with this purely empirical approach, a combination theoretic-empiric

approach has been recommended by some researches [12,20,21] to develop a standard

transducer. With this approach, any transducer becomes a "standard" transducer by

having its electro-mechanical transfer function, determined. The approach is to

excite the transducer with a known voltage and current pulse, measure the voltage

and current of the received echo signal from a known target, and having established

the propagation losses between target and transducer, to deconvolve the transfer

function. A crucial requirement of this approach is that the target be known, i.e.

possess a theoretical solution for its scattering profile. The implementation of

this approach will require the development of sophisticated computer software to

effectively handle the deconvolution process.

4.3 Pulser/Receiver

The electronic pulser used to drive the transducer, and the receiver used to amplify

the target echoes, comprise the principal electronic components in the ultrasonic

calibration system. Pulse width and shape variations, together with receiver

nonlinearity, can substantially affect system performance [19]. Specifically, RF

pulse width variations have been observed to cause echo amplitude changes on the

order of one dB (refer to Table 1 in Section 4.4). While such behavior is usually

ascribed to changes in the spectral content of the pulsed wave form, and the

scatterers' distinctive frequency dependent responses to that pulse [13, 22, 23],

mode conversion subtleties may also be playing a significant role [24]

It is not clear how these pulse variables may be controlled in practice, except that

at least the pulse width and shape should be kept constant. The NBS does use a constant pulse width for its reference block calibration work, and monitors the

shape and spectral content of the output pulse [25]

The ASTM is considering in a draft document, the use of a square wave pulse of specific width related to the wave frequency [18].

The presently allowed tolerance for receiver nonlinearity according to ASTM may be as large as + 5 percent of full scale [2]

For lower echo value block readings this

results in a very large tolerance. For example, + 5 percent out of a 20 percent

reading is + 2 dB. Present equipment design should permit a requirement of +1

percent or less nonlinearity.

11
4.4. Primary Reference Standards

The primary reference standard used in ASTM E127 for initializing system sensitivity

is the echo from a spherical steel target (i.e. , ball bearings ) of a specific

diameter depending on the block hole size. The difficulty in obtaining reproducible

results with this approach has apparently been due to differences in transducer

response to spherical and flat-bottom-hole reflectors, according to the arguments

presented above. This means that the amplitude ratio for the responses of different

transducers from a spherical target may not be the same as those from a flat-bottom-hole. Possible solutions to this problem may be (1) keeping the

transducer constant (or compensating for transducer differences), (2) using a different target, or (3) substituting an electronic source for the target echo. The

difficulty of the first approach was discussed above. A discussion of the other two

approaches follows.

With regard to the second approach, the effect of pulse width on the two targets’

sensitivities to beam characteristics may be exemplified by the data of Table 1.

The receiver gain was adjusted to keep the echo amplitude from a 5-0050 reference

block constant at 80 percent of full scale for three discrete settings of pulse

width, all of which allowed for ample resolution of the hole echo. The resultant

one dB variation (from minimum to maximum pulse width) in the spherical target echo

demonstrates a difference in sensitivity to the two targets which clearly is not

desirable in block calibration. Results like these have led to the conclusion that

the flat-bottom-hole provides a better primary reference standard for purposes of

block calibration than does the steel sphere [4,7].

Table 1. Relative echo amplitude responses of a referecnce

block (5-0050) and a steel sphere (5/16 inch diameter)

as a function of pulse width.

Pulse

Width

Reference

Block

Steel

Sphere

Min. 80.0 76.4

Med. 80.0 81.3

Max. 80.0 85.0

With regard to the third approach, various attempts to achieve reproducible results

by substituting an electronic source for the echo signal have not been very

successful. Some of the original work in this area was performed by the Navy to

develop an "Electronic Test Block" [26]. More recently, digital circuitry was applied to semi-automate and computerize system sensitivity initialization [27] with

a similar scheme, but proved to be inherently limited by coupling variations between

the transducer and the standard signal source.

12
We add here an important note regarding the inconsistency in the prescription of

ASTM E127 for initializing system sensitivity for different hole sizes. For each

block hole diameter, a specific diameter target sphere is prescribed in an effort to

normalize the echo data from different hole sizes to one set of calibration values.

The prescribed sphere diameter for each hole size is given in Table 2. Since echo

amplitude is proportional to the hole diameter squared [28, 29] and directly

proportional to the sphere diameter [29] , the ratio of hole diameters squared should

be equal to the ratio of sphere diameters. However, from Table 2, one finds that

relative to the No. 5 hole size, the sensitivity (as determined by a 1/8 inch

sphere) for the No. 3 holes will be about ten percent lower, and for the No. 8 holes

(as determined by an 11/16 inch sphere) will be about sixteen percent higher.

Table 2. Steel Sphere Diameters Used to Set System

Sensitivity for the Respective Block Hole Sizes

According to ASTM El 27.

FBH

No.

FBH Diameter

(inches)

Sphere Diameter

(inches

(FBH Diam.) 2

Sphere Diam,

3 3/64 1/8 0.90

5 5/64 5/16 1.00

8 8/64 11/16 1.16

Finally, we note that the use of check standards, for example a specific block

or group of blocks (two are used in the NBS procedure for each hole size [25]),

would serve several important purposes:

a. Check for proper system alignment and sensitivity initialization;

b. Detect system instability and long term drift;

c. Provide a data base to statistically evaluate measurement

repeatability.

4.5. Operator Judgment and Procedures

Operator judgment contributes to measurement error in two principal ways: screen

readings and alignment procedures. Errors associated with the first may be

significantly reduced by replacing video display readings with digital readout.

Errors associated with the second may be reduced by means of procedural automation

via microprocessor control, but at some cost.

When performing manual calibrations, without the advantage of mechanical automation,

at least two and preferably three independent readings of each block are recommended. This not only provides a check for possibly mistaken readings, but

also allows for a statistical statement of repeatability.

13
A salient consideration for avoiding mistaken block readings is the elimination of

air bubbles from the target surfaces and the transducer face. Very small bubbles

that may not be noticed in routine procedures can significantly affect echo

amplitude, especially if they should form on the primary reference standard when

adjusting system sensitivity. For example, a ten percent of full scale reduction in

signal, caused by a barely perceptible bubble forming on a 5/16 inch diameter steel

sphere, has been observed. The problem is lessened by allowing fresh or recently

agitated water to outgas for several days. (Heating the water will enhance

outgassing). Backlighting the inspection area, easily done for a tank with

transparent walls, greatly enhances the detection of any bubbles that might

interfere with the measurements.

4.6. Block Geometry

The principal geometric features (Fig. 1) of concern in block fabrication are the

flat-bottom-hole and the entry surface. The hole flat, its parallelism with the

entry surface, and its diameter are all crucial parameters for block response, as is

the roughness of the entry surface itself. In particular, block manufacturers

indicate that small deviations from a right angle at the hole-flat perimeter will

affect block response. Furthermore, effective quality control of these fabrication

parameters requires meticulous machining efforts, and sometimes a trial and error approach.

The effects on echo amplitude have not been well quantified for all these variables.

A simple calculation can be- made, however, to determine the effect of hole diameter

variation on block response. Since echo amplitude is proportional to

flat-bottom-hole area, the ASTM E127 tolerance of + .0005 inch on the diameter will

produce a one to two percent variation in echo amplitude for a No. 3 size hole.

This error will be proportionately less for the larger hole sizes. However, one may

question whether this tolerance is, in fact, a feasible machining tolerance for the

hole top diameter where a right angle at the perimeter of the hole flat must be

met.

5. RECOMMENDATIONS

Based on the above studies, specific recommendations to minimize the net tolerance

for ultrasonic reference block calibration follow. The categorization is that used

for the principal system variables Identified above.

1. Block material: Define a (arbitrary) level of ultrasonic material scattering

that is consistent with today's manufacturing processes, and as much as possible

with the present requirements of ASTM El 27, by assigning some value to the echo

amplitude from a material standard reference block. An 8-0575 block, fabricated

from the candidate material for the block set, would be a consideration. Hopefully,

the variation in echo amplitude between any two "identical" blocks, due to material

property variations, could be reduced to +5 percent of full scale with this

approach. This may require, however, that all extruded rod be taken from the same manufacturing lot and/or that other quality control restrictions be placed on the

material. An effective screening for acceptable material should be possible by the use of sampling.

14
2. Transducers: A significant improvement for transducer evaluation would be to

supplement the requirements of ASTM E127 with a far-field center-axis beam profile

analysis. The interplay of target shape (e.g., sphere vs. flat-bottom-hole) with

this analysis must be carefully considered. Based on the empirical approach

described above, and the discussion that followed, a tolerance of + 3 percent of

full scale in block response should be realizable for this variable. To extend this

practice to include other piezoelectric material types such as ceramic may be

feasible using a correction factor approach, but would require further study.

3. Pulser/Receiver : The effects of variations between pulser/receiver units can be

minimized by using pulses of the same shape and amplitude (and, therefore, spectral

content) to drive the transducer. If, in addition, the receiver linearity were kept

within + 1 percent of full scale, it should be feasible that the combined pulser and

receiver stay within + 4 percent of full scale bounds for reference block

measurements.

4. Primary Reference Standard: The use of a spherical steel target as the primary

reference standard is considered problematic. A more suitable target, sensitive to

the same beam characteristics as flat-bottom-hole blocks, and one that would be

reproducible, is not known, however. In lieu of that ideal target, it is

recommended that each laboratory performing calibrations identify a specific 0050

block with controlled or known characteristics for use at least as a check

standard.

Since the variations in system response due to the primary reference standard used

can be eliminated by using identical transducers and driving pulses, assigning an additional tolerance to this variable would be duplicative. However, it should be

borne in mind that in practice identical transducers and pulsers are not the case,

and the search for a more suitable target to minimize the effects of these and other

system variables should be made.

Finally, the source of systematic error attributable to ASTM El 27, as it is

presently written, should be recognized. The discrepancy between the spherical

target diameter ratios and the hole area ratios requires that the tabular data of

recommended block responses be corrected for different hole sizes.

5. Operator Judgment and Procedures: By eliminating screen reading judgment errors with digital readout of signal amplitude, the principal operator-dependent

variations would be limited to alignment errors. The detection of bubbles on the

transducer and/or block surface, a potential source of large error, is greatly

facilitated by back-lighting an immersion tank having transparent siding. Finally,

to minimize operator-dependent errors, all reported amplitude values should be an average of three repeated measurements; and a difference of more than two percent of

full scale within these three measurements should raise a caution flag, and be

investigated.

These factors combined should not contribute more than + 1 percent of full

scale to system tolerance for the first half of the calibration curve, and +2

percent for the second half.

6. Block Geometry: So long as reference blocks are fabricated by hole drilling,

possible variations of + 3 percent in echo amplitude due to this process will have

15
to be ascribed to the smaller diameter hole sizes. This assumes the machining

tolerance discussed above to be feasible, and includes variations due to the other

principal geometrical features of parallelism and front surface roughness.

Variations in the larger holes (No. 5 and No. 8) will be proportionally less.

6. CONCLUSIONS

By assuming a random accumulation of the six source errors listed above, it is

reasonable to develop a specification with a net system tolerance equal to the

square root of the sum of their squares. Using the individual tolerance values

identified and/or estimated there, the net system tolerance could be expected to

meet a value of plus or minus eight percent of full scale.

In decibels, a figure of +8 percent of full scale is less than +1 dB for the

shortest blocks, but scales up to nearly +3 dB for the longest blocks. We note that

the largest contribution to this tolerance is that due to the estimated material

variation, which may in practice be significantly less. It is ironic that a given

percent of full scale tolerance expressed in decibels imposes the most stringent

requirements on the longest blocks, which are most prone to ultrasonic response

variati ns to begin with. In light of these analyses, therefore, a +1 dB tolerance

using the approach of ASTM El 27 may not be feasible for all blocks.

ACKNOWLEDGEMENTS

The partial support of the Army Materials and Mechanics Research Center,

Watertown, MA, is gratefully acknowledged. The laboratory help of Mr. Jimmy

James, an NBS Cooperative Student from Prairie View A & M in Texas, is also

gratefully acknowledged.

16
REFERENCES

1. Tentative recommended practice for fabricating and checking aluminum alloy

ultrasonic standard reference blocks, E127-58T, in Annual book of ASTM

standards, part 3. Philadelphia, Pennsylvania: American Society for Testing and

Materials; 1958.

2. Standard practice for fabricating and checking aluminum alloy ultrasonic

standard reference blocks, E127-81, in Annual book of ASTM standards, part 11.

Philadelphia, Pennsylvania: American Society for Testing and Materials; 1982.

3. Blessing, G.V.; Eitzen, D.G. Ultrasonic standard reference blocks — what

future?, in 1982 paper summaries of the ASNT national conference; 1982 March

22-25; Boston, Massachusetts. Columbus, Ohio: American Society for

Nondestructive Testing; 1982. 9-12.

4. Blessing, G.V.; Eitzen D.G. Variables affecting ultrasonic reference block

calibration, in 1982 October 4-7; Pittsburgh, Pennsylvania. Columbus, Ohio:

American Society for Nondestructive Testing; 1982. 287-291.

5. Golan, S. A comparison of American and European ultrasonic testing standards.

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17
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19
APPENDIX

We stated It was not the purpose of this report to judge the flat-bottom-hole

reference standard per se. Here, however, we mention those qualities which an ideal

calibration standard might possess [30]

1. Fabrication reproducibility

2. Measurement reproducibility

3. Primary standard functioning to calibrate secondary or field

type standards

4. Theoretical traceability

5. Flaw simulation

6. Ease in setting system sensitivity

7. Performance for equipment evaluation

8. Ease of automation

9. Low cost

10.

Compatibility with existing standard.

Perhaps the most frequently discussed limitation of the flat-bottom-hole standard

relative to the above criteria is its shortcoming in simulating real defects. While

this criticism may be valid, it should be recognized that it is only one of many

criteria to judge a standard by, and for many purposes may not be the most

important. It should also be pointed out that the original rationale for choosing

the flat-bottom-hole was that it best simulated the more critical flaws — lamellar

defects and cracks, whereas other artifact candidates such as spheres and cylinders

best simulate relatively benign pores. Hopefully, an improved general purpose

primary reference standard is forthcoming, but until it does, flat-bottom-hole

fabrication and calibration procedures deserve optimization.

20
NBS-114A [rev. 2 -ac) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

4. TITLE AND SUBTITLE

PUBLICATION OR 2 REPORT NO.

NBSIR 83-2710

. Performing Organ. Report NoJ 3. 737.03

Publication Date

June 1983

An Assessment of Ultrasonic Reference Block Calibration Methodology

5. AUTHOR(S)

G.V. Blessing

6. PERFORMING ORGANIZATION (If joint or other than NBS, see instructions) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE 8. Type of Report & Period Covered

WASHINGTON, D.C. 20234 Final

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10. SUPPLEMENTARY NOTES

| | Document describes a computer program; SF-185. FIPS Software Summary, is attached.

11. ABSTRACT (A 200-word or less factual summary of most si gnificant information. If document includes a significant

bibliography or literature survey, mention it here)

The state of the art in aluminum ultrasonic reference block calibration

practices is reviewed, especially as it has been guided by the recommended

practices of ASTM for aluminum blocks. The principal system variables in the

calibration procedure are identified, and recommendations for reducing their

associated measurement errors are made. Quantitative evaluations of the

limitations to improving measurement precision are made in light of present

technology. Suggestions for improving present practices are made, and extensive

reference to the relevant technical literature is made.

12. KEY WORDS (Six to twelve entries; alphabetical order; capitalize only proper names; and separate key words by semicolon s) ASTM E127; ASTM reference block calibrations; ultrasonic aluminum reference

blocks; ultrasonic system calibration; ultrasonic transducer calibration.

13. AVAILABILITY 14. NO. OF

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