NBSIR 78-1454

The Evaluation of Search Units

Used for Ultrasonic Reference

Block Calibrations

Daniel J. Chwirut and Gary D Boswell

Ultrasonic Standards Program Team

National Bureau of Standards

Washington, D C. 20234

February 1 978

Interim Report

6/1/77 through 12/31/77

Prepared for National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

NBSIR 78-1454

THE EVALUATION OF SEARCH UNITS

USED FOR ULTRASONIC REFERENCE

BLOCK CALIBRATIONS

Daniel J. Chwirut and Gary D Boswell

Ultrasonic Standards Program Team

National Bureau of Standards

Washington, D C. 20234

February 1 978

Interim Report

6/1/77 through 12/31/77

Prepared for National Bureau of Standards

Department of Commerce

Washington, D.C. 20234

U S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman. Under Secretary

Jordan J. Baruch. Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS. Ernest Ambler. Director

CONTENTS

Page

1.0 Introduction 1

2.0 Ultrasonic Search Units 3

3.0 Equipment 4

4.0 Reference Block Readings 6

5.0 Search Unit Characteristics 10

6.0 Discussion 17

7.0 Summary 23

8.0 Acknowledgement 24

9.0 References 25
Abstract

The effects of using different (nominally identical) quartz search

units in the evaluation of ASTM-type standard reference blocks are determined.

Various characteristics of the search units are measured and correlated

with the amplitude of the ultrasonic response from reference blocks to

determine which characteristics must be specified if reproducible results

are to be obtained. It is shown by a series of experiments that the exact

shape of the distance-amplitude curve in water (axial profile) is a primary

characteristic that must be considered. When operational corrections

for differences in axial profiles are made, the variability in ultrasonic

responses from reference blocks, measured with different search units, is

reduced from about 25 percent to 4 percent.

Key Words: ASTM standards; calibration; nondestructive evaluation; reference blocks; search units; specifications; transducers; ultrasonics
1.0 Introduction

What characteristics of an ultrasonic search unit* must I specify, and

furthermore, what values for these characteristics are. annropriate , in order to assure reproducible results from an ultrasonic test? These are the general

questions being asked by the consensus standards organizations representing

the two largest groups in this country using ultrasonics for diagnosis--

the nondestructive testing community (through the American Society for

Testing and Materials Subcommittee E7.06 on Ultrasonic Nondestructive Testing)

and the medical diagnostic community (through, for example, the American

Institute of Ultrasound in Medicine, Transducer Characterization Subcommittee).

In the long term, it appears that at least two standards documents

will be necessary. One document should consider the search unit as part of

the total system and specify various system characteristics such as axial profiles,

front and back surface resolution, center frequency and bandwidth, etc.

that reflect the nature of the pulser, cable, search unit, receiver, display,

etc. These measurements are necessary to define the operating characteristics

of a complete system being used for a specific test and are similar to

those described in ASTM E 317 [10]. The other document should specify

the characteristics of the search unit alone when excited by a "standard"

or easily reproducible waveform such as a spike pulse (broadband)

or continuous sine wave or gated (burst) sine wave. These measurements

*For the purposes of this paper, the ASTM E 500 (Standard Definitions of Terms

Relating to Ultrasonic Testing) definitions for transducer and search unit are implied. Transducer refers to the piezoelectric crystal and electrical con- nections only. Search unit includes the transducer, case, backing material,

wear plate, etc.
could be used to characterize the search unit alone. This type of standard

would be extremely useful in purchasing a search unit to replace one proven

useful for a particular application. These measurements should be

repeatable in different laboratories on different, though similar, equipments.

In either case, the goal of the characterization measurements is to

determine definitively that two search units that appear to be identical,

based on the measured quantities, will indeed yield the same results in an

ultrasonic test. This is necessary for quantitative flaw definition, either

in materials inspection or in medical diagnosis. Without this assurance, the

comparison of successive inspection data on a flawed structure, be it a crack

in a reactor pressure vessel or a brain tumor, to determine flaw growth over

some time period, is not fully meaningful.

Of particular interest to the authors, as well as many others in NDE,

are the search units used to "calibrate" or "evaluate" ASTM-type ultrasonic

reference blocks [1-3]. Earlier studies at this laboratory [4] have shown

that the readings obtained from a particular aluminum reference block exhibited

a spread as high as 17 percent when measured with five nominally identical

quartz search units. The current ASTM document for checking aluminum blocks

[1] has a search unit specification written into it, but the working section

v'; '.df'.d/'} recognizes that this specification is not suf fl<.

and is currently investigating a revision. The aluminum block working section

has set a target of + 1 dB (down from the current + 2dB) variability in

blocks as the acceptance limit. Obviously, if search unit variability

now can contribute as much as 17 percent (about + .75 4B) to the

variability in one block's reading, the goal of + 1 dB spread for all blocks

2
is not attainable with the presently specified measurement procedures. The

data presented herein were taken in support of that effort. Of equal importance to the authors is the system used in our reference block calibration

service[3]. The objective of this program is to expand this service into

a Measurement Assurance Program (MAP) wherein a measurement process (algorithm)

is carefully specified and the accuracy of field measurements using the same

algorithm is assessed. Obviously, a well characterized search unit is an

important part of the measurement algorithm.

The objectives of this study were to re-evaluate the extent of the

variability among reference block readings using different search units

(more thoroughly and carefully than reported in [4]), to measure as

many pertinent characteristics of the search units as possible, to correlate

the results to determine the important parameters, and to specify, if

possible, appropriate tolerances on the parameters that would allow the

+ ldB acceptance criteria to be met.

2.0 Ultrasonic Search Units

Most ultrasonic inspections today are performed with search units

incorporating ceramic transducers. The advantages of these ceramic materials,

such as lead-zinconate-titanate, barium titanate, and lead meta-niobate,

include cost, availability, high sensitivity, and low Q (broadband response).

The primary disadvantages, as compared to quartz, are difficulty of exact

reproducibility, and depolarization caused by aging of the transducer

material. Because of these two disadvantages, ASTM E 7.06.02 has chosen,

for the near term, to specify that search units used for checking aluminum

3
reference blocks incorporate quartz transducers. A future goal is to develop

a search unit specification that will allow the use of ceramic transducers

for checking reference blocks.

The search units specified in E 127 [1] are nominal 5-MHz , 0.375-in

(9.52-mm) effective diameter quartz units. The characteristics of acceptable

search units, per E 127, include center frequency (5.0 + 0.5 MHz), location

of Y

q+

point in water (3.2 to 3.5 in, 8.1 to 8.9 cm), general shape of the

axial profile (fig. 1), Y

q+ beam symmetry (minimum 0.75:1 ratio of - 6dB beam

diameters from four scans), and beam symmetry (peak amplitudes vary by

less than 15 percent among four scans) . Other quantities not specified in

E 127, but sometimes shown in other search unit specifications, include

sensitivity, resolution, frequency bandwidth, damping factor, waveform distortion, and absolute output power (especially in the medical field). Characteristics that are rarely specified include the exact shape of the pressure field

(either axial or lateral profiles) and electrical impedance. The details of

procedures for measuring these parameters are fairly well documented in the

literature [ 5-9 ]

.

3.0 Equipment

Six nominally identical search units were used in the study, all

reportedly 5~MHz , 0.375-in (9.52-mm) quartz. These consisted of three different

styles from two different manufacturers. Two pairs (LS-1 and LS-2, and

LS-3 and LS-4) were purchased in pairs and had sequential serial numbers.

The other two (LS-5 , LS-6) were loners. For the reference block measurements

a commercial flaw detector with a tuned pulser/receiver was used. This is the

type specified in E 127 and used in our calibration service for the blocks.

4
LIWIT A1IHV3NIT UBddfl %

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5

FIGURE

1

-

AXIAL

PROFILE

OF

AN

"ACCEPTABLE"

SEARCH

UNIT

PER

ASTM

E

127-75.

REPRINTED

BY

PERMISSION

OF

THE

AMERICAN

SOCIETY

FOR

TESTING

AND

MATERIALS.

COPYRIGHTED.
The vertical linearity of this flaw detector was checked with all six

search units per ASTM E 317 [ 10 1 and was determined to be better than 5

percent over the range of interest in all cases. For the search unit

characterization, either the commercial flaw detector or a broadband

pulser/receiver was used as noted below. The output pulse waveform and

frequency spectra for these two pulser/receivers are shown in figures

2 and 3. The reference blocks were all ASTM E 127 aluminum reference

blocks, with hole diameters of 3/64, 5/64, and 8/64 in (1.2, 2.0 and 3.2mm)

and metal travel distances of 0.5, 1.0, and 2.25 in (13, 25, and 57 mm).

4.0 Reference Block Readings

Initially the nine reference blocks were read in accordance with the

procedures outlined in ASTM E 127-75. That is, the sensitivity wad set such

that the signal from the appropriate size ball, at 3.5-in (8.9-cm) water

distance, read 80 percent of vertical linear limit on the CRT. The blocks

were then read, keeping the water distance at 3.5 in (8.9 cm). Three

independent readings were taken on each block with each of the six search

units. The data are summarized in table 1. Statistical analysis of similar

data at this laboratory indicated that the standard deviation of the mean

of three readings is about 0.8 units.

The data are very interesting in some respects. The extent of the

variability is much larger than reported earlier [4]. However, all the

earlier data were taken on 0.5-in (13-mm) metal distance blocks. The

data shown in table 1 for-0050 blocks agree well with the earlier data.

6
RELATIVE

VOLTAGE

(linear

scale)

VOLTAGE

TIME

0 5.0 10.0

FREQUENCY. MHz

FIGURE 2 - OUTPUT PULSE WAVEFORM AND FREQUENCY SPECTRUM FOR TUNED

PULSER/RECEIVER. TAKEN FROM CENTER PIN OF "RECEIVE" JACK.

NORMAL MODE, 5 MHz. NO SEARCH UNIT, ATTENUATED BY 36dB.

7
(31 VO S fcl V3 Nil ) 39V110A 3AI1V13H

oCSI

( 31 VO S UV3NI1) 39V110A 3AI1V13U

LO o

aUJcc

o

39V110A 3 9 V 11 0 A

8

FIGURE

3

-

OUTPUT

PULSE

WAVEFORM

AND

FREQUENCY

SPECTRUM

FOR

BROADBAND

PULSER

TAKEN

FROM

"T/R”

JACK.

MODE

1

.

NO

SEARCH

UNIT.

ATTENUATED
Table 1 - Ultrasonic Reference Block Readings

Using Six Nominally Identical Search Units

LS-1 LS-2 LS-3 LS-4 LS-5 LS -6

Spread /Average

Percent

' 8-in ball 80 80 80 80 80 80

, 2 -mm ball)

•0050 93.0 82.2 97.70) 92.3 91.2 (a) 94.80)

+ 6.3

-10.5

0100 49.7 50.0 58.5 54.5 51.7 61.3

+12.9

- 8.4

•0225 20.3 20.2 24.5 23.2 21.7 25.5

+13.0

-10.5

'16-in ball 80 80 80 80 80 80

7.9-mm ball)

•0050 78.7 74.7 90.8 85.5 78.8 84 . 5 O)

+10.5

- 9.1

0100 47.5 48.8 61.3 56.8 50.5 56.8

+14.3

-11.4

•0225 18.2 18.2 24.8 22.3 19.8 22.7

+18.1

-13.3

L/16-in hall 80 80 80 80 80 80

L7.5-mm ball)

•0050 78.8 77.7 91.0 86.5 80.8 86.0 + 9.0- 6.9

•0100 50.3 50.8 63.5 59.0 53.0 59.7

+13.3

-10.3

•0225 20.5 20.2 26.8 24.7 21.2 24.8

+16.3

-12.3

All values are in percent of upper linear limit

All values are averages of three independent readings

(a) Not 100% resolved from front surface echo

9
! Secondly, the variability among readings tends to increase with increasing

block length in all three cases. Thirdly, there seems to be no systematic

relationship between hole size and variability. These last two trends

lead one to believe that the search unit characteristics contributing

to the variability are distance related characteristics, such as frequency

spectrum (which influences attenuation) or axial pressure distribution,

rather than other characteristics such as lateral beam symmetries, sensitivity

impedance. However, for completeness, most of the characteristics mentioned

earlier were measured for all six search units to show more fully which

* are the "important" characteristics for this application.

5.0 Search Unit Characteristics

Using the broadband pulser/receiver , a 100-MHz bandwidth oscilloscope,

stepless gate, and spectrum analyzer, we recorded the signals reflected from a

flat quartz plate and from a 3-0050 E 127 reference block for each search

unit (figs. 4-9). From these, the center frequency, half-power bandwidth, damping, and resolution were determined. Additionally, the center

frequency and half-power bandwidth were determined by the modulated radiation

force ®ethod [11]. Using the tuned pulser/receiver in the commercial flaw

detector, we measured the axial profile landmarks Y

q+

and , -6dB

beamwidth at Y +

, and checked for beam symmetry. We also carefully

o measured the values of the axial pressure amplitude (amplitude of the

signal reflected from a 0.500~in (12.7- mm) steel ball) at the water distances

*0bviously, if the application is the inspection for small defects near

the surface of a thick section of a highly attenuating material, sensitivity, penetration, and resolution are "important". Here we are concerned

only with specifying search units for reference block calibration.

10
11

FIGURE 4-

SEARCH

UNIT

LS1

(a)

WAVEFORM

AND

(b)

FREQUENCY

SPECTRUM

OF

PULSE

REFLECTED

FROM

FLAT

QUARTZ

PLATE,

(c)

REFLECTIONS

FROM 3

0050

REFERENCE

BLOCK.

BROADBAND

PULSER.
39V110A (31V3S HV3NI1) 39V110A 3AI1V13U

12

FIGURE

5

-

SEARCH

UNIT

LS2

(a)

WAVEFORM

AND

(b)

FREQUENCY

SPECTRUM

OF

PULSE

REFLECTED

FROM

FLAT

QUARTZ

PLATE

(c)

REFLECTIONS

FROM

3-0050

REFERENCE

BLOCK.

BROADBAND

PULSER.