Figures 1-3

Introduction
Calibration blocks are used to assess the performance of ultrasonic flaw detection equipment, including transducers. They are also used to calibrate the equipment (setting up of gain, range, sweep rate, etc.) for the examination of materials. The blocks are supposed to be made of materials having a specified composition and heat treatment, and of a given surface texture and geometrical form. Variation in any characteristic from block to block may lead to different interpretation of results. It is desirable that all the calibration blocks, used anywhere in the world, have the same general properties.

While it is not difficult to verify the dimensions and the geometry of blocks, the test for other properties such as composition, heat treatment, and surface texture is not straightforward. These other properties cast appreciable influence on ultrasonic velocity and attenuation. Fortunately, these two ultrasonic parameters are desired to be nearly the same in blocks, and if these parameters have the same values, one need not go to the extent of evaluating composition, grain size, surface texture, etc.

The ultrasonic velocity can be evaluated by any of several methods having an accuracy of 0.1 percent or better. The evaluation of ultrasonic attenuation, however, poses several problems. The transducer characteristics (diameter, frequency, bandwidth, acoustic impedance of facing material), couplant characteristics (thickness, acoustic impedance), velocity in sample (diffraction correction), etc., can affect the measurement of attenuation. Fortunately, the task is not so much to determine the absolute value of attenuation but to check the variation in the attenuation value from block to block. A method of evaluating the relative attenuation, or a parameter depending on the relative attenuation, should therefore serve the purpose very well. The method, of course, should be simple and provide results having dependence on very few variables and yet have an integrated influence of grain size, composition, and surface texture. A change in any of these characteristics should change the result.

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Variation in any characteristics from block to block may lead to different interpretation of results.

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The American Society for Testing and Materials has described a method for the quality evaluation of aluminum standard reference (cylindrical) blocks (ASTM, 1989). However, no method for the quality assurance of another widely used calibration block VU (also known as the IIW or A2 block) has been given in the literature available (IS, 1990; BS, 1978). In the method described by ASTM, a steel ball of 4.8 mm (0.18 in.) diameter is kept at a distance of 150 mm (6 in.) from the transducer while the distance between transducer and cylindrical block is kept at 50 mm (2 in.).

The height of the echo from the ball is used to set up the gain of the flaw detection equipment. If the ball echo height is 50 percent full signal height and the noise level between interface echo and first backwall echo is 10 percent full signal height, the block is said to be of acceptable quality. It is also desired that five backwall echoes should remain observable while scanning the block.

In the present paper, a method is described which in essence is similar to the method described in the ASTM method but is simpler and independent of the quality of ball and water. It also gives the quality evaluation of the material of the V1 block.
 

Theory
In the case of a large plate (such as a V1 block) the acoustic pressure amplitude of first interface echo can be given as

(1) 

and that of seventh backwall echo as

(2) 

where P₀ = initial pressure amplitude,  = attenuation coefficient for water,  = attenuation coefficient for material of the block, L_w = water path, D = diffraction correction term for first interface echo, D’ = diffraction correction term for seventh backwall echo, R = reflection coefficient, T = transmission coefficient, and L_m = thickness of the block.

Dividing P₁ by P₂

(3) 

The diffraction correction term D does not depend upon block material and hence can be treated as the same for all the blocks. The term in the denominator (D’T²R⁸) is dependent on transducer characteristics and acoustic impedance of block material, besides other factors. The effect of transducer characteristics can be obviated if the same type of transducer is used for the evaluation of all the blocks. The variation in acoustic impedance of the material from one block to another is usually very small and its effect on the ratio P₁/P₂ will be insignificant as compared to the effect of am on P₁/P₂. In other words, maximum change in the ratio P₁/P₂ will be due to the change in attenuation of the material.

Taking Equation 3 into consideration and by some mathematical manipulation, we can define the relative attenuation (RA) as given by the term

(4) 

This can be read from the ultrasonic flaw detector directly as the difference in echo heights in dB.

In the case of cylindrical blocks (ASTM, 1989), though the expressions for the pressure amplitudes of the echoes will change, the discussion remains unaltered.
 

Recommended Procedure
The block diagram for the experimental procedure is shown in Figure 1. The block under test is immersed in a water tank. An immersion transducer of 5 MHz frequency, 10 mm (0.75 in.) diameter is placed in a position where the interface echo from the front face is of maximum height.

In the case of the V1 block, the transducer is moved to a distance where the second interface echo falls halfway between the seventh and eighth backwall echoes (Figure 2). This nearly fixes the distance between transducer and reflector. The first interface echo is brought to 50 percent full signal height and gain is noted in dB. Then the seventh backwall echo is brought to the 50 percent full signal height by increasing gain of the flaw detector and new gain is noted in dB. The difference in gain in dB is the required relative attenuation.

In the case of the cylindrical reference blocks (ASTM, 1989), the transducer is kept at a distance of 150 mm (6 in.) to avoid the beam profile effects of the transducer (Figure 3). The transducer is positioned to get maximum height of the interface echo. The relative attenuation is evaluated using first interface echo and second backwall echo, taking both at 50 percent full signal height. To avoid confusion because of the presence of the echoes due to mode conversion, higher backwall echoes are not considered.
 

Acceptance Levels
On the basis of results obtained from several blocks, we find that in the case of the V1 blocks made of low carbon alloy steel having grain size of Mcquaid-ehn No. 5, the relative attenuation should be 35 dB. In the case of the cylindrical block made of aluminum alloy (DTD, 1973), the relative attenuation should be 26 dB. A block having relative attenuation more than the above specified value may not be acceptable, while one within 2 dB down may be acceptable. It may be noted that the above acceptance levels have been arrived at by the experimental observations for specific materials. In case the recommendations are for different material, the acceptance level has to be worked out.

Acknowledgment
The authors thank Professor E.S.R. Gopal, Director of the National Physical Laboratory, New Delhi, for taking interest in this work. They are also grateful to Om Prakash Vaish and Jagdish Lal for rendering help at various stages during the experiment.
 

References
ASTM E 127: Recommended Practice for Fabricating and Checking Aluminum Alloy Ultrasonic Reference Blocks. ASTM, 1989.

BS 2704: Calibration Blocks for Their Use in Ultrasonic Flaw Detection. British Standards Institution, 1978.

DTD 5124: Aerospace Material Specifications on Bars and Extruded Section of Aluminum-Zinc-Magnesium-Copper-Chromium Alloys. Ministry of Defence, UK, 1973.

IS 4904: Calibration Blocks for Use in Ultrasonic Nondestructive Testing - Specification. Bureau of Indian Standards, 1990.

 

* Ultrasonic Section, National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110 012, India; 91-11-5787161; fax 91-11-5752678; e-mail npl@sirnetd.ernet.in.

Copyright © 1997 by the American Society for Nondestructive Testing, Inc. All rights reserved.

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