T.O. 33B-1-15-54irregular shapes and rough surfaces, determination of the actual size of small discontinuities in general may not bepossible with ultrasonics. Therefore, estimating the size of small discontinuities by comparing their signal amplitudewith the signal amplitude of reference standard discontinuities is subject to errors. When making such comparisons(only to be used for rough estimates) the transfer technique should be used (see paragraph 5.3.5). If, after applyingtransfer, the test part discontinuity signal is as large or larger than the signal from the reference standard discontinuity,it can be concluded that the test part discontinuity is at least as large as the reference standard discontinuity. Thetransfer technique adjusts for differences in material attenuation, not for differences in discontinuity surfaceirregularities. Estimating the size of discontinuities larger than the sound beam is done by moving the search unit overthe discontinuity and mapping the extremities of the discontinuity. The outer edges of a discontinuity can be estimatedby noting the positions of the center of the search unit when the signal amplitude from the discontinuity is reduced to1/2 its peak value. This procedure estimates the projected area of discontinuities in a plane perpendicular to theincident sound beam.5.3.6.3 Orientation.In evaluating discontinuities it is helpful, if possible, to evaluate the discontinuities from several different directions.This can be accomplished by using a combination of angle and straight beam methods, and/or sound entry fromdifferent surfaces. Inspecting in these various directions reveals more about the discontinuity. The direction where thehighest amplitude signal is obtained is most nearly perpendicular to the plane of the discontinuity for equivalentdistances. If the discontinuity signal changes very little with changing direction, the discontinuity is probably rounded.The sound scattered from a rounded discontinuity is independent of the incident direction. A flat discontinuity gives amaximum reflection when the incident sound beam is perpendicular to the discontinuity.5.3.6.4 Spacing.Closely spaced small discontinuities may produce multiple indications that are often accompanied by the loss of backreflection. Figure 5-60 shows an example of how large grain size porosity can each produce multiple indications andreduce the amplitudes of back-reflection multiples. It is necessary to change the A-scan settings to check for both theeffects, because the back surface signal probably saturates the display at the gain setting that shows the multipleindications. By lowering the gain and lengthening the sweep range, the decreasing amplitude of multiple backreflections is observed. The rate of decrease in the amplitudes of the back reflection signals will be greater than for anarea with no discontinuities.5.3.6.5 TypesOfIndications.Several different types of indications will be encountered in ultrasonic inspections. Some of these indications can causeconfusion, resulting in false conclusions. It is important that the operator be familiar with paragraphs 5.1.6 through5.1.8.3 and the additional information below. This will help the operator in evaluating inspection results and avoidingerroneous conclusions.5.3.6.5.1 LossofBackReflectionAnd/OrMultipleIndications.Loss of back reflection with no other indication can be caused by a number of factors such as the following:a. Large grain sizeb. Porosityc. Dispersion of precipitated particles in the material.d. Overheated structureHowever, these features may produce multiple indications as well (Figure 5-59). Lowering the frequency will generallyreduce the multiple indications. When either multiple indications and/or loss of back reflection is noted, the test partshould be compared with the reference standard using transfer in accordance with paragraph 5.3.5. The results shouldbe evaluated in accordance with the limits in paragraph 5.3.5.4.
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