T.O. 33B-1-15-49standard and test part may differ, and/or the internal structures (grain size, etc.) may differ. Such differences cause thesignal from a discontinuity in the test part to differ from the signal from the same size discontinuity in the referencestandard. In order to obtain consistent results from ultrasonic inspections, it is necessary to correct for these differencesby using transfer.5.3.5.2 General Procedure.a. Transfer shall be accomplished by noting the dB or gain difference in the responses received fromreflectors in the reference standard and the part or piece of material to be inspected.b. Use the echo signals from the same type of reflector in both the reference standard and the test part toestablish transfer. For example, use back surfaces, flat-bottom holes, side-drilled holes or "V" notches(for angle beam inspections). If possible, a minimum of four reflections from different locations in thepart or piece of material to be tested shall be noted, and the lowest response shall be used forcomparison with the response from the reference standard. In practically all cases, any alteration of thetest part is prohibited. Therefore, transfer must be accomplished using reflectors already included in thetest part. Typical reflectors are the back surface or a fastener hole.5.3.5.3 Examples.5.3.5.3.1 StraightBeamInspectionofATwo-InchPlate.a. Suppose that a specification requires that any material with a discontinuity signal greater than thesignal from a 5/64- inch diameter FBH is unacceptable. The inspection is set up by establishing a DACcurve in accordance with paragraph 5.3.4.2. Use ASTM blocks with 5/64-inch diameter FBHs andmetal travel distances of 1/8, 1/4, 1/2, 3/4, 1-1/2, and 3 inches. Assume the curve shown in Figure 5-52is obtained. Note that the 1/8-inch point is not shown. This is because the dead zone extends beyond1/8 inch. Note that the near field appears to end around 3/4 inch.b. After constructing the DAC curve, the amount of transfer is established through use of back surfacereflections. The search unit is placed on the 1-1/2 inch metal travel ASTM standard as shown in Figure5-54. This gives 2-1/4 inch metal travel to the back surface. The gain control is set to bring the backsurface signal to the DAC curve as shown in Figure 5-55. This gain setting is maintained, and thesearch unit is placed on the test part. Assume the first signal shown in Figure 5-57 is obtained. This is50% lower or 6 dB lower than the DAC curve at the 2-inch metal travel distance. This is the amount oftransfer; the amount by which the gain must be increased after calibration.Figure 5-54. Search Unit on ASTM Block for Determining Transfer Amount.
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