Table 1-5. Reference Standard Metal Travel Tolerances.

T.O. 33B-1-1 1-53 c. Hollow cylindrical standards for inspection of hollow cylindrical parts or sections shall be fabricated in accordance with a particular specification. d. International Institute of Welding (IIW) blocks shall be used as specified for determining certain characteristics of angle beam and straight beam search unit and may be used for distance calibrations. e. Holes, notches, and other reflectors shall be protected against corrosion and mechanical defacing that would alter the ultrasonic echo signal. For example, it is recommended that all holes be sealed to prevent corrosion of the hole face. f. For most inspections performed to locate cracks, an effective reference standard can be made by electric discharge machining (EDM) notches. Notches must be replicated for verification. The notch of appropriate size should be placed in the expected location of cracks (or per drawing) with the plane of the notch in the expected plane of cracks. Information on the expected location and orientation of cracks shall be obtained from the depot-level engineering activity. Other reflecting surfaces meeting the requirements of MIL-STD-2154 are permitted. All standards should be clearly identified so that the material, hole or notch size, angles and dimensions are clear. 1.8.1.3 Metal Travel Distance. The metal travel distance (distance from sound-entry surface to a discontinuity) for the test part and the reference standard must be the same within the tolerances shown in Table 1-5; or else, as an option, distance amplitude correction could be used. Table 1-5. Reference Standard Metal Travel Tolerances. Metal Travel Distance to Discontinuity in Test Part (inches) Tolerances on Metal Travel Distance to Discontinuity in Reference Standard (inches) Up to ¼ 1/4 to 1 1 to 3 3 to 6 Over 6 ± 1/16 ± 1/8 ± 1/4 ± 1/2 ± 10% of metal travel 1.8.1.4 ASTM Blocks. The ASTM test block configuration is shown in Figure 1-10. Two sets of ASTM test blocks, one for aluminum and one for steel, are included in TA 455. Two ASTM specifications cover manufacturing and verification of these reference standards. They are ASTM E 127 (aluminum test blocks) and ASTM E 428 (steel test blocks). ASTM E 428 also allows the use of reference standards of other materials such as titanium. For more information see ASTM E1158, “Standard Guide for Material Selection and Fabrication of Reference Blocks for the Pulsed Longitudinal Wave Ultrasonic Examination of Metal and Metal Alloy Production Material.