QQ-N-286G between the part to be inspected and the specific reference standard to be used. For longitudinal wave inspections, the attenuation check shall be based on a comparison between a back reflection obtained from similar geometrical areas on the part to be inspected and the reference standard. For shear wave inspections, the attenuation check shall be based on a comparison between a corner reflection signal or a pitch-catch signal obtained from similar geometrical areas on the part to be inspected and the reference standard. A minimum of three areas shall be evaluated on the component to be inspected. Differences in attenuation, measured in decibels (db), shall be compensated
for by adding or subtracting the appropriate number of db from the instruments gain setting after the calibration reflector height or DAC curve is determined and marked on the screen. The most attenuative area evaluated shall be used
to determine the attenuation correction value. If the attenuation checks indicate less than a 1 db difference, no correction is required. When acoustic compatibility check is performed, the reference standard does not need to meet the signal amplitude comparison test in the acoustic similarity requirements of NAVSEA T9074-AS-GIB-010/271 or MIL-STD-2132.
4.3.3.2 Procedure for ultrasonic inspection of large size bars, rods and forgings. Coarse grained material may result in an unacceptable ultrasonic signal to perform the required inspections. A single, distinct signal which is clearly discernable from noise shall be used for the inspections. If such a signal is not obtainable using the lowest frequency allowed and with the transducers positioned for the shortest sound path, the material shall be considered uninspectable and rejected. Inspection shall be performed using either longitudinal wave end scan or shear wave pitch-catch methods as specified in 4.3.3.2.1 through 4.3.3.2.2. When the procedures of 4.3.3.2.1 are applied to long bars/rods, ultrasonic instruments with high-energy pulses and extended range capability may be required.
4.3.3.2.1 Longitudinal wave end scan. Inspection shall be performed from the end of the form as illustrated on figure 1. A 1 inch diameter, 2.25 megahertz (mhz) transducer shall be used whenever possible. If the 2.25 mhz transducer does not result in a valid calibration, a one inch diameter, 1 mhz transducer may be used. A distance calibration shall be made for the length tested. This shall be accomplished using multiple reflections across the cross-section of the test piece or by the use of a nickel-copper-aluminum reference standard. The signal from the opposite end shall be set to a minimum of 80% full screen height. Ensure that the location of the signal from the opposite end represents the form's full length. Both ends of the bar, rod or forging shall be scanned 100% with an overlap of at least 25%. Additionally, caution must be used to ensure that only the reflected longitudinal wave signal is used for evaluation as opposed to the mode converted signals which trail the primary longitudinal wave signal.
A) Qualification of inspection. The ultrasonic inspection shall be qualified by demonstrating the ability to reject (see
4.3.3.4.1.1) a maximum one inch diameter flat bottomed hole machined in an acoustically similar bar, rod or forging with the same or greater diameter as the item to be inspected at a test metal distance at least as long as the item to be inspected.
4.3.3.2.2 Shear wave pitch-catch scan. Inspection shall be performed
with two scans, 90 degrees apart, along the length of the bar, rod or forging.
For hexagonal bar, three scans are required. The scans shall be directed so
that the sound beam passes through the center of the form. The pitch-catch
technique illustrated in figure 2 shall be used. The maximum dimension of the
transducer active element shall not exceed 1 inch. The nominal transducer
frequency shall be a minimum of 1 mhz. The pitch (sending) transducer and the
catch (receiving) transducer shall be from the same manufacturer and of the
same type, size, and frequency. The sound beam angle shall be as shown on
figure 2 (plus or minus 2 degrees). The 41 degree transducers may be used for
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