"New" Candidates for Ultrasonic NDE Standards and Calibrations
The National Bureau of Standards program in acoustic-ultrasonic calibrations and standards, aimed at solving some of the immediate problems, is reviewed. Work on acoustic emission transducers is directed at the determination of sensitivity and spectral response by the use of a reproducible stress impulse. Also in the area of acoustic emission is a program to develop a theoretical basis for acoustic emission signal analysis to characterize moving cracks or defects. Work on the characterization of ultrasonic transducers which should lead to formal calibration services in the near future includes determination of spectral characteristics by measuring the pressure of the ultrasonic radiation field, determination of the radiation pattern from near field measurements and total power by calorimetry. Although the current NBS program is oriented toward standards and calibrations, instrumentation problems are being addressed including the improvement of signal-to-noise ratio by methods such as pulse compression and signal averaging, and the characterization of the important variables in ultrasonic instrumentation. The reliability of flat-bottom hole aluminum reference blocks, which are in wide use, has been improved and a calibration service is now available. Further directions for this effort will include calibration services for steel and titanium blocks, the development of material independent blocks and the development of well-characterized fatigue cracks that could provide calibration for many NDE tests. The use of theoretically characterized scattering from spheres as a standard has recently come into prominence, and Rockwell and NBS have begun to explore this possibility. However, many other standards and calibration procedures for ultrasonic NDE have been proposed or are in use. To provide a fresh look at this area, particularly as it applies to DoD systems, NBS has initiated a program funded by ARPA to assess the status of the field, determine current and future needs, and propose a plan for realizing these needs.