Fast Signal-Averaging Unit for Ultrasonic Testing: Characterization of Materials Properties and SNR-Improvement for Coarse Grained Materials

Abstract

<p>The scattering of ultrasonic waves depends on the relation scatterer diameter to wavelength. Coarse grained materials and high frequency ultrasonic waves therefore ere exciting high scattering amplitudes. During pulse-echo testing a backscattering signal is the result of superimposing all the single scattering processes in the sound beam for a given time of flight. Rectifying, digitizing and adding up several A-scans from several different probe positions, angles of incidence or frequencies equalizes interference maxima and minima. The resulting backscattering curve can be evaluated qualitatively and quantitatively for the materials structure characterization. Additionally, an improvement of the signal-to-noise ratio (SNR) is achieved for a defect surrounded by a coarse grained structure. The application in practice depends among other things on the time in which a sufficient number of digitized A-scans is averaged and on the dimensions and operating conditions of the equipment. With these limits a prototype instrument was developed. Up to 1024 digitized A-scans (each 400 ps long, digitizing rate 20 MHz) are averaged in 0.4 sec. The result is recorded on a CRT-display and the whole unit is microcomputer-controlled. The equipment consists of an ultrasonic instrument (USIP 11) and the averager unit (averager, ADC,display, μP). Examples are given of applications to the characterization of materials structure (detection of heterogeneities in steel, grain size determination) and to the ultrasonic testing of coarse grained materials( austenitic welds, castings, fibre reinforced plastics).</p>