Flaw-to-grain echo enhancement

Frequency wobbling techniques in an ultrasound random signal correlation system leading to the enhancement of ultrasound echoes from flaws relative to those from grain boundaries in large-grained materials are investigated. Mechanisms for the suppression of grain echoes by variations in the transmitted frequency coupled with the time averaging of the signal are discussed, and a computer simulation demonstrating the improvement of the flaw-to-grain echo ratio by the use of frequency wobbling techniques in five separate frequency bands 400 kHz wide with center frequencies from 3 to 5 MHz is presented. Split spectrum techniques for the processing of a wideband signal are then shown to allow the acquisition of individual correlation functions from the correlation function of a single wideband transmitted signal without requiring the transmission of separate narrowband signals. It is concluded that the coupling of the signal-to-noise ratio advantages of random signal flaw detection with the flaw-to-grain echo enhancement of frequency wobbling should greatly improve defect detection in many industrial large-grained materials.