Survey of Ultrasonic Grain Noise Characteristics in Jet Engine Titanium

In ultrasonic inspections of titanium billets and forgings, grain noise echoes are routinely observed. These arise from the scattering of the incident sound beam by the metal microstructure, and can limit the detection of small or subtle defects. We report on a survey of grain noise characteristics in fourteen billet and forging specimens supplied by aircraft engine manufacturers. All specimens were examined in a similar manner using a 5-MHz focussed transducer, with pulse/echo noise measurements made through three orthogonal sides of each specimen. Emphasis is placed on describing two related probability density functions (PDF’s) which characterize aspects of the backscattered noise seen in a scanning experiment. The first PDF describes the RF noise voltages seen at a fixed observation time t; the second describes the gated peak-to-peak noise voltages seen for time gates of various durations. The PDF for the RF noise voltages is expected to be Gaussian if a large number (>10) of grains contribute appreciably to the noise at time t [1], but non-Gaussian behavior is seen in some specimens. The use of K-distributions to describe the non-Gaussian cases is examined. This work is in support of efforts described in a companion article [2] to develop methods for predicting gated peak noise (GPN) distributions.