Stress and the ultrasonic detection of fatigue cracks in engineering metals

Abstract

The authors describe investigation on surface fatigue cracks penetrating part of the Nay through flat plates of steel, titanium and aluminium. The cracks were induced by tension-tension loading the plates in a three-point bending configuration at a frequency of 28 Hz and under constant load and R-factor. The metals were D6ac steel heat-treated to 1 447 MN m −2, annealed Ti-61-4V and 2219-T87 Al. The investigators then found the response of the cracks to ultrasonic shear waves by delta-scan. Elox slots of 1.52 × 0.25 × 0.76 mm length, width and depth served throughout the experiments as reference reflectors. The investigators found that stress relieving affected the ultrasonic response as did tension, cyclic compression and time. In fact, the ultrasonic echo in the aluminium and titanium was greater after many cycles of small compression or few cycles of great compression. But steel gave no such result. Tension increased the ultrasonic response to a degree related to the load. The echo lessened with time, however, because of creep.