Ultrasonic evaluation of fatigue crack growth of high strength steel at ambient temperature and below

By means of an ultrasonic testing device, the crack length in compact tension specimens is monitored. Four ultrasonic probes are used to cover the region of crack extension by transmission waves. The echo of the crack is amplified during fatigue crack propagation. By a knowledge of the initial crack length after precracking and the critical crack length, the alteration of the ultrasonic echo provides the information for determining the intermediate crack length which can be used to calculate the stress intensity range ΔK with respect to the elapsed cycles and the crack growth rate, da/dN.