Comparison of three real time techniques for the measurement of dynamic fracture initiation toughness in metals

Three techniques for measuring dynamic stress intensity factor time histories of dynamically loaded stationary mode-I cracks are compared as applied to dynamically loaded pre-cracked 6Al-4V titanium alloy specimens. The three techniques are crack opening displacement (COD), dynamic strain gage measurement, and coherent gradient sensing (CGS). The stress intensity factor histories are inferred from each measurement technique and are used to obtain the critical dynamic initiation toughness as a function of loading rate (KIcd(K̇Id)). There are significant differences in implementation and information obtained from each of the three measurement techniques, though for the tests performed all are found to yield very similar results.