Analysis of incremental cracking by the stress-wave emission technique

One of the most fruitful areas for application of the acoustic emission technique lies in characterization of yielding and fracture processes. For example, what are the microscopic details of the macroscopic slow crack growth process that precedes crack instability? Fundamental to answering this is the ability to detect and quantify the microcracking phenomena. It was found that the emission of a single elastic stress wave could be correlated to a load drop, ΔP, occuring during crack growth. Furthermore, this load drop could be interpreted via a theoretical compliance analysis in terms of area swept out by the advancing crack. It is proposed that any discrete stress wave emission associated with a fracture process can be interpreted in terms of an incremental fracture area, ΔA. This is given by g= 2·5 m K W ^{1 2}δA YLB where g is the amplitude of the emission. K is the applied stress intensity, L is the distance between grips, W is the specimen width, B is the specimen thickness. Y is f(a/w) and m is the proportionality constant between stress wave amplitude and ΔP B. Crack-line loaded or compact tension specimens of 7075-T6 aluminum were used for the experimental investigation. A theoretical relationship between the load drop and the crack growth step was derived for this test specimen configuration. Over fifty experimental observations verified the linear relationship between g and ΔP B. For the above relationship, m was found to be 0.05 in²/lb sec² from the compact tension data.