The objective of this study was to investigate and develop/adapt acoustic emission (AE) methods for detection and characterization of fatigue damage growth in metals. The materials investigated were 2024-T3 aluminum and 4340 steel. Edge-notch and compact tension specimens were tested under cyclic tensile loading. The investigation consisted of signal/noise discrimination, direct crack growth monitoring, acquisition of acoustic emission data, analysis of AE data and correlation of AE data and damage growth. In the case of aluminum the AE output consists of three characteristic parts corresponding to three stages of crack propagation; the first stage of high but decreasing AE rate, the second stage with a low and nearly constant rate and extending over 80% of the specimen lifetime and the third stage with increasing AE rate up to failure. The rate of AE output in the second stage can be described by a power law in terms of the stress intensity factor range, analogous to the Paris law for crack growth rate. In the case of steel AE results from crack extension, notch tip plasticity and closure, and plasticity dominated rapid crack propagation. The various sources of AE activity were analyzed based on the phase of the loading cycle at which they occur. A high rise in AE activity in the first half of the fatigue life has been attributed to a transition from plane strain to plane stress crack propagation.
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