4. Eddy Currents, Techniques and Phenomena
Identification of mechanisms generating acoustic emission during deformation of materials is often difficult because several mechanisms may be potentially or actually operating simultaneously. Identification of sources which are actually contributing significantly to the acoustic emission can often be accomplished by testing material with different process histories, by microstructural examination before and after deformation, and by using different stress states. Mechanisms which operate simultaneously in one stress state may· operate predominantly in different strain ranges in another stress state. Further confirmation of the mechanisms involved can be obtained by measurement of physical parameters, other than acoustic emission, during deformation which are sensitive to the proposed generation mechanisms for the acoustic emission. Several examples of the use of these techniques will be shown. The sources of acoustic emission in 7075 aluminum were identified by testing in the T6 and T651 tempers, by testing in both tension and compression, and by measurement of internal friction as a function of strain. Dislocation motion was shown to be the major source of acoustic emission in beryllium by testing beryllium of different purity, heat treatment, and origin (powder metallurgy or cast and worked) in both tension and compression combined with microstructural observations. Confirmation that the source was dislocation motion and identification of the type of dislocation activity involved was made by internal friction measurements during deformation. Acoustic emission from hydrogen assisted crack growth in an austenitic stainless steel was separated from other sources of emission by holding at constant load. Cracking was also monitored by observing changes in the apparent elastic modulus of a sample as hydrogen-assisted cracks propagated in it.
Heiple, C. R. and Carpenter, S. H., "Identification of Acoustic Emission Source Mechanisms" (1980). Proceedings of the DARPA/AFML Review of Progress in Quantitative NDE, July 1978–September 1979. 14.