Harmonic Generation for Measurement of Internal Stresses as Produced by Dislocations

Absrmcr-The present knowledge on the state of microscopic internal stresses, as produced by dislocations, and the relation of these internal stresses to materials properties such as flow stress and the state of fatigue are briefly reviewed. It will then be shown that ultrasonic harmonic generation is sensitive to the dislocation density and loop lengths and thus to microscopic internal stresses Some experimental data on harmonic generation in a deformed and fatigued aluminum specimen, which are in agreement with the above theory, are presented. Thus the technique of ultrasonic harmonic generation is shown to be a new and potentially powerful technique for the nondestructive evalua- tion of technologically important materials. N A RECENT meeting on nondestructive evaluation of internal stresses (l) the need for additional work in this area became quite apparent. The presence of either a compres- sional or a tensile macroscopic internal stress in a structural component is a very important parameter, in particular in de- sign work, which very effectively determines crack initiation and crack propagation under external loads. Strong efforts are being made at present to determine this kind of internal stress, mainly using X-ray techniques and, lately, acoustic birefringence and magnetic techniques. The role of micro- scopic internal stresses on mechanical properties, on the other hand, seems to be not quite clear to the designer or mainte- nance. The present paper, therefore, is addressed specifically to this subject. In particular, the internal stresses due to dis- locations which are the defects responsible for plasticity ef- fects in metals will be discussed. A literature survey as well as some preliminary experiments to be reported in this paper seem to indicate that the flow stress as well as a part of the fatigue life of a material are reflections of the state of the microscopic internal stress. These experiments have been per- formed using acoustic harmonic generation, and it is this tech- nique that is proposed in the present paper as a possible non- destructive tool to study the microscopic internal stress due to dislocation or, in other words, plasticity effects. The work reported here by no means is complete, and many basic questions remain to be answered. It is hoped, however, that the present work will lead to a better understanding of the materials' reliability and product lifetime.