Acoustic Interactions with Internal Stresses in Metals

Abstract

This chapter discusses two techniques that show promise of measuring different aspects of internal stress distributions in metals. The present knowledge of states of stress and their relation to failure mechanisms is first briefly reviewed, noting the distinction between long and short range stresses. Electromagnetic transducer efficiency tests for long range stresses are then discussed. It is demonstrated that the efficiency of the electromagnetic generation of ultrasonic waves is very sensitive to the deformations of ferromagnetic materials. Experiments defining the general features of the effect in three iron-nickel alloys, and its sensitivity to minor variations in material properties such as texture, composition, and heat treatment in low carbon steels are presented. The results are interpreted in terms of a magnetostrictive model of the generation process and the previously known strain dependence of the magnetostriction of these materials. Ultrasonic harmonic generation is then shown to be a technique sensitive to dislocation parameters. This measurement can thus be used to define the microscopic internal stresses which control such material properties as flow stress and state of fatigue. Some experimental data on harmonic generation in deformed and fatigued aluminum specimen, which demonstrates this effect are presented. Limitations of this technique, as observed in Al alloys, as well as results obtained on various other structural materials, are also discussed.