Ultrasonics

Abstract

The presence of stress within a material slightly alters the speed of acoustic waves traveling within the material. This is called the acoustoelastic effect. Thus, accurate measurement of the acoustic wave speed can provide an evaluation of the stresses present. Such measurements are typically done using ultrasonic waves and provide a practical method for evaluating residual stresses.Ultrasonic stress measurement is accomplished using one of two common probe arrangements: the critically refracted longitudinal L CR wave and the shear wave in birefringence mode. Parameters that are important in the stress measurement are the direction of the stress field being investigated and the particle motion and propagation path of the ultrasonic wave. The L CR wave travels parallel to the surface of the specimen shown in Figure 10.1(a) and has particle motion also parallel to the surface. This velocity is designated V 11 , where the first subscript designates the direction of travel and the second the particle motion. This wave has the maximum sensitivity to stress, as reported by Egle and Bray [1]. For the shear wave propagating across the thickness of the block, Figure 10.1(b), the greatest sensitivity to stress is for the wave with particle motion parallel to the stress field, V 21 . The L CR wave, traveling in the 11 direction is more sensitive to stress than the shear wave travelling across the field. The least sensitivity is for the particle motion perpendicular to the stress field, V 13 and V 23 .Shear wave birefringence uses two contact shear probes acting across the thickness of the part. Shear waves are polarized so that the particle motion is perpendicular to the direction of propagation. With this method, the velocity (or the measured arrival times) associated with V 23 and V 21 can be compared. For homogeneous isotropic material, the stress effect would be shown by the travel time for V 21 and the material zero stress travel time shown by V 23 . Real materials are seldom isotropic and homogeneous, but Practical Residual Stress Measurement Methods, First Edition. Edited by Gary S. Schajer.

Residual stresses in cold-wire gas metal arc welding

“…In this case, it has been shown that one of the principal acoustic axis is aligned with the RD and a second is perpendicular to it [9, 10, 27]. Therefore, the difference between the stress in the RD, , and the direction perpendicular to it, , can be calculated as [12, 29] where RD indicates the rolling direction, LD is the direction perpendicular to RD. is the acoustic birefringence in the unstressed state of the material and represents the acoustic anisotropy of the material [12]; k is the material acoustoelastic constant.…”

“…In this case, it has been shown that one of the principal acoustic axis is aligned with the RD and a second is perpendicular to it [9,10,27]. Therefore, the difference between the stress in the RD, σ RD , and the direction perpendicular to it, σ LD , can be calculated as [12,29]…”

Application of Al-Cu-W-Ta graded density impactors in dynamic ramp compression experiments

“…Other studies have confirmed a complex dependence on pore morphology parameters that may be unknown [42][43][44][45] and on a residual stress state, which can affect acoustic velocity by up to 30%. 46,47 MacKenzie 41 noted that an empirical approach incorporating experimentally determined values enables reliable estimates of porosity corrections in the presence of such combined effects.…”