Contrast enhancement of ultrasonic imaging of internal stresses in materials.
The ultrasonic methods, which detect applied or residual stress in materials, are based on nonlinear interaction of a small dynamic disturbance (acoustic waves) with the pre-deformed state of the solid. This weak phenomenon (acoustoelasticity) leads to a dependence of acoustic wave velocities on the initial stress, and a stress-induced anisotropy in the acoustical properties of the material. In anisotropic media, the transversal wave velocity depends on its polarization. The amplitude of the conical polarized shear wave, propagating through a plate specimen, is sensitive to pre-stress due to acoustoelastic birefringence. The resulting scan image is created by variations of the amplitude. The previous description is a basic principle of the approach used for stress mapping in Al-alloys by time-resolved acoustic microscopy. Disk specimens with central stress concentrators are loaded step by step. The acoustic scans are created during each loading step. Thermal stress detection is also shown on specimens with an Invar core. The original image processing procedure has been developed to improve edge detection of obtained stress maps. The acoustic images are compared with theoretically predicted isocline contours. The inherent material anisotropy and the structural inhomogeneities influence significantly the acoustoelastic measurements. Advantages and limitations of the nondestructive technique are summarized on the basis of presented experimental results.
Landa
Michal
M
Institute of Thermomechanics, Czech Academy of Sciences, Praha, Czech Republic. ml@it.cas.cz
Plesek
Jirí
J