Analysis of Thermal Stressing Techniques for Flaw Detection with Shearography

Abstract

Electronic shearographic interferometry [1,2] and time-resolved infrared radiometry (TRIR) [3,4] are both nondestructive evaluation (NDE) techniques that have been shown to be sensitive to subsurface and surface breaking defects. With TRIR, the location and type of defect can be determined by measuring the development of the surface temperature of an object during heating. Electronic shearography is a full-field optical technique that is sensitive to changes in out-of-plane displacement derivatives of a deforming object. The method is based on the evolution of a speckle fringe pattern formed by laser light scattered off the object surface. Various stressing methods have been employed to produce characteristic deformations which may be monitored shearographically [1]. Most of these techniques including vibration, pressure, mechanical, and thermal loading produce wide-field stressing of the test object. Controlled heating with a laser source described in this work provides several advantages for flaw detection. This noncontact, localized stressing method allows defect information to be obtained while heating. In addition, the beam profile can be tailored to aid in the detection of different defect types. This paper presents results of simultaneous observations of material response to an applied thermal load using both TRIR and shearographic detection methods. Of particular importance is the demonstration that the depth of a defect can be determined accurately by measuring the time-dependence of the shearographic fringe development during heating.