Decorrelation of Laser Speckle Patterns for Crack-Tip High Strain Field Determination and Use in Brittle-Ductile Fracture Analysis

Abstract

The uses of laser speckle photography are widespread in mechanics and metrology[1][2]. Most, if not all, of the applications of laser speckle involve the comparison of two correlated speckle patterns of a given field before and after some perturbation to the system has occurred. From the relative displacement of the correlated patterns either Young’s fringes or isothetic fringes may be produced when the specklegram is processed[3]. It is from these fringes that one quantitatively determines the speckle spacing from which such information as displacement and displacement gradient (strain) can be calculated. However, if the displacement, displacement gradient, surface tilt, or surface out-of-plane displacement is excessive, or if the surface morphology changes, the correlation of the speckle patterns is lost and the fringes are either distorted or simply no longer visible. In this case the speckle patterns are said to be decorrelated and accurate quantitative information about changes to the system is no longer available.