A coherent gradient sensor for crack tip deformation measurements: analysis and experimental results

Abstract

Abstract.A first order diffraction analysis of an optical interferometer, Coherent Gradient Sensor (CGS), for measuring surface gradients is presented. Its applicability in the field of fracture mechanics is demonstrated by quantitatively measuring the gradients of out-of-plane displacements around a crack tip in a three point bent fracture specimen under static loading. This method has potential for the study of deformation fields near a quasi-statically or dynamically growing crack.

Nondestructive Evaluation Using Shearing Interferometry

“…A number of shearing schemes have so far been proposed by a variety of researchers. The method of Coherent Gradient Sensing [1][2][3] is one such tool that uses diffraction grating elements to provide the shearing mechanism. The method works weIl in transmission and specular reflection modes for use with transparent or polished surfaces.…”

“…(1) n = 0,1,2 ... (2) In the above, h is the nominal thickness of the plate specimen; c is a stress-optic property of the material of the specimen; p is the pitch of the diffraction gratings; t'1 is the spacing between the two gratings; 0'11 and 0'22 are the in-plane stresses in the plate; and m,n are integers representing fringe orders for xl-and xr gradient fringes respectively. Note that the sensitivity of the technique depends on the spacing t'1 and the pitch p of the diffraction gratings, the first of which is very easily adjustable.…”

“…In this case, the fringes obtained using CGS can be shown to be related to the gradients of the surface shape (or displacements) of the object. The details of CGS in reflection mode are given in references [2,3]. In this paper, only results of the use of CGS in transmission mode are presented.…”

“…This information can be used for nondestructive evaluation applications using optical shearing methods in order to identify defects such as cracks and dis bonds. In this paper, we will first give a brief review of various shearing methods, and then describe in detail the use of Coherent Gradient Sensing, a diffraction grating shearing technique that was developed by Tippur, Krishnaswamy and Rosakis [1,2,3], for the optical detection of cracks in bodies.…”

Crack Tip Field Mapping and Failure Characterization of Functionally Graded Composites

“…The working principle of CGS has been reported by Tippur, et al, (1991). An expanded and collimated beam of laser light (typically 50 mm in diameter) was used to interrogate the specular surface of the specimen.…”