A Photoelastic and FEM Analysis of Interfacial Crack Propagation by I. Miskioglu, C.R. Villmann, J.S. Pawloski and D.M. Pariseau ABSTRACT--The linear elastic solution for the bimaterial interface crack is applied to determine the direction of crack propagation during surface-layer removal by scraping. The direction of the toad which must be applied to the free end of the surface layer for continued interfacial crack propagation is determined using photoelasticity and finite elements, and results are found to be in good agreement. Introduction When removing a surface layer which is bonded to a substrate, scrapers are often used. It seems reasonable to assume that a scraper which would promote continued crack growth at the interface between the layer and the substrate would be very efficient. It is the purpose of this paper to explore the fracture mechanics associated with layer removal. A method using photoelasticity to deter- mine the loading which a scraper would need to deliver to the surface layer for continued interfacial crack propaga- tion is developed and compared with finite-element results. During surface-layer removal in a bimaterial system (Fig. 1), it is desirable to propagate the interfacial crack along the interface.l Since the state of stress at the crack tip will depend on the direction of the load applied to the free end of the surface layer (Fig. 2), this loading direc- tion must be adjusted to promote continued interfacial crack growth. When a crack exists in a homogeneous material, knowledge of stress-intensity factors (SIF) alone allows the prediction of crack-propagation direction. Although the logarithmic singularity predicted by elasticity complicates matters for interfacial cracks in bimaterial systems, knowledge of SIF is again essential to describe the state of stress at the crack tip. An experimental method of SIF determination using photoelasticity was I. Miskioglu (SEM Member) is Assistant Professor, C.R. Pillmann is Associate Professor, and J.S. Pawloski and D.M. Pariseau are Research Assistants, Michigan Technological University, ME-EM Department, 1400 Townsend Drive, Houghton, MI 49931. Paper was presented at the 1990 SEM Spring Conference on Experimental Mechanics held in Albuquerque, N M on June 3-6. Original manuscript submitted: August 7, 1990. Final manuscript received: February 15, 1991. proposed by Chiang et al. 2 based on Rice's definition of SIF of classical type2 In this paper, their method coupled with nonlinear least-squares analysis ~ is used to extract SIF from photoelastic data. The direction of the applied load for crack propagation is determined both by the use of the SIF and also by the finite-element method and the results from the two are compared. Throughout both analyses frictionless contact between wedge and layer is assumed. Determination of the Stress Field from Photoelastic Data For the coordinate system shown in Fig. 2, the stress field near the tip of a bimaterial interface crack can be Fig. 1--Surface layer removal by a wedge Fig. 2--Components of the load applied at the free end of the surface layer Experimental Mechanics 9 135