Evidence of fracture surface interference for cracks loaded in shear detected by phase-shifted speckle interferometry
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I.
INTRODUCTION
THE effects of crack surface interference on shear crack growth are relevant to the analysis of crack propagation mechanisms, where closed or partially closed cracks are subjected to shear. Such mechanisms are found in the compression and shear-induced tensile fracture of rock or ceramics, rolling contact-induced fatigue cracks in metals, and interface cracks in electronic packages and composites.[1–8] In experimental fatigue crack growth studies of cracks loaded in shear, crack surface interference effects are believed to be the underlying cause of reported violations of similitude.[9,10,11] Crack surface interference occurs when microscopically rough crack faces are displaced relative to one another in shear. The contacting asperities of opposing crack surfaces interact through some combination of sliding, climbing, sticking, slipping, and deforming. The transfer of the resultant contact forces resists shear displacements and wedges the crack faces open.[12,13] The resistance to shear displacement shields the crack tip from the applied mode II (or mode III) stress intensity factor (SIF). For the case of pure remote mode II loading, the effective SIF at the crack tip is as follows: KII eff 5 KII app 2 KII res
[1]
where KIIeff, KIIapp, and KIIres are the effective, applied, and resistance SIFs, respectively. The wedging of the crack faces imposes an induced mode I SIF to the crack tip, KIind. It is proposed that the shear and opening displacements around the crack tip and the KIIeff and KIind SIFs at the crack tip are coupled through the elastic/plastic hinge at the crack tip and the crack surface interactions. This article describes
R.U. GOULET, formerly with the Department of Mechanical Engineering, University of New Hampshire, is Professional Consultant, Consultech Inc., Biddeford, ME 04005. T.S. GROSS, Professor, is with the Department of Mechanical Engineering, University of New Hampshire, Durham, NH 03824-3591. D.A. MENDELSOHN, Associate Professor, is with the Applied Mechanics Program, The Ohio State University, Columbus, OH 43210. Manuscript submitted October 16, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
an experimental effort to estimate the KIIeff and KIind by measurement of the displacement field around a crack tip and to describe the changing state of crack face contact as a function of the applied mode II SIF. The displacement fields around a mode I fatigue crack remotely loaded in shear were measured using phaseshifted speckle interferometry. The KIIeff and KIind values were estimated from crack tip displacements (CTDs) using single mode and mixed mode Irwin plastic zone corrections. A numerical simulation of the state of interpenetration of opposing crack faces subjected to observed crack face displacement gradients was used to describe the changing state of crack face contact as a function of the applied mode II SIF.
II.
EXPERIMENTAL METHOD
A three-illumination beam, phase-shifted speckle interferometer was used to measure the three-dimensional incremental displacemen
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