Postfailure subsidiary cracking from indentation flaws in brittle materials
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Vickers indentation sites in ceramics have been examined after specimen failure from median/radial indentation cracks. Evolution of a new cracking pattern of "ortho-lateral" cracks, originating at the intact corners of the Vickers indentation and running orthogonal to the classic-lateral cracks and parallel to the new fracture surface, has been observed. In some instances postfailure extension of the classic-lateral cracks toward the surface was also observed. Enhanced residual tensile stress from relaxation of constraints on the indentation-plastic cavity by the generation of a fracture surface is postulated to drive the subsidiary cracking. A simple qualitative model to explain this phenomenon is presented. Possible implications of such postfailure subsidiary cracking on residual-stress-driven flaws, postmortem fractography, and wear in ceramics are discussed.
I. INTRODUCTION It is now well known that indentation of brittle solids by sharp indenters (such as Vickers) gives rise to the formation of cracks, in addition to the plastically deformed hardness impression. The recoverable (elastic) and the unrecoverable (residual) stress fields generated during the loading-unloading of the indenter are responsible for the ensuing cracking. In most brittle solids, the cracks, which are driven by the residual component of the stress field, develop during the unloading of the indenter. The resulting cracking patterns are broadly classified into radial and lateral. (For a detailed description of indentation fracture, see reviews by Lawn and Wilshaw1'2 and Cook and Pharr3 and references therein.) In this paper we focus our attention on the lateral cracking pattern when the indenter is fully withdrawn. According to the elastic/plastic expanding-cavity model for indentation, the stress-free specimen surface relieves constraint on the indentation-induced plastic deformation zone giving rise to a residual tensile stress normal to the specimen surface.4'5 This residual stress provides the driving force for classic laterals.5'6 The residual stress reaches its maximum value at full contact load and persists at a constant value ar as the indenter is unloaded.5'6 In most brittle solids classic laterals have been observed to initiate during the unloading cycle at the base of the plastic deformation zone below the contact and extend laterally on a plane closely parallel to the specimen surface.1'7"10 Since these cracks
a) Guest
scientist on leave from the Department of Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania 18015.
lie subsurface, their detection is limited only to light optical inspection in transparent or highly translucent materials. In this paper we report a modification of the classical cracking pattern associated with indentations which occurs after stressing of indented specimens to failure. New cracks were observed to originate from the intact corners of the Vickers indentation and run somewhat parallel to the new fracture surface and orthogonal to both the specimen surface and classic l
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