Pressurized internal lenticular cracks at healed mica interfaces
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The equilibrium states of internal penny cracks at interfaces in thin-sheet bodies are investigated. Consideration is given to cracks held open by a center-loading force from an entrapped particle in combination with a uniform pressure from a fixed mass of entrapped gas. A fracture mechanics analysis indicates that under these conditions the cracks are stable, but are amenable to growth from an enhancement in net pressure (increase in internal pressure or decrease in external pressure) or effective particle size. Essential details of the theory are confirmed by experiments on lenticular cracks at healed interfaces in muscovite mica. The results are pertinent to flaw responses in brittle ceramic systems where structural integrity is an issue.
I. INTRODUCTION It has long been recognized that inherent cracklike flaws from material fabrication processes can have a profound influence on the performance of structural ceramics. The most immediate manifestation of such flaws is a degradation in bulk strength.1'2 In the special case of internal cracks parallel and adjacent to a free surface, the attendant fracture mechanics bear strongly on such issues as coating/substrate integrity,3 subsurface damage and associated wear in tribological applications,4 and internal flaw growth from particle- (electron or proton) and photon- (laser) radiation-induced damage.5"7 The study of near-surface internal cracks is, therefore, of potential interest in the strength characterization of brittle materials. In the present paper we study the mechanics and thermodynamics of internal cracks formed at healed interfaces in muscovite mica sheets. Results of earlier observations on this system8'9 have demonstrated the advantages of mica as a model material for such a study. On recontacting thin, freshly cleaved, atomically smooth mica flakes, axisymmetrical lenticular cracks can be formed at the adhered interface, even with the mica sheets in mutual angular misorientation. These cracks are primarily held open by spurious mica flakes wedged between the opposing walls. Additionally, air trapped at the interface during closure can cause an excess pres-
a
TJoctoral Student, Department of Physics and Astronomy, University of Maryland, College Park, Maryland 20742. b) Now at Schools of Applied Physics and Chemical Technology, University of South Australia, The Levels, South Australia 5095, Australia. c) Work done while on leave from Ecole Superieure de Physique et de Chimie Industrielles de la Ville de Paris, F-75231 Paris.
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sure within the crack cavity.9 In extreme cases, as we shall see, such a pressure buildup may constitute the only opening force keeping the crack open. The radii of the cracks formed are generally more than an order of magnitude greater than the thickness of an individual mica sheet, satisfying a necessary condition for the application of simple thin-plate elasticity theory to fracture mechanics problems.10 Moreover, whereas the mismatch dilation of the healed interface is not sufficient to allow major constituents of the
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