Effect of an adhesive interlayer on the fracture of a brittle coating on a supporting substrate
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Pedro Miranda Departamento de Electro´nica e Ingenierı´a Electromeca´nica, Escuela de Ingenierı´as Industriales, Universidad de Extremadura, 06071 Badajoz, Spain
Do Kyung Kim Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Yusong, Taejon 305-701, Korea
Brian R. Lawn Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 5 August 2002; accepted 31 October 2002)
The role of a compliant adhesive interlayer in determining critical conditions for radial fracture at the undersurfaces of brittle coatings bonded to substrates of dissimilar materials is investigated. Semi-empirical relations for the critical loads are derived by treating the adhesive as part of an effective substrate, thereby reducing the problem to that of a bilayer. A finite-element analysis of a model silicon/epoxy/glass system is used to evaluate adjustable parameters in the analytical relations. In situ experimental observations of crack initiation on the same material system are used to verify these relations. The critical loads depend sensitively on the adhesive thickness and modulus. Delamination at the interface in poorly bonded specimens greatly reduces the critical loads. This analysis affords a basis for predicting the prospective fracture resistance of brittle coatings joined by adhesives.
I. INTRODUCTION
Adhesives are commonly used to join plates in laminate structures. Because they are soft and compliant, adhesives can absorb energy from external loads and restrict damage in any one layer from spreading to the next (“crack containment”).1 However, they can also enhance flexural stress states from concentrated loading at the top surface, leading to fracture at the plate interior undersurfaces.2 This kind of fracture can lead to rapid loss of function, and ultimately failure, of the laminate system. It is considered to be a primary source of failure in dental crowns from occlusal loading of the brittle layer jacket on its dentin support3–6 and in laminated windows,7 among other applications.
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The present work was done at Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899.
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http://journals.cambridge.org
J. Mater. Res., Vol. 18, No. 1, Jan 2003 Downloaded: 30 Mar 2015
A problem of general practical interest is the influence of an adhesive interlayer on the loads to initiate transverse radial cracking at the undersurface of a brittle coating adhesively bonded to a supporting substrate of different material, subject to a concentrated load at the top surface. Chai and Lawn8 considered the special case of an epoxy adhesive between two like glass plates and derived working relations for the critical loads for radial fracture in terms of adhesive thickness and modulus mismatch. (Near-contact cone cracking at the top glass surface was shown to be a competitive fracture mode only in extreme cases of very thick coatings or sharp contacts.)
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