Cracking in brittle laminates from concentrated loads

A study is made of the crack resistance of multilaminates consisting of brittle layers interleaved with compliant interlayers and bonded to compliant substrates. Specific attention is paid to flexure-generated radial cracks in the undersurfaces of individual glass layers from concentrated loads applied at the top surfaces. A proposed condition for acceptance of such multilayer systems is that radial cracking in the upper layers should not occur before analogous radial cracking at the undersurface of an equivalent monolithic coating. Model multilayer systems constructed from glass plates laminated with polycarbonate interlayers and glued onto a polycarbonate base are used to test this condition. These systems enable in situ viewing of radial cracks through polished side surfaces. Critical loads to initiate such radial cracks are measured for multilayers with selected numbers of layers and interlayer/glass thickness ratios. Simple, approximate, semi-empirical closed-form relations are derived for the critical loads, and validated against predictions from finite element computations. We demonstrate the general existence of optimum conditions for multilayer design in terms of numbers of brittle layers, ratios of adhesive interlayer to brittle layer thickness, and modulus mismatch ratios.