The mechanics of contact-induced damage in brittle layers on soft support substrates are of interest for their relevance to lifetime-limiting failures of technological laminate systems in concentrated loading. This is particularly true of biomechanical applications-dental crowns and hip prosthesis-which must withstand intense biting or body weight forces under exacting cyclic contact loading and in vivo environmental conditions. Such prostheses may consist of several different material types-ceramic, metal, polymer, and composite-in layered configurations. Other, more engineering-oriented layer applications include cutting tools, thermal barrier coatings, electronic multi-layer devices, car windscreens and eyeglasses. The recent advances in the understanding and analysis of damage initiation and evolution in brittle layer structures will be presented. The results of experiments model bilayer that enable in-situ viewing during loading with simple concentrated loads (spherical indenters) and yet simulate the essential features of damage in a wide range of engineering layer structures are described. The experiments reveal a competition between damage modes-cone cracks or quasiplasticity at the top (near-contact) brittle-layer surfaces and laterally extending radial cracks at the lower surfaces due to the brittle coating layer thickness.
Brittle coating structures may be subjected to complex concentrated loading, including tangential component. Results of Hertzian contact tests investigating the effects of superposed tangential loads on the critical conditions for radial cracking at the undersurfaces of brittle coatings on compliant substrates are reported. Bilayer specimen is prepared from glass coating bonded to polycarbonate substrate and blunt indenter is load with tangential motion as well normal motion. In order to change the friction coefficient the loading geometry is changed to the inclined fixture. Changes in near-surface stress fields from tangential force are well documented in Hertzian contacts on monolithic solids, and markedly diminish critical loads to initiate cone cracking and other damage modes in brittle surfaces. Nonetheless, the depth of the cracks is not so sensitive to tangential force so that remaining strength are only slightly affected. The implication is that the influence of superposed non-normal load remains concentrated in the near-contact region, with only secondary effects on events in the far field. This explanation is also verified from the finite element analysis. It demonstrated that these effects are minor, so that conventional normal indentation remains an appropriate test procedure for characterizing this highly deleterious mode of coating fracture under a wide range of complex loading conditions.
Rate effects in the Hertzian contact of loading of model glass-polycarbonate and silicon-polycarbonate bilayers bonded by epoxy adhesive are examined. Glass is used because of its high susceptibility to slow crack growth, making this conventional contribution to the rate dependencies easy to distinguish. Silicon is used as a control material with effectively no slow crack growth. Critical loads are measured as a function of loading rate. The glass-polycarbonate bilayer critical load data show pronounced rate dependence, suggesting as extraneous contribution to the kinetics from the adhesive-substrate. The silicon-polycarbonate bilayer data also show as loading-rate dependence, although much smaller, confirming this last conclusion. Data from cyclic contact test on the glass-polycarbonate bilayers correspond with the loading-rate data on lifetime plots, eliminating the probability of a mechanical component in the fatigue response. It is concluded that the adhesive-substrate contribution is viscoelastic in nature, from energy-dissipation (but non-cumulative) an-elastic deformation during the cyclic loading. Critical load tests on bilayers with different exposures to external water shows no influence of external environment, suggesting that internal moisture is responsible for the slow crack growth in the glass-coating bilayers.
In the engineering laminate structures and brittle coated layer structures, curved surface is as usual as flat structure. Nevertheless, the role of curvature was not fully studied nor evaluated. In this study, bilayers of different curvatures were prepared and loaded with the blunt indenter. With the aid of in-situ experimental fixture and the capturing of load drop, the change of critical load for radial cracking at the bottom surface of brittle coating was evaluated. The difference of load between in a flat and a curved bilayer was verified and quantified using the finite element analysis. Over the wide range of curvature and thickness, the critical load change could be expected from the fitting equation using numerical method. Even though the maximum change of critical load was about 50% in the practical application range, this amount of change was relatively small, compared with the change of thickness and elastic modulus, which can result in the change of order of magnitude. As a conclusion, in the case of determining the optimum design parameter of layer structures, the effect of curvature is not so dominant as other variables for the change of fracture load of radial cracking.
