Effect of off-axis concentrated loading on failure of curved brittle layer structures

A study is made of the effects of superposed tangential force by off-axis indentation loading on curved bilayers consisting of brittle shells filled with polymer support material. Such loadings are pertinent to all-ceramic crown structures on tooth dentin in occlusal function. Layer flexure places the ceramic undersurfaces in tension, leading to fracture by initiation and propagation of radial cracks. Following an earlier study, model specimens with curved surfaces are prepared by pressing glass plates 1 mm thick onto steel spherical dies with radius of curvature 20 mm to 8 mm at elevated temperatures, and bonding the resultant hemispherical shells onto an epoxy support base. The specimens are tested by indentation with spheres loaded vertically but off-center, with the contact center located at 30 degrees to the hemisphere axis. The applied loads to initiate radial cracks are little affected by the resultant tangential component, but the loads to propagate the same cracks to the specimen edges are substantially reduced. Finite element calculations are used to evaluate stress states in the specimens for correlation with the experimental data.