A MODEL FOR MICROPACK INITIATION AND PROPAGATION BENEATH HERTZIAN CONTACTS IN POLYCRYSTALLINE CERAMICS

A fracture mechanics model of damage evolution within Hertzian stress fields in heterogeneous brittle ceramics is developed. Discrete microc racks generate from shear faults associated with the heterogeneous cer amic microstructure; e.g. in polycrystalline alumina, they initiate at the ends of intragrain twin lamellae and extend along intergrain boun daries. Unlike the well-defined classical cone fracture that occurs in the weakly tensile region outside the surface contact in homogeneous brittle solids, the fault-microcrack damage in polycrystalline ceramic s is distributed within a subsurface shear-compression zone below the contact. The shear faults are modelled as sliding interfaces with fric tion, in the manner of established rock mechanics descriptions but wit h provision for critical nucleation and matrix restraining stresses. T his allows for constrained microcrack pop-in during the loading half-c ycle. Ensuing stable microcrack extension is then analyzed in terms of a K-field formulation. For simplicity, only mode I extension is consi dered specifically here, although provision exists for including mode II. The compressive stresses in the subsurface field constrain microcr ack growth during the loading half-cycle, such that enhanced extension occurs during unloading. Data from damage observations in alumina cer amics are used to illustrate the theoretical predictions. Microstructu ral scaling is a vital element in the microcrack description: initiati on is unstable only above a critical grain size, and extension increas es as the grain size increases. Internal residual stresses also play a n important role in determining the extent of microcrack damage. Impli cations of the results in the practical context of wear and fatigue pr operties are discussed.