Indentation stress-strain curves for ''quasi-ductile'' ceramics

A micromechanical model describing ''quasi-ductile'' Hertzian contacts in otherwise brittle ceramics is developed. The elemental basis of the model is a discrete ''fault'' along an internal weak interface, constrained at its ends by an elastic matrix and subject to frictional sliding, in the subsurface zone of high shear stress in the Hertzian field. By summation over a prescribed density of shear faults within the active plastic zone, the analysis leads to a constitutive indentation stress-strain function, with special provision for the incorporation of microstructural variables. Experimental data from a series of mica-containing glass-ceramics with contiguous platelet microstructures are used to confirm the essential predictions of the model. It is demonstrated that plasticity increases with volume Fraction and aspect ratio, but not size, of the platelets. Parametric evaluations by curve fitting the indentation stress-strain data allow for predictions of intrinsic stress-strain responses for the glass-ceramics in conventional uniform stressing states.