Fracture mechanics studies of thermal mismatch using a four-point bending specimen

Layered composites develop thermal residual stresses during cooling from processing temperature to room temperature. The thermal stresses reduce fracture toughness data measured in four-point bending tests. To obtain a material parameter characterizing the interface fracture toughness the measured data must be corrected for the influence of thermal stresses. Thermal stresses often lead to kinking of an interface crack out of its initial plane. This tendency can be quantified by two parameters: (i) the ratio G/G₀ of the energy release rate of the kinked crack G and the energy release rate of the interface crack G_O and (ii) the ratio of the local stress intensity factors at the tip of the interface crack, K_II/K_I. Both quantities have been computed for a variety of material combinations using the finite element method. They are found to be strongly affected by thermal and elastic mismatch. Fracture experiments have been performed using brittle glass/glass composites with different thermal mismatched. The obtained fracture toughness values and crack deflection angles have evaluated on the basis of the numerical results. Measured and calculated kinking angles are in excellent agreement. The contribution of residual thermal stresses to the interface fracture toughness K_c has been elaborated.