Effect of residual stress on the fracture toughness of Al2O3/Al gradient materials

Fully dense Al2O3/Al functionally graded materials (FGMs) with continuous gradients were fabricated by casting a slurry into polyurethane foams with graded density, thermal decomposition of the polymer foam and subsequent infiltration of Al into the resulting Al2O3 preform with graded porosity. The R-curve behaviour for Al2O3/Al FGMs was determined using compact tension specimens with different gradation profiles in the direction of crack propagation. A special weight function adapted to graded specimens was used for a precise calculation of stress intensity factors, The fracture toughness of Al2O3/Al FGMs was compared with that of uniform composites. It was found that the fracture toughness for a particular metal content was always higher for the graded material. As bridging stresses of the metal ligaments could not account for these differences in fracture toughness, residual stresses along the crack path were investigated. It was found that residual stresses in the samples play an important role for the apparent fracture toughness observed. Thus, appropriately chosen gradation profiles can improve the performance of metal/ceramic composites considerably.