EFFECTS OF THICKNESS AND PRECRACKING ON THE FRACTURE-TOUGHNESS OF PARTICLE-REINFORCED AL-ALLOY COMPOSITES

The effect of specimen thickness on the fracture toughness of a powder metallurgically processed 7093 Al/SiC/15p composite was evaluated in different microstructural conditions. The fracture toughness in the un deraged condition increased significantly with a decrease in specimen thickness, even at thicknesses well below the value specified by ASTM- E 813 for a valid J(Ic) test. The influence of thickness was considera bly lower in the peak-aged (PA) condition. This relative insensitivity is believed to be due to the low strain to failure associated with se vere flow localization in the PA condition. The effect of precracking on the fracture toughness of discontinuously reinforced aluminum (DRA) was also evaluated. The dependence of fracture toughness on specimen thickness and precracking is explained in terms of the hydrostatic str ess, which has a strong influence on the plastic straining capability of the DRA material. The fracture toughness was modeled using a critic al strain formulation, with the void growth strain dependent on hydros tatic stress through the Rice and Tracey model. The predicted toughnes ses for the thick and thin specimens were in good agreement with the e xperimental data.