Crack tip damage development and crack growth resistance in particulate reinforced metal matrix composites

A study of crack tip damage development and crack growth resistance of aluminium 359/20% V-f silicon carbide and aluminium 6061/20% V-f Micral (TM) particulate reinforced metal matrix composites has been conducted. Observations of crack tip process zone development at the specimen surface have been compared with the results of fractographic examination of the centre of the specimen. Both materials were found to fracture by a process of void nucleation, growth and coalescence. Void nucleation was found to be by fracture or debonding of reinforcement particles, and/or fracture or debonding of secondary matrix particles. The preferred mode of void nucleation was found to vary depending on the constituents of the PR MMC and even the heat treatment state of the material. It was found that in these materials fractured particles identified on the fracture surface fractured during loading rather than being pre-cracked during fabrication. It was further found that observations of damage development from the specimen surface did not necessarily reflect the mechanisms prevailing in the specimen bulk. Under plane strain conditions, both materials were found to exhibit decreasing crack growth resistance as crack extension proceeded, due to the ''anti-shielding'' effect of damage accumulated in the process zone ahead of the crack tip. In thin specimens of the Comral-85 composite, however, dramatically improved toughness was obtained, and K-R curves have been obtained for such specimens. The method of measuring crack length was found to have a profound effect on the K-R curve; it was concluded that the K-R curve determined using the crack length measured at the specimen surface best reflected the true crack growth resistance of these materials.