THE EFFECT OF GRAIN-SIZE ON THE FATIGUE OF COMMERCIALLY PURE ALUMINUM

Fully reversed uniaxial fatigue tests were performed on polished hour- glass specimens of commercially pure aluminium with three different gr ain sizes, in order to examine the effect of grain size on fatigue. Th e growth of surface cracks was monitored by a plastic replication meth od. An improvement in fatigue strength was observed, as the polycrysta l grain size was refined. The endurance limit stress was shown to depe nd on the inverse square root of the grain size as described empirical ly by a type of Hall-Petch relation. The effect of refining grain size on fatigue crack growth is to increase the number of microstructural barriers to the advancing crack and to reduce the slip length ahead of the crack tip, and thereby lower the crack growth rate. Multiple crac k initiation and growth is a feature of the fatigue of aluminium, whil e the grain size influences the specific detail of crack coalescence. Crack path deviation is greatest in the coarse grained microstructure and crack surface roughness is more pronounced. SEM fractography revea ls that crack initiation and early crack growth takes place along crys tallographic slip planes, and that fatigue striations, characteristic of stage II cracking, extend to the very edge of the specimen section, suggesting extensive crack tip blunting.