Abstract
The effect of grain size on fatigue damage in pure aluminium was studied in fully reversed plane bending by using the X-ray microbeam technique and optical microscopy. The results are summarized as follows:
(1) The fatigue strength was found to be affected by the grain size in aluminium, although the substructure was well developed. This is interpreted on the basis that the crack initiation is affected by the grain size and the crack propagation takes place mostly along the grain boundary at higher stress amplitudes, but is retarded in the vicinity of the grain boundary at lower stress amplitudes. The grain size dependence of fatigue strength also seems to be associated with the purity of material.
(2) The Petch's type relation did not exist between the fatigue strength and the grain size. The fatigue strength plotted against the inverse square root of grain size did not fall on a straight line but on a downwards bent line. This appears to have resulted from the facts that the stress amplitude at the transition from intergranular to transgranular cracking is also affected by the grain size as is the fatigue strength, and that the substructure develops more in the specimens having smaller grain sizes.
(3) The subgrain size was found to be inversely proportional to the stress amplitude. But the subgrain size was also affected by the grain size and became smaller in the specimens with grains less than 50μm. On the other hand, both (Db)max and (Db)mim increased with an increase of stress amplitude and, furthermore, the former increased a little with increasing grain size.
