Al-2.4%Mg合金溶体化処理材の疲労強度に及ぼす結晶粒度の影響

Abstract

The rotating bending high cycle (cyclic frequency: f=50 Hz) and the push-pull low cycle (f=0.2 Hz) fatigue tests were performed, and the grain size dependence of the fatigue strength was examined for the solution-treated Al-2.4%Mg Alloy.The fatigue strength of the plain specimen depends on grain size in the long life range, while in the short life range it is almost independent of grain size. Such a grain size dependence, which is in contrast with that of copper and aluminum, is discussed from the strain ageing effect and the behavior of initial fatigue cracks.The fatigue life of the notched specimen is scarcely affected by grain size. This can be attributed to that a great percentage of fatigue life is occupied by the non-crystallographic fatigue crack propagation process in the same way as other metals. Moreover, a non-propagating crack is always formed in the notched specimen stressed cyclically up to about 2×107 cycles at the stress amplitude corresponding to the fatigue limit. This crack developes through the crystallographic path around the circumference of the notch root, but it stops at a depth of about 50 μ independently of grain size.The low cycle fatigue life controlled by the plastic strain range is almost independent of grain size. Therefore, the low cycle fatigue life does not always correspond to the fracture ductility influenced by grain size. This might result from that the defects such as inclusion and void are contained in the test material and the fatigue induced plastic deformation is localized in the vicinity of the grain boundary.The shapes of S-N curves suggest that the fatigue limits exist both in the plain and notched specimens. The dynamic strain ageing induced by cyclic stressing might contribute to the formation of the fatigue limit.