Abstract
Reversed plane bending fatigue tests were carried out on two kinds of forged Al-25Si P/M (Powder Metallurgy) aluminum alloys having different sizes and distributions of Si particles. The initiation and growth behaviors of small surface fatigue cracks were continuously monitored by the replica technique and investigated in detail.
It was found that the crack initiation strength of the material with fine Si particles was higher than that with coarse Si particles. Although little difference in strength was observed between the forged and transverse directions in the former material, some difference was found depending on the specimen orientations in the latter material. When the stress intensity range was calculated by considering the difference of the projected area of Si particles to the plane perpendicular to the loading axis, the difference in fatigue strength among the specimens oriented differently could be explained.
The macroscopic crack growth rate, da/dn, could be expressed by the Paris equation in terms of the maximum stress intensity factor, Kmax, irrespective of stress level and specimen orientation. The fatigue crack growth rate was found faster in the coarse grained material than that in the fine grained material, and as the crack propagated along the Si particles, more deflective behavior of crack growth was observed in the material with fine Si particles, which was thought to result in the increase in crack growth resistance.
