EFFECT OF LOADING RATE ON GROWTH RATE OF CRACK OF A MMC UNDER IMPACT-FATIGUE LOADING

摘要

MMC (metal matrix composites) have been widely used in various fields. However, their weakness in impact fatigue prevents them from application to some important impact engineering fields. This appeals to a detailed experimental investigation of the materials under impulsive loadings. This paper presents a comparative study of an Al_2O_3 particle reinforced Al alloy and its Al alloy matrix under impact-fatigue and cyclic fatigue loadings. For comparison, the two loadings have the same load amplitude and frequency but the former has shorter load duration than the latter and then has lower input momentum . At first sight, the loading with shorter duration would lead to lower crack extension rate. But, it is found that the growth rate of fatigue cracks under impact-fatigue loading is higher than that under cyclic loading in both materials. Furthermore, the effect in the MMC is more significant than its Al alloy matrix. According to dimensional analysis, the lower crack resistance in the materials subjected to shorter load duration may come from lower dynamic toughness. Hence, we measured dynamic toughness and did find lower toughness of the two materials under impact loading. The microscopic observations show that this still needs a further study of the coupling effect of loading rate and micro structure of the materials.