Microstructurally-short fatigue crack in SiC whisker reinforced 6061 aluminum alloy composite

Short crack propagation behaviour of 6061 aluminum alloy with and without SiC whisker was investigated. Short fatigue cracks initiate and grow in a much lower ΔK range than the threshold stress intensity ranges, ΔK_th, of long fatigue cracks in both materials. In addition, there exists a growth dip in the MMC, and it could be explained by measured abrupt awakening of crack closure which is mainly induced by asperity contact. Statistic scatter in the growth rates of microstructurally short crack is observed in both of the materials. The distribution of crack growth rates well follows a three parameter Weibull distribution function. The shape parameters of this distribution function which reflects the extent of scatter in the crack growth rates are initially small in both of the materials and they increase and close to the respective asymptotic lines with increasing crack length. The crack length range when the scatter converges is 110 to approximately 183 μm for the unreinforced alloy and 25 to approximately 40 μm for the composite. This is considered as upper bounds of microstructurally short cracks. The differences in crack length may be attributed to the respective crack growth mechanisms that crystallographic crack growths such as slip band cracking and intergranular cracking, which are generally observed in the unreinforced alloy, are suppressed in the MMC due to the existence of closely spaced reinforcement, and a microstucturally short crack mainly interacts with reinforcement itself and a packet of whiskers.