Statistical Characteristics on the Microcrack Growth Behavior of a Squeeze Cast Al Alloy
Masahiro GOTO, Seung-Zeon HAN, Takaei YAMAMOTO, Hironobu NISITANI, Norio KAWAGOISHI and Tomoshi MATSUI
Abstract:Fatigue tests of smooth specimens cut from squeeze cast Al alloy car wheels were carried out in controlled laboratory air (t = 30, h = 60%). To clarify the statistical aspects for fatigue behavior, 3 pieces of a region whose area is 4mm2 (2~2mm) were set on the surface of a specimen. In addition to a major crack, the behavior of all cracks initiated within these regions was monitored by a plastic replica technique. Statistical characteristics of fatigue cracks were analyzed by assuming a Weibull distribution. In this material, fatigue cracks were initiated principally from the eutectic Si particles and sometimes from a slip band in the matrix. But, no initiation from microscopic defects such as pinholes and shrinkage porosities was observed. The crack density increased due to the stress cycling, but it saturated at a later stage of the cycling (N/Nf = 0.6 - 0.7), then it decreased with an increasing the cycling. Distribution characteristics of the crack initiation life, crack propagation life, crack growth rate and crack length were represented by a Weibull distribution. Through the values of Weibull parameters and coefficient of variance, some statistical characteristics were studied quantitatively. Key Words:Fatigue, Statistical treatment, Squeeze cast Al alloy, Car wheel, Microcrack, Crack initiation, Crack growth rate, Laboratory atmosphere