Influence of casting surfaces on fatigue strength of ductile cast iron

Abstract

A B S T R A C T Quantitatively evaluating the fatigue strength of ductile iron (DI) with casting surfaces involves several complicated factors such as surface roughness, transition of microstructures from surface to interior, several types of defects and residual stresses. Tension-compression fatigue tests have been performed using DI having casting surfaces composed of a ferritic structure, a ferrite-pearlitic structure and a pearlitic structure. Residual stresses were relieved by annealing in order to separately evaluate each factor. The √ area parameter model was applied for quantitative evaluation of fatigue strength. Surface roughness was considered to be mechanically equivalent to a defect, and the effective defect size √ area eff due to the interaction between the surface roughness and a defect was defined. The present study proposes a method of evaluating the maximum defect size using statistics of extremes and the lower bound of the scatter of fatigue strength, for practical design.Keywords casting defects; ductile cast iron; fatigue; fatigue strength; statistics of extremes; surface roughness. I N T R O D U C T I O NBecause the structure of ductile cast iron (DI) contains graphite and casting defects, its fatigue strength is lower than that of steel, which has a hardness almost equal to that of DI. Several studies on the fatigue strength of DI have been conducted, and the influences of graphite, matrix structures and defects on fatigue strength have been examined. 1 However, a majority of these studies were conducted using smooth specimens, i.e. without casting surfaces, and only a few studies have been conducted on DI with casting surfaces. 2 This is due to the fact that the fatigue strength of DI with casting surfaces is influenced by various factors. The factors that should be considered are as follows:1 Surface roughness 2 Work hardening layer due to shot blasting, transitional layer near surfaces produced by casting 3 Several types of defects 4 Residual stresses caused by shot blasting and casting As these factors work together and influence fatigue strength in a complex manner, the quantitative evaluation Correspondence: Junichiro Yamabe. of the fatigue strength of DI having casting surfaces is very complex, resulting in a different fatigue strength from that observed for smooth specimens. With regard to these factors, a few studies on two-dimensional surface roughness produced by machining have been performed, although these studies did not use DI. 3-6 Some quantitative methods of predicting the fatigue limit have been proposed based on these studies. 5,6 However, only a few studies on three-dimensional surface roughness, such as that on casting surfaces have been conducted. 7 In order to thoroughly evaluate the fatigue strength of DI having casting surfaces, the present study focused on the three factors mentioned above ((1)-(3)) and the √ area parameter model, 8 which is one of the methods for quantitatively evaluating the fatigue strength of specimens containing small cracks, small defects or ...

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