Monosemousness of Thermal Plastic Strain on Thermal Fatigue Life in Ferrite Ductile Cast Iron

Abstract

In this study, the monosemous effect of thermal plastic strain on the thermal fatigue life is newly found on ferrite ductile cast iron around the alpha phase field. At first, the monosemousness is defined and its meaning described. Next, the monosemousness of thermal fatigue is demonstrated by its conditional equation and its existence is verified by the thermal fatigue test on ferrite ductile cast iron. By doing so, the feature on the thermal fatigue of ferrite ductile cast iron is clarified. Generally, it is considered that fatigue life in ferrite-matrix temperature range can be expressed at least by two or more different Arrhenius equations, namely there are two or more different activation mechanisms to govern the thermal fatigue life corresponding to various ferrite temperature ranges. In this case, for determining the life in any various ferrite temperature ranges, it must have at least four or more unknown quantities. If there is the presence of a general equation which is able to replace above described plural equations, then the life can be determined by simple one variable. Here, by introducing conditional equations, it is verified that the general equation is a Coffin and Manson's equation of low cycle fatigue and whole thermal fatigue life can be determined by a variable of thermal plastic strain occurred in thermal cycle. As a result, the law can apply to describe thermal fatigue phenomenon and predict thermal fatigue life monosemously from cyclic thermal plastic strain on ductile cast iron with ferrite matrix.