A B S T R A C T High speed steels, such as the alloy H-13, when used as forging dies are subjected to both wear and cyclic loading, and both of these factors can affect the useful life of such dies. It follows that it is of some importance to determine the fatigue characteristics of such steels. However, fatigue studies of such alloys are limited, especially with respect to fatigue crack propagation (FCP) behaviour as a function of mean stress, and therefore more detailed studies are necessary.In the present study, the fatigue lifetimes and the crack propagation behaviour of a high speed steel were experimentally investigated in laboratory air under different stress ratios, R. A modified linear-elastic fracture mechanics (LEFM) approach was applied to analyze the experimentally-obtained FCP behaviour. The predicted S-N curves and crack growth behaviour for a wide range of R ratios agree well with the experimental data, and the modified LEFM approach is therefore considered to be useful for evaluation of the fatigue behaviour of this class of high strength steels. A = Material-environmental constant a = Half crack length da/dN = Rate of crack propagation F = Elastic-plastic correction factor f = Stress frequency K = Stress intensity factor K T = Stress concentration factor K max = Maximum stress intensity factor K max th = Threshold maximum stress intensity factor K min = Minimum stress intensity factor K op = Crack-opening stress intensity factor K op max = Maximum crack-opening stress intensity factor K = Stress intensity factor range K eff = Effective stress intensity factor range K eff th = Threshold effective stress intensity factor range k = Material constant M = A modified linear elastic fracture mechanics parameter N = Number of cycles N f = Number of cycles to failure N/N f = Fatigue life ratio Correspondence: S. Ishihara.

In this paper, the mechanical properties of die cast aluminium alloys made by various die casting technologies were examined. To create high quality aluminium alloy die castings, two die casting processing technologies were employed. These were (a) ultra slow speed filling cold chamber die casting and (b) high speed hot chamber die casting. Significant improvements of the fatigue and mechanical properties were obtained for both die casting systems compared to the normal high speed cold chamber die casting technique. By comparing ultra slow die casting with hot chamber die casting, it was found that the fatigue and mechanical strengths from hot chamber die casting were higher than those for ultra slow filling die casting. The differences in material strength were attributed directly to the material properties, e.g. microstructural morphology and internal defects. Spherical fine dendritic cells in the hot chamber die casting sample gave rise to high fatigue crack growth resistance; the low crack growth resistance for cold chamber die cast aluminium is mostly due to the growth of aluminium rich a phase and the presence of eutectic silicon fibres. The fatigue strength was also related to the number of internal defects, e.g. the lower the defect rate on the fracture surface, the higher the fatigue resistance and mechanical strength. The characteristics of the principal internal defect were different depending on the die casting technology: this showed fine porosity for hot chamber die casting but solidification shrinkage and the scattered chill structure for slow and high speed cold chamber die castings. The reasons for the change of material strength were therefore influenced by the die casting process.

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