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Effect of copper alloying on fatigue life and fatigue crack growth behaviour of ductile cast irons

Specimens cut out and machined from Cu alloyed (about 1.6%) and unalloyed (less than 0.012%) ductile cast iron ingots were tested under cycling loading to reveal the effect of copper addition on fatigue performance. The influence of solidification rate and hence the microstructure were also studied by preparing sand cast and die cast ingots from each composition. Some high copper die cast specimens were annealed to spheroidize and homogenize the matrix's pearlite structure. All experiments were carried out by subjecting CT type specimens to constant load amplitude and tensile to tensile sinusoidal load cycles. Electrical crack foils were employed to measure instantaneous crack sizes. Cu alloyed samples showed higher fatigue lives. Similarly, crack growth rate diagrams pointed out lower fatigue crack velocities and higher near threshold stress intensity factors in high Cu specimens. The spheroidization of pearlitic matrix resulted in an insignificant increase in fatigue crack growth rate and a 10% decrease in near threshold stress intensity value. The fatigue performance of sand cast pieces was better than die cast ones. It is concluded that the fatigue life of ductile cast irons can be improved by copper alloying.