EFFECT OF TEMPERATURE ON THE THRESHOLD FATIGUE CRACK-GROWTH BEHAVIOR OF SPHEROIDAL GRAPHITE CAST-IRON

The influence of temperature at various R ratio values on the fatigue crack propagation response of a ferritic spheroidal graphitic cast iron has been studied. It has been established that the influence of R ratio on DELTAK(th) is strongly dependent upon the test temperature. At elevated temperatures the influence of R ratio is significantly less than that at ambient temperature. At low to intermediate R ratio values temperature initially decreased, then, with increasing temperature, increased the DELTAK(th) levels, causing a minimum in DELTAK(th) to occur at 250-300-degrees-C. At high R ratio, however, DELTAK(th) exhibited a small but consistent increase with temperature. The influence of temperature on the DELTAK(th) at various R ratio values could be adequately explained in terms of crack closure. Much intergranular failure was observed on the fatigue fracture surfaces at ambient temperature, whereas at elevated temperatures there was little evidence of this particular failure.