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 iro n has been studied. It has been established that the influence of R ra tio on DELTAK(th) is strongly dependent upon the test temperature. At elevated temperatures the influence of R ratio is significantly less t han that at ambient temperature. At low to intermediate R ratio values temperature initially decreased, then, with increasing temperature, i ncreased the DELTAK(th) levels, causing a minimum in DELTAK(th) to occ ur at 250-300-degrees-C. At high R ratio, however, DELTAK(th) exhibite d a small but consistent increase with temperature. The influence of t emperature on the DELTAK(th) at various R ratio values could be adequa tely explained in terms of crack closure. Much intergranular failure w as observed on the fatigue fracture surfaces at ambient temperature, w hereas at elevated temperatures there was little evidence of this part icular failure.