Fatigue crack initiation as function of temperature and strain rate

Thirty-four specimens of pure (0.9999+) aluminum were cyclically strained under a variety of plastic strain ranges, plastic strain rates and temperatures in an atmosphere of dry nitrogen. Stable cyclic stress strain data was recorded for a variety of plastic strain rates and temperatures. A time-temperature parameter method of analysis was found to give an adequate representation of the stable cyclic stress-strain relationships for the conditions tested. The specimen surface was observed with both light and electron microscopes to determine when and where fatigue crack initiation occurred. Electropolishing was used to discover the depth and relationship of the crack to slip bands and grain boundaries. The results showed that three forms of crack initiation were present. At low temperatures, grain boundary splitting was common. At room temperature, prominent slip band initiation was prevalent, and at high temperature vacancy coalescence at the grain boundary was the major source of crack initiation. The number of cycles to initiate a crack 0.002 deep at a given strain range was found to be independent of the nature of the crack and both test temperature and plastic strain rate.Advanced Research Projects Agency, ARPA Order No. 2169, 74/0