Study on Fatigue Crack Growth Behavior of Small and Long Cracks at Elevated Temperatures

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Fatigue crack growth (FCG) was investigated at elevated temperatures up to 500゜C. The emphasis was made on examining the role of crack closure on the near threshold FCG behAvior at elevated temperatures. FCG tests were made on the samples of low carbon steels (SM41A and SS41), a stainless steel (SUS304) and an aluminum alloy (A2218-T6) in air environment. Some additional tests were made in SM41A and SUS304 in Ar gas in order to examine the effect of oxidation ot the FCG behavior. The results are summarized as follows.(1) The FCG rate was accelerated at elevated temeratures at far threshold or higher <delta><kappa> region.(2) The FCG rate, on the other hand, was decelerated at elevated temperatures near threshold region, and the fatigue threshold, <delta><kappa>th, increased with increase in temperature in air environment.(3) The increase in <delta><kappa>th at elevated temperatures was found to be caused both by the enhanced crack closure due to the oxide products induced on the fracture surface and the increase in <delta><kappa>eff.th, the intrinsic FCG resistance of the material.(4) The contcibution of the crack closure to the increase in <delta><kappa>th was different for different materials. The crack closure was an important role on the increase in <delta><kappa>th in SM41A, but it was a less important role in SUS304.(5) Much increase in <delta><kappa>eff.th in SUS304 was observed in air environment in comparison withe the value in Ar gas at 300゜C. The intrinsic FCG resistance itself was found to be much influened by oxdation in this material, although the detail mechanisms is still unknown.(6) It was concluded that the oxide products could contribute to the increase in <delta><kappa>th only when the relative thickness of the crack tip opening displacement memained small.