Distinctive Features of Fatigue Cracks. Application to Estimating Fatigue Life for Two-Step Temperature Test

Abstract

Fatigue under varying temperature conditions and thermal fatigue are important problems for designing machines and plants used at elevated temperatures. In order to deduce a fatigue life rule under these complex conditions, it is necessary to find the physical changes which correspond to the“damage”, and then to clarify the effect of applied conditions on these changes.The authors examined the feature of cracks in the partially fatigued specimens and the effect of two-step temperature level on fatigue life.As the result of this investigation, the following conclusions have been obtained:(1) The number of cycles to failure of a low carbon steel decreases monotonously with increasing temperature level from R. T. to 600°C for a total strain range of 1%.(2) Submacro-cracks (30∼50μm depth) initiate and grow in the early stage of fatigue test at the temperature level of 500°C, but at R. T. level they do not appear until the later stage.(3) The order of application of the two temperature levels affects the fatigue life only when the upper temperature exceeds 400°C.(4) The total crack area seems to be a more realistic measure of damage, and it is useful for estimating fatigue life under the two-step temperature level test.