CYCLIC PLASTIC STRAIN-ENERGY AS A DAMAGE CRITERION AND ENVIRONMENTAL-EFFECT IN NB-BEARING HIGH-STRENGTH, LOW-ALLOY STEEL

Abstract

Low cycle fatigue behaviors in Nb-bearing high strength, low alloy (HSLA) steel have been studied at plastic strain amplitudes Delta epsilon(p)/2 ranging from 1x10(-5) to 1x10(-2) in air and in ultrahigh vacuum (UHV). Microcracks are initiated along slip lines both in air and in UHV, and grain boundaries act as effective barriers to the propagation of microcracks. Coffin-Manson relations are reasonably satisfied for the microcrack initiation life N-i and the fatigue life N-f both in air and in UHV. The accumulated total cyclic plastic strain energy W-i required for fatigue crack initiation and the accumulated total cyclic plastic strain energy W-f required for fatigue failure are expressed mathematically in the form of K(Delta epsilon(p)/2)(N). The computed values of W-i and W-f are in good agreement with the values determined experimentally. The vacuum environment enhances N-i, N-f, W-i and W-f. The environmental effect becomes greater as the plastic strain amplitude decreases.