Effect of T-stress on fatigue crack closure in 3-D small-scale yielding

Abstract

This paper investigates the effects of in-plane constraint on 3-D fatigue crack closure in the small-scale yielding regime. The finite element analyses grow a sharp, straight-through crack in a modified boundary layer model under mode I, constant amplitude cyclic loading with prescribed but independent peak values of stress intensity factor, K max, and the T-stress, T max. A purely kinematic hardening law with constant modulus represents the material constitutive behavior. The computational results demonstrate that a two parameter characterization of crack tip fields in terms of K max /σ 0 B and T max/ σ 0, where σ 0 denotes the yield stress of the material, correlates successfully the normalized opening load K op/ K max across variations of thickness ( B), constraint level and material flow properties. Both negative and positive T-stress reduce the through-thickness variation in local opening load levels along the crack front. A negative T-stress increases K op/ K max values, particularly at low peak loads where the plastic zone size remains a fraction of the thickness; a positive T-stress has limited effect on K op/ K max values. The fringe plots of individual plastic strain components reveal (a) in the absence of T-stress ( T max/ σ 0=0), plastic contraction in the thickness direction compensates primarily for permanent stretching in the direction normal to the crack plane required for closure, (b) for negative T-stress ( T max/ σ 0<0), plastic contraction in the in-plane transverse direction contributes the larger share of material flowing into the normal direction, and (c) for positive T-stress ( T max/ σ 0>0), both in-plane directions experience permanent stretching and the thickness direction alone undergoes plastic contraction.