A First Stage in the Development of Micromechanical Simulations of the Crystallographic Propagation of Fatigue Cracks Under Multiaxial Loading

Abstract

Simulations of the nucleation of dislocations, glide and annihilation ahead of a fatigue crack growing along a localized slip band (a ‘long’ Stage I crack or a Stage II crack with a K value close to the threshold) are performed for the case of push–pull or reversed torsion loadings, ignoring, in a first approach, the effect of grain boundaries. The crack growth rates are deduced from the dislocation flux at the crack tip. An influence of the normal stress on the friction between the crack flanks as well as on the condition for dislocation emission is introduced. A slower Stage I growth rate is then predicted for reversed torsion, consistent with experimental data.

Micromechanical simulations of microstructure‐sensitive Stage I fatigue crack growth

“…In the second case, although dislocations that individually pass the first GB glide at once to the next one, slip transfer is more progressive than in the model of Navarro and De Los Rios 3 for which the blocked slip band, with all its dislocations, jumps to the next GB when a critical stress concentration is reached. It also appears that, due to the strong repulsive force of the pile up that pushes leading dislocations away, a second dislocation‐free zone (in addition to that present between the crack tip and the trailing dislocation 1 ) forms behind the grain boundary, as already mentioned by Pippan. This is in contrast with the predictions of dislocation distribution for a ‘propagated slip band’ of Tanaka et al ., 2 according to which the dislocation density is infinite on both sides of the GB.…”

“…Because real Stage I cracks are not straight, allowance has to be made for asperity‐induced friction that tends to reduce the crack driving force, and is either enhanced or reduced by a compressive (respectively tensile) normal stress. Experimental information concerning crack flanks frictional interactions for long cracks loaded in mode II, in the presence of a static normal stress, has been obtained through combined tension and torsion tests performed inside a scanning electron microscope on precracked tubular specimens of maraging steel 1 . In the present study, an attempt is made to reproduce qualitatively these experimental data using empirical equations.…”

“…In the first stage of their development, fatigue cracks are driven by cyclic shear, but their growth rate is influenced by the normal stress: it is increased by an opening stress and reduced by a compressive one. In a previous paper, 1 simulations of dislocation dynamics at the tip of a crystallographic mode II crack, ignoring at first the influence of microstructural obstacles, have been presented. The crack growth rates were deduced from the dislocation flux at the crack tip.…”

A critical plane fatigue model with coupled meso‐plasticity and damage

“…It is now well‐known that the normal stress to a crack face can have a strong influence on its development 25 . For the mode II behaviour considered here, the normal stress applied on the active slip system can either promote or delay mesocrack initiation and growth.…”

“…It is now well-known that the normal stress to a crack face can have a strong influence on its development. 25 For the mode II behaviour considered here, the normal stress applied on the active slip system can either promote or delay mesocrack initiation and growth. In particular, the normal stress can have a strong influence on the friction mechanisms acting on the crack lips, and thus change the balance between the dissipated energy part and the stored energy part.…”