In this paper, Comprehensive progressive damage analyses are performed to evaluate the effects of patch sizes and layups on re-starting crack growth of single-side repaired panels with central inclined cracks under high cycle fatigue loading. Complicated nonlinear damage behavior of adhesive bonding, composite patch, mixed-mode cracks and fatigue loadings require precise numerical tools to consider and analyze the coupling effects of various damages on fatigue life of such complex component. For this purpose, two user-written materials subroutines added to a commercial finite element software to capture adhesive debonding and composite patch damages under high cycle fatigue. In progressive damage analysis, high cycle fatigue cohesive zone modeling used for debonding of patch and high cycle fatigue continuum damage model used for composite patches. Practical composite patch layups and sizes are selected in huge number of models and the restarting crack growth life are predicted. The results shown that the composite plies angles attached to the cracked panels have dominant effect on restarting crack growth life. Different combinations of the same layup angles show completely different restarting crack growth lives. The obtained results show the possibility of increasing the restarting crack growth life using an appropriate composite patch layup and geometry dimensions.

A numerical and experimental study was conducted on fatigue crack growth (FCG) of metallic components to investigate the effect of redistribution of mechanical residual stresses induced by single overloading during FCG on the fatigue life of notched specimens. For this purpose, the compact tension specimens of an aluminium alloy were used. In addition, mechanical residual stresses were introduced near the crack tip by applying tensile overload (OL), followed by fatigue loading of the specimens. In the numerical simulation, the modified cyclic J-integral was used as the crack growth fracture parameter and a good agreement was observed between the numerical and experimental results. The results of the finite element method demonstrated a clear redistribution of mechanical residual stresses and so the variation of the overload plastic zone size during FCG. After a few cycles, the residual stress field around the crack tip reached a lower magnitude value confined in a smaller zone, although this zone was stable during the remaining fatigue process. Finally, the present study evaluated the effects of stress ratio, load amplitude, and overload ratio on the redistribution of residual stresses and the size of the plastic zone around the crack tip. It was observed that the residual stresses are mainly released during the first steps of fatigue loading.

This paper provides a further understanding of the peak load effect on the damage mechanics and residual stress relaxation. The comprehensive numerical simulations using the finite element method are applied to take into account simultaneously the effect of the surface roughness and residual stresses on the crack formation in sandblasted S690 high strength steel surface under peak load condition. A ductile fracture criterion is introduced for the prediction of damage initiation and evolution. This study investigates especially the influences of compressive peak load, effective parameters on fracture locus, surface roughness, and residual stress on damage mechanism and the formed crack size. Results indicate that under peak load conditions, surface roughness has a far more important influence on microcrack formation than residual stress. Moreover, it is shown that the effect of peak load range on damage formation and crack size is significantly higher in comparison with the influence of residual stress. It is found the crack size has been developed exponentially with increasing peak load magnitudes.