The Fatigue Crack Growth Behavior And Mechanisms Under The Application Of Retardation Methods

The failure due to the fatigue of the structural components is one of the key issues in engineering.It is revealed that some retardation methods can decrease the fatigue crack growth rate,leading to the extension of the fatigue life.In this study,the relationship between the fatigue crack growth and the loading conditions was analyzed in detail with the application of overload,stop-hole drilling and their combination by conducting a series of experiments and finite element analyses of QSTE340TM automobile steel.The fatigue crack propagation under single overload was studied in detail under different loading conditions.The analysis revealed that the fatigue crack growth rate patterns were different between the higher load ratio(R=0.7)and the lower load ratio(R=0.1?0.5).A significant crack closure effect was found in the lower load ratio cases.The crack closure load decreased linearly after it reached the peak,while the fatigue crack growth rate increased as a curve.By introducing the crack closing region ratio to the effective stress intensity factor,the Alpha expression was proposed considering the contributions from crack closing region ratio,K_min and K_OP.On the other hand,the mismatch of the maximum crack closure load and the minimum fatigue crack growth rate was found in the experimental results.The Wheeler model was combined with the Alpha expression,achieving good estimates of the fatigue crack growth rate under residual stress effect and crack closure effect.The Wheeler-Alpha model also achieved good agreements in DP780 steel and 6082T6aluminum.Based on the analysis of the single overload,a series of experiments were conducted with double overloads and mixed-mode overload.The crack closure effect of the second overload had the same pattern in general.The Wheeler-Alpha model achieved good estimates in double overload experiments.In turns of the crack closure level,when the second overload was applied in the residual stress dominating region,the new crack closure effect weakened.When the second overload was applied in the crack closure dominating region,the new crack closure effect strengthened,which led to a larger extension of fatigue life.In the mixed-mode overload cases,the retardation effect declined as the angle increased.The fatigue crack growth rate and the fatigue life were studied with the application of stop-hole drilling,and the combined application of overload and stop-hole.The extension of the fatigue life was found to have a positive relationship with the stop-hole diameter and a negative relationship with the stop-hole location.An interaction effect was found when the overload was applied before the crack re-initiated from the stop-hole edge.A model was proposed based on the short crack effect and residual stress effect to estimate the fatigue crack growth with stop-hole retardation.The model was also merged with the Wheeler-Alpha model to estimate the fatigue crack growth rate when the interaction was working in the overload and stop-hole combined experiments.