基于位错演化的疲劳门槛值双驱动力模型OA

[Objective]Fatigue failure remains a primary mechanism of catastrophic damage in engineering structures,necessitating highly accurate prediction of the fatigue crack growth threshold(ΔKth)for ensuring structural integrity and performing life assessment.Traditional two-parameter models,particularly the widely used Vasudevan model,are based on the fundamental assumption that the maximum stress intensity factor(Kmax)and the stress intensity factor range(ΔK)contribute independently to the crack driving force.However,this assumption often leads to significant prediction inaccuracies across varying stress ratios(R),particularly in heterogeneous materials such as dissimilar metal welded joints(DMWJs).This study aimed to minimize these inaccuracies by developing a physically grounded and modified dual driving force model.By incorporating micromechanical dislocation interactions,this research aimed to bridge microscale damage mechanisms with macroscale fracture mechanical parameters,thereby enhancing predictive precision.[Methods]Systematic fatigue threshold investigations were conducted on a DMWJ consisting of base metals A and C and weld metal B.Compact tension specimens were prepared in accordance with GB/T 6398-2017 to assess the heat-affected zones and weld metal.Testing was performed at ambient temperature(23℃)and at an elevated temperature(550℃)at stress ratios(R)of 0.1,0.5,and 0.7.Crack length was precisely monitored using the direct current potential drop method.Analysis of the experimental data revealed a clear deviation from the classical Vasudevan"L-shaped"curve.Accordingly,a new model was developed based on crack-tip plasticity analysis.This theoretical model proposes that fatigue damage is governed not only by independent parameters but also by the synergistic interaction of forward dislocations,governed by Kmax and reverse dislocations governed by ΔK.Crack extension is initiated only when the product of forward and reverse dislocation densities reaches a critical threshold ρ*,resulting in a new hyperbolic predictive relationship.[Results]The experimental results demonstrated that the relationship between ΔKth and Kmax.th does not conform to the rigid"L-shaped"boundaries predicted by the Vasudevan model,confirming the inadequacy of this model for complex welded structures.In contrast,the proposed modified model accurately captured the continuous,nonlinear variation of the fatigue threshold over the full range of stress ratios.The model exhibited significantly improved predictive accuracy,particularly near the critical stress ratio(R*),where conventional models frequently fail.In addition,the model redefined the crack growth boundaries,indicating that certain loading conditions previously considered sufficient for crack propagation are,in fact,insufficient due to inadequate dislocation interaction.The robustness of the model was further validated using independent literature data for Ti-6Al-4V,AZ31B,and IN720 alloys,for which it consistently outperformed the original two-parameter model.[Conclusions]This study establishes a refined dual driving force model for the accurate prediction of fatigue thresholds.The results demonstrate that although crack-tip forward and reverse plasticity are governed by ΔKth and Kmax.th respectively,their effects are intrinsically coupled.Fatigue crack extension depends critically on the interaction of dislocations,requiring the product of their densities to reach a specific threshold.Compared with existing models,the proposed model provides more accurate,physically consistent predictions across a range of stress ratios.