Study On Very-High Cycle Fatigue Characteristics Of High Strength Aluminum Alloy

The question of whether a real fatigue limit of materials exists is of great value for the design life of engineering equipment.Centainly,it is not easy to prove the real fatigue limit because we can not prove infinity experimentally,and because of the diversity of fatigue failure mechanisms in the ultra-long life stage,it is difficult to establish a universal fatigue model.However,it can be confirmed that materials will inevitably experience very-high cycle fatigue（VHCF）before approaching the fatigue limit.Therefore,the study of VHCF is a necessary prerequisite to answer the limit question.Research on VHCF has been conducted for more than 30 years,and important consensus has been reached on its failure characteristics:for Type-Ⅱ materials containing inclusions,after entering VHCF,the fatigue crack source tends to transfer from the surface to the internal inclusions,that is,the notch effect on the material surface will disappear with the reduction of stress level,and VHCF will eventually become a process of crack initiation and propagation dominated by internal defects.Therefore,it is reasonable to suppose that the same material in different processing states may exhibit common life characteristics at a certain stress level while keeping the internal defects unchanged.Based on this,the failure modes,fracture mechanisms and S-N curve characteristics of the VHCF were studied with different processing states of 7075aluminum alloy,in order to reveal the general law of the VHCF stage.In this dissertation,the stress-controlled axial fatigue tests under multiple stress ratios（R）were performed on the coarse grain specimens（CG）,the CG subjected to surface mechanical rolling treatment specimens（CG-SMRT）,the as welded joint specimens（AW）and the friction stir welded joint subjected to surface mechanical rolling treatment specimens（FSW-SMRT）of 7075-T651 aluminum alloy.The characteristics of S-N curve and fatigue fracture were studied in detail and depth.Fatigue test results show that the CG-SMRT specimens exhibit S-mode,I-mode and IFGA-mode fatigue failure modes,and S-N curves do not show multi-stage characteristics even when the failure mode transition occurs.The CG and FSW-SMRT specimens contain S-mode and IFGA-mode failure modes,and S-N curves show multi-stage characteristics due to the transition of failure modes.AW specimens only show S-mode single failure mode within the stress range selected in this study.Under different stress ratios,with the decrease of stress,the S-mode is transformed into the fatigue failure led by internal inclusions(Al₁₈Ti₂Mg₃ crystal phase).In addition,the S-N curves of different processing states present the characteristics of all paths to the same destination when the fatigue failure is in IFGA-mode.The mean stress model was used to normalize fatigue data with different stress ratios to reveal the universal characteristics of VHCF failure of aluminum alloy.By comparing and analyzing multiple classical mean stress models,the indexγrelated to material properties in Walker model was optimized by using Matlab,and the mean stress model suitable for different processing states in this study was obtained,in which the optimal solution ofγwas 0.4595.Furthermore,a fitting equation was established to describe the stress-life relationship.In Walker model,all processing states except for the CG-SMRT specimens exhibit a multi-stage failure characteristic on the S-N curve,and each stage corresponds to a failure mode.The fatigue data are distributed on the fitting curve of CG-SMRT specimens when IFGA-mode failure occurs,and the S-N curve presents smooth and continuous distribution characteristics.According to this characteristic,theσ₀ of the CG-SMRT+IFGA-mode fitting curve may be the true fatigue limit of 7075 aluminum alloy,which is approximately 117 MPa.Furthermore,the essential characteristics of VHCF failure of aluminum alloy were revealed through the strain characteristics during cyclic loading.The strain amplitudes of different stress ratios were normalized by combining the equivalent strain model,and the strain-life relationship was described by constructing a fitting equation.In the ε_ar-N curve,the fatigue data are distributed along the fitting curve of CG-SMRT specimens when IFGA-mode failure occurs in different processing states,showing the characteristics of all paths to the same destination.For the CG-SMRT+IFGA-mode fitting curve,ε₀ tends to 0,indicating that no fatigue behavior appears,which corresponds to the fatigue limit（117 MPa）of stress-life relationship in Walker model.According to the strain characteristics,the damage-life relationship（ΔW-N）considering the effect of mean stress was derived,which was another important basis for the study of fatigue characteristics of aluminum alloy.In the ΔW-N curve,the fatigue data are distributed along the fitting curve of CG-SMRT specimens when IFGA-mode failure occurs in different processing states,showing the same characteristics as the strain-life relationship.TheΔW₀ of the continuous fitting equation tends to 0,indicating that no fatigue failure occurs under this stress（117 MPa）.The cumulative ratcheting strain during cyclic loading shows that ε_r is beneficial to fatigue performance and does not significantly cause deviation in the S-N curve under high stress due to its relatively small influence.By analyzing the fracture characteristics of VHCF,the mechanism of internal crack initiation and propagation was revealed.For IFGA-mode,the region inside the FGA is the crack initiation zone,while the FiE outside the FGA is the stable crack propagation zone.By calculating the range of stress intensity factors of the inclusions and FGA in crack source,it was concluded thatΔK_FGA can be used as the threshold value for fatigue crack propagation.By modifying the Paris–Hertzberg law,a calculation formula for the fatigue crack growth life of all R was obtained.The IFGA-mode crack initiation life accounts for more than 95%of N_f,and when N_f>10⁷,it accounts for more than 99%.Within the FGA region,the crack propagation rate is quite slow,about 10^-11～10^-12 m/cycle,while the crack propagation rate in the Fi E region is about 10^-8 m/cycle.Based on the above analysis,the multistage fatigue failure mechanism of different machining states and the general law presented by VHCF were explained from the perspective of fracture mechanics.Based on the above test results,for 7075 high-strength aluminum alloy:when the stress is at a higher level,the fatigue failure is S-mode,and the S-N curves of the materials under different processing conditions are significantly different.With the decrease of stress,all processing states within the stress ratio range of-1 to 0.5 in this study will be transformed into Al₁₈Ti₂Mg₃ crystalline phase led fatigue failure（IFGA-mode）sooner or later.The fatigue failure dominated by internal inclusions is the essential characteristic of high-strength aluminum alloy in VHCF.