DIC/DSI-based Studies On The Mechanical Behavior At Crack Tip And Crack Growth Mechanisms Of Ti-6Al-4V ELI Alloy Under Fatigue Overloads

The aviation structures usually undergo complex variable amplitude loading in their service processes,which make the damage tolerance analysis and crack growth life prediction involve the transient processes(including acceleration and retardation effect)under variable amplitude loading and their related description.The extra-low interstitial damage tolerance Ti-alloy Ti-6A1-4V ELI,which has the advantages of high strength,high corrosion resistance and high heat resistance,has been obtained more and more applications in aircraft bearing components.Though the basic crack growtfh behavior has been known,there still lacks of systematic and deep research on the crack growth behavior and the affected mechanism.The studies in this paper mainly focus on the crack growth law as well as intrinsic mechanism of Ti-6A1-4V ELI alloy.And for this purpose,experimental and theoretical investigations combing the macro-and micro-tests are performed,which are of importance for understanding the damage tolerance behavior of Ti-6Al-4V ELI alloy and developing fatigue life prediction method.The crack growth experiments were firstly carried out with three tensile overload ratio(Rol-1.375,1.685,2.6)and four overload patterns(single tensile overload,single compressive underload,tensile overload-compressive underload,compressive underload-tensile overload)based on the MT specimen of Ti-6Al-4V ELI alloy.The effects of overload ratio and overload pattern on the crack growth transient behavior were studied systematically.The results revealed that the crack growth retardation occurred after tensile overload and increased with overload ratio,while compressive underload hardly affected crack growth transient behavior.Besides,the crack growth behavior also corrected with overload/underload sequence,among which,the retardation effect after underload-overload was more significant than overload-underload.Crack profile observation showed that tensile overload,overload-underload and underload-overload would induced crack growth deflection,bifurcation and introduced more second cracks.The digital image correlation(DIC)technique was used to measure the crack tip strain distribution and the plastic zone size of Ti-6A1-4V ELI alloy under variable tensile overload ratios and overload patterns.The results showed that:the tensile overload introduced large crack tip residual strain and plastic zone,which would increase with the increasing of tensile overload ratio linearly.While the compressive underload has no effect on the distribution of crack tip strain.Besides,overload/underload also influenced the crack tip strain filed,among which,the crack tip residual strain and plastic zone after overload-underload was obviously smaller than that after underload-overload.Based on the DIC-measurements,the virtual extensometer technique was introduced to in-situ monitor the crack closure response under typical overload case.The results showed that the crack closure response was clearly related with the overload ratio and overload pattern,among which,the tensile overload would cause the crack surface to open and produce crack tip blunting effect,which the compressive underload would crush the opened crack surface.The depth-sensing indentation(DSI)technique was also utilized to systematically measure the indentation load-depth(P-h)curves of Ti-6A1-4V ELI alloy under three tensile overload ratios and four typical overload patterns.Combing the M.Dao algorithm and the Suresh model,the correlated material properties(including the elastic modulus,the yield stress and strain hardening exponent)and residual stress distribution near crack tip were calculated.On the basis of original material property and the DSI-measured material property,in this paper,the elastic-plastic finite element method was used to simulate the crack tip strain and residual stress field under different overload ratios and overload patterns and the influence of variation of material property on the residual stress distribution was discussed.The results showed that the simulated crack tip residual stress field based on real crack tip material property was more consistent with the measured results.The macro crack growth behavior and DIC/DSI-measured mechanical properties within the crack tip micro-area for the Ti-6A1-4V ELI alloy under typical fatigue overload were synthesized to investigate the major mechanism affecting crack growth transient behavior of this damage tolerance titanium alloy in terms of crack tip material hardening,crack tip blunting,crack closure as well as crack tip plastic zone(residual stress field).The results showed that,although the crack tip material hardening,the crack tip blunting and the crack closure would affected crack growth transient behavior to some extent,the crack tip plastic zone(residual stress field)was the major mechanism influencing crack growth behavior of Ti-6Al-4V ELI alloy under fatigue overload.The modified Wheeler model on the basis of DIC-measured crack tip plastic zone size was proposed to predict the crack growth rate and crack growth life under variable amplitude loading,which obtained an reasonable prediction accuracy.The research work of this paper provided physical mechanism-based crack propagation prediction model for the damage tolerance design and fatigue life prediction of this Titanium alloy under variable amplitude loading,and explored a DIC/DSI-combined new method to investigate the crack tip mechanical behavior.