A Study On Crack Propagation Behavior And Deformation Field Around The Crack Tip Of 7050 Aluminum Alloy Under Typical Fatigue Load

As for aerospace,a large number of engineering structures in service are often subjected to variable amplitude load such as tension-compression mean load and instantaneous overload.What's more,instantaneous overload leads to the changes in the fatigue damage and inhomogeneous deformation around the crack tip which are responsible for retardations or accelerations during the crack propagation and thus seriously influence the damage tolerance behavior of engineering structures.Therefore,adequate study and analysis of the relationship between fatigue crack propagation behavior and stress-strain distributions in the crack tip zone is of great importance and value for better understanding of fatigue overload damage mechanism and improving the damage tolerance design.In this paper,a series of fatigue crack growth experiments were conducted on 7050 aluminum alloy(MT)specimens under tension-compression mean load and single tensile overload.The results showed that mean load had a significant impact on the crack propagation behavior under constant amplitude load.As the increasing mean load,fatigue crack propagated faster.The influences of single tensile overload were also analyzed which denoted retardation effect.Larger overload ratios induced much more obvious retardation behavior.In addition,the crack propagation exhibited zigzag and bifurcate phenomenon under tensile overload.Digital image correlation technique and micro grid method were carried out to systematically study the deformation field around the crack tip before and after single tensile overload.Further analysis implied that symmetrical deformation zone and large strain gradient remained in front of the crack tip,while serious deformation existed in the wake of the crack tip.The strain field tested by two methods was basically in accordance with that obtained by finite element simulation.And the size of the plastic zone tested by digital image correlation technique was much more close to that calculated by finite element simulation.Based on the results of the deformation field in the crack tip zone,Krottenthaler model was used to calculate the residual stress field around the crack tip.It was found that residual compressive stress existed in front of the crack tip and the value in the crack tip seemed to be biggest.The peak value and the region of compressive residual stress both increased with larger overload ratios.The residual stress field in front of the crack tip seemed to be one of the key factors account for the crack propagation retardation behavior after tensile overload.Four representative models(Walker model,Forman model,Wheeler model and Elber model)were developed to predict the fatigue crack propagation behavior of 7050 aluminum alloy under typical fatigue load.The contrast of the results denoted that the predictions of Walker model were much more accurate than Forman model and the predictions of Wheeler model matched with the experimental data,that relative error was less than ± 15%,with a higher correlation coefficient than Elber model.Therefore,Walker model and Wheeler model could be used to in damage tolerance design and fatigue life prediction for 7050 aluminum alloy preferentially.