Study On Mixed-mode Fatigue Property Of AA7075-T6

With the development trend of lightweight,energy-saving and environmental protection in aerospace,high-speed transportation and automobile fields.AA7075-T6(American 7075 aluminium alloy under T6 treatment)is used as ultra-high strength aluminium alloy.It has become the preferred material for key structures in aerospace and high-speed transportation for its high specific strength,stiffness and corrosion resistance.These key structures are often under alternating loads and fatigue fracture is the main failure mode of brittle fracture.So,the study of fatigue characteristics is very important.At present,the research on fatigue properties of AA7075-T6 is mainly focused on single mode loading mode at home and abroad,and research on the fatigue propagation of mode I crack,its advantage is that there are ready-made national standards and mature analytical formulas in the process of fatigue crack propagation,so it is easy to analyze and obtain the propagation law of fatigue crack.In practical engineering,the structure is often subjected to mixed loading.At present,there are few studies on the fatigue characteristics of mixed cracks.The main difficulty lies in the difficulty of testing,and there is no analytical solution of K factor for the propagation of mixed cracks which needs to be obtained by numerical calculation.So study of the characteristics of mixed cracks has more engineering value.In this paper,?-? mixed mode high cycle fatigue tests of finite width AA7075-T6 butterfly-shaped plate with unilateral crack were carried out by using improved Arcan clamp.Combining with the finite element simulation,explore the fatigue crack growth law,propagation mechanism and influencing factors of material mixed ?-? mode,in order to better tap its potential and develop development prospects.The research contents in this paper have important engineering application value.(1)Through unidirectional tensile test,to obtain the basic mechanical properties of AA7075-T6,verify the reliability of materials,provide an effective basis for selecting stress levels for fatigue testing.(2)?-? mixed fatigue crack test of AA7075-T6 at loading angles ?=0°,15°,30°,45°,60°,75°.Seven-Point Incremental Polynomial Method is used to process the original data of the experiment.Obtain and analyze Aa-N?da/dN-N?da/dN-?a curve and its changing law under various load conditions.(3)Exploring the ?-? mixed mode fatigue crack propagation path of AA7075-T6.Comparative analysis for cracking angles obtained by theoretical prediction,experimental measurement and simulation calculation,It is found that the cracking angles obtained by the three methods are basically the same in size and trend.However,the cracking angles measured and simulated are slightly smaller than those predicted by maximum circumferential stress,five possible reasons for this result are summarized.(4)Based on the pure mode I crack,the possibility model is simulated by different finite element methods.Through the error analysis of the calculated value and the analytical solution,to obtain a more accurate and reliable simulation scheme for realizing ?-? mixed mode fatigue crack.(5)By using Abaqus software,simulate the fatigue cracks of ?-? mode with six cracking angles,and study on the variation of mode ?,mode ?,equivalent stress intensity factor and composite ratio of stress intensity factor of different ?-? mixed mode fatigue cracks in the process of crack propagation.(6)The mechanism of fatigue crack growth in ?-? mixed mode is studied by the influence of mode ? factor on crack growth and the law of stress intensity factor,stress and displacement field at crack tip with fatigue crack growth.The comprehensive analysis shows that mode ? factor plays a key role in changing the fatigue crack growth path.The direction of fatigue crack growth is determined by both mode ? and mode ? factors.Equivalent stress intensity factor ?K*at the crack tip of?-? mixed fatigue crack determines the crack initiation and crack growth rate.(7)Based on curves da/dN-?a and ?K*-?a,obtain the fatigue crack da/dN-?K*and lg(da/dN)-lg(?K-)curves of AA7075-T6 ?-? composite mode with different cracking angles,and obtain the fatigue crack growth law of material ?-?composite mode.Obtain the Paris formula of the linear region of stable fatigue crack growth by least square fitting.Obtain the macro fracture characteristics of AA7075-T6 via analyzing the tensile fracture and mixed ?-? fatigue fracture of AA7075-T6.