Crack Propagation Behavior Of Aluminum Alloy Welded Joints For High Speed Trains

A7N01 aluminum alloy has been widely used in parts of high speed trains because of its excellent extrusion and welding properties.Due to the limitation of welding technology,the welding defects and residual stress occurred at the joint of A7N01 aluminum alloy after welding.The fatigue crack of welded joint is most likely to occur because of the cyclic alternating load in the process of train running.In order to improve the anti fatigue strength,prolong the service life and guide the welding process of A7N01 aluminum alloy weldment,it is of great significance to study the fatigue crack propagation characteristics of welded joints.In this paper,the finite element numerical simulation technique is used to study A7N01 aluminum alloy plate and its welded joints.The variation characteristics of crack propagation rate and propagation path of joints under different stress ratios and different loading angles are analyzed emphatically.The main research contents and achievements are as follows:In this paper,a finite wide plate with unilateral penetrating mode I cracks is studied.The method of extrapolation of mode I stress intensity factor is discussed,and the stress intensity factor expressed by node displacement is obtained.Taking A7N01 aluminum alloy plate as the research object,the mode I crack of A7N01aluminum alloy plate was numerically analyzed with the finite element software ABAQUS,and the effect of stress ratio R on the stress intensity factor K_?was discussed.Under the same crack size ratio a/W,the K_?is increased with the increase of R.Under the same stress ratio R,the K_?increases with the increase of a/W.The change of stress intensity factor increment value is as follows:when a/W?0.6,the increase of stress intensity factor is almost the same under different stress ratio,and when a/W>0.6,the increase of stress intensity factor is obviously increased,and the bigger the stress ratio is,the greater the stress intensity factor increment value is.Based on the theory of near threshold region propagation of fatigue crack,ABAQUS is used to simulate the butt welding of A7N01 aluminum alloy plate.The stress field and temperature field perpendicular to the weld are obtained,and the predefined field is set to simulate the crack growth of the joint.Then the crack propagation is simulated and the effect of stress ratio R on the inflection point in the near threshold region of the joint crack is discussed.The amplitude of the stress intensity factor?K_t corresponding to the inflection point decreases with the increase of R,and when R is greater than 0.5,?K_t does not change much,and the expression of?K_t about R is analyzed by fitting.The slope k of the fitted straight line before and after the inflection point changes with R:the change of k ₁ with R is more obvious in L₁ region,and decreases with the increase of R,and the change of k ₂ with R in L₂region is not obvious.Based on the maximum circumferential stress criterion,ABAQUS is applied to simulate the crack propagation paths of A7N01 aluminum alloy welded joints under different loading angles.Then,under the condition of?-?composite loading,when the crack of the joint extends to the interface between the base metal and the weld,discuss the effect of loading angle?on crack initiation angle?.When?=30°,crack initiation angle?is about 0°,crack propagation along interface direction;when?=45°,crack initiation angle?is about 30°and the crack passes through the interface into the base metal.;when?=60°,crack initiation angle?is about 41°and the crack passes through the interface into the base metal.