Research On Fatigue Life Of Welded Joints With Defective Aluminum Alloy Under Welding Residual Stress

In the service process of high-speed train with 7N01 aluminum alloy as the main body,welding defects such as unmelted,pores and cracks are easy to form fatigue crack sources,which greatly reduces the time required for fatigue crack initiation.Therefore,the theory of fracture mechanics for cracked bodies can be used to evaluate the fatigue crack growth life of the welded structure.Based on fracture mechanics,combined with the theory of welding structure and material mechanics,this paper systematically studied the fatigue propagation behavior of aluminum alloy with defects under welding residual stress.Since the mechanical properties of the material are indispensable parameters in the fatigue life assessment model,this paper tested the mechanical properties of 7N01 aluminum alloy and obtains the yield strength,tensile strength,Young's modulus and fracture toughness of the typical area of the welded joint(weld seam,heat affected zone,base metal).The accurate characterization of the welding residual stress distribution is a prerequisite for quantifying the influence of welding residual stress on fatigue propagation behavior.The welding residual stress distribution of the middle tensile(MT)specimen was tested by laser speckle method,and the distribution function was given by data processing.The finite element simulation method was used to simulate the welding process of MT specimens and the distribution of post-weld residual stress field,which verifies the accuracy of laser speckle measurement results,was obtained.In addition,the MT specimen was processed by an overload tensile test to obtain specimen of different residual stress levels.The fatigue crack growth rate test was carried out.By controlling the test variables,the effects of stress ratio and residual stress on the growth rate were investigated.The results showed that the higher the stress ratio is,the faster the crack growth rate is in the stress ratio range from 0 to 0.5;the stress ratio continues to increase,the crack growth rate slows down,but it is still much larger than the data of low stress ratio;the higher the welding residual stress is,the faster the crack growth rate is.At the same time,the static stress gauge was used to measure the redistribution of residual stress during crack propagation,and the residual stress distribution function after redistribution was obtained.Furthermore,the finite element simulation of the crack propagation process was carried out,and the residual stress field in the crack tip region under different half crack lengths was obtained,and the results of the redistribution test were verified.In order to accurately characterize the crack propagation driving force,the stress intensity factors caused by the working load and the welding residual stress are calculated by the analytical method and the weight function method.Then,based on the Kujawski formula,the relevant parameters in the model are corrected and solved to obtain the formula of the subject.The results show that the welding residual stress mainly affects the fatigue crack growth rate by changing the crack propagation parameters.The fatigue crack growth rate of 7N01 aluminum alloy welded structure was predicted by the formula proposed in this paper,and the fitting curve close to the test results was obtained.Finally,the evaluation process of the fatigue life of the defective component under the welding residual stress field is established.