Damage Behavior Of Welded Joints With Defects On Fusion Line

Welding is a process that occurs under non-uniform thermal cycling condition,which goes with complicated physical and chemical metallurgy reaction. Therefore, there exist remarkable organization gradient and mechanical property heterogeneity in welded joints area,and the welded zone always becomes the weak site for cracks to initiate. It is doubtless that the research on damage behavior and ductile fracture of welded joints with mechanical property heterogeneity is practically significant for the assessment of the integrity of welded structures.Based on the general theory of continuum damage mechanics, the fully coupled strain-isotropic damage elasto-plastic constitutive equations and a finite element code are used to simulate the elasto-plastic damage behaviors and fracture behaviors. The comparison between the calculated and experimental results indicates that the finite element program not only can simulate the mechanical properties of structures with damage exactly,but also can reflect the effects of the damage on the mechanical behaviors. Two kinds of welded joints damage models,with notch and crack on fusion line,were applied to analyse the influences of the mechanical property heterogeneity and geometric heterogeneity such as intensity matching, base metal rupture strain,base metal strain-hardening exponential and welding line width on the damage distribution and the relationship between damage parameter and rupture paremater. In addition, in the presence of entire damage zone method, the crack extension behaviors of the welded joints were investigated.The numerical calculations of notched bar welded specimens reveal that base metal mechanical property has great effects on the damage distribution of the welded joints. On condition of similar load,the welded joints damage value increases with the decreasing base metal rupture strain and the increasing base metal strain-hardening exponential and intensity matching. For under-matched welded joints,the damage concentration zone of the welded joints near the notch will transfer from the welded metal side to the base metal side as the base metal rupture strain decreases and strain-hardening exponential increases. but for over-matched welded joints, the damage concentration zone will transfer from the base metal side to the welded metal side as the base metal rupture strain increases and strain-hardening exponential decreases. On the other hand,the varied welded line width has little effect on the damage distribution of the welded joints.For the welded joints with crack on fusion line,when loaded and when the crack mouth opening distance reaches the same level,the maximum damage value in front of the crack increases as the base metal strain-hardening exponential and intensity matching increase,but decreases as the base metal rupture strain increases. The establishment of the failure principle on the welded structure should take mechanical property heterogeneity into serious consideration. The numerical calculations of the crack extension behavior which is based on the entire damage zone viewpoint reveal that the crack extension path is parallel to the fusion line on condition of plane stress but on condition of plane strain the angle between the crack extension path and the fusion line is approximate 45°.The numerical calculations and anlysis indicate that micro-damage parameter and macro -rupture parameter both can describe the stress-strain behaviours,and their relationship intensively relies on mechanical property heterogeneity. The investigation on the micro-damage parameter and macro -rupture parameter is undoubtedly supplement to the safety evaluation and life-span prediction of the welded joints,which is of great theory significance.