Fracture Failure Characteristics And Damage Analysis Of DP780 High-strength Steel Weld-bonded Joints

Weld bonding technology is a new type of composite joint technology that combines adhesive and spot welding.The quality and strength of weld bonding joints are concerned,but there are few studies on the fracture failure characteristics and damage analysis of weld-bonded joints.Based on the Box-Behnken Design(BBD)method,the experiment of weld bonding was carried out for 1.2 mm thick DP780 highstrength steel.According to the simulation model and experiment,the fracture failure characteristics of the joint were identified.Based on the cumulative characteristic parameters of the acoustic emission,the damage model of the joint during the tensile process were established.The main research work and conclusions are as follows:The failure load,nugget diameter and energy absorption value of the joints were viewed as the target quantities.The welding process parameters were used as independent variables.The nonlinear multiple regression model of the weld-bonded joints for DP780 high strength steel was established.The experimental verification of the established multiple nonlinear regression model shows that the model has high saliency and high degree of fitting,which can effectively predict the failure load,nugget diameter and energy absorption value of the joints.The optimal process parameters are welding current 8 k A,welding time 150 ms,electrode pressure 0.3 MPa,which are obtained by the regression model,and the actual value of joint's failure load under the shear test is 18163 N.The simulation model of the weld bonding tensile process was established,and the failure characteristics of the joint from the elastic stage to the fracture stage was analyzed.The results show that the welding spot and the adhesive layer bear together before the tensile joint fails.When the load reaches the peak,the adhesive layer and the substrate edge deform significantly,and the welding spot and the adhesive layer have obvious stress concentration.As the displacement continues to increase,the adhesive layer at the edge of the lap joint zone first breaks and fails,and the adhesive layer gradually fails from the edge to the center.After the adhesive layer completely fails,the weld nugget starts to bear separately and cracks begin to initiate around the weld metal edge and the base material edge,and the cracks propagate along the weld nugget edge.Finally,the welding nugget is pulled out from base material,and the images collected by the test are consistent with the simulation results.With the aid of the material testing machine and the acoustic emission acquisition system,the signal was collected during the tensile process of the test piece,and the signal characteristics,frequency spectrum and energy spectrum of the welded joint during the tensile process were analyzed.The results show that the three types of joints have less signal in the elastic stage and the amplitude is low.In the yielding and strengthening phases the signals are frequent and the amplitude is large.The amplitude of the signal in the fracture phase is the largest.The energy spectrum of spot welding is similar to that of weld-bonded joints during the extraction of the weld nugget from the substrate.Furthermore,the energy spectrum can distinguish the fracture characteristics of the adhesive layer and the welding spot: the fracture of the adhesive layer of the joint is mainly concentrated in the low frequency band.The welding spot fracture of the spot welding joint is mainly concentrated at the low frequency.The adhesive layer of weldbonded joint which the welding spot and the adhesive layer are carried together is fractured,the bandwidth concentrated at low frequency is more than that of the bonded joint.By analyzing the regression model of the cumulative characteristic parameters of acoustic emission,the weld-bonded joints quality are shown by the number of the cumulative characteristic parameters of acoustic emission.Based on the cumulative characteristic parameters of acoustic emission,the damage model of the weld-bonded joint were established.Furthermore,the damage model was tested and verified.The results show that the error of the damage factor calculated by the damage model and the result of the test calculation are within 10%.Based on accumulated ring count and accumulated energy,the damage model can well reflect the damage degree of the material.