Research On The Applicability Of Ductile Fracture Criterion Under Complex Loading Conditions In Forming Process

With the maturation of plastic flow theory and plastic forming law,the metal plastic processing technology has occupied an important role in the domain of manufacturing industry at home and abroad.As the most common failure mode in metal forming,cracking has attracted much attention.Fracture prediction of ductile metal during forming process has also become a hot topic.Dozens of fracture prediction models have been put forward in succession,among which,the uncoupling ductile fracture criterion have been widely employed due to its simple functional form,easy and feasible parameter calibration process,as well as the accuracy of which could meet the requirements of industrial production.On one hand,depending on numerical simulation technology,simulation has become an effective tool for failure analysis in material processing.In terms of appropriate fracture criterion and hardening model,high prediction accuracy of fracture initiation could be realized.On another hand,a breakthrough with respect to the fracture prediction under monotonic loading conditions has been achieved in recent years.Abundance of fracture models suitable for monotonic loadings,nevertheless,remains challenges in fracture prediction in cases of complex loading conditions.The performance of fracture model under actual variablepath loading conditions still needs further improvement.According to the above analysis,variable-path loading tests were designed and the hardening model of experiment material was built.Based on the parallel simulations,the evaluation of the existing uncoupled fracture prediction model(DF2012)was executed.Further,the fracture model was modified,extended and validated.The main contents are as follows:(1)The hardening model of AA2024 were established.According to the stress-strain hysteretic curve of uniaxial tension-compression cyclic test with equal strain control,the kinematic law of Chaboche combined hardening model was obtained.Then based on the flow curve of smooth round bar tension test,the combined hardening model suitable for large strain-variable path loading conditions was calibrated.Besides,the corresponding flow equation of AA2024 was constructed for comparing.Subsequently,in terms of a series of variable-path loadings including torsion-tension and compression-torsion tests,the hardening behavior of materials with different hardening assumptions under complex loading conditions was discussed.(2)The applicability of uncoupled fracture criterion(DF2012)was evaluated,and the prediction performance of integral-based damage accumulation model as well as the fatigue-based damage accumulation law were explored.By analyzing the influence of preloading on fracture locus,the pre-strain was introduced to adapt the fracture model,which improved the prediction accuracy of fracture initiation in cases of torsion-tension tests.(3)A combined stress-strain fracture prediction model was put forward,by integrating stress-based fracture limit and fatigue-based nonlinear damage accumulation law.Specifically,the theoretical strategy and parameter study were discussed intensively.In order to verify the effectiveness of this model,a range of variable path loading tests were performed numerically and theoretically.It was concluded that this combined model proposed could eliminate the influence of strain-path sensitivity and improve the prediction accuracy under complex variable-path loading conditions.