Multi-scale Numerical Simulation Of Intergranular And Transgranular Fracture Behavior Of NiTi Alloy Based On Cohesion Model

Nickel-titanium(NiTi)alloys have shape memory effect and super-elasticity,and exhibit excellent thermodynamic properties,such as high damping capacity,good corrosion resistance,and wear resistance.These properties make it widely used in aerospace,medical,electronic machinery,and some other fields.In addition,NiTi alloys are commonly used in engineering fields,such as bridge and construction.Owing to complexities of loading condition in work environments,materials usually are required to exhibit excellent mechanical properties,such as high strength and high resistance to deformation.Therefore,to ensure the safety and reliability of NiTi alloys during the process of application,more detailed work on mechanical properties of NiTi alloys are required,especially for the micro mechanism behind fracture phenomenon.As yet,an investigation on NiTi alloy property characterization and prediction has become a hot but difficult research point.Although classical experiments could investigate material fracture behavior by fracture morphology and fracture toughness,it is short of understanding the stress state of crack tip.Also,the influences of microstructures,such as microcracks,micro-pores,and crystal structure,on the crack propagation need to be further discussed.In recent years,with an improvement of computer technology,numerical simulation methods are helpful to understand material fracture performance.It is expected to propose a model that connects micro-mechanism with macro-performance and predict material properties,which is surely a novelty to present work.Herein,this work is to present a multi-scale simulation method combined with experimental technologies for investigating the crack propagation of NiTi alloys and predicting fracture properties.With the help of molecular dynamics,the effects of different initial defects on intergranular and transgranular fracture of NiTi alloys were studied.In this paper,four types of models with different initial defects were made.These defects were blunt crack,blunt crack and void,sharp crack,sharp crack and void.By computing the traction force-separation(T-S)curve at the crack tip,it was shown that the model with blunt crack had the maximum traction peak,demonstrating that it was most difficult to crack propagation,and thus the ability of cracking resistance was intensified,as compared with other models.For sharp-crack model,there were over stress concentration at crack tip,which promotes further cracking.The introduction of voids was useful for crack propagation,because the traction peak values were less.Interestingly,such peak values in transgranular mode were larger than those of intergranular one,indicating that under the same loading conditions,the intergranular fracture was more easily occur in NiTi alloys.Many studies have shown that the information of material microstructures is helpful to explain micro mechanism of macroscopic fracture properties.In this paper,EBSD experiments were used to observe micro-structures,grain morphology and grain distribution,and to obtain the statistics of grain size of NiTi alloy.The results showed that there were texture phenomena in grains and wide-angle grain boundaries.In addition,grain sizes exhibited a statistical character.Based upon such information,the Voronoi diagrams with statistical character were generated to describe polycrystalline structures that obey grain size distribution in an actual state,and then they were inserted finite element simulations to accurately reproduce the fracture behavior.After obtaining the details of microstructures,intergranular and transgranular fracture were simulated by finite element method to analyze the influences of microdefects on fracture toughness and cracking growth.After calculating critical stress intensity factor(K_c)and damage dissipation energy,it was found that a higher K_c and lower damage dissipation energy lead to higher fracture toughness,indicative as better anti-cracking performance.The results also showed that the model with blunt crack was with higher K_c and lower damage dissipation energy,which meant that its fracture toughness was stronger.During the cracking process,the crack always expanded along the direction with the maximum normal stress.Transgranular fracture was more difficult to occur in NiTi alloys due to a higher K_c.In addition,there was a longer propagation path with a more rugged manner.It was noteworthy that under the framework of the stress-displacement at the cracking interface,the cohesive zone model was employed to obtain the T-S curve obtained by MD calculations.Accordingly,the cohesive element parameters were achieved to insert finite element method.Thus,numerical simulations on polycrystalline NiTi alloy for intergranular fracture and transgranular fracture were conducted by a computational method across micro-and macro-scale,by combining finite element method and molecular dynamics.The analysis of the influence of grain size and other factors on fracture toughness was investigated.A comparison between predicted value and experimental data on K_c highlighted the efficiency of the proposed method for predicting fracture toughness.Furthermore,the influence of grain size on the mechanical properties was analyzed.Based on the statistical information obtained from experiments,twenty groups of models with different grain sizes were built to simulate intergranular fracture and transgranular fracture of polycrystalline NiTi alloys.Besides,the effect of micro-defect on fracture was studied.The results showed that the model with smaller grain size had a higher K_c and lower damage dissipation energy,indicative of superior fracture toughness.Thus,it is more difficult for transgranular fracture to occur compared with intergranular fracture.The fracture toughness of model with blunt crack was higher than that with sharp crack,suggesting that the latter is beneficial to crack propagation.In this paper,through a hybrid method by the combination of experiments,-molecular dynamics,and finite element method,the effects of microstructures,such as grain size and microdefects,on the fracture toughness of materials are investigated,and this leads to a deep understanding of the fracture behavior of NiTi alloys.It is believed to promote further development and application of such materials.