| Super-elastic NiTi alloy exhibits superior super-elasticity and shape memory effect due to thermoelastic martensitic solid-solid phase transformation.In the process of crack propagation,the stress field at the crack tip is different from that of conventional metal materials due to the stress-induced martensitic transformation and constraint of martensitic plastic yield on the phase transformation.The simulation methods based on the traditional linear elastic fracture mechanics have certain limitations.Therefore,it is necessary to evaluate the applicability of existing crack propagation simulation methods to the crack propagation simulation of super-elastic NiTi alloy,and then develop the constitutive model considering transformation-plasticity to conduct analysis of the stress field at the crack tip of super-elastic NiTi alloy in-depth,and discuss the influence of material parameters of super-elastic NiTi alloy on the fracture parameter J-integral.Finally,it is necessary to develop a finite element plug-in based on ABAQUS finite element analysis software for the crack propagation of super-elastic NiTi alloy to quickly evaluate the fracture problems of super-elastic NiTi alloy devices,which it is very importance to reliability evaluation of the alloy.The main research work of this paper is as follows:(1)The static crack propagation and fatigue crack propagation experiments of super-elastic NiTi alloy are carried out.The characteristics of strain field and temperature field at crack tip under different loading rates are analyzed,and the stress intensity factor K_I and J-integral of super-elastic NiTi alloy under different loading rates are calculated,the influences of loading rates on fracture parameters are discussed.Moreover,based on the Paris formula,the relevant parameters of fatigue crack growth rate and stress intensity factor amplitude are determined.(2)Based on the linear elastic fracture mechanics theory,the finite element model of compact tensile specimen is established.The static crack propagations are simulated by the VCCT method and XFEM method,respectively.The advantages and disadvantages of two simulation methods for crack propagation are analyzed and the changes of J-integral with loads are studied,the fatigue crack propagation simulation is simulated by the XFEM method.(3)Finite element analysis of compact tensile specimens is carried out by using the constitutive model considering transformation-plastic its of super-elastic NiTi alloy.The changes of stress,strain field and martensite volume fraction at crack tip are analyzed,and the influence of super-elastic NiTi alloy material parameters on J-integral at crack tip is revealed.(4)Based on the birth-death element method and PYTHON scripting language,a finite element plug-in for crack propagation of super-elastic NiTi alloy based on ABAQUS platform is developed to simulate the I and I-II mixed static crack propagation of super-elastic NiTi alloy. |
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