Study On Thermo-mechanically Coupled Cyclic Constitutive Model In Finite Deformation

Metallic structural components used in the aerospace, pressure vessel, nuclear reactor and high speed railway areas are often subjected to a complicated cyclic loading. Fatigue failure caused by cyclically accumulated deformation is the most common failure mode of this kind of structures. Meanwhile, during the cyclic plastic deformation of metals, especially for that with relatively large plastic strain amplitude, the heat accumulation generated from the plastic work will result in an increase of material temperature. Such a temperature rise should be realized and strictly controlled since some thermo-sensitive elements are often connected to the loaded metallic structural components. Moreover, when the temperature rise is high enough, a thermal softening occurs in the metals, which will reduce the strength of metals greatly and make the metals produce larger plastic deformation during the subsequent cyclic loading. The coupling effect of thermal softening (caused by the temperature rise) and cyclic plastic deformation (producing more plastic work and causing further temperature rise) can significantly degrade the fatigue life of such metallic components. Therefore, the themo-mechanically coupled effect during cyclic loading of metals should be considered reasonably in the design and assessment of engineering structures. In the last few decades, the cyclic deformation of metals was extensively observed and modeled by many researchers. However, most of the existing models were limited to the framework of isothermal and small deformation, and few of them concerned the large deformation and thermo-mechanically coupled effect. In order to improve the design of metallic components and assess their reliability, it is necessary to carry out the experimental observations about the thermo-mechanical deformation of metals under uniaxial and multiaxial cyclic loading, and then develop a thermo-mechanically coupled cyclic constitutive model in the framework of finite deformation and implement it into into a finite element code (e.g., ABAQUS). Moreover, this work also enriches the theoretical results in solid mechanics.Therefore, to reveal the cyclic deformation and heat generation of metal materials, and consider it in the construction of constitutive model more comprehensively, this thesis has carried out the following studies:1. The thermo-mechanical deformation of 316L stainless steel is experimentally studied under the monotonic tension, and uniaxial and non-proportionally multiaxial cyclic loading conditions at room temperature. The effects of rate-dependent feature, stain amplitude-dependent cyclic hardening-softening-hardening, applied mean stress, stress amplitude, stress rate and multiaxial loading paths on the ratchetting of the material are discussed. Some significant conclusions useful to construct the thermo-mechanically coupled cyclic constitutive model in finite deformation are obtained.2. Based on the logarithmic stress rate, a constitutive model is developed to describe the material deformation under cyclic loading histories in the framework of finite plasticity by using combined nonlinear isotropic and kinematic hardening rules. The nonlinear kinematic hardening rule is extended from that proposed by Abdel-Karim and Ohno [1] for infinitesimal plasticity. The cyclic hardening/softening feature of the material is reflected by using a nonlinear isotropic hardening rule. Then, the proposed model is implemented into a finite element code (e.g., ABAQUS). Finally, some numerical examples are carried out to verify the prediction capability of the proposed model by comparing the predictions with the corresponding experiment results in referable literature. It is shown that the axial stress effect in simple shear. Swift effect in cyclic finite torsion, uniaxial and biaxial ratchetting are described well by the proposed model. Such a work provides a base for construting the thermo-mechanically coupled cyclic constitutive model in finite deformation.3. A framework of thermo-mechanically coupled elasto-plasticity (rate-independent or rate-dependent one) is first presented based on the thermodynamic laws and logarithmic stress rate. Then, a specific thermo-mechanically coupled elasto-viscoplastic constitutive model is constructed from the framework in order to describe the thermo-mechanically cyclic deformation of 316L stainless steel by using combined nonlinear isotropic and kinematic hardening rules and considering the thermal effect. The nonlinear kinematic hardening rule is extended from that proposed previously for finite plasticity; and a cyclic hardening-softening-hardening feature observed in the cyclic tests of 316L stainless steel is reflected by using a nonlinear isotropic hardening rule consisting of several components. The strain amplitude dependence of cyclic hardening is considered by introducing a memory surface of plastic strain proposed by Chaboche et al.[2], and the additional hardening effect caused by the nonproportionally multiaxial cyclic loading path is involved by using the nonproportionality parameter defined by Tanaka. Furthermore, the proposed constitutive model is implemented into a finite element code (e.g., ABAQUS) by combining the user subroutines UMAT and UMATHT. Finally, the proposed model is verified by comparing the predictions with the experimental results of 316L stainless steel. It is shown that the rate-dependence, stain amplitude-dependent cyclic hardening-softening-hardening and unixial and multiaixal ratchetting, and related thermal responses are well described by the proposed model.