Study On The Micromechanism Of Ratchetting Of Polycrystalline Metals And Corresponding Constitutive Model

Ratchetting, a cyclic accumulation of inelastic deformation occurred in the engineering materials and structures subjected to asymmetrical stress-controlled cyclic loading, can make the deformation of structure components exceed the designed limitation or shorten the fatigue life of engineering components. It should be considered reasonably in the design and assessment of engineering structures. In the last three decades, the uniaxial/multiaxial ratchetting of polycrystalline metals was extensively observed and modeled by many researchers. However, most of the existing models were phenomenologically constructed from the macroscopic experimental results, and no microscopic physical mechanisms of ratchetting were explicitly considered by them due to the lack of systematic investigation to the micro-mechanism of ratchetting. Thus, many variable and material parameters must be used in the models to obtain a reasonable prediction to the uniaxial/multiaxial ratchetting, which limits the application of such models in the design and assessment of engineering structures. Recently, based on the multi-mechanism approach or crystal plasticity, some attempts were conducted to include the micro-mechanism information of ratchetting deformation into the proposed models. Although the established multi-mechanism and cyclic crystal plasticity models can provide a reasonable simulation to the ratchetting of specific materials with fewer variables and parameters, the micro-mechanism of ratchetting associated with the dislocation slip has not been involved yet. A comprehensive understanding to the micro-mechanism of ratchetting deformation is extremely necessary to improve the capability and applicability of constitutive model.Therefore, to reveal the micro-mechanism of ratchetting for the cubic polycrystalline metals and consider it in the constitutive model more comprehensively, this thesis has carried out the following studies:1. The strain cyclic characteristics and ratchetting of two polycrystalline metals with different crystal structures (i.e., the face-centered cubic 316L stainless steel with low stacking fault energy and body-centered cubic 20 carbon steel with high stacking fault energy) were experimentally studied under the uniaxial and non-proportionally multiaxial cyclic loading conditions and at room temperature. The effects of cyclic softening/hardening feature, applied mean stress, stress amplitude, stress ratio and multiaxial loading paths on the ratchetting of the two metals were discussed.2. The dislocation patterns and their evolutions during the uniaxial/multiaxial ratchetting deformation were observed by transmission electron microscopy (TEM) for 316L stainless steel and 20 carbon steel. The evolution features of dislocation were discussed by comparing the dislocation patterns observed at different stages of ratchetting deformation with those obtained during the monotonic tension and symmetrical strain-controlled cyclic loading. The micro-mechanisms of uniaxial/multiaxial ratchetting for 316L stainless steel and 20 carbon steel are qualitatively explained in terms of observed dislocation patterns and their evolution during the ratchetting deformation. Some significant conclusions useful to understand the micro-mechanism of polycrystalline metals with cubic crystal structure and construct the dislocation-based constitutive model are obtained.3. Based on the obtained micro-mechanism of ratchetting, in the framework of crystal plasticity, a dislocation-based cyclic polycrystalline visco-plastic constitutive model is constructed to describe the ratchetting of the metals with cubic crystal structure. In the proposed model, a new rate-dependent flow rule considering the thermal activation energy of dislocation slipping is developed, and a dislocation-based Armstrong-Frederick non-linear kinematic hardening rule is introduced to provide a better prediction to the ratchetting. The isotropic hardening associated with the short-ranged interactions of dislocations is described by the evolution of critical shear stress in each slip system. Comparing the predicted results with corresponding experimental ones shows that the uniaxial and multiaxial ratchetting of polycrystalline 316L stainless steel and 20 carbon steel are reasonably described by the proposed model, and the dependence of the intra-granular ratchetting on the crystallographic orientation of grains and stress level can be also reasonably described by the model.