CRACK INITIATION AND PROPAGATION USING DAMAGE MECHANICS AND XFEM UNDER CREEP FATIGUE ENVIRONMENT

Abstract

Failure of any engineering component may become the reason for human and economic loss. Therefore, it should be investigated carefully. The components may fail either due to service conditions or the growth of inherent flaws such as voids, cracks, etc. The flaws in the engineering components may exist at the micro-level or macro-level due to material imperfection and/ or manufacturing processes. The service conditions increase the dimensions and number of flaws and thus reduces the life of the component. Therefore, several attempts have been made to predict the life of the component before failure so that the unprecedented occasions can be avoided. Continuum damage mechanics (CDM) is one such approach which can be used to predict the life of a component. CDM can estimate the life during crack initiation and propagation. Thus, it can be used to determine the complete life of a component. CDM uses a damage evolution equation to represent the loss of load-bearing capacity of the component, hence, it predicts life. In this thesis, CDM has been applied to determine the complete life under high cycle fatigue, low cycle fatigue, creep, and creep-fatigue conditions. To perform the simulations, CDM has been clubbed with numerical techniques. In the literature, CDM and finite element method (FEM) are combined to determine the life of the component. However, the combination of CDM and FEM is incapable of modeling the surface separation with crack propagation. Moreover, it may provide the mesh dependent results with crack growth. Although several approaches have been developed to reduce the mesh sensitivity. Yet, the CDM-FEM framework is mainly limited to model the crack initiation. In literature, numerous advanced methods such as extended finite element method (XFEM), element-free Galerkin method, extended isogeometric analysis and many more have been developed to model the crack separation along with crack propagation. XFEM is an extension of FEM to model the crack and its propagation. XFEM uses enrichment functions in the framework of FEM to imitate the crack characteristics. In literature, CDM and XFEM have been combined to model the elastic and plastic crack growth problems. This thesis is focused on the development and implementation of the CDM-XFEM framework to determine the life of components under high cycle fatigue, low cycle fatigue, creep, and creep-fatigue conditions. Furthermore, the mathematical methodology should be developed such that the results of laboratory specimens can be applied to model the behavior of actual components. It has been observed that the existing CDM-FEM framework does not consider constraint effects while predicting life. Hence, the application of such a framework to model the real-world component becomes questionable. Therefore, in this thesis, the constraint effects are incorporated while developing the methodology based on CDM and XFEM. To consider the constraint effects, a new definition of stress triaxiality is proposed, which considers the constraint effect through a triaxiality parameter. Furthermore, it has been observed that the existing damage models do not replicate the experimental results properly. Hence, the continuum damage models have been developed and modified so that the experimental results can be explained. Firstly, the CDM-XFEM framework is developed to determine the complete life under high cycle fatigue conditions. A stress-based fatigue damage model is introduced for this purpose. The proposed damage model is applied in the CDM-XFEM framework to predict the complete life of the components. Further, the developed new stress triaxiality definition is incorporated to consider the constraint effects. Regularization schemes are also implemented in the CDM-XFEM framework to suppress mesh sensitivity. The results obtained by developed methodology are compared with the experimental data available in the literature and found in good agreement. Next, the CDM-XFEM framework is developed to determine life under low cycle fatigue conditions. A strain-based fatigue damage model is introduced which considers the effect of strain ratios. Further, the proposed damage model is applied in the CDM-XFEM framework to predict the complete life. The simulated results of the proposed methodology are compared with the experimental data available in the literature and found in excellent agreement. Further, the CDM-XFEM framework is developed to model the crack initiation and propagation under the creep environment. It has been observed that the existing Liu-Murakami creep damage model does not consider the variation in constraint effects for crack propagation. Hence, the Liu-Murakami damage model is modified. The constraint effects are incorporated through developed new stress triaxiality definition. The simulated results are compared with the experimental results, which shows that the modified Liu-Murakami damage model captures the constraint effects effectively. Finally, the CDM-XFEM framework is developed to evaluate the complete life under a creep-fatigue environment. To model the creep-fatigue interaction, a simple creep-fatigue interaction damage model is proposed. The proposed damage model is applied in the CDMXFEM framework to perform the simulations. The simulated results are compared with the available experimental results and are found in good agreement. From the studies performed in this work, it can be concluded that the proposed CDM-XFEM framework is a straightforward and reliable approach to determine the complete life of components under high cycle fatigue, low cycle fatigue, creep, and creep-fatigue conditions.