Fatigue Damage Quantification and Life Estimation with Provision for Microstructure

This dissertation provides methodologies for the quantification of different forms of fatigue damage by considering the role of microstructure. The laws of thermodynamics in conjunction with Finite Element Method (FEM) are utilized to devise thermodynamically-base frameworks for the estimation of cyclic damage in polycrystalline metals. Accordingly, a microstructure-sensitive platform for fatigue life estimation is introduced. The proposed framework is assessed for 3D printed metallic alloys at room and elevated temperatures. Further, it is used to analyze the fatigue performance difference between additively manufactured Inconel 718 in three print orientations. This research is of fundamental vital interest because it ensures that manufacturers are able to provide the best possible products, structures/parts/products are not prone to failure, and society has access to safe and reliable technology. A plasticity model for quantifying the cyclic damage accumulation in metals, known as the Statistical Estimation of Plastic Strain Energy (SEPSE) is introduced. The model is based on determining the cyclic slip irreversibility of individual microstructural units (one or several polycrystalline grains) and subsequently calculates the total permanent deformations under cyclic loading by summing them up. Considering the true crystalline nature of metal grains and their orientation in modeling the cyclic damage makes this approach potentially applicable to a wide spectrum of metals. By incorporating the SEPSE in finite element simulations, a computer code is developed to estimate the temperature evolution of cyclically-loaded components. Then, using laws of thermodynamics (in particular the concept of entropy), a novel method is devised for estimating the failure or service life of materials subject to fatigue. Such a practical method, which is cognizant of microstructure type, offers a novel platform to the study and safe implementation of advanced materials used in additive manufacturing of 3D printed metals. The developed microstructure-sensitive algorithm is used to analyze the fatigue damage and life of additively manufactured C-18150 copper alloy at room and elevated temperatures, where the results are verified with the hysteresis stress-strain loop measurements. In addition, the fatigue performance of additively-manufactured Inconel 718 specimens with three different build orientations is assessed. The Fatigue Fracture Entropy (FFE) of each printing orientation is estimated via the proposed methodology and used to estimate their fatigue lives. Moreover, a new phenomenon called temperature-induced buckling (TIB) is introduced, which causes the premature failure of mechanical components subjected to cyclic loading. For this previously unknown phenomenon, a state-of-the-art infra-red (IR) camera is used to accurately measure the limiting failure temperature. Accordingly, using the thermodynamically-base model of Fracture Fatigue Entropy (FFE), a method is presented for determining the critical stress level below which such a failure can be prevented. This approach contributes significantly to the science of structural failure and enables machine designers and users to create and safely operate mechanical components