Fatigue of Structural and Additively Manufactured Steels: Damage Assessment and Modelling Techniques

Abstract

The topics of damage assessment by in situ measurements before a macroscopic crack has been initiated, nonlinear damage modelling and additive manufacturing are herein discussed and researched from the perspective of the maintenance of ageing structures and mechanical equipment subjected to fatigue loading. In relation to the aforementioned topics, the thesis can be categorized into three parts, all of which are related to the maintenance of ageing structures and mechanical equipment. These are: 1) Fatigue damage monitoring prior to macroscopic crack initiation, 2) Damage modelling based on S-N curves, and 3) Mechanical properties of additively manufactured steel. 1. Fatigue damage monitoring prior to macroscopic crack initiation A conceptual framework is developed to select a method to assess small fatigue crack propagation and fatigue damage accumulation. The framework is focused on the underlying fatigue mechanism which is correlated with the measurable change for each of the identified parameters throughout the fatigue life, and how the parameter will change. The framework is based on the literature and is useful for both research and development communities, for those who are involved in structural integrity assessment. An experimental study is also performed regarding the feasibility of adopting macroscopic hardness indentations and their correlation with fatigue damage accumulation. Both Brinell and Vickers were adopted, with Brinell being found to exhibit a more continuous change, in addition to being statistically significant. 2. Damage modelling based on S-N curves A new damage function is proposed herein, based on the damage theory known as the theory of the S-N fatigue damage envelope. The work can be considered a framework to develop S-N curve-specific damage functions, as the underlying theory can be applied to any S-N curve for materials or structural details. Furthermore, the proposed damage function is compared with other recently proposed functions, including Miner’s rule, using experimental data. The proposed function generally exhibits better prediction than Miner’s rule and some of the other models. A generalized expression/function for fatigue damage was also proposed by investigating the functional forms commonly adopted in the literature. The relation of the parameters or variables of the functional form can be represented in three dimensions, resulting in it being able to be evaluated in three dimensions with experimental data, where it will result in a surface. Herein, it is also shown that two special cases of the proposed function exist, where the parameters are reduced to a single ratio, which is subsequently evaluated based on experimental data. 3. Mechanical properties of additively manufactured steel An experimental study is performed regarding the mechanical properties of additively manufactured steel printed by the technology of Bound Metal Deposition. Specimens were printed in various directions, which were subsequently tension tested to develop stress-strain curves. It was found that the printing technology would result in the final product exhibiting anisotropic behaviour correlated to a mesh of crack-like defects. The crack like defects also result in the conclusion that the printed specimens are expected to exhibit very poor fatigue capacity in comparison to their traditionally manufactured counterparts and also to exhibit anisotropic behaviour in relation to fatigue. Keywords: Fatigue damage assessment, hardness-based fatigue damage, nonlinear damage modelling, additively manufactured steel