Mikroyapi - mekanik özellik i̇lişkisi yardımıyla yapısal ve biyomedikal alaşımların çevrimsel ve ağır plastik deformasyonlar altında modellenmesi

The first aim of the work presented herein is to investigate thickness reduction in steel plate heat exchangers utilized in combi boilers. A multi-scale modeling approach was utilized to predict thickness reduction, as well as crack initiation sites due to severe plastic deformation. Specifically, texture and microstructural effects were considered with appropriate coupling of crystal plasticity and finite element analysis. The crystal plasticity simulations were carried out in order to attain appropriate multi-axial hardening rule to define the material strain hardening during forming operation. The multi-axial hardening rules were successfully incorporated into the finite element metal forming simulations. The analysis not only showed the success of multi-scale modeling approach but also demonstrated that failure of plate heat exchangers can be prevented at the design stage by predicting crack initiation sites. Furthermore, crack initiation analysis was extended to rail materials for complicated loading condition and the multi-scale modeling approach was also applied to biomedical and structural materials, in order to further verify both approaches in different materials and under different conditions. Initially, crack initiation in rail materials (bainitic and pearlitic steel) was analyzed under rolling contact loading. In particular, analytical Jiang – Sehitoglu model was performed to predict the ratcheting and multiaxial fatigue damage in these rail steels. It was observed that bainitic rail steel withstood higher loads as compared to its pearlitic counterpart at low shear tractions. As the shear tractions ascended, the performance of rail steels became similar. Moreover, maximum damage followed by rapid crack initiation was visible in both steels when ratcheting and fatigue damage coincide on the surface. The outcome of this analysis evidences a methodology for the realistic prediction of crack initiation under the action of rolling contact loads. However, it is noted that microstructure was not considered in rolling contact loading simulations as it plays a key role on determining mechanical properties. For investigating the microstructure-mechanical property relationship, author developed a model that bridges micro and macro scales. For this purpose, impact response of a biomedical niobium-zirconium alloy was investigated incorporating geometric and microstructural aspects utilizing multi-scale modeling approach. Specifically, the roles of texture and stress-strain distribution regarding geometry under loading were successfully accounted for by implementing a multi-axial hardening rule acquired from crystal plasticity simulations into the finite element analysis. It was observed that more reliable results were obtained with multi-scale modeling approach when compared to those of the classical finite element approach. This analysis constituted a considerable guideline for the design stage of dental and orthopedic implants under impact loading. In order to further investigate the effects of microstructure on the impact response, the impact behavior of high-manganese austenitic steel was analyzed utilizing multi-scale modeling approach incorporating strain rate effects. This specific steel was analyzed due to the fact that it has a very complex microstructure. A similar modeling procedure was applied to predict the impact response of this steel. The findings emphasized the need of multi-scale modeling approach for reliable impact predictions. Overall, the results presented in this thesis shed light on the importance of multi-scale modeling approach considering microstructure for a reliable assessment of failure of plate heat exchangers. The significance of this analysis is that plate heat exchangers can be manufactured with lower costs and improved service performance. Furthermore, current work also demonstrates that implementing the gained knowledge to the biomedical and structural materials are of great importance for design procedure of new applications. With this motivation, long term usability of biomedical and structural materials is ensured. Specifically, this leads to safety of patients by eliminating serious injuries as well as bone fractures during service of biomedical materials. Moreover, the strength of structural materials in ballistic and bearing applications may be significantly enhanced.