Atomistic and Microstructural Computational Fatigue Design and Integrated Creep-Fatigue Theory for High-Temperature Alloys

在汽车、飞机、压缩机、泵、涡轮机等经受循环载荷的部件中，疲劳至少占所有服务故障的90%。传统上，工程师和研究人员必须通过测试来确定材料的疲劳性能，这导致了高昂的时间和金钱成本，从而延长了机械/结构系统的材料和组件设计的开发周期。最近的研究表明，材料的低周疲劳（LCF）寿命可以用材料的物理特性来表示，包括汉堡矢量、剪切模量和表面能。高周疲劳本质上是一个微结构LCF裂纹形核加裂纹扩展的过程。在这些认识的启发下，本研究提出了一种计算疲劳设计方法，旨在寻找具有优异抗疲劳性能的新型高温合金。结合第一性原理密度泛函理论（DFT）建立了基于微观结构的数值模型，并将其用于预测从LCF到HCF的宽载荷谱下多晶金属的裂纹成核寿命。此外，利用位错在晶粒内部和沿晶界滑动和爬升的变形机制，提出了高温合金蠕变-疲劳综合理论框架。基于第一个基石建立的理论模型得出了包含所有可能变形机制的本构规律，基于第二个基石建立的理论模型描述了导致断裂的损伤积累过程，从而定义了疲劳、蠕变和热机械疲劳等一般载荷条件下的寿命。利用这一理论，新材料的开发只需要有限的实验室测试，这样材料在复杂载荷条件下的行为就可以与基本的变形机制联系起来。一旦建立，所提出的理论和模型将加快材料的开发和应用，以满足21世纪和未来快速增长的技术需求和严格的环境要求。它们在现有或新设计的机械系统的预后和健康管理中也有很大的意义，具有准确的寿命预测，为生命周期管理提供了巨大的节省。除了对研究领域的好处外，拟议的研究还计划在认真考虑公平，多样性和包容性（EDI）的情况下培训高素质人才（HQP）。利用这一研究成果来设计新材料和维护机械系统以满足燃气轮机行业不断增长的要求，节省时间和成本，必将有利于加拿大的经济和社会。

Fatigue accounts for at least 90 percent of all service failures of components subjected to cyclic loading, as experienced by automobiles, aircraft, compressors, pumps, turbines, etc. Traditionally, engineers and researchers have to perform testing to determine a material's fatigue property, which results in high costs in time and money, thus prolonging the development cycle in material and component design for mechanical/structural systems. Recent research has reported that the low cycle fatigue (LCF) life of a material can be formulated in terms of material's physical properties including Burgers vector, shear modulus and surface energy. High cycle fatigue (HCF) is, in essence, a process of microstructural LCF crack nucleation plus crack propagation. Enlightened by these understandings, a computational fatigue design approach is proposed in the present research, aiming to search for new high-temperature alloys with superior fatigue resistance. A microstructure-based numerical model combined with first-principles density functional theory (DFT) will be developed and then used to predict the crack nucleation life of polycrystalline metals for a wide spectrum of loading from LCF to HCF. Furthermore, utilizing the deformation mechanisms involving glide and climb of dislocations within grain interior and along grain boundaries, a holistic theoretical framework - the integrated creep-fatigue theory (ICFT) for high-temperature alloys will also be proposed in the present research. The theoretical model established based on the first cornerstone leads to constitutive laws encompassing all possible deformation mechanisms and that based on the second cornerstone describes the damage accumulation process leading to fracture, and thus defines the life under general loading conditions that involve a combination of fatigue, creep and thermomechanical fatigue. Using this theory, only limited laboratory-testing will be required for a new material development such that material behavior under complicated loading conditions can be linked to the fundamental deformation mechanisms. Once established, the proposed theory and model will speed up material development and application to meet the fast-growing technological demands and stringent environmental requirements in the 21st century and future. They also have a great implication in prognosis and health management of existing or newly designed mechanical systems, with accurate life prediction, offering tremendous savings in life cycle management. In addition to the benefits for the research field, the proposed research has also planned the training of Highly Qualified Personnel (HQP) with serious consideration of Equity, Diversity and Inclusion (EDI). The time and cost saving using this research outcomes to design new materials and maintain mechanical systems to meet continuously increasing requirements for the gas turbine industry will certainly benefit and contribute to the economy and society of Canada.