Ratcheting and fatigue response of materials undergoing asymmetric multiaxial and multistep loading spectra in the absence and presence of notches

在北美，在役工程部件的灾难性故障每年造成的损失超过1500亿美元。汽车、航空航天、发电厂和压力容器等各个行业的零件设计都严格依赖于材料的完整性、复杂的几何形状和负载条件。由于孔、缺口等应力源的存在是不可避免的，因此必须通过选择影响参数来适应机械零件的可靠设计；材料性能，循环硬化/软化，应力水平和应力提升器形状/尺寸，以减少棘轮变形和提高疲劳寿命。应用应力循环高度集中在缺口根部附近，导致不对称加载循环中塑性应变的严重积累，称为局部棘轮。当棘轮和疲劳现象耦合在一起时，它们会逐渐促进损伤。与工程部件故障相关的安全问题和费用，需要通过开发稳健的棘轮疲劳方法，在设计和分析中具有高度的可靠性。该研究计划开发了一个强大的框架来评估SS316钢和Al7075-T6样品在不同应力水平、速率、步骤、序列和保持时间事件下经历非对称疲劳循环的多轴棘轮。为了考虑棘轮的速率依赖性，在申请人监督下开发的Ahmadzadeh-Varvani （A-V）模型将与统一的粘塑性流动规则一起使用。在提出的框架中，一个不可或缺的步骤是通过硬化规则来控制超载循环的塑性应变。将开发一种分析方法，将应用应力/应变与缺口根超载循环时的局部应力/应变分量联系起来。所提出的算法将能够评估(i)通过框架的应力循环的棘轮进展，以及（ii）缺口根部附近和给定塑性应变增量的应力松弛。在拟议的研究过程中，将使用申请人在2000年由NSERC资助的Ryerson大学的多轴疲劳测试设备，在未缺口和缺口样品上进行几种不同加载步骤/序列的棘轮试验，直至失效阶段。申请人相信，在接受该建议后，未来五年的研究成果将在很大程度上有助于制定设计指南和安全规范，引导该领域的工程师对承重部件进行更可靠的设计，并显著节省工程部件过早失效的成本。在申请人的指导下，学生将参与整个拟议的研究，培养能够在加拿大和世界范围内服务的高素质人才，包括汽车，航空航天，压力容器和管道。这将使使用安全和耐用的制成品和部件的人民和社会受益。

Catastrophic failure in engineering components in-service annually costs over $150 billion dollars in the North America. Design of parts in various industries such as automotive, aerospace, power plant and pressure vessel are rigorously hinged upon materials integrity, complex geometry, and loading conditions. As the existence of stress raisers such as holes and notches is inevitable, reliable design of machinery parts must be adapted through choice of influential parameters; material property, cyclic hardening/softening, stress level, and stress raiser shape/ size to lessen ratcheting deformation and to improve fatigue life. Applied stress cycles are highly concentrated at the vicinity of notch root causing a severe accumulation of plastic strain over asymmetric loading cycles, referred as local ratcheting. Ratcheting and fatigue phenomena progressively promote damage when they are coupled. Safety issues and expenses associated with failure of engineering parts, necessitate a high degree of reliability in design and analysis through developing a robust ratcheting-fatigue approach. The research program develops a robust framework to assess multiaxial ratcheting of SS316 steel and Al7075-T6 samples undergoing asymmetric fatigue cycles at various stress levels, rates, steps, sequences, and holding-time events. To account for rate-dependency in ratcheting, the Ahmadzadeh-Varvani (A-V) model developed under supervision of the applicant will be employed along with the unified viscoplastic flow rule. An indispensable step in the proposed framework is to govern plastic strain over loading cycles through the hardening rule. An analytical approach will be developed to relate applied stress/ strain to local stress/ strain components at notch root over loading cycles. The proposed algorithm will enable to evaluate (i) ratcheting progress over stress cycles through the framework, and (ii) stress relaxation at the vicinity of notch root and for given plastic strain increments. Through the course of the proposed research, several ratcheting tests with different loading steps/ sequences will be conducted up to failure stage on unnotched and notched samples using the applicant's multiaxial fatigue testing facilities at Ryerson University funded by NSERC in 2000. The applicant believes that upon acceptance of the proposal, outcomes of the proposed research over the next five years will largely contribute to develop design guidelines, safety codes leading engineers in the field toward a more reliable design of load-bearing parts, and noticeably saves cost involved with premature failure of engineering components. The involvement of students throughout the proposed research under the applicant's supervision will train highly qualified personnel capable of serving in Canadian and world-wide industries including automotive, aerospace, pressure vessel, and pipeline. This will benefit people and society employing safe and durable manufactured goods and components.