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

在北美，每年在役工程部件的灾难性故障造成的损失超过1500亿美元。汽车、航空航天、发电厂和压力容器等各种行业的零件设计严格取决于材料完整性、复杂几何形状和载荷条件。由于孔、缺口等应力集中点的存在是不可避免的，因此机械零件的可靠设计必须通过选择影响参数，如材料性能、循环硬化/软化、应力水平、应力集中点形状/尺寸等来减小棘轮变形，提高疲劳寿命。外加应力循环高度集中在缺口根部附近，在不对称的加载循环过程中会引起塑性应变的严重积累，称为局部棘轮效应。当棘轮效应和疲劳现象耦合时，它们逐渐促进损伤。与工程部件故障相关的安全问题和费用需要通过开发鲁棒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，以评估SS 316钢和Al 7075-T6样品在各种应力水平，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效应。为了解释棘轮效应中的速率依赖性，将采用在申请人监督下开发的Ahmadzadeh-Varvani（A-V）模型以及统一粘塑性流动规则沿着。在所提出的框架中，一个必不可少的步骤是通过硬化规则来控制加载循环中的塑性应变。将开发一种分析方法，将施加的应力/应变与缺口根部过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中的棘轮进展，以及（ii）缺口根部附近和给定塑性应变增量的应力松弛。通过拟议的研究过程中，几个不同的加载步骤/序列的棘轮试验将进行到失败阶段的无缺口和缺口样本使用申请人的多轴疲劳试验设施在瑞尔森大学由NSERC于2000年资助。 申请人认为，在接受该提案后，未来五年的拟议研究成果将在很大程度上有助于制定设计指南、安全规范，引导该领域的工程师设计更可靠的承载部件，并显著节省工程部件过早失效所涉及的成本。学生在申请人的监督下参与整个拟议的研究，将培养能够在加拿大和世界各地的行业，包括汽车，航空航天，压力容器和管道服务的高素质人才。这将有利于使用安全耐用的制成品和部件的人和社会。

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.