Fracture Mechanics Methodologies for Structural Integrity Assessments and Fatigue Life Predictions under Multi-axial Non-proportional Loading

核压力容器、天然气管道、海洋和航空工业等高性能结构部件的安全运行具有重要的实际意义。任何这些部件的故障都是主要问题，因为它们可能导致对公众的伤害，对环境的破坏和高昂的维修成本。结构完整性评估和疲劳寿命预测旨在量化构件缺陷的影响，是保证高性能结构安全使用的关键。目前基于断裂力学的程序使用小尺寸样品的实验室测试结果来关联和预测全尺寸结构部件的响应。近年来，基于约束的断裂力学取得了重大进展，以确保从实验室测试中获得的性能可可靠地转移到全尺寸部件。在这些发展中，裂纹尖端三轴应力状态（所谓的“约束”）之间的差异，在试件和全尺寸部件进行了量化。****然而，所有这些发展都集中在按比例施加外载荷的情况下，因此结构体中的应力分量在加载过程中保持成比例。目前的方法不能正确地处理非比例施加的多轴载荷的影响。多轴非比例加载在工程结构中经常发生，对其对断裂和疲劳行为的影响进行定量分析具有重要的现实意义。本文建议发展断裂力学方法，以考虑多轴非比例加载的影响。特别是，非比例加载对约束的影响将被广泛研究。双参数方法将扩展为一个额外的参数，用于量化非比例加载。采用微力学模型进行数值模拟，并对多轴非比例加载下的各种实验室试样进行实验测试，以量化其对脆性、韧性断裂和疲劳行为的影响。失效评估和疲劳寿命预测的程序将开发，能够解决非比例加载的影响。研究结果将为复杂载荷条件下的高性能工程结构部件提供更可靠的评估和疲劳寿命预测

Safe operations of high-performance structural components such as those in nuclear pressure vessel, gas pipeline, offshore and aircraft industries are of significant practical interest. Failures of any of those components are major concerns since they can result in injury to the public, damage to the environment and high repair costs. Structural integrity assessments and fatigue life predictions aimed at quantifying the impact of defects in those components play a key role in ensuring the safe service of those high-performance structures. Current fracture-mechanics based procedures use results of laboratory tests on small-scale specimens to correlate and predict the response of full scale structural components. Significant developments in constraint-based fracture mechanics have taken place in recent years to ensure reliable transferability of properties obtained from laboratory tests to the full scale components. In those developments, the differences between the crack tip triaxial stress states (so-called "constraint") in test specimens and full scale components was quantified.****However, all these developments have been focused on the cases where external loading are proportionally applied, and thus the stress components in the structural body remain proportional during the loading process. The present methodologies cannot properly address the effects of multi-axial loading that are applied non-proportionally. Multi-axial non-proportional loading occurs routinely in engineering structures, and the proper quantification of its effect on fracture and fatigue behaviors is of practical significance. It is proposed here to develop fracture mechanics methodologies that accounted for the effect of multi-axial non-proportional loading. In particular, the effect of non-proportional loading on constraint will be studied extensively. Two-parameter approach will be extended with an additional parameter that quantifies the non-proportional loading. Numerical simulations, employing micromechanical models, and experimental testing of various laboratory specimens under multi-axial non-proportional loading will be carried out to quantify the effects on brittle, ductile fracture and fatigue behaviors. Procedures of failure assessment and fatigue life predictions will be developed that is capable of addressing the non-proportional loading effects. The outcome of the research will enable more reliable assessments and fatigue life predictions for high performance engineering structural components under complex loading conditions.**