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

高性能结构部件的安全运行，如核压力容器、天然气管道、海上和飞机工业等，具有重要的实际意义。这些部件中的任何一个发生故障都是主要问题，因为它们可能导致公众受伤、环境破坏和高昂的维修费用。结构完整性评估和疲劳寿命预测旨在量化这些部件中缺陷的影响，在确保这些高性能结构的安全使用方面发挥着关键作用。目前基于断裂力学的程序使用小规模试件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，在基于约束的断裂力学方面取得了重大进展，以确保从实验室测试获得的特性可以可靠地转移到全尺寸部件。在这些发展中，量化了试件和足尺构件中裂纹尖端三轴应力状态(所谓的“约束”)之间的差异。然而，所有这些发展都集中在按比例施加外部载荷的情况下，因此结构体内的应力分量在加载过程中保持成比例。目前的方法不能适当地解决非比例施加的多轴载荷的影响。多轴非比例加载是工程结构中经常发生的现象，合理量化其对断裂和疲劳行为的影响具有重要的现实意义。本文建议发展考虑多轴非比例加载影响的断裂力学方法。特别是，非比例加载对约束的影响将得到广泛的研究。两参数法将被扩展，增加了一个量化非比例加载的参数。采用细观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件进行实验测试，以量化其对脆性、延性断裂和疲劳行为的影响。将开发能够处理非比例加载影响的失效评估和疲劳寿命预测程序。研究结果将使高性能工程结构部件在复杂载荷条件下能够进行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.**