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

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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.**