Determination of Residual Stress Based on the Estimation of Eigenstrain   Michael Ralph Hill, Ph.D. Stanford University, 1996 Advisor: Drew V. Nelson, Professor   Inspection of older welded structures often reveals weld defects buried deep within the welded joint. To determine the amount of time the structure can remain safely in service, a lifetime assessment is then performed, assuming crack growth will initiate from these buried defects. Residual stress in the vicinity of the defect is a necessary component of the lifetime assessment. The objective of this thesis is to investigate and develop sectioning-based methods for residual stress determination in thick welded plates. Residual stress in a defective weld may be different from that in a sound weld. Further, the difference in stress may not be well predicted by linear superposition, since the development of residual stress is a non-linear process. To investigate the coupling of residual stress and weld defects, a welding process was developed by which a buried defect of pre-determined size, shape, and orientation was introduced. Residual stress existing in defective welded plates could then potentially be compared to sound welds made under similar conditions. However, the task of uncovering these residual stresses is a difficult one. The remainder of the thesis is devoted to an investigation of subsurface residual stress determination methods. Finite element based simulation of competing sectioning techniques revealed that the eigenstrain method (or inherent strain method), developed by Ueda, provided the most accurate estimates of subsurface residual stress. Experimental results further demonstrated that the principal assumptions of this technique are well-founded. Even though the eigenstrain method is attractive, its complexity and need for a large experimental effort have hindered its adoption. In response to these concerns, the localized eigenstrain method was developed in this thesis. This technique retains the benefits of the eigenstrain method while reducing the effort required to produce estimates of residual stress. To support conclusions based on numerical simulation, a physical verification of the eigenstrain approach was performed by investigating residual stress within a swage-autofrettaged tube. Stress estimates provided by the eigenstrain method were compared to an elastic-plastic simulation of the swage process. Limitations of both the eigenstrain method and the elastic-plastic simulation are revealed, but both produce comparable results.

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