Modeling of Welding-Induced Distortion Effects on Fatigue Behaviors of Lightweight Shipboard Structures

Abstract

In the design and construction of modern lightweight shipboard structures, wide-spread welding-induced distortions have become a major structural producibility issue and an increasing structural integrity concern over secondary bending stresses caused by interactions of distortions with cyclic service loads. The goal of this dissertation is to develop an effective methodology for evaluating the secondary bending effects caused by complex welding-induced distortions on fatigue behaviors of lightweight structures. A novel analytical approach based on a divide-and-conquer approach is taken to obtain the solutions to complex distortion problems in closed-forms through an assembly of its solution parts achievable through a decomposition technique. A notional load method for providing analytical treatment of distortion curvature effects on fatigue behaviors of lightweight shipboard structures within the context of beam theory is first presented. Using this method, closed-form analytical formulae can be developed for analyzing secondary bending stresses caused by nonlinear interactions between several common distortion types and remotely applied load. Then, an analytical method for computing the secondary bending stresses at weld locations caused by both axial and angular misalignments without curvatures. The model enables a consistent definition of each type of misalignment commonly observed in practice. As such, the secondary bending stresses caused by misalignments at each weld toe location can be appropriately combined for fatigue evaluation purposes. All closed-form analytical solutions derived are validated by direct finite element computations in various cases. Moreover, the developed analytical solutions are used for interpreting fatigue test data of welded components with misalignments and distortion curvatures. An excellent agreement is achieved not only between thin plate lab specimens and full-scale stiffened panels but also with the traction structural stress-based master S-N curve scatter band adopted by ASME Div. 2 since 2007, further validating their effectiveness in fatigue evaluation of welded structures exhibiting general forms of misalignments and distortion curvatures. These new closed-form solutions offer some significant insights not only on what types of distortions are more detrimental to fatigue performance than others but also on the validity limits of the empirical equations stipulated in current Codes and Standards. In addition, parameterized limits can now be clearly stated on conditions when straightening effects should be considered based on the closed-form solutions. Finally, a general distortion mode decomposition-and-assembly procedure is presented. By introducing a consistent reference framework, complex distortions regarding both butt-welded joints and fillet-welded joints in panel structures can be readily decomposed into various elementary distortion modes studied in this dissertation. The final assembly of the constituent secondary stress solutions is accomplished through superposition. To facilitate real-world engineering applications and support future adoptions of Codes and Standards, the closed-form formulae are presented in tabular form for following the workflow of the proposed decomposition-assembly procedure. Two examples are provided for illustrating how the procedure and closed-form solutions are used in real engineering applications. In summary, this dissertation presents a series of novel analytical treatments for computing secondary bending stresses caused by various elementary distortion modes, accompanied by a comprehensive distortion decomposition-and-assembly procedure based on a consistent framework. These new solutions offer a comprehensive suite of tools to engineers and researchers for a consistent and effective treatment of secondary stresses caused by distortion types unique to lightweight shipboard structures in performing fatigue evaluations.