Date of Award

2008

Level of Access Assigned by Author

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Advisor

Senthil S. Vel

Second Committee Member

Arthur O. Willey

Third Committee Member

Vincent Caccese

Abstract

Laser welding for ship structural connections is a maturing technology that offers much promise for increasing productivity and reducing fabrication costs. Past fabrication methods for ship structures have relied upon fastening methods such as periodically spaced bolts or rivets, adhesive bonding or conventional welding. Laser welding provides an extremely robust method to connect steel components and it can be performed at speeds 5-10 times that of conventional welding, thus requiring less manufacturing time. Furthermore, dimensional accuracies far superior to that obtained by conventional welding have been realized through laser welding. Since ships are complex structures which are subjected to dynamic loads, a thorough study of the static and dynamic load resistance is essential to the proper design of laser welded connections. Accordingly, the major objective of this thesis is to analyze the static and fatigue response of laser welded connections. We focus on continuous laser welded lap joints under different loading conditions. The analyses are based on the principles of linear elastic fracture mechanics due to the singular stress fields at the edges of the laser welds. The computational model presented here will enable engineers to design robust laser welded lap joints for prescribed loads. A static stress analysis is performed in the first part of the thesis to obtain the stress intensity factors and the failure load of laser welded lap joints. The geometry and nonlinear contact conditions of lap joints make it difficult to obtain an analytical closed-form solution. Therefore, the commercial finite element software ABAQUS is used to perform a stress analysis of various joint configurations and obtain the stress intensity factors and joint strength. It is found that the overlap length, weld width and plate thicknesses play a major role in determining the joints behavior under different loading conditions. The results are utilized to construct a metamodel that can be used to estimate the fracture strength of lap joints of arbitrary dimensions without having to perform a detailed finite element analysis every time. The parameters in the metamodel are optimized using a real-coded single objective genetic algorithm. The metamodels, which are validated and found to be quite accurate, are very useful for designing lap joints for prescribed static loads. The fatigue response of laser welded lap joints is analyzed in the second part of the thesis. Fatigue failure is often a complex process due to numerous factors that influence crack growth. Nevertheless, it is important to develop a fatigue model for purposes of scheduling inspection at regular intervals, determining required maintenance, conducting a failure analysis, and minimizing the problem of crack propagation in future designs. A fracture specific finite element program FRANC2D is used to analyze crack growth, obtain the crack propagation trajectories and compute the corresponding stress intensity factor histories for the lap joint geometries under consideration. The Paris fatigue growth law is used to obtain the S-N curves for constant amplitude cyclic loads. It is found that the weld width has a significant effect on the crack trajectory and fatigue life.

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