Swaged Bulkhead Analysis Verification: Study on Full-scale Testing of Swaged and Bulb Stiffened Bulkheads

Thin steel plates with welded stiffeners are used extensively in ship structures where a high strength-to-weight ratio is important. In conventional construction of bulkheads in the U.S., bulb, angle, or tee stiffeners are widely used. Recently, General Dynamics NASSCO (GD NASSCO) studied the use of swaged bulkheads as non-load bearing components in order to reduce the overall cost of ship design, construction, and life cycle maintenance. To investigate their application as load bearing structural components, it is necessary to conduct both experimental and analytical studies so that design guidelines can be developed. In Phase 1 of this research project to develop such design guidelines, full-scale experimental test conducted.
Six sets of bulkheadsthree steel (DH36) and three aluminum (A5083-H116)for a total of 12 full-scale specimens were fabricated by Marinette Marine Corporation and shipped to UCSD for testing. Each set included one conventional bulb stiffened bulkhead and one swaged bulkhead. The steel bulb stiffened bulkheads were designed by GD NASSCO based on ABS (American Bureau of Shipping) rules, while the swaged bulkheads were designed such that the moment of inertia (Ixx) about the weak axis was the same as the bulb stiffened bulkheads. A similar approach was applied to design the aluminum bulb stiffened and swaged bulkheads, ensuring they have the same Ixx.
One set of steel and one set of aluminum specimens were tested to failure in vertical compression. The steel specimens first experienced plate buckling, followed by plate-induced overall buckling when the maximum load was reached. The steel specimens showed a 16% difference in elastic axial stiffness, and the swaged specimen had compression strength 36% higher than that of the bulb specimen. The aluminum specimens also showed a 12% difference in elastic stiffness, but the swaged bulkhead showed a 63% increase in maximum strength compared to the bulb stiffened one. Testing also showed that stiffener tripping as a result of low torsional rigidity of the bulb stiffeners caused a drastic reduction (60% loss) of strength upon buckling in the bulkhead.
One set of aluminum specimens was tested to failure under lateral load (shear). The bulb stiffened specimen failed due to stiffener tripping in the compression zone. The swaged bulkhead failed prematurely due to weld failure, and, thus, the maximum strength could not be compared. But the elastic stiffness of swaged bulkhead was 11% higher than that of the bulb stiffened bulkhead.
One set of steel and one set of aluminum specimens were tested at low load level to evaluate the elastic out-of-plane stiffness. Testing showed that the swaged bulkheads had an elastic stiffness about 17% higher than the bulb stiffened bulkheads.
The objective of Phase III was to evaluate the effects of hole opening and cabinet attachment on the shear strength and stiffness of steel swaged panels. Monotonic racking shear tests were conducted on three full-scale specimens. The results showed that, as expected, hole openings decreased both the ultimate lateral strength and elastic stiffness of the swaged panels. The attachment of an electrical cabinet had a minimal effect on the lateral strength and stiffness, although the rate of strength degradation in the post-buckling region was more gradual. A comparison of the test results in this phase with one specimen in Phase II showed that the ultimate lateral strength can be significantly affected by the direction of buckling and the actual yield stress of the steel; further research including nonlinear finite element parametric study is need to confirm this postulation.