Structural Testing and Analysis of Hybrid Composite/Metal Joints for High-Speed Marine Structures

The implementation of composite materials in engineering applications has observed a rapid increase over the last few decades. The U.S. Navy currently has an objective to enhance its future naval capabilities by developing hull-forms with advanced materials, including composites. Speed can be increased by reduction in weight and by incorporating innovative shaping of the hull-form, which can potentially be achieved with composites. Any naval vessel using composite materials for structural components will require hybrid connections of some sort, where composite sections are joined to metallic sub-structures. Joints are critical regions in the design of hybrid systems, as failures typically occur at joints and interfaces and rarely within the bulk of the structure. Accordingly, this effort presents the development of a watertight, hybrid composite/metal bolted joint, for cases where the panels are removable. An experimental study was conducted to quantify the performance of numerous hybrid joints with various geometries, loaded in flexure. The test results showed that, for resisting bending loads, joints with doubler plates can be made stronger and rotationally stiffer than standard bolted joints while also mitigating opening of the joint, thereby improving the ability to seal the connection for watertight integrity. Based on these results, a joint geometry was selected and incorporated into the hydrostatic testing of a large-scale, four-panel assembly. A linear response of the system was observed up to its design pressure load of 82.74 kPa. Damage initiated as stiffener delamination at 1.4 times the design load. After failure of several stiffeners, the hybrid assembly withstood up to 3 times its design load without leakage. Hence, the response of the hybrid joint employed was deemed successful. Numerical analysis of the joints and the hybrid assembly are also presented. Simplified shell finite element models were developed at both local and global levels. These models were used to estimate the joint stiffness and good correlation with the test results was observed. To study the local stresses at the joint region and to provide an estimate of failure initiation, detailed generalized plane strain contact models were created to capture the three-dimensional effects of the connection