Available at: https://digitalcommons.calpoly.edu/theses/898
Date of Award
MS in Aerospace Engineering
Salt water environments are very harsh on materials that are used within them. Many issues are caused by either corrosion and/or internal degradation to the materials themselves. Composites are better suited for this environment due to their high strength to weight ratios and their corrosion resistance, but very little is known about the fracture mechanics of composites. The goal of this study is to gain a better understanding for the behavior of a composite boat hull under a shear loading, similar to the force water applies on the hull as the boat moves through the water; then attempt to strengthen the composite sandwich panel against the shear loading.
A parametric study was conducted to investigate monotonic in-plane shear loading for composite sandwich panels used in commercial naval vessels. In order to model a conventional composite boat hull, test specimens were composite sandwich panels made of a Divinycell H100 foam core with four layers of fiberglass on both sides of the core. Specimens were tested under a monotonic loading with a rate of 0.2 in/min, and tested until complete failure using the standard test.
Seawater specimens were manufactured in the same manner as the original test specimens, but then were submersed in either filtered seawater or the ocean. The differences between the filtered pieces and the ocean allowed us to determine if any changes found in the composite sandwich panels were related to environment conditions or if the changes were related to the saltwater interaction itself. To create these different environments the seawater specimens were taken to the Avila pier where 36 specimens were placed in a tub that was fed filtered saltwater, while 30 specimens were placed in a plastic mesh with weights and lowered to a depth of approximately 30 ft. in the ocean. Three specimens were then removed at monthly intervals from both filtered and ocean environments.
Shear Keys were created as a method to strengthen the composite sandwich panels against the shear force that the previous specimens had been tested to. Eight Shear Keys were then placed into groves cut into the foam core (four on each side) and the four fiberglass layers were laid on top.
Testing showed that the seawater did have an initial effect on the composite sandwich panels. The filtered pieces showed a decrease in yield strength and stiffness the longer they were subjected to the seawater. The raw unfiltered pieces placed in the ocean saw an even higher decrease in their yield strength and decrease in stiffness. However, for both the unfiltered and raw specimens there was an increase in the ultimate strength and fracture point of the specimens. The effects of the sea water seemed to taper off after the 3rd month however.
The Shear Key specimens were tested with a 4mm and an 8mm Shear Key. The 8mm Shear Keys showed a decrease in shear strength, which was primarily due to removing too much material from the core and weakening the specimen. It was concluded that the decrease in area created a force concentration at the deepest part of the Shear Key causing the premature failure. The 4mm Shear Key showed an increase in the yield strength, ultimate strength, and fracture point. A finite model was built to simulate the original test specimen along with the 4mm and 8mm Shear Key cases, and the results were compared to the experimental results.
The numerical results showed that it was possible to relate the experimental results to the linear or elastic portion of the plots. There was a difference between the maximum displacement of the model and the actual specimens, but this was attributed to potential inaccurate comparison of the loading on the model compared to the actual specimens. The correlation between the model itself and the experimental data was close enough to conclude that it could be used for predicting baseline trends.
Further investigation of the specimens should include looking into the effects of a cyclic shear loading on the specimens. This combined with the seawater element used in this thesis would provide further insight to the initial degradation seen in the seawater specimens, and could potentially provide a closer relation to current hull failures. In addition to including a cyclic loading another numerical model should be created. A model that could be constrained both locally and globally would provide more accurate results. The FEM should also include the ability to run a crushable foam core model within the solver which would also increase the accuracy of the numerical solution.