Impact Test Development for Large-Scale Composite Hull Panels for the "Mako" Patrol Craft
The Mark V patrol craft is used by the U.S. Navy Special Forces to carry Sea, Air and Land (SEAL) combat swimmers to and from operations. The boat is a high speed planning craft, designed for low to medium threat insertions and extractions. However, during a 2001 study the Navy found that Mark V crews were 5 times more likely to be injured under normal operating conditions than traditional navy crews. Most of these injuries were caused by high impact, repetitive wave slamming loads felt during high speed operations. The current vessel is aluminum, and it is hypothesized that changing the hull material from a metal to a composite designed to be impact absorbing could help mitigate the slamming loads felt by the crew. This thesis seeks to help address this problem by developing a test methodology to select the hull material for a patrol craft prototype called the Mako. The test fixture and protocol for impact testing hull panels were developed based on earlier work done at the Florida Institute of Technology. Four material combinations were considered: cored and single-skin carbon fiber/epoxy and cored and single-skin E-glass/vinyl ester. The material selection testing predicted that cored carbon would be the most efficient material selection for the Mark V.l from a weight and impact absorbance standpoint. Overall, the cored E-glass panels were the most impact absorbent; however there were no significant weight savings over the aluminum baseline with the cored E-glass panels. The aluminum panel was the heaviest and the least impact absorbent of all the four possible material options. A 250 Hz low-pass filter revealed that removing the high-frequency noise lead to an even greater advantage for the cored carbon panel over the aluminum. The filtered results show that the cored carbon panel is almost ten times more effective at filtering out the low frequency vibrations that cause damage during an impact event. This will not directly translate into a l0x reduction in the impact felt by Mako's crew, but it does provide a comparison of the effects of just the material change. A FFT analysis revealed that the significant frame mode is picked up by the panel accelerometers. Both the cored carbon and aluminum panels showed peaks at the frame frequency along the fore and aft accelerometers. However, the aluminum panel also picked up the frequency along the centerline accelerometers. This indicates that the vibration from the frame is transmitted through the entire aluminum panel, without any attenuation. The cored carbon panel, on the other hand, damps out the vibration before it transmits to the centerline accelerometers.