Master of Science in Mechanical Engineering (MSME)
Composite sandwich structures have been widely used in aerospace structures, ship building, infrastructure, etc. due to their light weight and high strength to weight ratio. Traditionally, light-weight core materials such as foam core, truss core, honeycomb core have been used in fabricating sandwich structures with limited success. In this study, a new composite sandwich structure with a hybrid core was proposed, fabricated, tested, and modeled. The hybrid core consists of a fiber reinforced polymer grid skeleton that is filled in by extremely light-weight syntactic foam in the bay areas. The new sandwich structure was manufactured using a new manufacturing process. The behavior of these structures under impact and compression was investigated. Experimental results show that the grid structure with smaller bay area has higher impact resistance compared to grids with larger bay areas. For the hybrid core with a smaller bay area, the damage was localized to a much smaller area as evidenced by ultrasonic inspection. The nodal region was observed to have the highest impact resistance while the bay area was the weakest and had the least impact resistance. Compression testing was done after subjecting the specimens to impact testing. The results obtained show that the residual load carrying capacity decreases as the bay area increases. However, compared to the control group, which was the traditional laminated composite, the sandwich structure with grid stiffened core shows better impact tolerance and higher residual strength, although the fiber volume fraction used was the same. SEM images of impact region were taken and the mechanisms involved in the failure of foam, ribs and nodes were observed. From impact analysis it was found that grid stiffened sandwich structures with a smaller bay area have a higher impact loading capacity. The damage was localized to a smaller area. The residual load carrying capacity of grid structures was found to be higher than that of the laminates. Finite element analysis was conducted on the hybrid structure using ANSYS. A 3-D finite element model was developed and appropriate material properties were given to each component. Boundary conditions similar to those used in compression testing were utilized. The model was firstly validated by the experimental results. Parametric analysis was then performed on the validated model by varying important design parameters like skin thickness, skin modulus, rib width, rib modulus and bay area. Results obtained by changing these parameters were analyzed. The experimental and finite element results were discussed and general conclusions were drawn. From the impact results it was observed that grid structures with small bay area have a higher impact tolerance. The damage caused by impact was confined to a much smaller area compared to laminate composites. The residual load carrying capacity of grid structures with small bay area was found to be higher compared to large bay area specimens. It was found that for higher energy impacts the grids with smaller bay areas were able to retain higher load carrying ability.
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Muthyala, Venkata Dinesh, "Composite sandwich structure with grid stiffened core" (2007). LSU Master's Theses. 552.