Available at: https://digitalcommons.calpoly.edu/theses/955
Date of Award
MS in Aerospace Engineering
The use of composite sandwich panels is rapidly increasing in the aerospace industry. In these applications, thin composite face sheets are typically co-cured to foam or honeycomb cores. One of the greatest concerns in these types of applications is face-core delamination because it can ultimately lead to catastrophic failure of the structure. Further, the effects of foreign object damage on composite sandwich structures are often catastrophic. Damage to the structure is often undetectable through visual inspection, leading to concerns of unknown interlaminar damage through the life of the structure, which can cause a catastrophic failure. This research determines the effectiveness of a damage arrestment device (DAD) on the structural integrity of composite sandwich beams. The effects of these DADs on the fatigue life of these beams following a low-velocity impact will be considered.
Composite sandwich beams were constructed utilizing two layers of Advanced Composite Group LTM45EL/CF1803 bi-directional woven carbon fiber face sheets with a General Plastics Last-A-Foam FR-6710 rigid polyurethane core, and were cured utilizing an autoclave. Static 4-point bend testing following the ASTM standard D6272 was investigated to determine the flexural behavior of composite sandwich beams. Six different beam cases, with dimensions 1-inch wide by 11.75-inch long, were investigated. The cases considered were beams with and without initial delamination and also, with DAD keys placed transversely and longitudinally with and without delamination. A drop test was employed to simulate a low-velocity impact to the composite sandwich beams with and without delamination. The flexural behavior of these two cases was determined and used in fatigue testing. Experimental results showed that the addition of DAD keys to initially delaminated composite sandwich beams improved the ultimate strength values significantly.
Fatigue testing was investigated to determine the fatigue life of composite sandwich beams following a low velocity impact. Similar to static 4-point bend testing, six different beam cases, with dimensions 1-inch wide by 11.75-inch long, were investigated. The cases considered were exactly those seen in static 4-point bend testing - beams with and without initial delamination and also with DAD keys placed transversely and longitudinally with and without delamination. The three cases without delamination were investigated in the 4-point bend fatigue test, with and without impact. Experimental results showed that the addition of DAD keys had a significant effect on increasing the fatigue life of composite sandwich beams with initial damage from impact, and even improved the fatigue life of beams with initial delamination.
A numerical investigation was completed using a finite element model (FEM) as a means to validate the experimental results found in this research. Static 4-point bend testing was modeled using Abaqus, where maximum deflection values at a specific load were used as a comparison between the experimental and numerical results. Experimental and numerical results showed that the addition of DADs significantly improved the integrity and fatigue life of the composite sandwich design. All cases showed close agreement between the experimental and numerical results.