A two part research study has been completed on the topic of compression after impact (CAI) of thin facesheet honeycomb core sandwich panels. The research has focused on both experiments and analysis in an effort to establish and validate a new understanding of the damage tolerance of these materials. Part one, the subject of the current paper, is focused on the experimental testing. Of interest are sandwich panels, with aerospace applications, which consist of very thin, woven S2-fiberglass (with MTM45-1 epoxy) facesheets adhered to a Nomex honeycomb core. Two sets of specimens, which were identical with the exception of the density of the honeycomb core, were tested. Static indentation and low velocity impact using a drop tower are used to study damage formation in these materials. A series of highly instrumented CAI tests was then completed. New techniques used to observe CAI response and failure include high speed video photography, as well as digital image correlation (DIC) for full-field deformation measurement. Two CAI failure modes, indentation propagation, and crack propagation, were observed. From the results, it can be concluded that the CAI failure mode of these panels depends solely on the honeycomb core density.
Thermal effects on the pin-bearing behavior of an IM7/PETI5 composite laminate are studied comprehensively within this paper. Pin-bearing tests of several lay-ups at the operating temperatures of −129, 21, and 177°C are conducted to generate data on the effect of temperature changes on the pin-bearing behavior of the IM7/PETI5 material. A unique computational study investigating the factors influencing pin-bearing strength is performed for the [90,0]8s lay-up. A detailed three-dimensional finite element model is developed including the modeling of resin layers between plies in the laminate with a pin hole. Using a previously established technique, the pin-bearing problem was modeled assuming a rigid frictionless pin and restraining only radial displacements at the hole boundary. An original three-dimensional solution, where thermal residual stresses were combined with the state of stress due to pin-bearing loads was evaluated. The presence of thermal residual stresses were observed to intensify the interlaminar stresses predicted at the hole boundary in the pin-bearing problem. Furthermore, the research shows that changes in material properties drives pin-bearing strength degradation with increasing temperature. The thermal residual cure stresses then somewhat affect the extent that the pin-bearing strength changes with temperature. In general, thermal residual stresses were determined to be significant in affecting pin-bearing strength.
A two part research study has been completed on the topic of compression after impact (CAI) of thin facesheet honeycomb core sandwich panels. The research has focused on both experiments and analysis in an effort to establish and validate a new understanding of the damage tolerance of these materials. Part 2, the subject of the current paper, is focused on the analysis, which corresponds to the CAI testings de-
A multifunctional hot structure heatshield concept is being developed to provide technology enhancements with significant benefits compared to the current state-of-the-art heatshield technology. These benefits can potentially enable future planetary missions. The concept is unique in integrating the function of the thermal protection system with the primary load carrying structural component. An advanced carbon-carbon material system has been evaluated for the load carrying structure, which will be utilized on the outer surface of the heatshield, and thus will operate as a hot structure exposed to the severe aerodynamic heating associated with planetary entry. Flexible, highly efficient blanket insulation is sized for use underneath the hot structure to maintain required operational internal temperatures. The approach followed includes developing preliminary designs to demonstrate feasibility of the concept and benefits over a traditional, baseline design. Where prior work focused on a concept for an Earth entry vehicle, the current efforts presented here are focused on developing a generic heatshield model and performing a trade study for a Mars entry application. This trade study includes both structural and thermal evaluation. The results indicate that a hot structure concept is a feasible alternative to traditional heatshields and may offer advantages that can enable future entry missions.
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