Damage Characterization and Analytical Modeling of Quasi-statically Indented Graphite/Epoxy-Honeycomb Core Composite Sandwich Stru

The research conducted for this dissertation focused on understanding the nature and extent of barely visible impact damage (BVID) in composite sandwich structures. This was part of a larger research effort that included studies on the influence of BVID on the compressive strengths and failure modes of composite sandwich structures. In this dissertation, the nature and extent of BVID is studied in aluminum honeycomb core sandwich panels with eight and sixteen ply, quasi-isotropic, graphite/epoxy face sheets. The damage in the sandwich structure is created quasi-statically using spherical indentors of two different sizes. Apart from the face sheet thickness and indentor size, other parameters that are varied in the experimental investigations include the core thickness, core density and face sheet layup. The effects of these parameters on the nature and extent of damage in the sandwich structure is evaluated. The different damage metrics of dent diameter, dent depth and planar area of delamination are used for damage characterization and these damage metrics are evaluated non-destructively. The damage resistance of the different sandwich configurations based on these damage metrics is then assessed. It is shown that when the extent of damage in a sandwich structure is determined based on the dent depth or the dent diameter, the sandwich structure that uses a higher density core is the most damage resistant. However, when the extent of damage is based on the planar area of delamination, the parameters that govern the damage size differ for the different face sheet thicknesses. An analytical model is developed in this dissertation to predict the quasi-static load versus displacement response of the sandwich structure during loading, including the onset of damage and the subsequent unloading behavior. It is shown that the analytical model is capable of predicting the residual dent depth and the residual dent diameter of the sandwich structure for damage within the vicinity of BVI

Characterization of Simulated Low Earth Orbit Space Environment Effects on Acid-spun Carbon Nanotube Yarns

The purpose of this study is to quantify the detrimental effects of atomic oxygen and ultraviolet (UV) C radiation on the mechanical properties, electrical conductivity, and piezoresistive effect of acid-spun carbon nanotube (CNT) yarns. Monotonic tensile tests with in-situ electrical resistance measurements were performed on pristine and exposed yarns to determine the effects of the atomic oxygen and UVC exposures on the yarn’s material properties. Both type of exposures were performed under vacuum to simulate space environment conditions. The CNT yarns’ mechanical properties did not change significantly after being exposed to UV radiation, but were significantly degraded by the atomic oxygen exposure. The electrical conductivity of the yarn was not significantly affected by either exposure. The piezoresistive effect did not significantly change due to atomic oxygen exposure, but was significantly enhanced as a result of the UV exposure. Scanning electron microscopy revealed significant erosion due to atomic oxygen exposure, but the UV exposure did not significantly change the appearance of the yarn’s external surface. Raman spectroscopy showed that both exposure types induced significant structural disorder in the surface level CNTs. Focused ion beam milling of a UVC exposed yarn revealed that the depth of the induced disorder was very shallow

In-situ Characterization of Bulk Carbon Nanotube Behavior in a Sheet under Tensile Load

Excerpt: The present study characterizes orientation changes of randomly oriented nanotubes in carbon nanotube (CNT) sheet under externally applied tensile load. The tensile loads were applied using a microtester placed inside a scanning electron microscope (SEM) that allowed in-situ observations of morphological changes in the CNT sheet as the applied load increased