Analyses of Failure Mechanisms and Residual Stresses in Graphite/Polyimide Composites Subjected to Shear Dominated Biaxial Loads

This research contributes to the understanding of macro- and micro-failure mechanisms in woven fabric polyimide matrix composites based on medium and high modulus graphite fibers tested under biaxial, shear dominated stress conditions over a temperature range of -50 C to 315 C. The goal of this research is also to provide a testing methodology for determining residual stress distributions in unidirectional, cross/ply and fabric graphite/polyimide composites using the concept of embedded metallic inclusions and X-ray diffraction (XRD) measurements

Silicone rubber aging in electrolyzed aqueous salt environments

In this work we suggest an entirely new and highly transformative model of silicone rubber aging caused by the presence of electrolyzed aqueous salt, leading to the formation of hypochlorous acid in-service on energized High Voltage (HV) transmission lines in coastal environments. The model was subsequently verified by aging one-component Room Temperature Vulcanized (RTV-1) rubbers in commercially produced hypochlorous acid and an electrolyzed aqueous salt environment at room temperature. Molecular dynamics models of the rubbers with silica fillers were conducted to offer at least partial explanations of the types and extents of aging observed in the RTV-1 samples tested in the aqueous salt solutions. We have shown that the above environments are highly damaging to the silicone rubber polymer network and significantly more destructive than non-electrolyzed standard aqueous salt solutions at higher temperatures. The main observations of this research should greatly contribute to a better understanding of complex degradation mechanisms of modern HV silicone rubber compounds, which are utilized in some of the most critical lines and substations all over the world

New Approach to Moisture Accumulation Assessment

We suggest an innovative approach to simulate moisture buildup on structures with arbitrary geometries and different surface adhesion characteristics. Using 3D particle physics, rain, snow and freezing rain buildup on flat and cylindrical surfaces was assessed. The numerically simulated moisture accumulations were subsequently successfully verified experimentally through a series of outdoor and indoor moisture accumulation experiments. We also conducted molecular dynamics simulations to assess how hydrogen bonds influence snow and rain adhesion on high and low energy substrates. The simulations supported the experimental observations, namely that freezing rain results in increased moisture adhesion and accumulation due to the spreading of the water droplets, resulting in more hydrogen bonds between water droplets and substrates than snow

Silicone Rubber Aging in Electrolyzed Aqueous Salt Environments

In this work we suggest an entirely new and highly transformative model of silicone rubber aging caused by the presence of electrolyzed aqueous salt, leading to the formation of hypochlorous acid in-service on energized High Voltage (HV) transmission lines in coastal environments. The model was subsequently verified by aging one-component Room Temperature Vulcanized (RTV-1) rubbers in commercially produced hypochlorous acid and an electrolyzed aqueous salt environment at room temperature. Molecular dynamics models of the rubbers with silica fillers were conducted to offer at least partial explanations of the types and extents of aging observed in the RTV-1 samples tested in the aqueous salt solutions. We have shown that the above environments are highly damaging to the silicone rubber polymer network and significantly more destructive than non-electrolyzed standard aqueous salt solutions at higher temperatures. The main observations of this research should greatly contribute to a better understanding of complex degradation mechanisms of modern HV silicone rubber compounds, which are utilized in some of the most critical lines and substations all over the world

Aging Resistant TiO2/silicone Rubber Composites

We have recently shown [1,2] that one component room temperature vulcanized (RTV-1) silicone rubbers (SIR) based on polydimethylsiloxane (PDMS) can be rapidly degraded by low voltage (LV) energized aqueous salt solutions by previously unreported aging mechanisms related to the formation of hypochlorous acid in high voltage (HV) transmission line applications. In this study, we are showing how to improve the resistance of the rubbers to extreme environmental aging by embedding TiO2 micro-particles. Molecular dynamics (MD) simulations were conducted to determine the combined effect of TiO2 and different concentrations of hydrophobic PDMS methyl groups on surface hydrophobicity of a TiO2/PDMS composite. In addition, the effects of both TiO2 and silica on the diffusivities of LV aqueous salt components in the PDMS were predicted and related to unique interfacial interactions between the particles and the methyl groups of the PDMS. Rutile TiO2 reoriented methyl groups away from the particles reducing the diffusivities of water and hypochlorous acid. This effect shielded the PDMS network against environmental chain scissions. On the other hand, silica attracted the groups accelerating acid and water migrations and thus enhancing damage to the network. In the experimental part, TiO2/RTV was subjected independently to hypochlorous acid and electrolyzed LV aqueous salt. As expected, TiO2 greatly increased the contact angle, reduced the surface energy and improved the hydrophobicity of the composite, mitigating the negative effect of the reduced concentrations of methyl groups. As a result, aging damage to the rubber was dramatically reduced by about 50% in highly oxidative environments

Determination of Gel Point and Completion of Curing in a Single Fiber/polymer Composite

By embedding both a single fiber Bragg grating (FBG) sensor and a thermocouple during the cure cycle of a room temperature cure epoxy, a novel in-situ approach was developed to identify certain properties of the polymer. Residual strains during curing and cooling of the epoxy were determined as a function of time for three different ambient curing temperatures. Comparing the thermocouple based temperatures to those of the ambient environment allowed for the calculation of the degree of cure and the full cure time of the epoxy. Analyzing the wavelengths of the FBG sensor and compensating for the temperature measured by the internal thermocouple, the gel point was precisely identified and validated using Flory-Stockmayer theory. As expected the residual strains increased with the curing temperature while the gel time and the completion of curing time decreased with temperature. The integrated approach of using both FBG sensors and embedded thermocouples presented in this study could be used for other polymer systems and polymer matrix composites during their manufacturing