Characterization of S-glass epoxy composite interface under various rates of loading
Date
2018
Authors
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Journal ISSN
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Publisher
University of Delaware
Abstract
Glass fiber reinforced epoxy composites are used extensively in military application due to their higher specific stiffness and strength and high levels of damage tolerance and specific energy absorption. When subjected to dynamic loading, a major portion of this energy is dissipated within the composite due to fiber matrix debonding along the interface and localized plastic deformation of the epoxy matrix that occurs at high strain rate. This dissertation involves development of novel experimental methods including carbon nanotube (CNT) based damage sensors to detect interface debonding and characterization of high strain rate resin behavior. A methodology using Finite Element (FE) modeling of the experiments is established to uniquely determine rate dependent Mode II cohesive traction laws of the composite interfaces. ☐ Interface properties were characterized using a microdroplet test specimen at loading rates spanning over six decades of magnitude. At higher loading rates, a tensile Hopkinson bar has been designed that can load the interface in the range of 1-10 m/s. Experimental results on S-2 glass and DER 353 epoxy system and post-failure inspection of the fiber matrix interface showed that the new test method is effective in measuring high rate interface properties of composites. The average interfacial shear strength (IFSS) increased by a factor of 1.6 when the loading rates were increased from 1 μm/s to 1 m/s. ☐ Crack initiation at the interface was studied by developing a CNT sensor at sub-micron length scale, which was achieved by modifying the fiber surface morphology through the deposition of CNT using electrophoretic deposition method. As CNTs are coated on a non-conductive substrate, electrically percolating nanotube network is formed in isolation at the interfacial region. This allows for the study of in-situ crack initiation at the interface through electrical resistance measurements. Experimental results confirms that crack initiation occurs near the peak load. ☐ To accurately model the microdroplet experiments, rate dependent resin properties were determined over a wide range of strain rates (0.001/s to 12,000/s). To obtain compressive stress strain response up to large strain under high strain rates, a specimen geometry was designed by studying the state of stress in the specimen using FE simulations. High strain rate tests were conducted in a traditional split Hopkinson pressure bar up to a strain rate of 12,000/s over a strain range up to 70%. This specimen geometry overcomes the Hopkinson bar’s limitation on maximum attainable strain without altering the bar design. For DER 353 epoxy resin, yield stress increased significantly with applied strain rate and exhibited a bi-linear dependency. Thermal softening was observed under high strain rates at large strains due to adiabatic heating. ☐ A methodology was developed to determine rate dependent traction separation law for composite interface through iterative method by simulating the microdroplet experiments using finite element analysis. A series of parametric studies were conducted by varying peak traction and fracture energy. By matching the peak load, crack initiation time and failure modes from the experiments for three different droplet sizes, a unique set of Mode II traction law parameters are determined for each loading rate. FE results show that both the peak traction and fracture energy associated with the interface are dependent on the rate of loading.
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Keywords
Applied sciences, Epoxy composites, Epoxy matrix, Fiber matrix debonding, Finite Element modelling, Plastic deformation