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Viscoelastic characterization of a non-linear, glass/epoxy composite including the effects of damage
Abstract
Isothermal creep and recovery tests were conducted on an epoxy resin and a glass fiber-reinforced composite made from the same bulk resin. The glass/epoxy which was studied included unidirectional and laminated (angle-ply) composites as well as samples removed from a Minuteman III solid rocket motor case. The creep and recovery tests were carried out at a series of stress levels well into the nonlinear region at temperatures of 20, 75 and 140°F for several fiber angles. Both the epoxy and glass/epoxy were found to be thermorheologically complex materials with a creep compliance which may be represented by a power law in time. The linear viscoelastic principal creep compliances were determined for the glass/epoxy using fourth-order tensor transformations. Using the Halpin-Tsai relationships and the "rule of mixtures", the principal creep compliances were compared with those predicted by micromechanics. The experimental results were found to agree very well with the Halpin-Tsai model except at the highest temperatures and were within the upper and lower theoretical bounds on compliance. Even at low stress levels the presence of micro-crack growth was found to produce appreciable softening at the highest temperatures. The nonlinear properties were found to depend primarily on the stress normal to the fiber, suggesting a crack opening mode as the essential mechanism of growth. Multiple cycles of creep and recovery showed a disproportionate amount of damage during the first cycle. Crack growth was found to reduce more rapidly and with less softening effect in the laminated (±θ) composites, probably as a result of the interfacial barrier between the layers. In general, the off-angle composites exhibit considerable softening due to micro-crack growth. Bending tests conducted on glass/epoxy beam and plate specimens brought out a strong influence of the strain gradient. It was found that linear theory can be used for most of the useful engineering range of application. Nonlinear theory based on tensile tests of unidirection specimens predicts considerably more reduction in bending stiffness than observed experimentally. Multiple cycling effects on the glass/epoxy composite were found to be more sensitive to the stress normal to the fibers than to the shear stress. The second-order Lebesgue norm (L₂) was found to approximately characterize multiple cycling effects such as seen in a solid rocket motor case. The L₂ norm which, at a given time, is proportional to the root mean square value of the stress is, therefore, proposed as a parameter for defining the damage produced by hydrotesting.
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Citation
Beckwith, Scott Williams (1974). Viscoelastic characterization of a non-linear, glass/epoxy composite including the effects of damage. Texas A&M University. Texas A&M University. Libraries. Available electronically from https : / /hdl .handle .net /1969 .1 /DISSERTATIONS -213405.
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