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The Effect of Time and Temperature on the Mechanical Behavior of Epoxy Composites. Part 2. Mode of Failure, Yield Stress and Yield Strain

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A crosslinked epoxy resin consisting of a 60 400 weight ratio of Epon 815 and Versamid 140 and composites of this material with glass beads, unidirec-tional glass fibers and air foams were tested in tension, compression and flexure to determine the effect of time and temperature on the mode of failure, yield strain. Un-idirectional continuous fiber-filled samples were tested at different fiber orientation angles with respect to the stress axis. Strain rates ranged from ten to the minus four power to ten to the minus one power and the temperature from -1 to 107 C. The material was found to change from a brittle-to- ductile-to-rubbery failure mode with the tran-sition temperatures being a function of strain rate, filler content, filler type and fiber orientation angle, indicating that the transition is perhaps dependent on the state of stress. The isotherms of yield stress versus log strain rate were shifted to form a practically linear master plot of yield stress versus log shifted strain rate that can be used to predict the yield stress of the composites at any tem-perature and strain rate in the ductile region. The time- temperature shift factors were found to be independent of the type, concentration and orientation of filler and the mode of loading. Thus, the composite shift factors seem to be a property of the matrix and not dependent on the state of the stress. The compressive-to-tensile yield stress ratio was practically invariant with shifted strain rate for the un-filled matrix, while fillers and voids raised this ratio and caused it to increase with a decrease in shifted strain rate. The yield strain of the composites is less than the unfilled matrix and is a function of fiber orientation and shifted strain rate.

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  • Laminates and Composite Materials
  • Mechanics

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