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It is well known that matrix failure in carbon/epoxy composites is influenced by multiaxial states of stress. However most of the experimental work to measure this interaction has not focused exclusively on matrix failure, and a general multiaxial stress criterion for matrix failure has not been established. To examine this question experimentally we have carried out a series of tests involving torsional shear combined with axial tension or compression of unidirectional hoop wound cylinders, using AS4/55A carbon/ epoxy lamina. The matrix failure stresses are seen to be well correlated with the Tsai-Wu quadratic polynomial. There is also a strong interaction between the strains at failure, with small amounts of transverse tension giving a significant reduction in the shear strain at failure. The effect of σ 2 on the nonlinear shear stress-strain curves is also presented.

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SynopsisThe effects of pressure on the a (ca. 7OoC, 1 kHz) and y (ca. -lOO°C, 1 kHz) relaxations of linear polyethylene were studied dielectrically between 0 and 4 kbar. Equation of state (PVT) data were also determined in the range of interest of these relaxations. The sample was rendered dielectrically active through oxidation (0.8 C=O per 1000 CH2). The a process (which occurs in the crystalline fraction) could he studied over a much wider temperature range than heretofore possible due to the effect of pressure in increasing the melting point. Examination of relaxation strength from 50 to 15OOC showed that there must be two crystalline relaxation processes: the well-known a relaxation plus a competing one. The a relaxation is believed to be due to a chain twist-rotation-translation mechanism that results in rotation-translation of an entire chain in the crystal. The relaxation strength of the a process decreases and therefore indicates the presence of a second (faster and not directly observed) process that increases in intensity with increasing temperature. It is postulated that the second process is due to motion of defects that become more numerous through thermal injection a t higher temperatures. Analysis of the relaxation data along with the PVT data allowed the constant volume activation energy for the a relaxation to be determined. It was found to be 19.4 f 0.5 kcal/mole. The constant volume activation energy is important in modeling calculations of the crystal motions and is significantly smaller than the atmospheric constant pressure activation energy of 24.9 kcal/mole. The effect of pressure on the activation parameters and shape of the y process was also determined. There has been controversy over whether the y process occurs only in the amorphous fraction or in both the amorphous and crystalline phases. Since the two phases have quite different compressibilities, increasing the pressure should change the shape of the loss curves (versus frequency and temperature) if the process occurs in both phases. The shape (but not location) of the loss curves was found to be remarkably independent of pressure. This finding strengthens the view that the y process is entirely amorphous in origin.