Journal of The American Ceramic Society -TrantinaV O~. 60, NO. 7-8 teresis, in direct contrast to the counter-clockwise hysteresis trends observed for microcracked FezTi0,.4 When the sample is cooled to room temperature, the hysteresis curve does not close, indicating a permanent change in the material. This behavior is consistent with the formation or growth of new microcracks as the result of a fatigue process during the thermal cycling. This effect will occur simultaneously with the expected healing of the microcracks at high temperatures. However, for the as-hot-pressed material, the effect of new crack formation or growth during the thermal cycling dominates any effects of crack healing. Further SEM examination revealed that no detectable grain growth had occurred during the thermal diffusivity measurement, thus the formation of additional microcracks cannot be attributed to a grain growth (size) effect.The competition between crack healing at high temperatures and new crack formation or crack growth during cooling during thermal cycling also explains the thermal diffusivity hysteresis of the annealed specimens (Figs. 2(B), 2(C), and 2(0)). Note that the hysteresis curves are in the form of a horizontal figure eight with a counter-clockwise behavior at the higher temperatures and a clockwise behavior at the lower temperatures, with a crossover at an intermediate temperature, -250 "C. A similar figure-eight type of behavior has been reported for the modulus of rupture of beta e~cryptite.~ This behavior is consistent with the hypothesis that in these microcracked materials crack healing is more pronounced than new crack formation or crack growth at higher temperatures (above the crossover point), with the reverse being true at temperatures below the crossover point. The second thermal cycle does not duplicate the first thermal cycle (Figs. 2 ( B ) and 2(C)). This behavior suggests that a given microcrack configuration or distribution (in terms of crack geometry, orientation, density, or size, as well as degree of cooling) is not necessarily an equilibrium configuration but can be modified by additional thermal treatment. As aresult, the properties of microcracked materials are expected to be a function of past thermal history.The observation that the relative effect of microcracking on thermal diffusivity is larger at the lower temperature than at high temperatures suggests that not only the thermal diffusivity but its temperature dependence as well is affected. Undoubtedly this effect is primarily the result of crack closure and/or healing at higher temperatures. However, heat transfer by thermal radiation across the cracks also contributes to the heat transfer process at higher temperatures, analogous to the contribution of radiative heat transfer to the total thermal conductivity of porous solids.'O*ll This effect is also related to the recovery of the thermal diffusivity during repeated thermal cycling, as indicated in Figs. 2(B) and 2(C), and is also observed for the microcracked Fe2Ti0,, reported previ~u...

Tensile and flexural tests on specimens cut from rectangular injection‐molded plaques show that long‐fiber filled thermoplastic composites are complex, non‐homogeneous, anistropic material systems. Like all fiber‐filled materials, they exhibit through‐thickness nonhomogeneity as indicated by differences between tensile and flexural properties. The in‐plane orientation of fibers in through‐thickness layers causes the material to have in‐plane anisotropic properties. However, these long‐fiber filled materials exhibit an unexpectedly large level of in‐plane nonhomogeneity. Also, the effective mechanical properties of these materials are strongly thickness dependent. The thinnest plaques exhibit the largest differences between the flow and cross‐flow tensile properties. These differences decrease with increasing thickness. A methodology for part design with this class of materials is discussed.

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To predict the fatigue performance of talc-filled polypropylene in engineering applications, a simple relationship using constant stress data to predict the cycles-to-failure (Nf) was developed:where k and n are constants from creep rupture tests, u is the cyclic frequency (11 Hz to avoid self-heating),f(n) is a function that accounts for the wave form, and u is the peak cyclic stress with a zero minimum stress. For stress concentrations produced by central holes, u is taken as the average net section stress and the effect of the stress concentration is ignored. Extensive tests were used to verify this expression for cyclic frequencies of 1 .O and 0.1 Hz, sinusoidal and triangular waveforms, stress concentrations produced by small and large holes, and 20 and 40 percent talc filler content.