Thus, the use of anti-distortion appliqués was demonstrated to have theoretical potential for order-of-magnitude improvement in dimensional stability in thermal environments. However, the practical potential may be limited in the near future for two reasons: (1) a large amount of computational modeling is needed to implement this approach and (2) at present, techniques for measuring distortional displacements of structures with the precision needed for further development of this approach are not yet available.

The other main approach followed in this project involved studying effects of damage of polymer matrices (and related effects associated with aging of polymer matrices) as sources of dimensional instability. In one example of such damage, in the course of cooling to the cryogenic environment of outer space, polymers become susceptible to microcracking. Even in cases in which damage may be so small as to degrade structural integrity insignificantly, it can cause dimensional instability. In this project, effects of matrix cracking on effective properties of composite laminae were studied in finite-element-model computational simulations. In comparison of results of the simulations with experimental data in the literature, it was found that the residual properties (defined here as the effective properties reduced by amounts that depend on degrees of damage) as predicted by use of the models closely approximate experimentally observed properties, except in experiments that were directed toward measuring extremely small changes in stiffness. The models were also found to expose several subtle dependences of the residual properties on the parameters of undamaged plies adjacent to the plies containing the cracks. These dependences were explained by the observation that the residual properties depend partly on the crack-opening behavior and that crack opening is restrained by adjacent plies to degrees determined by the properties of those plies.

This work was done by Mark R. Garnich, David Long, Akula M. K. Venkata, John F. Fitch, and Pu Liu of the University of Wyoming for the Air Force Research Laboratory.
AFRL-0060

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