A research project has yielded progress on several fronts toward the goal of minimizing thermal and aging distortions of composite-material (specifically, polymer- matrix/graphite-fiber) outer-space structures that are required to retain precise dimensions and shapes. The achievements of this project are also applicable to terrestrial composite-material structures to the extent to which various environmental effects can be properly taken into account. Examples include effects of expansion caused by absorption of atmospheric moisture (similar to effects of purely thermal expansion) and effects of outgassing of volatile constituents of polymers (effects of out-gassing are more pronounced in the outer-space vacuum).
One of two main approaches followed in this project involved the concept of using exterior material additions, called "anti-distortion appliqués," as means of deliberately introducing thermomechanical distortions to offset undesired thermomechanical distortions such that the magnitudes of the resulting net thermomechanical distortions are reduced as nearly as possible to zero. A composite Tjoint structure (see figure) was selected as the subject of a demonstration computational-simulation problem, and a finite-element model was developed to characterize the thermomechanical properties of the structure. (An experimental model of the structure was also constructed but the project ended before the structure could be tested.)
In the computational simulations, initially, some assumed thermal distortions were shown to be reduced by trial-and-error positioning of anti-distortion appliqués. Subsequently, optimization software was employed to automatically adjust the design parameters of the anti-distortion appliqués so as to minimize an objective function based on distortional displacements in the structure. It was found that an effective and practical way to effect the optimization was to introduce only a few appliqués and set their location coordinates as design parameters.
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.
This Brief includes a Technical Support Package (TSP).
Dimensional Stabilization of Composite Space Structures
(reference AFRL-0060) is currently available for download from the TSP library.
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