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Dimensional Stabilization of Composite Space Structures

Stability of terrestrial composite-material structures can be assessed for environmental effects.

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).

This T-Joint Composite-Material Structure served as a model for computational-simulation demonstration and testing of the concept of anti-distortion appliqués as means of reducing thermal distortions.
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.

AFRL-0060

Topics:
Composite materials Computer simulation Materials Optimization Polymers Research and development Simulation and modeling
This Brief includes a Technical Support Package (TSP).
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Dimensional Stabilization of Composite Space Structures

(reference AFRL-0060) is currently available for download from the TSP library.

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    Defense Tech Briefs Magazine

    This article first appeared in the April, 2008 issue of Defense Tech Briefs Magazine (Vol. 2 No. 2).

    Read more articles from the archives here.

    Overview

    The document appears to be a technical script related to finite element analysis (FEA) using a software platform, likely Abaqus, for modeling composite materials. It outlines various functions and procedures for setting up a simulation, including defining material properties, boundary conditions, loads, and mesh configurations.

    Key components of the document include:

    1. Material and Section Definitions: Functions such as lami_parti(), lami_mater(), and lami_section_1() are used to define the properties of the composite materials and their structural sections. This includes specifying the layers, orientations, and mechanical properties necessary for accurate simulation.

    2. Boundary Conditions and Loads: The script includes a section dedicated to imposing boundary conditions (lami_bc()) and applying loads (lami_load()). It specifies how the model will react to external forces, such as concentrated loads at specific vertices of the model, and how displacements are constrained.

    3. Job Setup: The lami_job() function prepares the simulation job, creating an input file for the analysis. This is crucial for running the simulation and obtaining results.

    4. Element and Node Management: The document contains code for reading input files and managing nodes and elements within the model. It initializes various parameters and arrays to store node and element data, which are essential for the simulation's computational processes.

    5. Equation Writing for Constraints: The script includes loops that write equations for edge constraints, indicating how different elements interact with each other. This is vital for ensuring that the model behaves correctly under simulated conditions.

    6. Output Management: The document specifies how results are written to output files, which is important for post-processing and analyzing the simulation results.

    Overall, the document serves as a comprehensive guide for setting up a finite element model of a composite material, detailing the necessary steps to define the model, apply loads and boundary conditions, and prepare for simulation. It emphasizes the importance of accurate material definitions and boundary conditions in achieving reliable simulation outcomes. The structured approach in the script allows for flexibility and adaptability in modeling various composite configurations, making it a valuable resource for engineers and researchers in the field of materials science and structural analysis.

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