The AstroMesh antenna is an example of a mesh reflector with perimeter truss for use as a large aperture space antenna. A quarter model of such an antenna, with the original mesh supplemented with a continuous membrane, was analyzed in nonlinear finite element code ABAQUS. In particular, we investigated requirements for maintaining the initial parabolic shape under thermal perturbation. An RMS error was calculated for different thermal loadings as a measurement of deviation of the perturbed surface from a reference parabolic shape. Thermal contraction or expansion of the membrane boundary springs was found to offer reduced RMS error, but at the cost of higher spring reaction force. Electrostatic pressure was also applied to different membrane regions to acheive better RMS error with minimized spring reactions.
Nickel Titanium (NiTi) film shape memory alloy (SMA) is integrated with space-qualified polymer and mesh materials for potential use as deployment mechanisms and actuation of flexible space apertures. SMA thin film is successfully applied to Astromesh metal mesh, Kapton, Upilex, and CP-1 polymer films. Sputter deposition of NiTi onto the substrate is used to validate the material system process and demonstrate the NiTi deployment capability. Although successful, the relatively high processing temperatures required to crystallize NiTi onto the substrates requires care. A second approach is demonstrated that deposits NiTi onto a silicon substrate, followed by coating the NiTi with the desired polymer, e.g. CP-1. Micro-electro-mechanical (MEMS) processing steps are then used to remove the silicon substrate beneath the NiTi, thus freeing up the composite membrane (i.e. NiTi + CP-1). Using MEMS fabrication techniques, a hot-shaped small dome shape structure is shaped into the NiTi before deposition of the CP-1 polymer. Activation of the integrated SMA/CP-1 produces deformation of this composite structure without damage. The test articles demonstrate the feasibility to both grossly deploy and locally actuate space-qualified polymer materials.
The boundary element method is applied to problems of 3D piezoelectricity. The continuum equations for the mechanical and electrical behavior are combined into one governing equation for piezoelectricity. A single boundary integral equation is developed from this combined filed equation and the Green's solution for a piezoelectric medium. The Green's function and its derivatives are derived using the Radon transform, and the resulting solution is represented by a line integral which is evaluated numerically using standard Gaussian quadrature. The boundary integral equation is discretized using 8-node quadrilateral elements resulting in a matrix system of equations. The solution of the boundary problem for piezoelectric materials consists of elastic displacements, tractions, electric potentials and normal charge flux densities. The field solutions can be obtained once all boundary values have been determined. The accuracy of this piezoelectric boundary element method is illustrated with two numerical examples. The first involves a unit cube of material with an applied mechanical load. The second example consists of a spherical hole in an infinite piezoelectric body loaded by a unit traction on its boundary. Comparisons are made to the analytical solution for the cube and axisymmetric finite element results for the spherical hole. The boundary element method is shown to be an accurate solution procedure for general 3D piezoelectric materials problems.