Composite tape springs, and a related design called a storable tubular extendable member (STEM), are used in aerospace structures as deployment mechanisms for various components on satellites, e.g., solar arrays, antennae, booms. Laminates used in their designs must be thin to prevent micro-buckling when deformed into compact stowed configurations. As a result, they have matrix dominated properties that viscoelastically evolve in time and can result in deployable space structure designs with reduced deployment torque and deployed stiffness. In order to understand how to mitigate the effects of viscoelasticity in STEM designs, this dissertation explores how laminate stacking sequence and fiber orientation affect composite viscoelastic behavior and incorporates this knowledge in a method to model the performance of a particular biaxially stowed and then deployed STEM design.
To explore the effect that stacking sequence and fiber orientation have on viscoelastic behavior, tensile and four-point bend creep tests are performed on neat resin, unidirectional and plain weave lamina, and 3-ply [45PW/0/45PW] and 4-ply [0/45PW/45PW/0] laminate samples, all composed of the same epoxy resin and carbon fiber system. The results of the creep tests indicate that fibers placed in the load path, e.g., surface plies in composites under flexure loads or fibers in the direction of tensile loads, mitigate the viscoelastic creep and relaxation of the composites. The presence of elastic fibers also increases the stiffness of the laminae and laminates and reduces the degree to which they creep. The proportional viscoelastic change in the transient properties of the neat resin, lamina, and laminate, e.g., percent strain change over time at fixed stress between the initial elastic response and the equilibrium response of the material, are compared to one another. The results from indicate that the viscoelastic behavior of the resin system composing the matrix of the laminae and laminates is unchanged by the presence of fibers.
An analytical model is developed to predict the viscoelastic behavior of a STEM after prolonged deformation in a state of biaxial bending, e.g., the stowed configuration. The model predicts the extent to which biaxial bending moments relax during the period of time a viscoelastic composite STEM is subjected to a state of constant biaxial bending curvature (i.e., stowage) and then predicts the resulting recovery curvature and moments after the stowed STEM is allowed to deploy. The predictions of the model for a one-dimensional beam in bending are shown to compare favorably to the predictions of validated one-dimensional beam models found in the literature.
Knowledge of a STEM’s laminate flexural creep and relaxation properties is needed for the model to predict the biaxial bending curvature and moments of a STEM design. Since the available test data provides information on creep performance only, viscoelastic inversion methods are needed to obtain relaxation information. Several approaches are explored for converting the creep compliance and flexure properties that are ascertained by the tensile and four-point bending creep tests. A review of one-dimensional viscoelastic inversion methods such as Gutierrez-Lemini’s exact method shows them not to be applicable for inversion of the multidimensional creep properties of the anisotropic laminae and laminates tested for this dissertation. To accomplish the task of inverting the creep data, a Laplace transform based numerical method is developed for multidimensional viscoelastic materials. The creep properties for the neat resin, unidirectional and plain weave laminae, and 3-ply and 4-ply laminates are inverted using the Laplace transform based numerical inversion method. The flexure properties cannot be inverted because the axial/transverse flexure creep properties could not be measured by the four-point bend test method utilized. To acquire the relevant information, the inverted unidirectional and plain weave tensile creep properties are used as inputs for a viscoelastic version of classical laminate theory to predict the relaxation flexure properties of the unidirectional and plain weave laminae, and 3-ply and 4-ply laminates.
