High Strain Composites Research Articles

Relaxation behavior of thin-ply high strain composites using a dynamic mechanical analyzer based column bending test

Deployable space structures made from thin-ply high-strain composite (HSC) laminates can be compactly stowed, but prolonged stowage under high curvature can alter their deployed shape due to relaxation. In this research, a dynamic mechanical analyzer-based column bending test (CBT) method, with a custom-developed fixture was used to characterize the relaxation behavior of thin-ply HSCs. Four laminate configurations were prepared from thin-ply unidirectional IM7/PMT-F7 and Astroquartz/PMT-F7 prepregs: (i) IM7/PMT-F7 [0°], (ii) Astroquartz/PMT-F7 [±45°], (iii) Flexlam [±45°/0°/±45°] and (iv) Flexlam [±45°/90°/±45°]. Surface strains of 0.5, 1.0, 1.5, and 2.0% were sequentially applied to the specimen for 100-min durations, each separated by a 1000-min recovery period. This stepped strain approach was performed at 30°C, 50°C, and 70°C. The relaxation results indicate that the fiber-dominated test configuration, UD IM7/PMT-F7 [0°] lamina, shows no measurable relaxation. However, the matrix-dominated configurations, Astroquartz/PMT-F7 [±45°] lamina and the Flexlam laminates, show measurable relaxation. The Flexlam laminates show less relaxation than the Astroquartz/PMT-F7 [±45°] lamina due to the inclusion of unidirectional IM7 carbon fiber. The result also indicates that the relaxation behavior is time and strain-dependent, not temperature-dependent.

Analysis of Geometric and Material Nonlinearity in the Column Bending Test

The column bending test (CBT) is an experimental method to measure the moment–curvature behavior and failure curvature of high-strain composites (HSCs), designed to apply a relatively constant bending moment through a sample up to very high curvatures. In this paper, we first calculate the errors involved in the simplified geometric analysis used to model the CBT by considering two different effects: variations in the bending moment through the sample and nonlinearities in the material response of the fibers. The first effect, modeled using Euler’s elastica, results in a set of design guidelines for the test geometry (i.e., length of the grips and the specimens) in order to achieve moderate errors in moment and curvature predictions. The nonlinearity in the fiber behavior (tension stiffening and compression softening) does not have a strong effect on the curvature but produces significant variations in the bending stiffness of HSCs as well as the maximum strain observed in the fibers due to the shift in the neutral axis. This indicates that accounting for the nonlinear behavior of fibers is necessary in order to accurately predict the failure properties of HSCs under bending.

Dynamic interferometric measurements of thermally-induced deformations on telescope based on high-strain composite tape-springs

Thin carbon fibre reinforced polymer (CFRP) tape-springs are attractive structures for use in space-based optical instruments because of their compact stowed form, and their high dimensional stability when deployed. In this paper we present, with examples, two inexpensive methods to assess the thermal expansion properties of tape-spring structures: one based on strain gauges to obtain coupon level values, and another based on laser interferometry for structure level measurements. The strain gauge technique is a versatile approach that exploits the thermal output characteristics of the sensors. The thermal expansion characterisation of thin-composite samples measured a longitudinal expansion of 4.44 ppm/∘C and transverse expansion 5.95 of ppm/∘C. The interferometry system is designed with a view to capturing the displacements and tilts that occur when a structure with a low thermal mass, like a tape-spring, experiences a rapid change in flux, as occurs in the space environment. The homodyne interferometer is developed for three degree-of-freedom (DoF) measurements with a resolution of 10−8 m for distances and 10−6 rad for angles. The interferometric setup is based on the classical Michelson architecture and consists of few inexpensive commercial optical components. The source is a 0.8 mW Helium–Neon laser with a wavelength of 632.8 nm. The other elements include two spherical singlets, a right-angle prism, a cubic beamsplitter and a CMOS camera. The recorded interference fringes are analysed by using an algorithm based on Discrete Fourier Transform (DFT). Spectral information on the light intensity signals can be used to determine relative displacements and tilts. The dimensional stability of an optical payload based on high-strain composites was tested. The telescope has a deployable Cassegrain design, which uses six extendable members for the separation of its secondary mirror. Axial deformations between 20–30μm along with angle variations of the order of 0.1 mrad were recorded with good repeatability.

Simulation of the column bending test using an anisotropic viscoelastic shell model

Finite element simulation of the column bending test (CBT) is carried out using shell elements with anisotropic viscoelastic section properties. The CBT is an experimental method for evaluating the bending behavior of thin-ply high strain composites (TP-HSC), to support development of deployable composite booms for space structures. A linear viscoelastic shell formulation is derived and implemented using two solution techniques: quasi-elastic (QE) and direct integration (DI). The model inputs, namely Prony series of the ABD matrix, are obtained using Mechanics of Structure Genome. The model is implemented in Abaqus as a UGENS user-subroutine. Recent CBT experiments are briefly reviewed, and a shell element model is developed including the geometrically nonlinear features of the test. Comparisons of the test and analysis results show that the model is capable of predicting most of the measured trends. Nonuniform deformation is captured and emphasized in data reduction considerations. Although relaxation can be captured with QE implementation of the viscoelastic model, to capture the residual curvature DI implementation is necessary. Residual curvature measured in the tests, but not predicted by the present model, suggests that viscoplasticity should be considered. A demonstrative study also shows the potential of material model calibration using the virtual CBT developed in this work.

Thermoviscoelastic behaviors of high-strain composites with shape memory polymers

The temperature and time-dependent behaviors of thin-walled high-strain composites including structural nonlinearity are experimentally and numerically investigated. The high-strain composites using woven fabric fibers with heat-softenable resin of shape memory polymers are fabricated. The numerical model based on the elastic properties of the woven fabric fiber considering its actual fabric geometry and the thermoviscoelastic properties of the shape memory polymer resin is developed. Experimental tests of the high-strain composite specimen are analyzed to acquire the parameters needed for its model. A thin-walled high-strain composite structure, which can be flexibly folded into an arbitrary configuration by heating, is fabricated. Furthermore, its folding and unfolding viscoelastic behaviors at elevated temperature are investigated for application in deployable space structures.

Use of Catalysts in the Synthesis of Carbon Nanocoils

Because of their unique 3 D structure, carbon nanocoils have potential applications to electromagnetic components, such as nanosolenoids and electromagnetic wave absorbers, and mechanical components, such as resonating elements and nanosprings or as a novel reinforcement in high-strain composites. The other potential application is to field emission devices. The stable synthesis of these nanocoils has been made possible by developing the catalysts containing Fe, In and Sn. Recently, the coil diameter has also been able to be controlled by the adjustment of the synthesis condition and the modification of the catalyst. The use of thin nanocoils, for example, enables us to develop the spatially-uniform field-emission-devices with the turn-on voltage as low as 30 V.