The research of the mechanical behavior of elastically transformable composite structures

The paper presents the computational and experimental studies of the stress-strain state of the composite flexible joint during transformation of the rod structure. A flexible joint consists of two co-acting tape springs and acts as an actuator for the self-deployment of large-scale space structures. Flexible joints that have different reinforcement patterns and shapes of cut-outs have been studied. Full-scale tests of the joint were carried out using a tailored facility that ensures its complete folding and unfolding and records joint moments for each folding angle. The strain pattern for each angle was recorded using a system of photo and video recording based on DIC (Digital Image Correlation) - VIC 3D. To identify elastic properties of the flexible joint, the materials mechanical testing was performed and tensile and compressive Young modulus and ultimate strength were determined. To reduce the volume of full-scale tests, a micromechanical model of the material taking in account the properties of reinforcement fibers, resin and weaving pattern was simulated in Digimat. Verification of micromechanical model was performed based on tensile and compressive properties. Other mechanical properties of the material were determined via virtual testing in Digimat system. Finite element modelling of joint folding and deployment processes was carried out in Ansys Workbench and LS-Dyna. Calculations were carried out for various structures of the joint based on the explicit and implicit methods. Dynamic behavior, geometric nonlinearity, progressive failure and self-contact of joints surface were taken into account in the computational model. As a result, strain pattern and maximal joint moment were determined. The computational strain and joint moment have a good agreement with the experimental data. Based on the results of the study, a comprehensive computational and experimental method to determine rational properties of flexible joints for transformable composite structures is suggested.