Shape-memory Polymer Composite And Its Study Of Mechanical Foundations

Shape memory polymers (SMPs) are a new type of smart materials, they perform large reversible deformation with a certain external stimulus. The properties (e.g., stiffness, strength and other mechanically static or quasi-static load-bearing capacity) are primarily considered for conventional resin-based composite materials which are mainly used for structural materials. By contrast, the mechanical actuating performance with finite deformation is considered for the shape memory polymers and their composites which can be used for both structural materials and functional materials. For shape memory polymers and their composites, the performance of active deformation is expected to further promote the development in smart active deformation structures, such as deployable space structures and morphing wing aircraft.In this thesis, a new type of epoxy shape memory polymer, which is proposed to be used for space structures, is discussed. The author characterized its fundamental thermomechanical properties, and highlighted the performance in extreme space environments. Furthermore, the carbon-fiber-reinforced epoxy shape memory polymer composite (SMPC) was prepared and characterized. On this basis, the analysis of microbuckling deformation for SMPC was proposed. Finally, using the SMPC, a space deployable hinge was designed, fabricated, and finally an on-ground deployment process was demonstrated.First of all, an epoxy SMP was synthesized in the author s team, on this basis, the author selected a type of epoxy SMP which is better for using in aerospace. It showed excellent thermomechanical properties, large recoverable strain (50%) at glass transition temperature. The properties in extremely terrible environments in space (-100°C to +80°C temperature environment, 106Gy ofγ-ray radiation and 8.7×10-4Pa vacuum environment) was evaluated, and it basically met the space requirements for polymeric materials. Furthermore, a carbon fiber reinforced epoxy SMPC was prepared. The mechanical properties at room temperature were evaluated. The microbuckling was observed under large deflection deformation. For the shape-recovery performances, it showed good shape recovery ratio (100% for 5 cycles, 97% for 25 cycles). There was no significant decay of recovery capabilities after radiation (106Gy).Secondly, an anisotropy Ni/SMP composite was prepared. In a weak static magnetic field during the curing process of SMP, the nickel powders are aligned along the magnetic field, forming a micro-anisotropy structure. Test revealed that the alignment of nickel powders enhanced the thermomechanical properties and electrical conductivity. This chain was able to maintain its natural configuration after several times of cyclic deformation at large-strains. That is to say, it maintained the stability of electroactive performance. This Ni/SMP composite embedded with aligned nickel powders is proposed to be used for the morphing skin on the morphing aircraft.Based on the actuation of fiber reinforced SMPC in large deflection deformation, this thesis discussed its mechanism which is required for design principle for deployable structure. The local post-microbuckling is required for the unidirectional fiber reinforced composite materials, at the conditions of its large geometrical deflection. The cross section of SMPC is divided into three areas: non-buckling stretching area, non-buckling compressive area, and buckling compressive area. Three variables are considered: critical buckling position, and neutral plane, the fiber buckling half-wavelength. Considering the condition of the small strain and large displacement, the strain energy expression of the SMP/fiber system was derived. According to the minimum energy principle, the expression for all key parameters were derived, including the critical buckling curvature, neutral plane position, the buckling half-wavelength, fiber buckling amplitude, and strain. Along with the increase in shear modulus of SMP matrix, the critical curvature, critical amplitude and critical strain increased, but critical half-wavelength decreases. The key parameters of the above theoretical prediction correlated well with the experimental measurement data, which demonstrated the accuracy of the theoretical prediction. Based on the microbuckling of carbon fiber, the SMP experiences small strain, but the SMPC receives large macroscopic strain in the compressive area, and the SMPC deployable structures realized large geometrical deflection deformation.Finally, a deployable hinge fabricated by fiber reinforced SMPC was designed, fabricated, and the deployment process was demonstrated. Optimization design of a SMPC hinge structure was conducted, including thickness curvature angle of SMPC thin shell, design of the fixed side and heating methods. Then a deployable hinge was fabricated, and electroactive actuation was realized (voltage 20V, power 28 W, deployment duration 100s, and deployment angle 140°). The moment decreased linearly and the torsion stiffness was about 47 Nmm.rad-1. Under the simulated ground-based weightlessness conditions, a solar array was deployed actuated by the SMPC hinge.