Investigation Of Thermomechanical Properties Of Shape Memory Polymer And Its Composite Structure

As a new class of functional materials, shape memory polymers (SMPs) provide several advantages over shape memory alloys and ceramics, such as low density, high shape-fixed strain, easy operation, tailorable critical transition temperature and part of them bio-compatible. Thus, there have great potential applications in engineering. Since heating is the primary stimulus to realize the shape fixing and recovery functions of SMPs, it is critical to have a thorough understanding of their thermomechanical behaviors. Moreover, as a class of polymeric materials, the lower stiffness is the inherit disadvantage of SMPs. Some reinforced composite structures are usually considered in practical applications. Thus, it is another interesting topic to design and research new SMP composite structures to improve their stiffness and shape recovery stress. Experimental, theoretical and numerical researches were carried out in this study to investigate thermomechanical and shape memory behaviors of SMP matrix and their composites structures. The details include the follows.Firstly, the viscoelastic properties of the SMP were experimentally investigated. In this part, uniaxial hysteresis and stress relaxation tests were performed. Mechanical parameters and viscoelastic effect of the SMP in rubbery, glassy and transition states were measured. The results provide a basis for further analysis of the mechanical response of the SMP under the thermomechanical cycle.Secondly, the shape frozen/recovery responses of the SMP were tested. Considering that it is often a thermal-mechanical coupling process in the application of the SMP, the thermal strain of the SMP near the shape transition temperature was measured. Then the shape memory properties of the SMP with different uniaxially pre-strains on conditions of free recovery and full constrains were studied. The shape memory properties of the SMP under three-point bending were also measured. The related results provided experimental basis for establishing thermodynamic constitutions of SMPs.Thirdly, a thermormechanical constitutive equation of the SMP was established. Based on the above experimental results, a new thermormechanical constitutive equation was proposed via introducing the concept of the frozen fraction from polymeric crystallization theories. The theoretical analysis results were compared with other models. Fourthly, the shape memory properties of SMP sandwiches were analyzed. The folding deformation of the SMP sandwiches were studied by the finite element method and the critical buckling theoretical solution was determined by the minimum potential energy principle. Then, theoretical analyses were performed on post-buckling responses of SMP sandwiches and their shape memory properties. The failure modes of SMP sandwiches were also discussed.Finally, the shape memory properties of SMP composite shell structures were analyzed. The folding properties of SMP composite shell structures were numerically analyzed and the influence of geometric parameters on the buckling behavior of the composite shells was studied by using the energy conservation principle. Moreover, the shape memory properties of SMP composite shells were investigated. Some discussions were presented about how to avoid the structure failure.The above results are useful to have a better understanding of the deformation mechanism of this class of functional materials and promote their engineering applications. Moreover, it is meaningful to design and research new types of SMP composite structures.