Research On Bending Properties Of Unidirectional Fiber Reinforced Shape Memory Polymer Composites

As a new kind of intelligent materials, shape memory polymers(SMPs) have many advantages like low density, large deformation and adjustable transition temperature. While keeping such advantages, fiber reinforced SMP composites, i.e. elastic memory composites(EMCs), also possess higher strength, stiffness and recovery speed. Thus, EMCs have broad application prospects in space deployable structures.The SMP matrix is softened at activating temperature and fiber can not be well supported. Fiber microbuckling tends to occur when a bending load is applied, which enables the laminate to reach higher bending strain without fracture. Two kinds of buckling modes, in-plane buckling and outof-plane buckling, are observed in former study. According to the minimum energy principle, total deformation energy for in-plane buckling mode is found to be always less than out-of-plane buckling mode. Hence, theoretically, a bent EMC laminate tends to buckle in-plane.EMC laminates can be regarded as transversely isotropic materials. The stiffness of compression area changes nonlinearly when the fibers buckle. A linear-nonlinear constitutive model which takes the critical buckling as demarcation point is derived by using energy methods. The position of the neutral strain surface is determined according to the balancing of stress. Further, relationships between the cross-section moment and bending strain are figured out. Results show that the model in this paper agrees with the experiment better than other existing models.A four-point bending finite element model of a EMC laminate based on the linear-nonlinear constitutive model is established to calculate the cross-section moment by using user-defined material(UMAT) subroutine. The finite element analysis is in good agreement with experiments and the results in small strain section are better than the theoretical model.This study can provide theoretical foundations for predicting the bending properties of EMC laminates, and promotes its application in engineering.