A Study On Packaging Deformation Mechanism Of Elastic Memory Composites

Elastic memory composite (EMC) materials are composed of shape memory polymer (SMP) matrix and continuous fiber reinforcements. They have not only high structural properties, but also very high strain capability and shape memory characteristics. Hence, this type of functional materials has great potentials in future space deployable structures. Experimental study has confirmed that the matrix is in the rubbery state and with high compliance at elevated temperature, which makes EMC realize high packaging strain through microbuckling of compressed fibres. Therefore, microbuckling is the fundamental deformation mechanism of such functional materials.Firstly, fiber microbuckling modes are analyzed when an unidirectional EMC laminate is bent at elevated temperature. A microbuckling element volume is chosen at arbitrary bending state of EMC laminate. The strain energies of the element volume under in-plane and out-of-plane microbuckling modes are respectively calculated. Compared with the expressions of two strain energies, their difference only exist in the shear modulus and Young's modulus of soft matrix. In-plane and out-of-plane microbuckling modes respectively correspond to shear modulus and Young's modulus. Shear modulus always is less than Young's modulus for same isotropic material such as shape memory polymer at elevated temperature, so the strain energy stored in element volume due to in-plane microbuckling always is less than the one due to out-of-plane microbuckling. The real microbuckling mode should be the one that corresponds to lower energy. Hence, in-plane microbuckling mode is theoretically actual deformation mode of an unidirection EMC laminate under bending.Secondly, the known neutral strain surface location and the half wavelength expressions are substituted into the total strain energy expression of packaging EMC laminate. According to the principal of virtual work, the real total strain energy is deduced to yield the nonlinear relationships between the bending moment and curvature of the plate. Before microbuckling, the neutral strain surface locates the geometric midplane and the traditional linear elastic plate theory is suitable. The theoretical linear and nonlinear bending predictions are basically agreeable with the 4-point pure bending testing results of EMC laminates reported at elevated temperature.Thirdly, the geometric application conditions are quantitatively studied for the traditional linear elastic plate theory and the nonlinear bending predictions proposed in this paper when an unidirectional EMC laminate is bent at an elevated temperature. In a soft matrix EMC laminate, the fiber microbuckling critical compressive strain is deduced by combining a new simplified 2-D model proposed in this study with Timoshenko's beam buckling theory. The plate microbuckling critical curvature as a function of thickness can further be derived when the nominal strain of the compressed bottom edge of cross section is just equal to the fiber critical microbuckling strain. The traditional linear elastic plate theory is suitable from the start of bending to critical micrbuckling curvature and the nonlinear bending theory proposed in this paper is suitable from critical microbuckling curvature to material damage, which is most of the whole bending process.Finally, several possible failure modes of EMC laminate under bending are considered at the elevated temperature. When an unidirection EMC laminate (or rod) is bent in free-space, kink-deformation damage rather than uniform microbuckling appears under a small bending level. When an unidirectional EMC laminate is bent around a cylinder, if appropriate tensions are applied EMC laminate can prevent the kink phenomenon and produces uniform microbuckling. On this basis, the damage limits of fibers and matrix are respectively analyzed. The results show that the reinforced fibers remain elastic and undamaged. Delaminating along the fiber/matrix interface at the compressive surface layer may be the only failure mode when equivalent bending strain is greater than 5%, which are agreeable with the testing results. At the same time, the analytical results also indicate that a suitable increase of the matrix volume fraction might be a method to reduce the maximum shear strain in EMC matrix and can make EMC materials realize higher packaging strain. In addition, micro-mechanisms of deformation in 2D weave EMC laminates under bending are roughly analyzied.This study can provide a theory for accurately predicting the packaging behaviors of EMC materials above Tg and promote their application in future space deployable structure. The work is applicable, not only for EMC materials, but to any composite composed of long-fibers and soft matrix.