Mechanical Characterization and Modeling of Composite Materials During Compression and Actuation

Composites have a wide range of applications due to their excellent mechanical properties. In order to investigate the mechanisms of the recently developed composites, it is critical to characterize and model the performance of such composites for load-bearing applications. The high tensile strength of polymer matrix composites is mainly derived from the high strength of the carbon fibers embedded in the matrix. Fibers typically have high strength in tension. However, their compressive strengths are generally much lower than tensile strengths due to weak fiber/matrix interfacial shear strength. A new approach was recently developed to overwrap an individual carbon fiber with carbon nanotube (CNT) sheet, which is subsequently impregnated into a polymer matrix to enhance the interfacial shear strength and therefore increase the compressive strength without degrading the tensile properties of the carbon fibers. A theoretical model is established to identify the appropriate thickness of the interphase region formed by CNTs embedded in matrix. Fibers are modeled as an anisotropic elastic material, and the interphase region and matrix are considered as isotropic. A micro-buckling problem is considered to take into account of the unidirectional composite under elastic micro-buckling. The formulated problem is solved numerically and the results are compared with finite element simulations for verification. It is determined that the critical load at the onset of buckling is lower in an anisotropic carbon fiber composite than in an otherwise isotropic composites due to lower transverse properties in anisotropic fibers. An optimal thickness for the CNT and matrix is determined and this finding will provide a guidance in the manufacture of composites using aligned CNTs as fillers in the interphase region. The other type of composite is made of CNT yarn and polymer, which can be used as artificial muscles. The topology of this type of artificial muscle is polymer coated on a twisted or coiled CNT core, which can provide higher performance than the muscles made of guest-filled, twisted and coiled CNT yarns. However, the mechanisms of torsion and tension of the two artificial muscles are unclear. A theoretical model considering the torque balance between the polymer and yarn, both before and after twisting actuation is established to predict the torsional stroke. The theory captures the two primary mechanistic contributions to the torsional actuation of a polymer coated CNT yarn artificial muscle, where both polymer swelling and softening combine to partially release elastically stored torsional energy in the core yarn. This theory shows that while a low polymer thickness to core diameter ratio limits the capability of the polymer to compress the core and maintain the initially inserted twist before actuation, a very high polymer thickness to core diameter ratio provides less release of such inserted twist after the polymer coated CNT yarns is actuated. Consequently, there is an optimum polymer thickness to core diameter ratio that maximizes torsional stroke. We next theoretically predict the stress dependence of tensile stroke and contractile work capacity for coiled polymer coated CNT yarns and polymer infiltrated CNT yarns for isobaric actuation. Another theoretical model established is for nylon artificial muscle. The torsional nylon artificial muscle is fabricated by twisting nylon wire into coil spring. When subjected to temperature fluctuations, nylon muscle fiber can sustain 300 000 heating-cooling cycles rotation to spin a magnet rotor in three phase coil. In this process, the rotor’s mechanical energy transfers to electrical energy. The theoretical model captures the entire process by considering all the torques acted on the magnet, hence temperature, electrical field will be incorporated. Transfer efficiency and parameters effect on kinetic energy are studied. Numerical results show that the kinetic energy increases with the artificial muscle diameter, which is consistent with experimental observation. Although the kinetic energy increase with the increase of magnet mass and radius, the torsional speed decreases as the magnet mass and radius become smaller. Hence there exist an optimum magnet mass and radius to maximize kinetic energy.