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Composite laminates with negative Posson's ratios (i.e., auxetic composite laminates) were experimentally found to demonstrate a three-fold increase in buckling strength under uniaxial compression in comparison with the equivalent non-auxetic ones. To investigate whether the enhancement is genuinely due to the negative Poisson's ratio (i.e., the auxeticity) or merely caused by the concurrent change in the bending stiffness matrix as the composite layup changes, a novel monoclinic plate-based composite laminate approach is proposed, which for the first time, allows to isolate the auxeticity effect from the concurrent change of the stiffness matrix. Results provided theoretical proof that the auxeticity plays an active role in enhancing the critical buckling strength of layered composite structure. However, such a role is dynamically sensitive to elements in the bending stiffness matrix, especially the bending-twisting ratio and the anisotropy of the bending stiffness between the longitudinal and lateral directions. Insights are expected to provide guidance in exploiting negative Poisson's ratio for improving the stability of layered composite structures. 

This research focused on testing the effect of the negative Poisson’s ratio of a carbon fiber composite on its critical buckling load. A secondary goal was to determine the accuracy of simulation compared to the experimental results for carbon fiber composites. In order to accomplish these two goals, both simulation and experimental testing were employed. For the simulation, ABAQUS software was used to determine predicted values for the critical buckling loads of auxetic and nonauxetic composites as well as the respective nonlinear force behavior of these composites. These results were then compared to experimental results of four auxetic and four non-auxetic specimens each experiencing uniaxial compressive tests. The results of simulation and experimentation showed that the critical buckling loads, and force sustained in general, of the auxetic composites were about three times higher than those of non-auxetic composites. While it appears that the negative Poisson’s ratio has a significant impact on the buckling strength of composite materials, further testing is required to determine the effects of other factors on the critical buckling loads. Along with this, the simulation was more accurate for the auxetic composites than for the non-auxetic composites. Therefore, further testing and simulation are required to determine the limits of simulation accuracy for composite structures. 

Carbon nanotubes (CNTs), as they possess outstanding mechanical properties and low density, are considered as one of the most promising reinforcements in composite structures. Due to their capability of transferring loads, CNTs in long continuous forms such as yarns and tapes can withstand 20 times as much load as steel can do at the same weight. In this research, carbon nanotube yarns were wound onto an aluminum plate using a custom-built fixture to fabricate a unidirectional strip. Then, by brushing epoxy resin on the strip and laminating four layers, the unidirectional CNT reinforced epoxy resin composite beam specimens were produced. The mechanical properties of the unidirectional CNT-reinforced composite (CNTRC) were determined using standard tensile tests. This study presents a method for manufacturing CNTRC out of CNT yarns, determining the CNTRC’s Young’s modulus as well as the tensile strength, and obtaining its strain field via digital image correlation (DIC) method. It is observed that the pressure due to sandwiching of the aluminum plates during the manufacturing process leads to nonuniformity of the specimen in the width along midspan of the longitudinal direction which results in the specimen’s not being perfectly unidirectional. This phenomenon can cause the matrix cracking in tensile test and reduce the ultimate tensile strength up to 78% in comparison with perfectly unidirectional specimens. However, the Young’s modulus of such composites is comparable with those obtained from previously existing research. Also, Results from DIC showed the possible failure prone areas in the specimens, as it presents a up to 64% difference between the highest and lowest strain in the tensile loading direction through the specimens. This study will serve as a foundation for future research involving CNT composites, particularly the use of their high anisotropy to produce auxetic composites with large negative Poisson’s ratios. 

for thermoset-based fiber-reinforced polymer composites (FRP) in comparison with the traditional autoclave/oven-curing method, due to its rapid curing process, low energy consumption, and low cost. Optimizing the weight contents of initiators relative to the resin’s mass is needed to adjust the mechanical properties of FRPs in industrial applications. This study investigates the effect of varying the photoinitiator (PI) weight content on tensile properties and the frontal polymerization characteristics, including the front velocity, front temperature, and degree of cure, in the FP process of the epoxy resin. Specifically, a dual-initiator system, including PI and thermal-initiator (TI), is used to initiate the polymerization process by ultraviolent (UV) light. The weight content of the TI is fixed at 1 w%, and the relative PI concentration is varied from 0.2 w% to 0.5 wt%. Results show that increasing the PI amount from 0.2 wt% to 0.3 wt% significantly improves the front velocity and the degree of cure by about two times. Increasing the PI content from 0.3 wt% to 0.4 wt% results in 15% and 26% higher degree of cure and front velocity, respectively. Moreover, due to the different front velocity in the top and bottom regions of the specimen, the specimens with 0.4 wt% PI exhibited a curved shape. The specimen with 0.5 wt% PI is thermally degraded and foamed. By comparing tensile properties, it is found that increasing the PI concentration from 0.2 wt% to 0.3 wt% improves the tensile strength and Young’s modulus by 3.91% and 7%, respectively, while the tensile strength and the Young’s modulus of frontal polymerized specimens are on average 8% and 14% higher than traditionally ovencured ones, respectively. 

Carbon fiber reinforced polymer (CFRP) matrix composites have become increasingly popular across industries such as aerospace and automotive industries due to its outstanding mechanical properties and significant weight saving capability. CFRP composites are also widely known to be highly tailorable. For instance, different laminate-level mechanical properties for CFRP composites can be achieved by varying the individual carbon fiber laminar arrangements, among one of them is the Poisson’s ratio. Conventional materials have a positive Poisson’s ratio (PPR), visualize any conventional materials in a 2D block shape, when stretching that material in longitudinal direction, contraction follows on the transverse direction, whereas for materials with a negative Poisson’s ratio (NPR), stretching in the longitudinal direction leads to expansion in the transverse direction. Materials with NPRs have been shown to improve the indentation and impact resistances, when compared to equivalent materials with PPRs. However, producing NPRs could potentially compromise other properties, such as tensile properties, which has not been reported. The current work investigates the effects of NPR on the tensile properties of CFRP composites. Specifically, a laminatelevel NPR of -0.4094 in the in-plane direction is achieved through ply arrangement of CFRP composites using classical lamination theory (CLT). The non-auxetic counterpart CFRP composites are designed to produce an PPR of 0.1598 in the in-plane direction while simultaneously match their elastic moduli in three directions with those of the auxetic composites. Results show that the predicted tensile modulus and in-plane Poisson’s ratio were in excellent agreement with the experiment results. It was found that the ultimate tensile strength and failure strain or ductility of auxetic specimens were on average 40% lower than those of the conventional CFRP composites. 

Introducing auxeticity or negative Poisson’s ratio is one potential solution to mitigate the low velocity impact damage of fiber reinforced polymer matrix composites, which can be achieved by tailoring the layup of an anisotropic composite laminate. This study aims to investigate the effect of laminate-level in-plane negative Poisson’s ratio on the low velocity impact behavior of carbon fiber reinforced polymer (CFRP) matrix composites using numerical simulations. The layups of the auxetic composites that allow them to produce negative Poisson’s ratios are identified based on the Classical Lamination Theory and verified through fundamental coupon-level experimental tests. To ensure meaningful comparisons, the non-auxetic counterpart composites are designed by allowing them to produce positive in-plane Poisson’s ratio while closely matching the longitudinal effective modulus of the auxetic laminate. The simulation results indicate that the auxetic laminates suffer smaller (12.6% on average) delamination area in top and bottom interfaces, much smaller (38% on average) matrix compressive damage in the top and bottom plies, and smaller (14.6% on average) fiber tensile damage area in each ply of the laminate at relatively higher impact energies (5 and 8 J).