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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. 

Abstract Mechanical metamaterials with negative Poisson’s ratio (NPR) have emerged as a novel class of engineering material, and have attracted increasing attention in various engineering sectors. Most studies available on the buckling problem of laminated plates with positive or NPR are those under uniaxial compression. Here, we report that the buckling phenomenon may occur for auxetic nanocomposite laminated plates under uniaxial tension when the unloaded edges of the plates are immovable. Two types of nanocomposites are considered, including graphene/Cu and carbon nanotube/Cu composites. Governing equations of the auxetic nanocomposite laminated plates are formulated based on the framework of Reddy’s higher-order shear deformation theory. In modeling, the von Kármán nonlinear strain–displacement relationship, temperature-dependent material properties, thermal effects, and the plate–substrate interaction are considered. The explicit analytical solutions for postbuckling of auxetic nanocomposite laminated plates subjected to uniaxial tension are obtained for the first time by employing a two-step perturbation approach. Numerical investigations are performed for tension buckling and postbuckling behaviors of auxetic nanocomposite laminated rectangular plates with in-plane NPR rested on an elastic substrate under temperature environments. 

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. 

Abstract Rapid and scalable production of high‐performance composites remains a key challenge in achieving sustainable manufacturing. Here, Exo‐press frontal polymerization (EPFP), a novel and transformative method for manufacturing fiber‐reinforced thermoset polymer composites, overcoming energy efficiency, scalability, and curing complex geometries, is introduced. Unlike conventional curing methods that require prolonged processing times and high energy, EPFP utilizes exothermic heat to reduce curing time from hours to minutes with minimal external energy. Combining exothermic heat with press molding, the novel EPFP enables the efficient fabrication of complex geometries, such as airfoil skin sections, with high fiber volume fractions (above 60%). In addition, EPFP is compatible with commercial off‐the‐shelf epoxy by integrating frontal resin, showcasing its versatility and adaptability for diverse industrial applications. Composites manufactured using EPFP exhibit superior thermomechanical properties while significantly reducing energy consumption by 80% and production costs by 40%. This makes it a sustainable and efficient solution for polymer composites manufacturing. 

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. 

Due to the incapability of one-dimensional (1D) and two-dimensional (2D) models in simulating the frontal polymerization (FP) process in laminated composites with multiple fiber angles (e.g., cross-ply, angle-ply), modeling a three-dimensional (3D) domain, which is more representative of practical applications, provides critical guidance in the control and optimization of the FP process. In this paper, subroutines are developed to achieve the 3D modeling of FP in unidirectional and cross-ply carbon fiber laminates with finite element analysis, which are validated against the experimental data. The 3D model is employed to study the effect of triggering direction in relevance to the fiber direction on the FP process, which cannot be studied using traditional 1D/2D models. Our findings suggest that triggering in the fiber direction leads to a higher front velocity, in comparison to cases where front was triggered in the direction perpendicular to the fiber. Moreover, the average front velocity in cross-ply laminates is on average 20~25% lower than that in unidirectional laminates. When triggered using two opposite fronts in the in-plane direction, the maximum temperature of the thermal spike in the cross-ply laminate, when two fronts merge, is about 100 °C lower than that in the unidirectional laminate. In cross-ply laminates, a sloped pattern forms across the thickness direction as the front propagates in the in-plane direction, as opposed to the traditionally observed uniform propagation pattern in unidirectional cases. Furthermore, the effect of thermal conductivity is studied using two additional composite laminates with glass (1.14 W/m·K) and Kevlar fibers (0.04 W/m·K). It is shown that the frontal velocity, degree of cure, and the thermal spike temperature decrease as the thermal conductivity reduces. 

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. 

Auxetic materials are those that exhibit negative Poisson’s ratios. Such a unique property was shown to improve the indentation and impact resistances. Angle-ply composite laminates can be designed to produce negative Poisson’s ratio at the laminate level due to the large anisotropicity of the individual layer and the strain mismatch between adjacent layers. This paper investigates the effect of through-thickness negative Poisson’s ratio on the low velocity impact behaviors of carbon fiber reinforced polymer matrix composite laminates, including the global impact behaviors, as well as the delamination, and the fiber and matrix damage. Results from numerical investigations show consistently reduced fiber and matrix tensile damage in the auxetic laminate in all plies, in comparison to the non-auxetic counterpart laminates (up to 40% on average). However, the auxetic laminate does not present a clear advantage on mitigating the delamination damage or the matrix compressive damage. 

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.