In this paper, theoretical and experimental studies are carried out to investigate the damping characteristics of carbon nanotube (CNT) reinforced epoxy nanocomposite beams. A modified Biot model is proposed to simulate the frequency-dependent damping characteristics of CNT reinforced epoxy nanocomposites and implemented into the finite element model for composite beams. The natural frequencies and modal damping ratios of CNT reinforced epoxy beams are predicted using the proposed finite element model. Then, CNT reinforced epoxy beam specimens are fabricated, upon which dynamic mechanical analysis and vibration tests are carried out. Comparison studies between theoretical and experimental results show that modified Biot model proposed here is more accurate than the original one in predicting loss factors of CNT reinforced epoxy nanocomposites. It is revealed that the damping ratios associated with the first three vibration modes of composite beam specimens initially increase and then decrease with the increment of CNT weight ratio. The first order damping ratio with 0.4 wt% CNT reinforcement has the maximum value of 0.591%, which is improved by 41% compared with that of pure epoxy.

Damping characteristics of carbon nanotube reinforced epoxy nanocomposite beams

This article describes an experimental study of the impact behaviour of a novel composite corrugated core sandwich structure under low-velocity impact. Influences of geometric parameters of the novel sandwich core, such as thickness, height, and short span length were studied. Four different configurations of composite corrugated core sandwich structures were prepared and tested. The impact events were monitored using a high-speed camera and measured by an impact force transducer and accelerometer attached to the impactor. The results revealed that the increase of core thickness improved impact capacity while the increase of core height decreased sandwich strength by increasing elastic deformation. The damage status of the novel composite core sandwich was simulated and insight into the damage mechanism was gained with finite element analysis. This study proposes an improved numerical model by incorporating the effect of the impactor head which was able to predict the impact capacity to within 10% variation of the experimental results. The results also identified that the multi-cell composite corrugated core increased the impact capacity due to the continuity of the fibres between adjacent cells. Moreover, the trapezoidal composite corrugated sandwich core showed higher specific strength compared to traditional honeycomb, truss and foam cores.

Behaviour of continuous fibre composite sandwich core under low-velocity impact

It is challenging to maintain the lightweight performance of sandwich structure while at the same time to obtain high bending stiffness and strength. In this study, sandwich beams with hierarchical honeycomb cores were proposed. Corresponding mechanical responses bsubjected to three-point bending load were explored and compared with that of the sandwich equipped with conventional honeycomb core by means of numerical simulation. Theoretical solutions of equivalent bending stiffness coincide well with numerical results. Scaling effect of vertex cell and cell size on the hierarchical honeycomb sandwich was investigated. Two tailored hierarchical patterns were proposed by replacing the hexagonal vertex cells with circular and square cells. Compared with the conventional counterpart, the hierarchical sandwich beams offer higher load-bearing capacity. Distinctions of bending strength of each configuration demonstrate different influences of tailored hierarchical cores on enhancing bending resistance ability. There are more local interactions of cells for hierarchical structures. The local improvement promotes more cells to withstand load and the whole sandwich beam can be globally strengthened.

Bending behavior of sandwich beam with tailored hierarchical honeycomb cores

Crashworthiness index of honeycomb sandwich structures under low-speed oblique impact

In this paper, the orthotropic damping behavior and mechanism of Nomex honeycomb composites are investigated. The needed specimen sizes for the measurement of the frequency-dependent transverse shear moduli (TSM) and fundamental damping coefficients of the honeycomb cores were analyzed at first. Then, the effects of cell side length and beam orientation on the orthotropic damping properties were explored. The results reveal that relatively high TSM (GLT) and damping values (ηWT) can be obtained by decreasing the cell side length without adding any additional weight. The analytical results of the damping mechanism indicate that the difference in the damping contributions of the interfacial phase to the honeycomb core in different directions leads to the orthotropic damping behavior of the honeycomb core. This study is helpful to guide the TSM measurement and structure design of honeycomb composites.

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Frequency-dependent orthotropic damping properties of Nomex honeycomb composites

This study investigates numerically and experimentally the effect of longitudinal ribbon reinforcement in conventional honeycomb structure on the various dynamic characteristics of a hybrid composite sandwich plate. Longitudinal strips are considered to be embedded along the transverse directions at various locations of a honeycomb core material without and with carbon nanotubes (CNT) reinforcement in a composite sandwich plate. The governing differential equations of motion of the various hybrid honeycomb composite sandwich plate configurations with strips inserted in conventional honeycomb structures as core layer having top and bottom face composite layers are derived using higher order shear deformation theory (HSDT) and solved numerically using a four noded rectangular finite element. Further, experimental investigations are performed by fabricating the various strips embedded hybrid honeycomb core material with and without CNT reinforcement to evaluate the shear and loss moduli using the alternative dynamic approach. The efficacy of the developed finite element formulation is demonstrated by comparing the experimental and numerical results obtained in terms of natural frequencies and loss factors for the various prototypes of honeycomb composite sandwich plates. Also, the effect of variation in CNT content and thickness of longitudinal ribbon reinforcement in conventional honeycomb core material and ply orientation of the face sheets on the various dynamic properties of hybrid composite structures are studied under various end conditions. This study proposes that the ribbon reinforcement location and the addition of CNTs in honeycomb core materials strongly influence the stiffness, damping and transverse vibration displacements of the ribbon reinforced hybrid composite sandwich plates.

A. Paul Praveen

Papers

Assessment of dynamic properties of hybrid ribbon reinforced multifunctional composite sandwich plates: Numerical and experimental investigation