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This paper investigates the compression properties and energy-absorbing characteristics of a carbon fiber-reinforced honeycomb structure manufactured using the vacuum-assisted resin transfer molding method (VARTM). The composite core materials were manufactured using a machined steel baseplate onto which hexagonal blocks were secured. A unidirectional carbon fiber fabric was inserted into the slots and the resulting mold was vacuum bagged and infused with a two-part epoxy resin. After curing, the hexagonal blocks were removed, leaving a well-defined composite honeycomb structure. Samples were then cut from the composite cores and inspected in an X-ray computed tomography machine prior to testing. Mechanical tests on the honeycomb structures yielded compression strengths of up to 35 MPa and specific energy absorption values in excess of 47 kJ/kg. When normalized by the density of the core, the resulting values of specific strength were significantly higher than those measured on traditional core materials. The unidirectional cores failed as a result of longitudinal splitting through the thickness of the core, whereas the multidirectional honeycombs failed in a combined splitting/fiber fracture mode, absorbing significantly more energy than their unidirectional counterparts. Increasing the weight fraction of fibers served to increase the strength and energy-absorbing capacity of the core. Finally, it was also shown that introducing a chamfer acted to reduce the initial peak force and precipitate a more stable mode of failure.

This paper presents a study on the potential offered by lightweight energy-absorbing foam structures reinforced with aluminium and steel cylindrical tubes subjected to compressive crushing. Initially, attention was focused on establishing the influence of the length as well as the diameter to thickness ratio of the metal tubes on the specific energy absorption (SEA) characteristics. The metal tubes were then embedded in a range of cross-linked PVC foams to investigate how the stiffness of the foam influences the SEA of the tubes and the failure modes. The results show that aluminium tubes offer a superior energy-absorbing capability relative to their steel counterparts and the SEA of the metal tubes remains roughly constant with an increase in the tube length. The energy-absorbing capability of the two types of metal tubes increased with decreasing value of inner diameter/thickness ratio. Small D/t tubes were also embedded into a range of polymer foams and tested at both quasi-static and dynamic rates of strain. It has been shown that that the stiffness of the foam does not modify the energyabsorbing behaviour of the metal tubes. This evidence suggests that it should be possible to predict the response of an energy-absorbing system containing an array of tubes using a simple rule of mixtures approach. The output of this study provides useful data on core materials for sandwich construction.

The energy-absorbing behaviour of an aluminium honeycomb core reinforced with unidirectional and woven carbon fibre reinforced plastic composite tubes has been investigated experimentally at quasi-static rates of strain. Small diameter carbon fibre reinforced plastic tubes, with chamfered ends, were inserted into the cells of an aluminium honeycomb in order to yield a lightweight energy-absorbing material. The resulting data are compared with crushing tests on arrays of free-standing composite tubes, supported on a specially designed compression test fixture. The study continues with an investigation into size effects in the energy-absorbing response of these cellular materials, where compression tests are undertaken on four scaled sizes of reinforced honeycomb core. Crushing tests on the multi-tube arrays have shown that woven carbon fibre reinforced plastic tubes absorb significantly greater levels of energy than their unidirectional counterparts. Here, the specific energy absorption did not vary with the number of tubes in the array, with values for the woven tubes averaging 110 kJ/kg and those for the unidirectional tubes averaging 75 kJ/kg. Inserting composite tubes into aluminium honeycomb served to increase the measured specific energy absorption of the core, resulting in values of specific energy absorption of up to 100 kJ/kg being recorded in the woven-based system. Tests on four scaled sizes of core have shown that the measured SEA does not vary with specimen size, indicating that data generated on small samples can be used to represent the energy-absorbing response of larger, more representative components.

In this work, the long-term creep response of high-performance carbon fiber PEKK (CF/PEKK) composites was evaluated by performing extrapolated short-term flexural creep tests at various temperatures. The time-temperature superposition principle (TTSP) with vertical as well as horizontal shifting was used to generate master curves at reference temperatures of 120°C. Satin weave-based CF/PEKK prepregs were used to manufacture eight-layer composites via compression molding, with three different stacking sequences: (a) zero-direction [0]8 (b) cross-ply [0, 90]4 and (c) quasi-isotropic [90, −45, 45, 0]2 s. The flexural properties under three-point bending arrangement in a universal testing machine were also evaluated. A dynamic mechanical thermal analyzer (DMTA) in three-point bending mode was used to evaluate the temperature-dependent viscoelastic properties of the three types of composites. The creep and creep-recovery behavior was evaluated at 40°C, 80°C, 120°C, 160°C and 200°C. To construct a master curve, extrapolated short-term isothermal creep tests were performed from 120°C to 180°C at the intervals of 10°C. The predicted master curve represents the creep behavior of composites over more than 10 years. It was shown that the quasi-isotropic CF/PEKK composites exhibited 27% and 12% higher creep resistance at 120°C as compared to zero-direction and cross-ply laminates, respectively. Higher flexural modulus (23%) and flexural strengths (33%) were also exhibited by the quasi-isotropic CF/PEKK composites. The final thickness of quasi-isotropic laminates was 8% lower than the 0o laminates. After analyzing the cross-sections of the composites, it was proposed that the superior mechanical properties of the quasi-isotropic laminates could be due to enhanced nesting between neighboring prepreg layers during the compression molding process, which resulted in closer packing of the fibers. It has been shown that the prepreg stacking sequence could affect the creep behavior and flexural properties of the compression-molded CF/PEKK composites.

The effect of varying strain rate on the compression strength and energy absorption characteristics of a carbon fibre-reinforced plastic honeycomb core has been investigated over a wide range of loading rates. The honeycombs were manufactured by infusing an epoxy resin through a carbon fibre fabric positioned in a dismountable honeycomb mould. The vacuum-assisted resin transfer moulding technique yielded honeycomb cores of a high quality with few defects. Compression tests were undertaken on single and multiple cells and representative volumes removed from the cores in order to assess how the compression strength and specific energy absorption vary with test rate. Crushing tests over the range of strain rates considered highlighted the impressive strength and energy-absorbing response of the honeycomb cores. At quasi-static rates of loading, the compression strength and specific energy absorption characteristics of the unidirectional samples exceeded those of the multidirectional cores. Here, extensive longitudinal splitting and fibre fracture were the predominant failure mechanisms in the cores. For all three stacking sequences, the single-cell samples offer higher compression strength than their five-cell counterparts. In contrast, the specific energy absorption values were found to be slightly higher in the five-cell cores. The experiments highlighted a trend of increased compression strength with loading rate in the multidirectional samples, whereas the strength of the [0°]4 samples was relatively insensitive to strain rate. Finally, the energy absorbing capacity of all structures studied was found to be reasonably constant at increasing rates of strain.

This article investigates the energy-absorbing behaviour of lightweight foam structures reinforced with aluminium and steel cylindrical tubes. Initial testing focuses on establishing the influence of the inner diameter to thickness ratio ( D/ t) of the metal tubes on their specific energy-absorption characteristics under quasistatic compression and low velocity impact loading. Following this, individual metal tubes are embedded in a range of crosslinked PVC foams, and the specific energy-absorption characteristics of these reinforced systems are determined. The effect of increasing the number of tubes on the energy-absorbing response of the tube-reinforced structures is also studied. The crushing responses of both aluminium and steel structures are then predicted using the finite element analysis package Abaqus/Explicit, and the predictions of the load–displacement responses and the associated failure modes are compared to experimental results. Agreement between the numerical predictions and the experimental data is good across the range of structures investigated, with the model accurately predicting the compression response and failure characteristics observed in the structures. It has been shown that the stiffness of the foam does not significantly alter the energy-absorbing behaviour of the stiffer metal tubes, suggesting that the density of the foam should be as low as possible, whilst maintaining the structural integrity of the part.

The energy-absorbing response of sandwich structures with exceptionally high levels of energy absorption is investigated. The sandwich panels are produced by fixing small composite tubes onto metal facings with surface features that reflect the internal geometry of the tubing. Small diameter tubes are employed to manufacture the cores, since it is well established that the specific energy absorption (SEA) characteristics of a composite tube increase as the inner dimension (diameter or wall-to-wall) to thickness ratio decreases. Tests have been undertaken on tubular arrays based on both circular and square composite tubes. The effect of varying the areal density of the tubular array within the core was investigated by systematically increasing the number of tubes from one to nine. An examination of the composites during the crushing process indicated that all of the tubes failed in a splaying process, involving significant fracturing of fibers and longitudinal splitting. The measured values of SEA remained relatively constant in most cases as the areal density of the tubular arrangement was increased, suggesting that cores could readily be designed to absorb known levels of applied external energy. Arrays based on circular tubes offered higher energy-absorbing characteristics than their square counterparts, with values in excess of 100 kJ/kg being recorded in some cases. It is believed that these tubular sandwich structures offer potential for use in components that are subjected to extreme dynamic loading, such as those associated with impact and blast.