An experimental study on energy absorbing structures made of fabric composites

Egg-box shaped energy absorbing structures made of fabric composites were fabricated to find out the compressive characteristics and energy absorption capacity. Various stacking sequences and boundary conditions (unconstrained and bonded) were examined to investigate the stress–strain curves during compression. Failure modes of composite egg-box panels were observed and investigated correlating each step of meaningful collapsing behaviour. In order to check out the possibility as an ideal energy absorbers foam filled composite egg-box panels were fabricated and tested. From the test results it was found that the foam filled composite egg-box panels had good energy absorption capacity with smooth stress–strain curves which resembles the ideal energy absorber. The energy absorption per unit mass of composite egg-box panels made of different types of material and stacking sequences was calculated and compared with.     

An experimental study on the effect of tow variations on compressive characteristics of plain weave carbon/epoxy composites under compressions

The effect of tow deformation on the static and fatigue characteristics of fabric composites under compression was investigated by experimental approach. Sheared specimens made of plain weave carbon/epoxy prepregs were prepared using a picture frame rig and the shear angles were 0°, 16°, 26°, 34°, 46°. To verify the effect of the tow variations of the fabric composites on compressive characteristics, the unidirectional composite specimens composed of the same fibre and matrix system with the same stacking sequences as the fabric composites were prepared for comparison. The static compressive test results showed that the static compressive strength of sheared fabric specimens was lower than that of the unidirectional specimens with the same stacking angle. On the other hand, the fatigue test results showed that fatigue life of sheared fabric specimens was higher than that of the unidirectional specimens for mild shear deformation cases. It was proved that these results were fully affected by the tow deformation caused by the shear deformation of the fabric specimens. The compression–compression fatigue behaviours of sheared fabric specimens were verified by appropriate unit-cell models.     

Compressive characteristics of foam-filled composite egg-box sandwich panels as energy absorbing structures

This paper describes compressive tests on foam-filled composite egg-box panels which were carried out to assess their performance as energy absorbers. Material type, number of plies and stacking angle were varied. Stacking sequences of CFRP [0]_nT, CFRP [45]_nT (n = 3, 4) and GFRP [0/90]_S, [±45]_S were used for foam-filled egg-box cores. Fracture modes of representative composite egg-box cores without foam, including crack initiation and propagation, were observed and analysed by multiply-interrupted compressive tests using transparent acrylic face plates on both sides of the egg-box core. The concept of premature buckling of the foam-filled egg-box panels is used to explain the small initial stress peak. Finally, the collapse curve of the core was used to estimate energy absorption capacity. It was found that the foam-filled composite egg-box sandwich panels had a good energy absorption capacity with a stable collapse response resembling the ideal energy absorber.     

Through-thickness compressive strength of a carbon/epoxy composite laminate

As carbon-fiber-reinforced composite materials are increasingly used for heavy-duty self-lubricating bearings, their through-thickness compressive strength (TTCS) has become an important parameter because the TTCS depends on the weave type and stacking sequence of laminates regardless of their tribological properties.In this work, the TTCS of a carbon/epoxy composite bearing material was measured with respect to weave type, stacking sequence, and direction of cut from the laminate. The tests showed that, for unidirectional laminate, cylindrical specimens resulted in the most reliable data of TTCS. However, for woven fabric laminate, cubic specimens with the edge length greater than twice the repeating unit gave reliable results.     

The relation between compressive strength of carbon/epoxy fabrics and micro-tow geometry with various bias angles

In this paper compressive tests of carbon/epoxy (plain weave, 3k) fabric composites were performed to investigate the relation between compressive strength and various bias angles and shear angles. Compressive properties such as chord modulus and maximum strength of the fabric composite materials are essential to analyze the drape behaviour and estimate the quality of the final products. Various specimens with different bias and shear angles which were fabricated by using autoclave de-gassing moulding process were prepared to estimate the strength and chord modulus with respect to the bias and shear angle variations. The stacking sequences of the compressive test specimens are [0]_10T, [15]_10T, [30]_10T and [45]_10T for bias specimens and [±37]_10T, [±32]_10T, [±28]_10T, [±22]_10T for sheared specimens. Micro-tow structures were observed to correlate the fabric compressive strength with crimp angle. Anti-buckling rig was involved to prevent specimens from buckling during the compressive tests. The compressive test was performed with 1.3 mm/min strain rates. Compressive test results were compared with calculation results. Facture modes which were classified in two different modes were analyzed using microscopic observation.     

Energy absorption in composite stiffeners

The energy absorption behaviour of composite stiffeners subjected to axial compression has been investigated. A semi-empirical analysis methodology has been developed for prediction of the energy absorption capability of composite stiffeners based on crush tests of flat plate specimens and an understanding of the fundamentals of the energy absorption process. Flat plate, angle and channel specimens were fabricated from T650-35/F584 graphite/epoxy plain-weave fabric using five different lay-ups that consisted of varying percentages of 45° and 0° plies. The specimens were crush tested under axial compression, and measured levels of sustained crushing stress were compared with model predictions.     

Modelling of composite sandwich structures with honeycomb core subjected to high-velocity impact

In this study the perforation of composite sandwich structures subjected to high-velocity impact was analysed. Sandwich panels with carbon/epoxy skins and an aluminium honeycomb core were modelled by a three-dimensional finite element model implemented in ABAQUS/Explicit. The model was validated with experimental tests by comparing numerical and experimental residual velocity, ballistic limit, and contact time. By this model the influence of the components on the behaviour of the sandwich panel under impact load was evaluated; also, the contribution of the failure mechanisms to the energy-absorption of the projectile kinetic energy was determined.     

A numerical study of ply orientation on ballistic impact resistance of multi-ply fabric panels

This paper presents a detailed finite element (FE) analysis aiming to investigate numerically the impact deformation of multi-ply fabric panels with angled plies. The purpose of the investigation described in this paper is to study numerically the way in which the multi-ply panels deform and to identify the energy absorption in different panel constructions. The FE model was created using ABAQUS to simulate the transverse impact of a projectile onto various woven fabric panels. Influencing factors such as the impact velocity, panel construction and the number of plies are taken into account in the FE simulations. The numerical predictions show that the orientation of plies significantly affects the energy-absorbing capacity of the multi-ply fabric panels. The angled panels always increase the energy-absorbing capacity, compared with the aligned panel, by as much as 20%, depending on the number of plies in the panel. In addition, the stacking sequence of oriented plies also plays an important role in absorbing the energy. For the multi-ply fabric panel with large numbers of plies, there is an optimised sequence of plies which can maximise the energy-absorbing capacity of the panel. An important aspect of the work is validation of the numerical technique. It is shown that the FE predictions are highly consistent with the experimental study [1].     

Energy absorption and failure response of silk/epoxy composite square tubes: Experimental

This paper focuses on natural silk/epoxy composite square tubes energy absorption and failure response. The tested specimens were featured by a material combination of different lengths and same numbers of natural silk/epoxy composite layers in form of reinforced woven fabric in thermosetting epoxy resin. Tubes were compressed in INSTRON 5567 with a loading capacity of 30 kN. This research investigates the influence of the wall lengths on the compressive response and also failure mode of the tested tubes are analysed. The load–displacement behaviour of square tubes recorded during the test. Since natural woven silk has been used as textile in centuries but due to rare study of this fabric as reinforcement material for composites, the results of this paper can be considerable. Outcomes from this paper might be helpful to guide the design of crashworthy structures.     

LOW VELOCITY PERFORATION BEHAVIOUR OF POLYMER COMPOSITE SANDWICH PANELS

The paper describes low-velocity impact tests on square panels made from two polymer composite sandwich constructions, namely woven glass vinyl ester skins with Coremat core and woven glass epoxy pre-preg skins with honeycomb core. The impact velocity was up to 8 m s^-1 with an impact mass of up to 30 kg giving a maximum impact energy of 882 J. This maximum energy gives full perforation of the panels. The panels were 0.5 m by 0.5 m with clamped but free to pull in boundary conditions. The impactor geometry considered was a 50 mm diameter hemisphere. Results are expressed in the form of energy and failure mode plots and it is shown that the energy absorbing capabilities of the panels increase with the velocity of impact. The increase in energy absorption is attributed to an increase in the core crush stress and skin failure stress at high strain rates. Some discussion is given on the influence of the energy absorbing capabilities of constituent materials on the overall energy absorption behaviour of the panel. Suggestions have also been made for increasing panel perforation energy.     

Crashworthy characteristics of axially statically compressed thin-walled square CFRP composite tubes: experimental

In this paper the results of experimental works pertaining to the crash behaviour, collapse modes and crashworthiness characteristics of carbon fibre reinforced plastic (CFRP) tubes that were subjected to static axial compressive loading are presented in detail. The tested specimens were featured by a material combination of carbon fibres in the form of reinforcing woven fabric in thermosetting epoxy resin, and they were cut at various lengths from three CFRP tubes of the same square cross-section but different thickness, laminate stacking sequence and fibre volume content. CFRP tubes were compressed in a hydraulic press of 1000 kN loading capacity at very low-strain rate typical for static testing. The influence of the most important specimen geometric features such as the tube axial length, aspect ratio and wall thickness on the compressive response and collapse modes of the tested tubes is thoroughly analysed. In addition, the effect of the laminate material properties such as the fibre volume content and stacking sequence on the energy absorbing capability of the thin-wall tubes is also examined. Particular attention is paid on the analysis of the mechanics of the tube axial collapse modes from macroscopic and microscopic point of view, emphasizing on the mechanisms related to the crash energy absorption during the compression of the composite tubes.     

Effect of silica nanoparticles on compressive properties of an epoxy polymer

The effect of nanosilica on compressive properties of an Epikote 828 epoxy at room temperature was studied. A 40 wt% nanosilica/epoxy masterbatch (nanopox F400) was used to prepare a series of epoxy based nanocomposites with 5–25 wt% nanosilica content. Static uniaxial compression tests were conducted on cubic and cylindrical specimens to study the compressive stress–strain response, failure mechanisms and damage characteristics of the pure and nanomodified epoxy. It was found that the compressive stiffness and strength were improved with increasing nanosilica content without significant reduction in failure strain. The presence of nanosilica improved ductility and promoted higher plastic hardening behaviour after yielding in comparison with the unmodified resin system. This result suggested that nanoparticles introduced additional mechanisms of energy absorption to enhance the compressive properties without reducing the deformation to failure.     

Performance analysis of fibre metal laminated thin conical frusta under axial compression

The crushing behaviour and energy absorption capacity of frustrated conical shells made-up of bare aluminium (AC) and E-glass fibre/epoxy resin composite overwrapped aluminium (CWAC) was studied under quasi-static axial compression condition. Using spinning process, the hollow frustrated conical specimens were fabricated with the help of wooden conical shaped mandrill with semi apical angles of 16° and 21°. Thin commercial aluminium sheets of average thickness 0.87 mm were obtained for making aluminium conical specimen. CWAC frusta were fabricated by wrapping glass fibre/epoxy resin over aluminium conical shell to form hybrid composite with required thickness by hand layup process. Quasi-static axial compression load was applied over top end of the specimen with cross head speed as 2 mm/min using Universal Testing Machine (UTM). From the experiment results, the load–deformation characteristics of different AC and CWAC frusta were investigated. Energy absorption capacities or crashworthiness and mode of collapse of all models of AC and CWAC are determined from load–deformation curve and the same was validated with finite element analysis package ABAQUS®.     

Self‐sensing damage in CNT infused epoxy panels with and without glass‐fibre reinforcement

The objective of the study is to utilise a material's inherent electrical conductivity as means of damage quantification and damage location detection. After determining the percolation threshold for a carbon nanotube (CNT)‐epoxy mixture, an optimum concentration was chosen to infuse it into glass‐fabric reinforced panels to make them electrically conductive. Two different multiwalled CNT‐epoxy composites were manufactured for this study: CNT enhanced epoxy resin and glass‐fabric reinforced CNT epoxy resin. Epoxy resin‐based glass‐fabric reinforced composite panels enhanced with carbon nanotubes were manufactured with embedded electrodes and then subjected to damages. Rectangular panels of various proportions were studied. Disks made out of copper foil were affixed to surfaces of CNT epoxy panel, whereas in glass‐fabric CNT epoxy specimen, total of 64 electrodes (grid of 8 × 8) were embedded inside the composite panel under the top layer of the 10‐ply fabric. The disks acted as electrodes, enabling voltage measurements using in‐line 4‐probe technique, which minimises contact resistance between the electrodes and the material. Two different configurations of electrode network were employed to scan voltage change in the entire composite panel. The networks included evenly spaced (3 in. for inner ones) electrodes that spanned the surface of the panel. To further investigate influence of electrodes distribution, finite element simulations were used to solve the electrical potential distribution in the panel for various damage sizes and location. Predamage and postdamage voltage field was used as gauge in sensing the damage and its extent for quantification. The finite element method simulation results matched the experimental data closely. The results indicate that there is a consistent behaviour that can be correlated to the size and location of the damage. As spacing between electrodes is increased, they become less responsive to smaller damages. Forty‐eight electrodes (out of 64) were actively used and were enough to confirm that the method can be used as an alternative to electrical tomography method where fewer (boundary) electrodes per area are employed but at a higher cost of computational cost. One important aspect of this study with embedded and distributed electrodes is the fact that the method can be applied to larger panels increasing its utility in practical applications.     

Quasi-static indentation tests on aluminium foam sandwich panels

Mechanical response and energy absorption of aluminium foam sandwich panels subjected to quasi-static indentation loads were investigated experimentally. These sandwich panels consisted of two aluminium face-sheets and a closed cell aluminium foam core (ALPORAS®). Quasi-static indentation tests were conducted using an MTS universal testing machine, with sandwich panels either simply supported or fully fixed. Force–displacement curves were recorded and the total energy absorbed by sandwich panels was calculated accordingly. Videos and photographs were taken to capture the deformation of top face-sheets, foam cores and bottom face-sheets. Effects of face-sheet thickness, core thickness, boundary conditions, adhesive and surface condition of face-sheets on the mechanical response and energy absorption of sandwich panels were discussed.     

Characterization of interlaminar fracture behaviour of woven fabric reinforced polymeric composites

An experimental study was conducted to characterize the interlaminar fracture behaviour of 2-D woven fabric reinforced epoxy composites under mode I loading using double cantilever beam tests. A large displacement, small strain non-linear beam model was used to calculate the interlaminar fracture toughness. The fabrics used included fibreglass and Kevlar woven structures with different weave patterns. An attempt was made to enhance the composite interlaminar toughness by adding different types of microfibres into the matrix. Toughening mechanisms of the composites were analysed using scanning electron microscopy. It was found that the weave patterns of fabrics exhibited a strong influence on the interlaminar fracture behaviour, and that the addition of the microfibres to the epoxy matrix could improve the interlaminar fracture toughness significantly.     

Investigation of the behaviour of a chopped strand mat/woven roving/foam-Klegecell composite lamination structure during Charpy testing

This paper presents the investigation of the characteristic behaviour of polymer matrix composites under Charpy impact conditions with different design configurations of the laminate structure. The aim of this study was to evaluate the capability of different lamination designs for composite materials, in term of contact load, energy absorption, deflection and damage behaviour. In this study, laminated panels were fabricated using chopped strand mat (CSM), woven roving fabric (WR) and foam-PVC Klegecell as reinforcement with a combination of epoxy or polyester resin, respectively. Structural panels of composite laminates were produced using a hand lay-up technique. Each configuration design was impact tested to failure. Finite element analyses (FEA) were employed in this study to correlate the experimental value of energy absorption with simulation results. The characteristics of different reinforcement types, matrix type, hybrid type, architecture and orientation type were studied. These characteristics need to be considered, due to their affecting the characteristic behaviour of the composite lamination structures. Based on the results, it was found that differences in configuration design of the lamination structure of the polymer matrix composites do influence the strength and weakness of the materials.     

Rubber Impact on 3D Textile Composites

A low velocity impact study of aircraft tire rubber on 3D textile-reinforced composite plates was performed experimentally and numerically. In contrast to regular unidirectional composite laminates, no delaminations occur in such a 3D textile composite. Yarn decohesions, matrix cracks and yarn ruptures have been identified as the major damage mechanisms under impact load. An increase in the number of 3D warp yarns is proposed to improve the impact damage resistance. The characteristic of a rubber impact is the high amount of elastic energy stored in the impactor during impact, which was more than 90% of the initial kinetic energy. This large geometrical deformation of the rubber during impact leads to a less localised loading of the target structure and poses great challenges for the numerical modelling. A hyperelastic Mooney-Rivlin constitutive law was used in Abaqus/Explicit based on a step-by-step validation with static rubber compression tests and low velocity impact tests on aluminium plates. Simulation models of the textile weave were developed on the meso- and macro-scale. The final correlation between impact simulation results on 3D textile-reinforced composite plates and impact test data was promising, highlighting the potential of such numerical simulation tools.     

Progressive failure analysis of 2/2 twill weave fabric composites with moulded-in circular hole

Micromechanical three-dimensional finite element models of 2/2 twill weave T300 carbon/epoxy woven fabric composite panels with moulded-in circular hole are established for stress analysis. In these models, the streamline equation is used as a shape function to simulate the fibre configuration. A progressive failure analysis together with a newly developed ‘maximum notched strength method’ are also proposed to predict the failure modes and notched strengths of the fibre dominated laminate with moulded-in hole. Perforated specimens of different hole sizes are prepared using a special procedure. Tension tests are performed to evaluate the stress–strain and failure characteristics. An increase in tensile strength with increasing hole size is observed within the experimental data range. Numerical results from progressive failure analysis provide good prediction to the failure phenomena of the fractured specimens. The notched strengths from the proposed numerical procedure are slightly higher than the experimental results.     

Development of a satellite structure with the sandwich T-joint

In this study, a monocoque satellite structure composed of many composite sandwich panels, which consist of two carbon fiber/epoxy composite faces and an aluminum honeycomb core, was designed to reduce structural mass and to improve static and dynamic structural rigidity. To join composite sandwich panels with T-shape joints, a new I-shape side insert, which was fixed inside the composite sandwich panel edge with film adhesive, was suggested. The composite sandwich panels were assembled with bolts using the through-the-thickness insert and the I-shape side insert. The flatwise tensile and compressive tests of the composite sandwich panels were performed with respect to the bonding pressure between the composite face and the aluminum honeycomb core to achieve an optimal bonding pressure. To investigate the joint characteristics of the composite faces and the I-shape side insert, cleavage peel tests were performed with respect to the bonding thickness. Also, a finite element model of the composite sandwich T-joint with the I-shape side insert was developed from experimental results of the impulse response tests and composite sandwich T-joint static tests. From the finite element analysis, the structural reliability of the monocoque composite sandwich satellite structure was verified.