Three-point bending of sandwich beams with aluminum foam-filled corrugated cores

Sandwich panels having metallic corrugated cores had distinctly different attributes from those having metal foam cores, the former with high specific stiffness/strength and the latter with superior specific energy absorption capacity. To explore the attribute diversity, all-metallic hybrid-cored sandwich constructions with aluminum foam blocks inserted into the interstices of steel corrugated plates were fabricated and tested under three-point bending. Analytical predictions of the bending stiffness, initial failure load, peak load, and failure modes were obtained and compared with those measured. Good agreement between analysis and experiment was achieved. Failure maps were also constructed to reveal the mechanisms of initial failure. Foam insertions altered not only the failure mode of the corrugated sandwich but also increased dramatically its bending resistance. All-metallic sandwich constructions with foam-filled corrugated cores hold great potential as novel lightweight structural materials for a wide range of structural and crushing/impulsive loading applications.     

Sandwich panels with Kagome lattice cores reinforced by carbon fibers

Stretching dominated Kagome lattices reinforced by carbon fibers were designed and manufactured. The sandwich panels were assembled with bonded laminate skins. The mechanical behaviors of the sandwich panels were tested by out-of-plane compression, in-plane compression and three-point bending. Different failure modes of the sandwich structures were revealed. The experimental results showed that the carbon fiber reinforced lattice grids are much stiffer and stronger than foams and honeycombs. It was found that buckling and debonding dominate the mechanical behavior of the sandwich structures, and that more complaint skin sheets might further improve the overall mechanical performance of the sandwich panels.     

Wet-sand impulse loading of metallic plates and corrugated core sandwich panels

We have utilized a combination of experimental and modeling methods to investigate the mechanical response of edge-clamped sandwich panels subject to the impact of explosively driven wet sand. A porthole extrusion process followed by friction stir welding was utilized to fabricate 6061-T6 aluminum sandwich panels with corrugated cores. The panels were edge clamped and subjected to localized high intensity dynamic loading by the detonation of spherical explosive charges encased by a concentric shell of wet sand placed at different standoff distances. Monolithic plates of the same alloy and mass per unit area were also tested in an identical manner and found to suffer 15-20% larger permanent deflections. A decoupled wet sand loading model was developed and incorporated into a parallel finite-element simulation capability. The loading model was calibrated to one of the experiments. The model predictions for the remaining tests were found to be in close agreement with experimental observations for both sandwich panels and monolithic plates. The simulation tool was then utilized to explore sandwich panel designs with improved performance. It was found that the performance of the sandwich panel to wet sand blast loading can be varied by redistributing the mass among the core webs and the face sheets. Sandwich panel designs that suffer 30% smaller deflections than equivalent solid plates have been identified.     

Impact damage characterisation of carbon fibre/epoxy composites with multi-layer reinforcement

Low-velocity impact tests were performed to investigate the impact behaviour of carbon fibre/epoxy composite laminates reinforced by short fibres and other interleaving materials. Characterisation techniques, such as cross-sectional fractography and scanning acoustic microscopy, were employed quantitatively to assess the internal damage of some composite laminates at the sub-surface under impact. Scanning electron microscopy was used to observe impact fractures and damage modes at the fracture surfaces of the laminate specimens. The results show that composite laminates experience various types of fracture; delamination, intra-ply cracking, matrix cracking, fibre breakage and damage depending on the interlayer materials. The trade-off between impact resistance and residual strength is minimised for composites reinforced by Zylon fibres, while that for composites interleaved by poly(ethylene-co-acrylic acid) (PEEA) film is substantial because of deteriorating residual strength, even though the damaged area is significantly reduced. Damages produced on the front and back surfaces of impact were also observed and compared for some laminates.     

Effects of the environmental conditions on the mechanical behaviour of the corrugated cardboard

Corrugated cardboard is very sensitive to atmospheric conditions. The aim of this work is to study the effects of these parameters, in fact the relative humidity (RH), on the mechanical behaviour of a sandwich structure of the corrugated cardboard type. For that, tensile tests were used under various rates of relative humidity. In high rates of moisture, the instrumentation must be adapted. A preliminary comparison of several measurement methods of the mechanical properties was carried out. It made it possible to validate the method of strain measurement using the measurement of the displacement which requires a minimal handling of the specimen. During this study, the effect of the water content on the mechanical properties is presented. Moreover, the viscoelastic character coupled with the damage of this material is high lighted.     

Quasi-isotropic bending responses of metallic sandwich plates with bi-directionally corrugated cores

In order to reduce anisotropic behaviors of sandwich plates with open channel cores under the bending load, bi-directionally corrugated cores were introduced. Bi-directionally corrugated core has two additional design parameters related with a corrugation pass than uni-directionally corrugated core, so that its properties with respect to core orientations can be controlled. Sandwich plate with bi-directionally corrugated core is designed optimally so that beam buckling of face sheets is reduced drastically and anisotropic buckling behavior in the face sheets is minimized. The cores fabricated by a sectional forming process were bonded with face sheets by adhesive bonding. Three-point bending experiments were carried out with respect to core orientations. It has been shown from the experiments that sandwich plates with bi-directionally corrugated cores exhibit quasi-isotopic bending behaviors and structural performances in sandwich plates.     

A parametric study on compressive characteristics of Wire-woven bulk Kagome truss cores

Compressive behaviors of the Wire-woven bulk Kagome (WBK) cores fabricated of stainless wires are experimentally investigated. Effects of geometrical parameters such as the wire diameter, strut length and number of layers on the compressive behaviors are studied. Also, behaviors of two different types of the specimen configurations, namely, pointed faced and flat faced specimens are compared to each other and analyzed. As the number of layers increases, the strength of the specimens slowly decreases. The compressive strength of WBK dominantly depends on the slenderness ratio. Difference in the specimen configurations results in the significant difference of the stress–strain responses for the specimens with slenderness ratio higher than a certain value. The compressive strength and energy absorption capability of WBK are as good as those of the regular truss type cellular metals, when the relative density is sufficiently high.     

Structural capacitor materials made from carbon fibre epoxy composites

In this paper, an approach towards realising novel multifunctional polymer composites is presented. A series of structural capacitor materials made from carbon fibre reinforced polymers have been developed, manufactured and tested. The structural capacitor materials were made from carbon fibre epoxy pre-preg woven laminae separated by a paper or polymer film dielectric separator. The structural capacitor multifunctional performance was characterised measuring capacitance, dielectric strength and interlaminar shear strength. The developed structural CFRP capacitor designs employing polymer film dielectrics (PA, PC and PET) offer remarkable multifunctional potential.     

Characterization of melded carbon fibre/epoxy laminates

‘Melding’ is a novel in situ method for joining thermosetting composite structures, without the need of adhesives. Laminate joining is achieved using uncrosslinked resin matrix of the pre-preg. This study used Hexply914C pre-preg material to characterize melded CFRP structures produced using the melding method. A designated area of a laminate was maintained at temperatures below 40 °C retaining uncured (B-staged) material, while the remainder of the laminate was cured at 175 °C. After a 2.5 h cure cycle, the cured region showed a high degree of cure (0.88) and glass transition temperature (176 °C). The uncured area of the same laminate was cured in a second stage, simulating an in situ melded joint. By controlling the temperature and duration of the intermediate dwell and affecting minimum viscosity values prior to final cure, low values of porosity (<0.5%) were achieved. The mechanical properties of the resulting joint were consistent throughout the melded laminate. Flexural strength (1600 MPa), flexural modulus (100–105 MPa) and short beam strength (105–115 MPa) values observed where equivalent or greater than those found in the recommended autoclave cured control specimens. After the entire laminate was post cured, glass transition temperatures of 230 °C (peak tan δ) were observed in all areas of the laminate.     

Nonlinear finite element analysis of axially crushed cotton fibre composite corrugated tubes

It is proven experimentally that introducing corrugation along a shell generator together with a proper advanced composite material will enhance the crashworthiness performance of energy device units. This is because corrugation along the shell generator will force the initial crushing to occur at a predetermined region along the tube generator. On the other hand, a proper composite material offers vast potential for optimally tailoring a design to meet crashworthiness performance requirements. In this paper, the energy absorption characteristics of cotton fibre/propylene corrugated tubes are numerically studied. Finite element simulation using ABAQUS/Explicit was carried out to examine the effects of parametric modifications on the tube’s energy absorption capability. Results showed that the tube’s energy absorption capability was affected significantly by varying the number of corrugation and aspect ratios. It is found that as the number of corrugations increases, the amount of absorbed energy significantly increases.     

Blast resistance of sandwich-walled hollow cylinders with graded metallic foam cores

The dynamic responses and blast resistance of all-metallic sandwich-walled hollow cylinders with graded aluminum foam cores are investigated using finite element simulations, and compared with those of conventional ungraded ones. After validating the numerical approach and introducing the computational model, sandwich-walled hollow cylinders with various graded aluminum foam cores are analyzed under air blast loading. It is demonstrated that the radial deflection of graded cylinders is smaller than and the blast resistance superior to that of ungraded ones when subjected to identical air blast loading. This can be further improved by optimizing the foam core arrangement. Finally, the influence of face-sheet arrangements on the dynamic behavior of graded cylinders is explored.     

激光对碳纤维及碳纤维/环氧树脂复合材料性能影响

采用高能激光束对聚丙烯腈（PAN）基碳纤维进行表面改性。利用SEM、EDS、FTIR、XRD、万能试验机等表征手段,对改性前后碳纤维微观形态、成分变化、物相结构、力学性能进行表征,系统地研究了激光束对碳纤维微观组织变化、性能变化等的影响规律,探索激光束对碳纤维的作用机制。结果表明,碳纤维经激光表面改性后,其表面的粗糙度和比表面积增加,碳纤维的浸润性得到提升,且激光束的功率越高、扫描速度越低,碳纤维浸润性越好。改性后的碳纤维化学成分、微观结构及官能团种类没有改变;改性后的碳纤维官能团种类没有改变,说明激光改性过程主要以物理过程为主;激光改性没有改变碳纤维的微观结构,改性后微晶尺寸略有减小,有利于改善碳纤维与环氧树脂的界面黏结性能。激光表面改性碳纤维/环氧树脂复合材料的拉伸强度和冲击强度均有不同程度的提高,当碳纤维质量分数为0.2wt%、激光改性功率为150 W时,碳纤维/环氧树脂复合材料的拉伸强度提高了59%,冲击强度提高了52%。     

Effect of fibre length on fatigue of short carbon fibre/epoxy composite

The effect of fibre length on the fatigue of a random short carbon fibre/epoxy composite containing 1, 5 or 15 mm length fibres has been studied. All laminates gave a sloping S-N curve with longer fatigue lives obtained at decreasing peak stresses. The fatigue life was independent of fibre length at any peak strain, within experimental variation. Damage accumulation during fatigue cycling is studied in terms of residual strength and modulus reduction. Both techniques suggest that fatigue failure is the result of a ‘sudden death’ mode of failure. Finally, the effect of matrix type on the fatigue life of laminates containing 5 mm length fibres was investigated by adding a greater quantity of flexibilizer to the epoxy matrix. Shorter fatigue lives were obtained for laminates having the more flexible matrix.     

Energy absorption during projectile perforation of lightweight sandwich panels with metallic fibre cores

This paper concerns energy absorption during projectile penetration of thin, lightweight sandwich panels with metallic fibre cores. The panels were made entirely of austenitic stainless steel (grade 304). The faceplates were 0.4 mm thick and the core (∼1–2 mm thick) was a random assembly of metallic fibres, consolidated by solid state sintering. The impact tests were simulated using ABAQUS. Faceplate behaviour was modelled using the Johnson and Cook plasticity relation and a strain rate-dependent, critical plastic strain failure criterion. The core was modelled as an anisotropic, compressible continuum, with failure based on a quadratic, shear stress-based criterion. The experimental data show that, with increasing impact velocity, the absorbed energy decreased from the ballistic limit, reached a minimum value, and then underwent a monotonic increase. The FEM modelling demonstrates that this increase arises from the kinetic energy of ejected fragments, while the energy absorbed by plastic deformation and fracture tends to a plateau. Normalised absorbed energies have been compared to values for single faceplates. The sandwich panels are marginally superior to single plates on an areal density basis.     

Pull-through performance of carbon fibre epoxy composites

An investigation of the through-thickness properties of carbon fibre prepreg laminates, Non-Crimp Fabric laminates and non-crimp 3D orthogonal woven composites by pull-through testing was performed. Influence of matrix system and curing temperature on the performance of the 3D woven composites was investigated.     

Experimental investigation of energy-absorption characteristics of components of sandwich structures

Two series of experiments are performed to investigate the dynamic response of various essential components of a class of sandwich structures, under high-rate inertial loads. One consists of dynamic inertia tests and the other involves dynamic impact tests. A split Hopkinson bar apparatus is modified and used for these experiments.     

Thermomechanical behaviour of interfacial region in carbon fibre/epoxy composites

A study of the thermomechanical stability of the fibre-matrix interphase in carbon/epoxy composites has been carried out. The thermodynamic work of adhesion has been evaluated at room temperature by wetting measurements. The interfacial shear stress transfer level τ for sized and desized carbon fibre has been measured as a function of temperature by means of a single-fibre fragmentation test. As the test temperature increased τ values were found to decrease, with values being higher for the desized carbon fibre. The dependence of interfacial shear stress transfer on bulk matrix mechanical properties (modulus and shear strength) has also been discussed. Dynamic mechanical measurements performed on single-bundle composites confirmed the better thermomechanical stability of the desized fibre interphase.     

The effect of fibre misalignment on the compressive strength of unidirectional carbon fibre/epoxy

The effect of a misalignment angle between the fibres and loading axis of a unidirectional composite is analysed by considering the shear strains induced by the misalignment. It is shown that shear instability in the matrix drastically reduces the predicted compressive strength even for very small misalignments. The same trend is predicted for composites with initial fibre curvatures due to the misalignment angle associated with the curvature. The reduction in compressive strength often attributed to initial fibre curvature may therefore actually be due to fibre misalignment angles. Small misalignments are hard to avoid during the manufacture and testing of unidirectional composites and so these results cast serious doubts on the possibility of measuring a true ultimate compressive strength for this kind of material.     

Energy absorption properties of multi-layered corrugated paperboard in various ambient humidities

This paper develops a mathematical model to depict the energy absorption properties of multi-layered corrugated paperboard (MLCP) in various ambient humidities. It is a piecewise function to model the energy absorptions corresponding to three deformation stages of MLCP (elastic stage, plateau stage and densification stage) separately. Simple formulas are derived for each stage which relating the energy absorption capacity of MLCP to the thickness-to-flute pitch ratio (_tc/λ$t_c/\lambda$) of corrugated-core cell, the mechanical properties of corrugated medium tested under a controlled atmosphere [23 °C and 50% relative humidity (RH)], and the RH. The theoretical energy absorption curves are then compared with experimental ones and good agreements are achieved for MLCP with wide range ratios of _tc/λ$t_c/\lambda$ in various ambient humidities. Results of this research can be applied in the optimum design and material selection of cushioning packaging with multi-layered corrugated paperboard.     

Stacking sequence effects on fracture of notched carbon fibre/epoxy composites

The purpose of this study was to investigate the ability of the so-called damage zone model (DZM) to predict the influence of stacking sequence on the strength of notched carbon fibre/epoxy composites. The DZM is in essence based on the unnotched tensile strength, σ₀, and the apparent fracture energy, G_c^*, and the damage zone is modelled as a crack with cohesive forces acting on the crack surfaces. The DZM predicts fracture loads for three-point bend (TPB) specimens and specimens with circular holes quite accurately. As an attempt to explain the difference in strengths, the damage zone extension in the TPB specimens with different stacking sequence was examined.