Transverse shear stiffness of thickness gradient honeycombs

This paper describes the transverse shear stiffness of a novel topology of gradient honeycomb structures. Opposite to classical honeycomb configurations, gradient honeycombs feature elements of their unit cells with a regular geometry variation across the whole honeycomb panel. The tessellation of the cells is not periodic, but is dictated by geometric constraints between adjacent units. Gradient honeycombs with wall thickness linearly increasing along the panel are described using experimental data and Finite Element models. The gradient behaviour of the cellular structure provides additional complexity, and the possibility of tailoring design properties, such as the stiffness per unit of weight. We observe a good agreement between the Finite Element and the experimental results, with maximum percentage errors <7% for the shear moduli of the honeycombs.     

Core material effect on wave number and vibrational damping characteristics in carbon fiber sandwich composites

A sandwich composite is typically designed to possess high bending stiffness and low density and consists of two thin and stiff skin sheets and a lightweight core. Due to the high stiffness-to-weight and strength-to-weight ratios, sandwich composite materials are widely used in various structural applications including aircraft, spacecraft, automotive, wind-turbine blades and so on. However, sandwich composite structures used in such applications often suffer from poor acoustic performance. Ironically, these superior mechanical properties make the sandwich composites “excellent” noise radiators. There is a growing interest in optimizing and developing a new sandwich composite which will meet the high stiffness-to-weight ratio and offer improved acoustic performance. The focus of this study is to investigate the structural–vibrational performance of carbon-fiber face sheet sandwich composite beams with varying core materials and properties. Core materials utilized in this study included Nomex and Kevlar Honeycomb cores, and Rohacell foam cores with different densities and shear moduli. The structural–vibrational performance including acoustic and vibrational damping properties was experimentally characterized by analyzing the wave number response, and structural damping loss factor (η) from the frequency response functions, respectively. It was observed that the relationship between the slopes of the wave number data for frequencies above 1000 Hz is inversely proportional to the core material’s specific modulus (G/ρ). The analysis also showed the importance of using a honeycomb core’s effective properties for equal comparison to foam-cored sandwich structures. Utilizing analytical modeling, the loss factors of the core materials (β) was determined based upon the measured structural loss factors (η) for a frequency range up to 4000 Hz. It was determined that low shear modulus cores have similar material damping values to structural damping values. However as the core’s shear modulus increases, the percent difference between these values is found to increase linearly. It was also observed that high structural damping values correlated to low wave number amplitudes, which correspond to reductions in the level of noise radiation from the structure.     

The transverse elastic properties of chiral honeycombs

This work describes the out-of-plane linear elastic mechanical properties of trichiral, tetrachiral and hexachiral honeycomb configurations. Analytical models are developed to calculate the transverse Young’s modulus and the Voigt and Reuss bounds for the transverse shear stiffness. Finite Element models are developed to validate the analytical results, and to identify the dependence of the transverse shear stiffness vs. the gauge thickness of the honeycombs. The models are then validated with experimental results from flatwise compressive and simple shear tests on samples produced with rapid prototype (RP)-based techniques.     

Predicting evolution of ply cracks in composite laminates subjected to biaxial loading

An energy-based model is developed to predict the evolution of sub-critical matrix crack density in symmetric multidirectional composite laminates for the case of multiaxial loading. A finite element-based numerical scheme is also developed to evaluate the critical strain energy release rate, G_Ic, associated with matrix micro-cracking, a parameter that previously required fitting with experimental data. Furthermore, the prediction scheme is improved to account for the statistical variation of G_Ic within the material volume by using a two-parameter Weibull distribution. The variation of G_Ic with increasing crack density is also accounted for based on reported experimental evidence. The simulated results for carbon/epoxy and glass/epoxy cross-ply laminates demonstrate the ability of the improved model to predict the evolution of multidirectional ply cracking. By integrating this damage evolution model with the synergistic damage mechanics approach for stiffness degradation, the stress-strain response of the studied laminates is predicted. Finally, biaxial stress envelopes for ply crack initiation and pre-determined stiffness degradation levels are predicted to serve as representative examples of stiffness-based design and failure criterion.     

Investigation into the changes in bending stiffness of a textile reinforced composite due to in-plane fabric shear: Part 1 – Experiment

A study to investigate the factors that contribute to the variation among the stiffnesses of consolidated composite plates reinforced by plain-weave fabrics with various degrees of in-plane shear is presented. The first part of the two-part study focuses on the experiments performed. Three-point bend tests were used to measure the effective stiffness of the composite plates along the global X and Y axes, which were aligned with the weft and warp orientations, respectively, in the undeformed configuration at 0° of shear. The warp yarns were sheared 0°, 10°, 20°, 25° and 30° toward the weft yarns. It was observed that as the shear angle in the plates increased, the thickness of the plates also increased. An increase in stiffness for bending in X-direction with increasing shear angle was observed as was expected, but the change in stiffness for bending in Y-direction was observed to be inconsistent with the expected decrease with increasing degree of shear.     

Structure characterization and oxidation mechanism study of porous biomorphic carbon template derived from basswood

A porous biomorphic carbon template (BCT), retained its biological feature, was prepared by carbonized basswood. Microstructure and component of BCT were performed by scanning electron microscope (SEM), energy dispersive spectrometry (EDS) and Raman spectroscopy. Non-isothermal oxidation mechanism of BCT was studied by thermogravimetry analysis (TGA). Experimental results show that BCT has a honeycomb structure and double-peaked porous diameter distribution. It is a typical turbostratic network structure, with increasing carbonization temperature, the ratio of the integrated intensities I_D/I_G increase, the lateral extension L_a of the graphene units decrease. The non-isothermal oxidation mechanism of BCT exhibits a self-catalytic characteristic, which is explained by molecular structure schematic model.     

Experimental determination of torsion and shear properties of sandwich panels and laminated composites by the plate twist test

A recently developed sandwich plate twist test is employed here for determination of the transverse shear modulus of the core and twist stiffness (D₆₆) of a sandwich panel consisting of a soft (H45 PVC foam) core and glass/vinylester face sheets. The shear modulus of the H45 PVC foam core extracted from the twist test was in good agreement with shear modulus obtained from ASTM plate shear testing of the foam core. D₆₆ values obtained from the sandwich twist test were in good agreement with predictions from classical laminated plate theory. In addition, the twist test was used to determine the in-plane shear modulus of glass/vinylester laminates isolated and as face sheets in sandwich panels with a stiff (plywood) core. The in-plane shear modulus of the face sheets, isolated and as part of a sandwich panel, was in good agreement with shear modulus determined using the Iosipescu shear test. The results point to the potential of the twist test to determine both in-plane and out-of-plane shear moduli of the constituents of a sandwich structure, as well as D₆₆.     

Fracture characterization of sandwich structures interfaces under mode I loading

The accurate prediction of failure of sandwich structures using cohesive mixed-mode damage models depends on the accurate characterization of the cohesive laws under pure mode loading. In this work, a numerical and experimental study on the asymmetric double cantilever beam (DCB) sandwich specimen is presented with the objective to characterize the debonding fracture between the face sheet and the core under pure mode I. A data reduction method based on beam theory was formulated in such a way to incorporate the complex damaging phenomena of the debonding due to the material and geometric asymmetry of the specimen, via the consideration of an equivalent crack length (a_e). Experimental DCB tests were performed and the proposed methodology was followed to obtain the debonding fracture energy (G_Ic). The experimental tests were numerically simulated and a cohesive damage model was employed to reproduce crack propagation. An inverse method was followed to obtain the local cohesive strength (σ_u,I) based on the fitting of the numerical and experimental load–displacement curves. With the value of fracture energy and cohesive strength defined, the cohesive law for interface mode I fracture is characterized. Good agreement between the numerical and the experimental R-curves validates the accuracy of the proposed data reduction procedure.     

Interactions between propagating cracks and bioinspired self-healing vascules embedded in glass fibre reinforced composites

This study considers the embedment of a bioinspired vasculature within a composite structure that is capable of delivering functional agents from an external reservoir to regions of internal damage. Breach of the vascules, by propagating cracks, is a crucial pre-requisite for such a self-healing system to be activated. Two segregated vascule fabrication techniques are demonstrated, and their interactions with propagating Mode I and II cracks determined. The vascule fabrication route adopted played a significant role on the resulting laminate morphology which in-turn dictated the crack-vascule interactions. Embedment of the vascules did not lower the Mode I or II fracture toughness of the host laminate, with vascules orientated transverse to the crack propagation direction leading to significant increases in G_I and G_II through crack arrest. Large resin pockets were found to redirect the crack around the vascules under Mode II conditions, therefore, it is recommended to avoid this configuration for self-healing applications.     

Out-of-plane effective shear elastic properties of a novel cellular core for sandwich structures

When designing sandwich structures, different types of periodic cellular cores have been considered trying to achieve higher overall bending stiffness and strength to weight ratios as a main objective. Although most of them have proved to offer advantages in certain specific applications, there is a common drawback for all: complexity of the manufacturing process reflected in higher manufacturing costs. This article aims at investigating the out-of-plane shear elastic properties G_xz and G_yz of a novel cellular core named ExpaAsym, produced by a potentially simple manufacturing method: sheet material expansion. Numerical analyses are carried out in order to study the way in which the out-of-plane shear elastic properties modify in terms of several geometric parameters that define the topology of the structure. The numerical results are validated through experimental tests.