Cork Core Research Articles

Manufacturing and Mechanical Characterization of Bio-Based Laminates and Sandwich Structures

Jute fabric is well-known reinforcing material in composite science, however, there is a necessity to treat these fabrics to reduce moisture uptake and improve properties. Nevertheless, every modification increases the cost and reduces the possible applications. Presented research deals with an investigation of possibility to use untreated jute in various fabric weight as a reinforcing material in sandwich structures facings. Untreated jute reinforcements and two types of cork cores were saturated in one step during vacuum infusion creating a lightweight sandwich composite. All samples were mechanically tested in three-point bending test. Experimental results showed the most appropriate material combination and produced sandwich structure are proposed for design applications.

Effects of carbon nanotube modified adhesive layer on low velocity impact and flexural properties of cork core sandwich structures

The traditional method of manufacturing sandwich structures using a structural adhesive to achieve face-sheets to core bonding maintains widespread use. Most research addressing experimental performance of sandwich structures focuses on core and face-sheet related parameters, and only marginal attention is directed to the mechanical properties of the thin, low content, adhesive layer – it usually suffices that it can provide effective face-sheet/core bonding. The present work studies sandwich structures with a cork agglomerate core and resin infusion processed skins consisting of +/-45° glass fibre fabric reinforced epoxy resin. A polyurethane structural adhesive was used to assemble the structure. The focus of the study was the influence on sandwich performance with respect to a modification of the adhesive with multiwall carbon nanotubes, exploring the possibility of an increase in both shear and adhesion strength of the modified adhesive. The sandwich structure was evaluated with respect to four-point bending and low velocity impact tests. In addition, scanning electron microscopy analysis was used to examine the adhesive layer morphology. The results were analysed to determine how the addition of ca. 0.4wt.% of carbon nanotubes to the adhesive effectively influenced failure behaviour and damage events in both flexural and impact testing.

Indentation experiments on novel sandwich composite tubes

This article introduces a novel energy absorber of sandwich tube with the agglomerated cork core and the composite tube during the indentation process under the applied quasi-static lateral loading by a solid cylindrical rigid indenter. Also, the corresponding cork specimens and the empty composite tubes are tested, too. The solid cylindrical cores made of agglomerated cork are prepared by punching a cork sheet with 200kg/m3 density and positioned into the composite tubes to manufacture the cork-filled tubes. The results show that the cork-filled composite tubes have an energy absorption capacity up to 5.65 times of the corresponding cork sample and up to 6.06 times of the corresponding empty tube. Also, it is found that total absorbed energy by a cork-filled composite tube is 174% higher than summation of absorbed energy by the corresponding empty tube and the cork sample. Also, specific absorbed energy by all the specimens is discussed.

Mechanical Behaviour of a Green Sandwich Made of Flax Reinforced Polymer Facings and Cork Core

This work investigates the flexural behavior of a composite sandwich made of flax fibers reinforced skin facings and an agglomerated cork core, to be employed as an eco-friendly solution for the making of structural components of small sailing boats. An experimental mechanical characterization of the strength and stiffness flexural behavior of the proposed sandwich is carried out, providing a comparison of performances from three implemented assembling techniques: hand-lay-up, vacuum bagging and resin infusion. Sandwich beams have been tested under three point bending (TPB) at various span lengths. A procedure is also proposed and implemented to consider the potential influence of the local elastic indentation in the experimental evaluation of the flexural stiffness. This procedure is based on the analytical solution of an indented beam resting on a fully backed Winkler foundation.

Influence of Processing Parameters Production of Sandwich Composite Structures Designed Especially for the Construction of Machine Tool Parts

All components of a huge machine tool need to work together perfectly to ensure not only a quality finished product, but operator safety. One of those many components is the material of support panels and moving parts in the machine construction. Mechanical structure of the machine, in most cases does not reach the technical level of regulatory techniques or electric drives and potential increase in dynamics, speed or accuracy of the machines remain unused. Used classic construction materials, steel or light alloy then zoom in to limit their physical capabilities. Designers have machine tools or robots are forced to solve the formidable task in the form of reducing the weight of the moving parts while maintaining or even increasing stiffness with respect to the thermal stability or damping. Laminates are for construction of production machines, robots and manipulators of crucial importance. One big advantage is the high rigidity while maintaining low specific weight. The presented paper is focused on the investigation of composite sandwich construction produced by prepreg technology, which is the suitable production choose for construction of machine tools parts. First of all, the production of sandwich structures with cork core, honeycomb core and foam cores are compared and described. In terms of the production process it is a method very friendly to the environment, since there are no leaks styrene vapours into the air. Since it is important from the perspective of a manufacturer of transport or machine industry manufactured from environmentally friendly materials, so naturally from materials that are from the perspective of safety of passengers (operators of machine tools or robots) at a high level, when any accident was lowest possible hazard due to fire (burn materials used) and terms of the strength of composite materials. This is one of the major scientific articles and to find the most ideal surveyed composite sandwich materials that would be the greatest possible extent fulfils the required properties. In the experimental part of the article are compared and investigated the mechanical characteristics of composite sandwich structures with regard to varying number of layers of predimpregnated skin materials due to the different types of core materials suitable for use in the machine industry. In addition, these characteristics are an important part of the article research because described the influence number of layers to values of the bending stiffness those composite structures.

Preparation and Characterization of Cork Layered Composite Plywood Boards

In the furniture and construction industries, there is increased demand for lightweight, high-performance, and low-maintenance materials with specific properties. Increased demand necessitates testing of new and composite materials to find a viable alternatives to classical materials. In this study, two different types of cork layered plywood composites (plywood board with a cork core (PLYW-K1), and plywood board with a cork core and cork face layers (PLYW-K2)) were prepared and tested for their basic mechanical properties as well as screw withdrawal resistance. The measured properties were compared with standard particleboard (PB) and plywood board (PLYW1) to determine the difference in properties and potential applications. The results presented include preparation parameters, mechanical properties, maximum withdrawal force, and withdrawal resistance. In addition, the effect of screw diameter and material density on withdrawal resistance was observed. Results indicate that cork-layered plywood possessed superior mechanical properties and withdrawal strength at a much lower density than particleboard. In comparison to classical plywood, the improved factors were a reduction in density and production cost.

Low velocity impact and flexural performance of sandwich structures with cork and polymer foam cores

The core material plays an important role in the static behavior of sandwich structures, but particularly in their dynamic behavior. Cork with its unique flexibility, elasticity and compressibility properties is an excellent natural energy absorber and motivates the study of its incorporation in sandwich construction. In this work, flexural and impact behaviors of sandwich structures with fiberglass/epoxy face sheets, different cork based core materials, and PMI, PU and PVC foam cores are experimentally studied by four-point bending and low velocity impact, and compared. Flexural properties of cork based structures are lower than those of structural polymer foams for the geometries explored in this work. Core indentation was the single collapse mode observed in this study; therefore, further studies including other specimen geometries for four and three-point bending are required to evaluate other collapse modes and understand the full behavior of cork under flexural loading. Damage tolerance exhibited by cork core structures subject to low velocity impact make cork an improved core material, particularly for sandwich structures likely to regularly suffer low velocity impact events. Ongoing work on cork based core materials modified with a nanoporous gel, and with a dispersion of carbon nanotubes in the bonding layer, is briefly mentioned.

The influence of equipment variations on sliotar–hurley impact in the Irish game of hurling

The game of hurling is ranked as one of the fastest and most skilful field games in the world. It is played by a stick, hurley, made of ash wood and a ball, sliotar, made of a cork core or similar viscoelastic materials. Better standardisation of the game equipment requires analysing impacts to quantify precisely the effect of variations in equipment design on the resultant impact force at low and high striking speeds. Therefore, the purpose of the present study was to use a high-speed camera to (1) characterise the impact dynamics in term of impact force magnitude and duration, and (2) investigate whether a relationship exists between the game equipment and the magnitude of the impact force. An air cannon unit (ACU) test rig was set up to cause the sliotar to strike the hurley at predetermined speeds. A high-speed camera operating at 12,500 frames per second (fps) was used to capture 32 experimental impacts covering a range of sliotar and hurley brands, impact locations and impact speeds. The factor “sliotar brand” was identified as the most significant factor for the impact force among other main factors and two-factor interactions at a constant speed and it had a significant effect with up to 27 % difference between sliotar brands. It was demonstrated that, at higher impact speeds, the force–deformation curves of the sliotar brands varied significantly, even though similar force–deformation curves have been reported at quasi-static compression and similar COR values at low impact speeds using the standard free-drop test. This is the first known study to characterise the unique, highly non-linear and anisotropic sliotar–hurley impact using high-speed camera technology. It highlights the need for a tighter standard for sliotar materials and manufacturing to increase uniformity between different brands and strongly suggests that the standard free-drop test is not sufficient to characterise and compare game equipment.

Natural Cork Agglomerate Employed as an Environmentally Friendly Solution for Quiet Sandwich Composites

Carbon fiber-synthetic foam core sandwich composites are widely used for many structural applications due to their superior mechanical performance and low weight. Unfortunately these structures typically have very poor acoustic performance. There is increasingly growing demand in mitigating this noise issue in sandwich composite structures. This study shows that marrying carbon fiber composites with natural cork in a sandwich structure provides a synergistic effect yielding a noise-free sandwich composite structure without the sacrifice of mechanical performance or weight. Moreover the cork-core sandwich composites boast a 250% improvement in damping performance, providing increased durability and lifetime operation. Additionally as the world seeks environmentally friendly materials, the harvesting of cork is a natural, renewable process which reduces subsequent carbon footprints. Such a transition from synthetic foam cores to natural cork cores could provide unprecedented improvements in acoustic and vibrational performance in applications such as aircraft cabins or wind turbine blades.

STUDY OF MECHANICAL AND HYGROTHERMAL PROPERTIES OF AGGLOMERATED CORK

The mode of development of sandwich panels with cork core by the method infusion cannot prevent the resin from penetrating inside cork. This study characterizes white cork agglomerated before and after the development of sandwich panels to see the influence of the resin on the mechanical and hygrothermic characteristics of cork. Two types of cork are considered, they are characterized by their density of 270 kg/m3 (C270) and of 190 kg/m3 (C190). One notes that the modulus of rigidity of C270 cork is 20% larger than that of C190.The presence of resin increases also the module of shearing 47% for C270, 64% for C190. The hygrothermic analysis out of drying oven at 100°C shows a fall of significant mass in the first 7 hours. The analysis thermogravimetry at 220°C, the fall of significant mass is recorded in the first 5 minutes.DOI: http://dx.doi.org/10.5755/j01.mech.18.1.1278

Finite Element Modeling of a Baseball

A baseball consists of a spherical cork core that is wrapped with two layers of rubber, three layers of wool, one layer of cotton, cement glue and one layer of cowhide. The respective nonlinear and time-dependent material properties of these layers make it challenging to capture the behavior of a baseball accurately over a range of impact speeds. As a result of this challenge, finite element models (FEMs) of the baseball typically treat the baseball as a single viscoelastic material where the respective material properties have been homogenized. These single-material models do not necessarily possess the same hyperelastic and viscoeleastic qualities as a real baseball, and these missing behaviors limit the applicability of these baseball models to simulate a bat-ball collision over a very narrow range of impact speeds. In the current work, a layer-by-layer approach is used to create a robust baseball FEM. The pill, which is a cork core wrapped in two layers of rubber, were subjected to bounce and compression testing at a wide range of impact speeds and compression rates. These data are used to tune the pill properties and ensure close agreement between experimental and FEM results. The remaining layers are added individually until the complete baseball is modeled and agrees closely with experimental results over a wide range of impact speeds. Possible applications for a new and improved baseball FEM include accurate simulations of baseball testing and bat-ball collisions. The improved ball model can also be used to predict performance of future bat designs not yet manufactured.

Experimental study of agglomerated-cork-cored structures subjected to ballistic impacts

The present paper examines the high-velocity impact behaviour of agglomerated cork-cored structures. The ballistic performance was studied by impact-perforation tests. Three different types of specimens were tested: an agglomerated cork, two spaced thin aluminium plates, and a pair of thin aluminium plates separated by an agglomerated-cork core. The behaviour of the agglomerated cork and the effects of the cork core were analysed in terms of the ballistic limit, residual velocity, and energy absorption. The ballistic limit of cork-cored structures increased slightly, whereas the absorbed energy was strongly augmented by the presence of the cork core.

Finite element analysis of Carbon composite sandwich material with agglomerated Cork core

Composite sandwich structures were developed for the hydrofoil sail boat hull using biaxial carbon fabric/epoxy composite facing and agglomerated cork core. These ultra-light weight structures were tested through four point bending tests to characterize their flexural behavior. The material exhibited an initial linear elastic behavior followed by non-linear elastic-plastic behavior. Finite element analysis of the sandwich beams was performed to analyze the flexural behavior of the structure. It was found that the transition from linear elastic to non-linear elastic-plastic behavior is the result of compressive yielding of core material which leads to indentation in the beam. This also causes initiation of failure in the core. A shift in the centroid axis of the beam towards the tensile face was also observed. The sandwich structures made of cross-linked PVC and PMI foam cores were also analyzed for comparison. Further, a parametric study on the effect of areal density and ply angle of the facing fabric and core thickness were performed using finite element method. The parametric study revealed that the transition from linear to non-linear trend is caused by different mechanisms with the change in the mentioned parameters.

Using natural cellular materials to improve blast-wave protection

Energy absorbing materials and systems incorporating materials such as foams or honeycomb structures are highly relevant in the design of blast protection systems. Systems with high strength-to-weight ratio and excellent energy absorption capacity under dynamic loading play an important role in defence systems. Sandwich structures, namely their cores, have the ability to sustain large deformations (strains) under constant load enabling them to absorb energy. After reaching their yield stress cellular materials exhibit a region of constant stress for increasing strain until densification is reached, allowing them to absorb considerable amounts of energy relative to their density. The authors incorporate a Micro-Agglomerate Cork (MAC) compound core between two aluminium alloy panels in order to combine the properties of light-weight materials with their high energy absorption capability. It is shown that the inclusion of the micro-agglomerate cork core increases the energy absorption capability of the protection structure under blast loading.

A contribution to the properties of combined plywood materials

The paper summarizes the results of institutional research aimed at new types of combined plywood materials. Under pilot plant conditions, three variants of combined plywood materials were pressed, namely with the layer of fibreglass, with a core cork layer and with a cork wear layer on one side of the plywood surface and a cork core. Tests of selected physical and mechanical properties were carried out on these materials including the basic statistical evaluation. Comparisons with plywood materials Multiplex 15 and 20 mm in thickness were also made.