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Mechanical Behavior of Sandwich Structures using Natural Cork Agglomerates as Core Materials
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Cork is a material of great value to the Portuguese economy. Unfortunately, its use is still restricted to traditional areas, with the agglomerate form in particular not being used to its full potential. The objective of this article is to analyze the viability of using cork-based material as core materials in sandwich structures in aeronautical and aerospace applications. The use of cork-based material is proposed because of its isolation properties (both thermal and acoustic) and there is no significant performance loss, when compared with the currently used materials. It presents other advantages, as well as, less wastage of energy in manufacturing and a better environmental integration, both in the transformation stage and in the end of life recycling stage. The objective of this work is to study the mechanical behavior of different sandwich specimens, with carbon/epoxy faces, and cores of different cork agglomerates and their comparison with the results obtained with similar specimens using current material cores. Experimental shear tests and three-point bending tests were carried out and the evolutions of the load— displacement curves of the different cork agglomerates/sandwiches were analyzed and discussed. The obtained results show that significant room for improvement still exists in use of cork-based core materials.
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... Often used as a core material in sandwich structures, agglomerated cork is produced from granules or the residue of cork, a natural material removed from the bark of cork oaks [9,10]. NL10 cork, used in this work as the core in the studied sandwich panels, is a specific variety of agglomerated cork supplied by Sicomin (Châteauneuf-les-Martigues, France), and designed for technical applications [11]. ...
This study investigates the flexural behavior of three sandwich panels composed of an agglomerated cork core and skins made up of cross-ply [0,90]2 flax or glass layers with areal densities of 100 and 300 g/m2. They are designated by SF100, SF300, and SG300, where S, F, and G stand for sandwich material, flax fiber, and glass fiber, respectively. The three sandwich materials were fabricated in a single step using vacuum infusion with the liquid thermoplastic resin Elium®. Specimens of these sandwich materials were subjected to three-point bending tests at five span lengths (80, 100, 150, 200, and 250 mm). Each specimen was equipped with two piezoelectric sensors to record acoustic activity during the bending, facilitating the identification of the main damage mechanisms leading to flexural failure. The acoustic signals were analyzed to first track the initiation and propagation of damage and, second, to correlate these signals with the mechanical behavior of the sandwich materials. The obtained results indicate that SF300 exhibits 60% and 49% higher flexural and shear stiffness, respectively, than SG300. Moreover, a comparison of the specific mechanical properties reveals that SF300 offers the best compromise in terms of the flexural properties. Moreover, the acoustic emission (AE) analysis allowed the identification of the main damage mechanisms, including matrix cracking, fiber failure, fiber/matrix, and core/skin debonding, as well as their chronology during the flexural tests. Three-dimensional micro-tomography reconstructions and scanning electron microscope (SEM) observations were performed to confirm the identified damage mechanisms. Finally, a correlation between these observations and the AE signals is proposed to classify the damage mechanisms according to their corresponding amplitude ranges.
... They found a thermal conductivity between 0.063 W/mK and 0.068 W/mK. Reis et al. [20] examined the mechanical strength of a multi-layers incorporating inner layer cork and carbon/epoxy outer layer. Their findings indicated that smaller grain sizes and higher densities yield the most favorable results. ...
... One of the species with the highest economic value is the cork oak (Quercus suber L.), associated with the extraction of its barkcork (Sierra-Pérez et al., 2015). Due to its recognized thermal and acoustic properties (Poeiras et al., 2021;Silva et al., 2013), its applications are varied from corks for wine bottles, clothing, surfboards and even in the aerospace industry (Mateus et al., 2017;Peña-Neira et al., 2000;Reis & Silva, 2009). Due to the growing economic value of cork in recent decades, there is a tendency to increase the areas planted with cork oak, where irrigation and fertilization are even tested to increase their productivity and reduce the time between extractions, which is currently at least nine years (Camilo-Alves et al., 2022;Ribeiro et al., 2020). ...
... These structures consist of two outer composite face sheets separated by a lightweight core material, providing enhanced mechanical properties and structural integrity. One promising core material is cork, which possesses desirable characteristics such as low density, high compressive strength, and good energy absorption capacity [13,15,10]. ...
Composite sandwich structures have gained significant attention in engineering applications due to their exceptional strength-to-weight ratio, stiffness, and energy absorption capabilities. One promising core material is cork, which possesses desirable characteristics such as low density, high compressive strength, and good energy absorption capacity. This study aimed to compare the impact performance of composite shells with composite sandwich shells featuring an agglomerated cork core, while investigating the influence of semicylindrical shell diameters. The results underscore the significant impact of specimen diameter on the performance of these structures. Larger diameter specimens (106 mm) exhibited notably higher maximum force, albeit with lower maximum displacement, energy absorption, and contact time compared to those with smaller diameter (82 mm). Similar trends were observed in composite sandwich shells. Furthermore, the inclusion of cork in composite sandwich shells marginally reduced strength of the specimens. For identical diameters, both configurations exhibited comparable maximum force and displacement. However, composite sandwich shells with a cork core absorbed less energy than those without a cork core. This behaviour persisted across both shell geometries, with slightly higher percentage differences observed in the larger diameter specimens.
... The presence of agglomerated cork allows for better impact resistance and high energy absorption compared with other synthetic or mineral materials [10][11][12]. Cork has been studied as a central material in sandwich structures for aeronautical and aerospace applications [13] and as insulation for buildings. ...
Agglomerated cork is a natural cork that has gone through a process of crushing and pressing using heat and binders. One of its applications is thermal insulator in construction. The design of these materials is becoming an essential part of building. The raw materials currently used to make insulators consume a large amount of energy, which has created the need to increase the use of renewable and ecological resources such as plant fibers to reduce the environmental problems generated. The objective of this study was to determine the different properties of experimental particleboard panels made from cork and Canary Island date palms without using any binder at minimum energy consumption. The produced cork–palm boards (density of 850 kg/m3, reached a MOR 8.83 N/mm2, MOE 794.5 N/mm2, and IB 0.38 N/mm2) are higher values than the traditional cork particleboards with UF made from cork. The thermal conductivity values obtained 0.069 to 0.096 W/m·K are higher than cork boards with UF. Ecological boards that can be used as rigid thermal insulators in the construction industry have been achieved to improve the mechanical properties of the traditional agglomerated cork.
... Cork is characterized by liquid impermeability, renewability, and the possibility of incinerating with energy recovery. Moreover, cork has exceptional elasticity and high vibro-, thermo-, electro-, and acoustic insulation, low friction coefficient, flame retardancy, and chemical and biological resistance [7][8][9]. Adding cork as a natural filler in various ratios to the polymer matrix gives the possibility of obtaining composites with different programmed properties. Thus, cork can be an ideal "carbon neutral" material in many applications, both common use and industrial application, with advantages mainly in short-life applications [10,11]. ...
... The thermal conductivities of the three samples were 0.068, 0.063, and 0.065 W. m 21 .K 21 . Reis and Silva (2009) studied the mechanical resistances of a sandwich structure containing a cork core and carbon/epoxy skin under bending and shear test. They found that the best results are obtained for specimens with small grain sizes and higher densities. ...
Lightweight eco-materials are in high demand in many sectors, such as aerospace, industry, and building due to their several characteristics. The present paper is an experimental investigation of the thermal characteristics of novel sandwich panels made with local and ecological materials namely agglomerated cork for the core and bio-composite materials for the skin. Three configurations (symmetric, asymmetric, and two layers) were studied with different cork core thicknesses. Density values have been measured and compared. Thermal characterization consists of determining thermal conductivity and specific heat using a HFM apparatus; whilst thermal diffusivity and thermal effusivity have been calculated using the experimental findings. The panels are lightweight and thermally insulating. The values of thermal conductivity are in the range 0.071 and 0.102 W.m ⁻¹ .K ⁻¹ . The comparison between experimental results of thermal conductivity to theoretical values highlights the accuracy of method for multi-layer thermal characterization and the good adhesion between layers. Finally, a life cycle assessment of the new sandwich panels has been carried out and compared with common insulation materials. The sandwich panels are efficient in terms of embodied energy and CO 2 emissions compared to commercialized insulators and some insulators based on recycled or natural materials, the embodied energy for symmetric configuration with 4 cm cork core are 79.73, 94.75, and 89.35 MJ/FU corresponding to an embodied carbon 5.33, 6.32, and 6.01 CO 2 /FU respectively. They can be classified in the middle between synthetic and natural insulators. Based on the findings, it was concluded that utilizing these sandwich panels as construction materials for interior paneling or partition walls could offer benefits in terms of being environmentally sustainable and cost-efficient.
This study examines the mechanical performance of bio- and bio-inspired composite sandwich panels under out-of-plane compressive loading. Using balsa wood face sheets and natural oak tree cupules for core reinforcement, the research aims to develop lightweight, sustainable structures. Three different cupule arrangements and geometries were tested, with and without polyurethane (PU) foam infill, to compare natural and commercial cores. The results show that cupule geometry significantly influences mechanical behaviour, demonstrating notable load-bearing and energy absorption capabilities, though with a tendency towards brittle fracture. Reinforcement techniques, such as cupule duplication and the integration of commercial PU foam, were explored to enhance load-bearing capacity and energy absorption while mitigating sudden failure. Panels reinforced with oak tree cupule-filled PU foam cores exhibited superior mechanical properties compared to non-reinforced cores. This study highlights the potential of using oak tree cupules to improve mechanical performance and sustainability in sandwich panel design. Furthermore, bio-inspired structures were fabricated via Multi Jet Fusion additive manufacturing, harnessing the unique properties of cupules to achieve superior mechanical performance in printed structures, emphasising their potential in structural engineering applications.
The great efforts to reduce the environmental impact of composite materials by replacing synthetic components with bio-based ones are involving also sandwich structures. Agglomerated cork proved to be a feasible alternative to traditional polymeric foam as core material. Sandwich structures are extremely prone to impact events which can cause the direct exposure of the core material to the external environment making fundamental to know how its out-of-plane properties can be influenced by external factors such as sunlight and rain. The present work assessed the effects of UV accelerated aging and freeze–thaw (FT) cycles on the compressive behavior of two agglomerated corks with different densities providing also a direct comparison with a traditional PVC foam. Agglomerated cork compressive properties increase when exposed to UV accelerated aging up to 600 h, determining an increase of 31 % and 9 % in the compressive modulus for the less dense and denser cork, respectively. No significant changes were observed for the synthetic foam. FT cycles on as-received specimens induce a decrease in cores compressive properties after 30-day cycling, with a reduction in compressive modulus of approximately 11 % for PVC foam and of 12 % and 11 % for the less dense and denser cork, respectively.
ASTM C 273 and BS 4370 single block tests for shear properties of sandwich cores are critically examined. A previous appraisal using finite element modelling is highlighted and extended to include the effects of stress concentrations. Overall performance is assessed by selected laboratory programs and comparisons with panel test and flexural test methods. The ASTM method is shown to give accurate results generally. Modifications are required to the BS 4370 method. Problem areas with regard to theoretical stress concentrations in both methods are identified in relation to the test for shear strength.
End grain balsa is widely used as a core material in sandwich structures, especially in marine applications where the shear properties are of major importance. In a recent study, a new balsa core structure that better utilizes the anisotropy of balsa was designed and numerically analyzed. An improved shear stiffness was predicted with this new core. In the present study, manufacturing methods for this core were developed and more extensive experiments were performed to verify the improvement in shear stiffness as well as to evaluate the shear strength.
A thorough investigation of the mechanical behavior of a closed-cell cellular foam (Divinycell) under multiaxial stress conditions was undertaken. Two types of Divinycell, H100 and H250, with densities of 100 and 250 kg/m3, respectively, were investigated. The uniaxial tensile, compressive and shear stress–strain curves along the in-plane and the through-the-thickness directions of both materials were obtained. The materials showed quite different stress–strain behavior in tension and compression. The H100 material showed a nearly isotropic behavior, while the H250 material showed orthotropic behavior with a higher stiffness along the through-the-thickness than the in-plane direction. A series of biaxial tests were conducted, including: (i) constrained strip specimens in tension and compression with the strip axis along the through-the-thickness and in-plane directions; (ii) constrained thin-wall ring specimens in compression and torsion; (iii) thin-wall tube specimens in tension and torsion; and (iv) thin-wall tube specimens under axial tension, torsion and internal pressure. From these tests, biaxial strength results in the stress plane of the through-the-thickness and in-plane directions for different values of applied shear were obtained. Failure envelopes were constructed by the Tsai–Wu failure criterion based on the strength values in uniaxial tension, compression and shear. The experimental results were described well by the Tsai–Wu failure criterion.
The material characterization described in the paper is aimed at the structural design of the front shield for a high speed train made of composite material. A sandwich structure was considered, made of glass fiber epoxy face sheets with a polymeric foam core. Initially, the material properties and the rate sensitivity of the skin and core materials were investigated through a series of static and quasi-static tests. Static and dynamic impact tests were then run on the sandwich structure. For all materials tested, no significant strain-rate effects were observed over the range of test conditions investigated in the study. Results show that the structural response of the sandwich depends primarily on the strength properties of the foam core material. The dynamic impact resistance of the sandwich structure was then substantially improved by adding a net of resin walls within the foam.
Effective transverse shear moduli in the principal material directions of corrugated board were examined experimentally for five corrugated board types using the ASTM block shear test and the three-point bend test. It was found that shear moduli determined by the three-point bend test are significantly lower than those obtained from the block shear test. The difference is probably caused by local indentation of the board at the supports and contribution from bending deformation of the facings in the three-point bend test. Experimental results were compared to shear moduli obtained by finite-element analysis (FEA) and analytical predictions. Shear modulus along the corrugations, determined with the three-point bend test, was about half of the predicted value using FEA, while the shear modulus transverse to the corrugations was substantially below that prediction from FEA, apparently due to delamination damage of the core material inflicted during the corrugation process.
Shearing Testing of Sandwich Panel Core Materials, http:// www.compositesworld.com/hpc/issues
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Estudo Experimental e Analı´ticoAnalı´tico, em Compressaõ, de Diferentes Espumas para Aplicac¸aõAplicac¸aõ em Construc¸aõConstruc¸aõ Sandwich
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Mecaˆnica Experimental, Ponta Delgada, Portugal, 27-29 Julho 2005.
Estudo Experimental e Analı´tico, em Compressa˜o, de Diferentes Espumas para Aplicac¸a˜o em Construc¸a˜o Sandwich
- M Leite
- J Lopes
- A Silva
Leite, M., Lopes, J., Silva, A., Estudo Experimental e Analı´tico, em Compressa˜o, de
Diferentes Espumas para Aplicac¸a˜o em Construc¸a˜o Sandwich, In: 6 Congresso Nacional de
Mecaˆnica Experimental, Ponta Delgada, Portugal, 27-29 Julho 2005.