Cork Agglomerate Research Articles

Mechanical characterization of sandwich composites with embedded sensors

Sandwich composites are known for presenting interesting properties such as high strength to weight ratio which explain the wide use of this kind materials throughout several applications. However, the formation of different failure modes in those materials is not fully comprehended. This problem can be overcome with the embedment of sensors into the material in order to assess its integrity. With this in mind, this study intends to assess how embedded sensors can change the mechanical properties of a sandwich composite material with cork agglomerate core and fibreglass and epoxy resin skins. The behaviour of the material was analysed during tensile, bending and creep tests, in three different situations: without sensors, with surface sensors, and with embedded sensors (strain gages and fibre Bragg grating). The sensors’ embedment can be performed during the manufacturing process of the composite and for that the hand lay-up process was selected. From the tensile tests, the analysis of the collected data showed that the highest stiffness was found at the sensor region. In the bending tests, it was observed that embedded sensors affected bending resistance and also affected the failure mode of this composite. In the creep test, the specimens were sensible to the highest temperature, and all the data collected by the sensors translated this difference. The stiffness analysis did not allow to draw conclusions due to the small number of specimens. However, some specimens with embedded sensor showed no significant changes in stiffness, compared with specimens without sensors and with surface sensors.

Development of an Eco-Friendly Head Impact Protection Device

Nowadays, the number of people practising contact sports has increased. In many of them, using head protective equipment is not mandatory, even if the use of headbands could increase the level of safety regarding several types of traumatic brain injuries. Many commercial solutions are currently available, based on plastic-based foams providing a decent level of protection and comfort to the user. This work introduces the use of agglomerated cork as an eco-friendly alternative to synthetic foams but at least keeping safety levels. Cork is a natural cellular material that has been showing excellent crashworthiness properties. In this study, cork agglomerate density is carefully chosen to be incorporated into a protective headband. Results are compared against three other commercial headbands. For each one, the risk of brain injury was analysed for different injury thresholds and impact energies. The results clearly demonstrate that the cork-based apparel may provide comparable, and in some cases, better performances, outlasting the commercial ones.

A stochastic approach for predicting the temperature-dependent elastic properties of cork-based composites

This work deals with the problem of predicting the temperature-dependent equivalent elastic properties of cork-based composites by investigating the influence of the variability related to the material properties of natural cork. A dedicated numerical homogenisation approach based on the strain energy of periodic and a-periodic media has been developed to this purpose. The proposed methodology is based, on the one hand, on an efficient modelling strategy capable to generate 2D and 3D finite element models which closely emulates the realistic meso-structure of the agglomerate. On the other hand, the strategy makes use of the Monte Carlo algorithm to study the influence of the variability in model inputs on the equivalent elastic behaviour of the cork-based composite at different temperatures. Firstly, the effectiveness of the modelling strategy in representing a realistic meso-structure of the agglomerate, in terms of geometry, has been evaluated by comparing the numerical results to digital images of real cork-based composites. Secondly, the homogenisation strategy has been applied to a realistic configuration of cork agglomerate. Numerical results show that the greater the temperature the lower the overall stiffness of the agglomerate, according to natural cork trends, but the variability related to the macroscopic equivalent elastic properties of the agglomerate is considerably lower than that affecting the elastic behaviour of the natural cork, which represents a positive result concerning the use of cork agglomerates in industrial applications.

Flexural fatigue behaviour of an asymmetric sandwich composite made of limestone and cork agglomerate

The ultimate goal of developing new composite materials is to predict their behaviour in service. In this work, the asymmetric composite studied comprises a layer of stone and a layer of reinforced-cork agglomerate, with glass fibres skins on both sides. The main goal is identifying the damage mechanisms leading to fatigue failure and understand the fatigue initiation phenomenon and the accumulation of damage in relation to the physical and mechanical properties of the materials used. Two configurations made of limestone (L1 and L2) having different modulus of elasticity, mechanical strength and porosity have been manufactured and tested.A four-point bending test was conducted with a load ratio of 0.1 and maximum load taken as a percentage of the maximum static load, i.e. from 0.70 and 0.85 of the static load for configuration L1C1 and from 0.40 to 0.70 of the static load for configuration L2C1.A continuous test was carried out up to 500,000 cycles or up to failure of the material, and a test with resting time after each block of cycles was used to evaluate the influence of the viscoelastic recovery of the cork.Different failure modes were observed for both configurations, and for the same configuration tested with and without interruptions. The failure modes vary according to the maximum load; stone indentation caused by the contacting rollers of the four-point bending roller was always observed in the composite with the stone showing the lowest mechanical strength (L1C1) and fibre breakage was observed for the first time on the other configuration (L2C1). Tests with resting time showed a dramatic increase in lifetime when compared with continuous tests and were used to confirm the occurrence of permanent damage in the cork layer.It has been possible to evaluate the occurrence of fatigue and to have a qualitative indication of the behaviour of the material in service.

Water retention and drainage capability of expanded cork agglomerate boards intended for application in green vertical systems

The expansion of urban centres, together with the increasing frequency of extreme weather phenomena, have prompted the need to study and implement Natural-Based Solutions that can enhance the sustainability and resilience of cities. In this context, green vertical systems (GVS) have been referred to as eco-efficient solutions with potential benefits in terms of building insulation and urban environment. Nonetheless, the sustainability of these solutions has been questioned due to the possible low environmental performance of some of the materials used and the irrigation needed to support vegetation.This work describes studies that have been performed to evaluate expanded cork agglomerate as an alternative eco-friendly material for building GVS. Expanded cork agglomerate is an insulation material produced with by-products from the industry, which may have benefits when used in GVS in terms of regulating the moisture in the planted facade. Experimental tests were carried out to evaluate the water retention and drainage capability of expanded cork agglomerate boards of varying density, height and thickness.For this, board specimens were fully laterally confined in a watertight box with open ends at the top and bottom, with successive water discharges being carried out on the top. Other tests were performed with the front of the box removed, thus allowing the water to drain from the facade too. The results suggest that expanded cork agglomerate is suitable for use in GVS, with the boards achieving water retention up to 20 kg/m3 and providing rapid drainage of excess of water.

Helmet Design Based on the Optimization of Biocomposite Energy-Absorbing Liners under Multi-Impact Loading

Cellular materials have been used in many applications such as insulation, packaging, and protective gear. Expanded polystyrene has been widely used as energy-absorbing liner in helmets due to its excellent cost-benefit relation. This synthetic material can absorb reasonable amounts of energy via permanent deformation. However, in real-world accidents, helmets may be subjected to multi-impact scenarios. Additionally, oil-derived plastic is presently a major source of societal concern regarding pollution and waste. As a sustainable alternative, cork is a natural cellular material with great crashworthiness properties and it has the remarkable capacity to recover after compression, due to its viscoelastic behavior, which is a desired characteristic in multi-impact applications. Therefore, the main goal is to analyze the applicability of agglomerated cork as padding material in safety helmets. First, a finite element model of a motorcycle helmet available on the market was developed to assess its safety performance and to establish a direct comparison between expanded polystyrene and cork agglomerates as liners. Secondly, a new helmet model with a generic geometry was developed to assess the applicability of agglomerated cork as liner for different types of helmets, based on the head injury risk predictions by the finite element head model, YEt Another Head Model (YEAHM), developed by the authors. Several versions of helmet liners were created by varying its thickness and removing sections of material. In other words, this generic helmet was optimized by carrying out a parametric study, and by comparing its performance under double impacts. The results from these tests indicate that agglomerated cork liners are an excellent alternative to the synthetic ones. Thus, agglomerated cork can be employed in protective gear, improving its overall performance and capacity to withstand multi-impacts.

Multi-scale foam : 3D structure/compressive behaviour relationship of agglomerated cork

This study focuses on the microstructural aspects of a cork-based by-product known as agglomerated cork and its influence on the compressive mechanical behaviour. The material consists in granulates of a natural polymeric foam - cork - mixed together with a small quantity of a bio-sourced resin.Optical and scanning electron microscopy (SEM) are first used to investigate on the bead geometry and placement and interfaces arrangement. Then X-ray computed tomography allows to study the spatial arrangement of agglomerated cork microstructure and hence to complete and confirm 2D observations. 2D and 3D observations show a transverse anisotropic material which is confirmed by the mechanical tests. SEM pictures demonstrate an intricate and heterogeneous material. Microtomography confirms the presence of macroporosities between cork granulates having a mean volume around 0.1 mm3. Cork cell specific geometry is also confirmed. The volume of those cells lies around 10−5 mm3. Finally quasi-static compression tests are run to establish a link between microstructure and mechanical behaviour thanks to digital image correlation (DIC). Cork agglomerate demonstrates strong strain localisation at its surface caused by its multi-scale structure.

Graphene-Enriched Agglomerated Cork Material and Its Behaviour under Quasi-Static and Dynamic Loading.

The use of cork for a variety of applications has been gaining significance due to environmental concerns and political agendas. Consequently, its range of applications is growing rapidly. In this work, aiming to improve its mechanical response for crashworthiness applications, cork agglomerates were enriched by small quantities of graphene oxide or graphene nanoplates in order to observe a resulting improvement of the mechanical behaviour during quasi-static and dynamic compressive loading cases. To produce homogenous cork agglomerates including graphene, the material was previously dispersed into granulated cork using stirrers to achieve a good distribution. Then, the typical procedure of compression and curing was carried out. Magnified images attest a good dispersion of graphene into the cork matrix. Mechanical testing was performed for a variety of graphene concentrations (0.1, 0.5 and 1.0 weight %), becoming clear that the beneficial effect of including graphene (either oxide or nanoplates) is related to a later densification stage while keeping the same stress plateau levels.

Innovative use of giant reed and cork residues for panels of buildings in Mediterranean area

The environmental impact of buildings has to be assessed not only in reference to the energy consumed by their use but also with reference to the energy inside materials with which they are made of. The "Sick Building Syndrome" (SBS) is increasing. It was discovered that the major causes are linked to chemical contaminants from indoor sources such as building materials, inadequate ventilation, excessive use of Heating, Ventilation and Air Conditioning (HVAC) and volatile organic compounds (VOCs). The insulation building materials have a relevant role in the SBS for the capacity, not only to limit the use of HVAC but also to limit the emission of pollutants inside a building environment. The present paper reports an up- to- date review of some innovative uses of wastes deriving from agricultural production in order to build walls and partitions for Mediterranean houses. Some test methods of building elements, made with giant reed and agglomerate cork which are two typical natural materials of the Mediterranean area, are illustrated. These vegetal materials are often residues deriving from agricultural production, the agricultural residues are often a problem for farmers or firms because the organic wastes are considered dangerous and the disposal of such material is very expensive, therefore the reuse of the wastes is the best way to recycle these materials. This paper analyzes a cavity wall panel made with a wood skeleton on which two double crossed layers of giant reed stems were fixed and a multilayer agglomerated cork wall with a double cavity multilayer BOTH 20CM THICK. The dynamic thermal analysis carried out for the houses with the proposed walls highlights a better environmental performance of buildings with agglomerated cork and with giant reed walls rather than brick walls. The production of CO2 for the indoor environmental thermal control of the house with giant reed walls is less than 1/2 and the house with agglomerated cork walls is less than 1/4 compared to the brick wall house.

Lightweight Construction of Aluminum-cork Sandwich Composites in the Transportation Industry

Multilayer Plycork Aluminum (Al) composites were produced via vacuum bonding using a vacuum membrane press. The cladding material of the Plycork Al composites was an aluminum sheet, while the core material consisted of a natural cork agglomerate with varying thicknesses. Fire resistance tests and comparative static four-point bending tests were conducted on the Plycork Al composite samples. In addition, the characteristics of the deformation caused by the bending stress were compared between the Plycork Al composite and birch plywood. It was evident that this light composite can be applied as a structural material for transport purposes in railway vehicles due to its specific desirable properties. Additionally, the use of Plycork Al composites posed no toxic risk regarding the gases released during a fire and ensured vital operational safety because sharp edges did not form when the material was destroyed. In conclusion, this light multilayer composite can be frequently used to replace conventional materials employed in the transportation industry.

Experimental study of expanded cork agglomerate blocks – Compressive creep behavior and dynamic performance

The main goal of the present work is to experimentally evaluate the behavior of expanded cork agglomerate blocks subjected to static and dynamic loads. The study focuses on the compressive and creep effects and on the vibration isolation capability.First, compressive creep behavior and deformation recovery capability (after loading) tests were performed on blocks made of expanded cork agglomerate. The dynamic characterization then involved performing transmissibility tests for different static and dynamic loading conditions and evaluating the dynamic transfer stiffness and damping ratio of the blocks. The resulting transmissibility curves indicate how much vibration is transmitted through the isolator to its base at a given frequency. These tests were also performed under different conditions of strain to better understand the effect on the dynamic performance of the material, and to assess whether it might be a limiting factor.The results show that expanded cork agglomerate blocks, especially those with higher mass density, have physical and mechanical properties that provide this material with great potential for withstanding heavy static loads since small compressive creep effects and low permanent deformation was exhibited.The results also indicate that expanded cork agglomerate can be a good option for mitigating vibration. It was found that specimens with lower mass density and larger thickness performed better, with higher vibration isolation and a wider range of frequencies isolated. However, this behavior is limited by the influence of compressive stress on the blocks.

Evaluation of aging behavior under salt-fog spray conditions of green sandwich structures

ABSTRACTIn this paper, the aging resistance of sandwich structures made with cork agglomerate as core and flax-reinforced polymeric laminates as face sheets was investigated for the first time. In order to evaluate their durability in a hostile environment, a sandwich plate was manufactured by vacuum bagging technique and aged under salt-fog spray conditions, according to ASTM B 117 standard. The evolution of the sandwich mechanical properties during the aging exposition was analyzed by carrying out quasi-static flexural tests and Charpy impact tests both on unaged specimens and on specimens aged for 10, 25, and 60 days, respectively.The quasi-static experimental results showed that despite the salt-fog exposition leads to clear decrements in both maximum strength and modulus, the deformation capability of the sandwich structure evidences a noticeable improvement. According to this last result, it was found that the impact strength of the sandwich specimens was clearly increased after the salt-fog exposition evidencing that the studied green sandwich maintains good toughness properties in a hostile environment such as marine.

Experimental assessment of the shield-to-salt-fog properties of basalt and glass fiber reinforced composites in cork core sandwich panels applications

Abstract In this paper, the effect of salt-fog exposition on the mechanical behavior of sandwich panels having cork agglomerate core was investigated. In particular, the aim of this research work is to assess the shield function of unidirectional basalt and E-glass reinforced polymer on the aging and failure mechanism of the core when such class of material are destined to operate in hostile environments (e.g., marine environment). To do this, the manufactured sandwich panels were exposed up to 60 days to salt-fog environment, according to ASTM B 117 standard. Unaged and aged specimens were analyzed through three point bending tests after aging time of 10, 25 and 60 days. Digital Image Correlation (DIC) was carried out to perform a strain field analysis of the longitudinal cross-sections. The differences in the mechanical behavior of the synergistic cooperation of the cork core with basalt and glass fiber were highlighted thorough this technique. Moreover, the failure mechanisms related to each combinations for different aging times were described. The experimental results showed that, the intrinsic conditions of the composites (e.g., presence of defects, poor fiber-matrix adhesion), that are strictly linked to the fiber-matrix compatibility and to the impregnation quality, may induce to differences in the local mechanical behavior of the sandwich panels and to a faster aging process of the cork core in the case of basalt fiber skinned sandwich, especially in the first phases of the exposition.

Influence of a period of wet weather on the heat transfer across a wall covered with uncoated medium density expanded cork

A building materials’ properties have a great influence on its energy performance, and this is particularly true of the hygrothermal properties of the insulation material. The variation in moisture content generated by weather conditions can have a strong impact on heat transport. The phenomena involved are very complex and difficult to fully simulate numerically, particularly when the moisture variation occurs under exposure to rain. In the case of the medium density expanded cork agglomerate that is increasingly used without any coating as an external thermal insulation layer of buildings, these phenomena can be particularly relevant. Thus, this paper evaluates the importance of varying moisture content on the thermal behavior of a wall covered with uncoated medium density cork agglomerate when exposed to rain. A simple analytical model that simulates the heat transfer phenomenon was used to quantify its importance by comparing the mathematical results with those obtained experimentally.Steady and unsteady state conditions were simulated. For comparison purposes, winter and summer environmental conditions for both dry and wet weather were assumed. The results show that the thermal behavior of the wall is only affected in the rainy period, until the first few hours when the expanded cork board starts to dry. When rain is simulated, the temperature of the water has a key effect on the wall's behavior. The moisture transport phenomenon and changes in the hygrothermal material properties are almost entirely confined to the upper layers of expanded cork board and to the rainy period.

Strain Rate Effect on the Compressive Behaviour of Reinforced Cork Agglomerates

The demand for bio-sourced materials is currently increasing. Cork material because of its unique properties (fire resistant, energy absorbing, …) is then an excellent candidate for a large set of applications. In order to widen its possible uses, cork agglomerates with reinforcements at a 0.48 density were studied to compare their mechanical performances with classical cork agglomerates. This paper investigates the effect of these foreign reinforcements on the properties of agglomerated cork under a compressive loading. The material behavior has been determined as a function of the average strain rate and the direction of solicitation. The microstructure was first observed through optical and scanning electronic microscopy, spotting charges between each cork bead. The characterisation of cork at different strain rates was then carried out. An electromechanical testing machine was used to apply an uniaxial compression at quasi-static strain rates. Reinforced agglomerated cork was found to be anisotropic and strain-rate dependant. Its micro-structure reveals at complex composite material influencing strongly mechanical properties. Both Young's modulus and absorbed energy density at 0.6 strain increase with the cross-head speed displacement. From 12.7 MPa and 0.77 J.mm-3 when compressed at 0.05 mm·min-1 to 19.9 MPa and 1.44 J·mm-3 at 500mm·min-1 in the Off-plane direction.

Natural Cork Agglomerate as an Ecological Alternative in Constructional Sandwich Composites

The investigations presented in this article include a comparative study of static and fatigue four-point flexural tests performed for sandwich composites. The investigated composites consisted of a glass-epoxy laminate as a cladding material and core materials, such as synthetic foams and natural cork agglomerates, in different densities. The sandwich composites were prepared with the vacuum bagging method using the same resin, reinforcement, and additives. Although using cork agglomerate in sandwich composites instead of synthetic foam resulted in a decrease of the static flexural strength in such composites, it increased their resistance to fatigue cycles considerably and benefitted their eco-friendly image. However, only the reproducibility of all the factors in the production process and testing of composites allows a direct comparison of their test results to be made.

Gas-diffusion microextraction coupled with spectrophotometry for the determination of formaldehyde in cork agglomerates.

In this work, a simple methodology was developed for the extraction and determination of free formaldehyde content in cork agglomerate samples. For the first time, gas-diffusion microextraction was used for the extraction of volatile formaldehyde directly from samples, with simultaneous derivatization with acetylacetone (Hantzsch reaction). The absorbance of the coloured solution was read in a spectrophotometer at 412nm. Different extraction parameters were studied and optimized (extraction temperature, sample mass, volume of acceptor solution, extraction time and concentration of derivatization reagent) by means of an asymmetric screening. The developed methodology proved to be a reliable tool for the determination of formaldehyde in cork agglomerates with the following suitable method features: low LOD (0.14mgkg-1) and LOQ (0.47mgkg-1), r 2 = 0.9994, and intraday and interday precision of 3.5 and 4.9%, respectively. The developed methodology was applied to the determination of formaldehyde in different cork agglomerate samples, and contents between 1.9 and 9.4mgkg-1 were found. Furthermore, formaldehyde was also determined by the standard method EN 717-3 for comparison purposes; no significant differences between the results of both methods were observed. Graphical abstract Representation of the GDME system and its main components.

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.

Performance of Expanded High-Density Cork Agglomerates

AbstractPrototype boards of high-density expanded cork agglomerates (230 and 290 kg m−3) were produced and tested for tensile strength (before and after moisture resistance cyclic tests), hardness, thermal conductivity, and reaction to fire. Increases in tensile strength and dimensional stability were observed with the increase in density. No significant change in thermal conductivity was noticed. The application of a varnish resulted in a noticeable increase in hardness for the higher-density cork agglomerate. Reaction to fire was improved from class Efl to class Cfl. The improvement of properties of expanded cork agglomerates from densification can enhance their application in the building sector.

Cork as a building material: a review

This review focuses on cork as a natural, renewable and sustainable construction raw-material. Cork has an unusual combination of properties making it suitable for application in buildings and infrastructures, for example insulation, wear-resistance and durability. The material properties combined with a favourable ecological footprint allow designers, architects and engineers to meet some of the Green Building demands. A summary on cork production, structure, chemistry and properties was made. The processing into cork-based products, for example cork agglomerates and composites, is detailed as well as their properties and applications in construction. The aptitude of cork-based products for compliance with sustainability and energy efficiency criteria is also addressed.