Comparing the mechanical performance of synthetic and natural cellular materials

This work compares the mechanical performance of agglomerated cork against synthetic materials typically used as impact energy absorbers. Particularly, the study will focus on the expanded polystyrene (EPS) and expanded polypropylene (EPP).Firstly, quasi-static compression tests are performed in order to assess the energy storage capacity and to characterize the stress–strain behavior cellular materials under study. Secondly, guided drop tests are performed to study the response of these materials when subjected to multiple dynamic loading (two impacts). Thirdly, finite element analysis (FEA) is carried out in order to simulate the compressive behavior of the studied materials under dynamic loading.Results show that agglomerated cork is an excellent alternative to the synthetic materials. Not only for being a natural and sustainable material but also for withstanding considerable impact energies. In addition, its capacity to keep some of its initial properties after loading (regarding mechanical properties and dimensions) makes this material highly desirable for multiple-impact applications.     

The application of fracture mechanics to the testing of wood

The problem of a mode I fracture toughness of wood is considered. After a short discussion of relevant literature the test results concerning mode I fracture on three types of wood and the obtained values of stress intensity factor, K _Ic, are discussed. The compressive and tensile strength of the wood fibres and flexural strength are also presented. A considerable variation of the stress intensity factor, K _Ic, has been found to depend on the wood species and the direction of taking specimens for tests. The character of a failure process and the obtained values of the stress intensity factor, K _Ic, were determined by interrelations of cohesion forces existing between particular components of the wood structure, and by anisotropy of the wood. Both the compressive and tensile strength tests performed along the fibres and the bending strength tests crosswise to the fibres have not confirmed the tendencies observed in the fracture toughness tests. The investigations performed show the usefulness of fracture mechanics for evaluation of the strength properties of wood. It is concluded that materials science must consider wood as a valuable and rewarding material upon which to focus research efforts.     

Development and properties of advanced composites based on cork and nanometric silicon carbide-filled phenolic resin

This paper presents the obtaining of advanced materials based on cork powder as reinforcement and phenolic resin (PR) with silicon carbide (nSiC) nanofiller as matrix with potential applications in aerospace industry. Three formulations were obtained: one control sample PR/cork with no nanofiller, two nanofilled samples with 1 and 2 wt% nSiC loadings into the resin. The materials were tested by flexural and compressive mechanical tests to determine their strength and stiffness, to determine their friction coefficient by tribological tests, to determine their thermal decomposition behaviour by TG-DSC analysis and to evaluate their thermal behaviour by thermal shock tests when subjected to extreme temperature directly from room temperature. The material structure was analysed by SEM visualizing the fracture cross-section after mechanical testing. The test results illustrate that silicon carbide nanoparticles improve flexural and compressive strength, but also stiffness and friction coefficient, delay thermal decomposition onset and improve thermal shock resistance. All these sustain the PR/nSiC/cork materials as potential advanced materials candidates for thermal protection applications.     

Modelling impact response of agglomerated cork

Cellular materials have been intensively used in engineering applications where a good energy absorption capability is a desired feature. Cork is a natural cellular material capable of absorbing considerable amounts of energy. When compared to synthetic cellular materials, cork also appears as a sustainable alternative, once it is fully recyclable. The purpose of this work is to simulate cork’s compressive behaviour when subjected to impact, including the material’s relaxation after dynamic compression. This study comprises experimental and numerical tests at quasi-static and dynamic strain rates under axial compressive loading. Numerical simulations are performed using Finite Element Analysis, and the material model developed is validated against experimental results. After validation, a dynamic test resorting to a drop tower is carried out successfully validating the model and representing adequately cork’s mechanical behaviour under dynamic compressions.     

Development of recycled strain-hardening cement-based composite (SHCC) for sustainable infrastructures

This paper presents the experimental results of an attempt to develop sustainable strain-hardening cement-based composite (SHCC) using recycled materials. SHCC exhibits desirable mechanical properties, including strain hardening and ductility. However, SHCC is composed of silica sand and a high volume of cement, which makes it more energy intensive than conventional concrete. The aim of this study is to promote SHCC sustainability in infrastructure design through the use of recycled materials. Alternative recycled materials – sand, fly ash, and polyethylene terephthalate (PET) fibers – are used to partially replace silica sand, cement, and polyvinyl alcohol (PVA) fibers, respectively, in SHCC specimens. The effects of the recycled materials on the mechanical behavior of the SHCC specimens are examined by conducting compressive tests, four-point bending (flexural) tests, and uniaxial tensile tests. Fundamental information is then used in the constitutive model to analyze and design infrastructures using SHCC with recycled materials. Test results indicate that fly ash improves both the bending and uniaxial tensile performance of SHCC due to an increase in chemical bond strength at the interface between the PVA fibers and cement matrices. However, SHCC that contains PET fibers does not perform well in the bending and uniaxial tensile tests due to the inferior material properties of the PET fibers, although its compressive behavior is similar to that of the PVA2.0 specimen. Also, it is noted that recycled sand increases the elastic modulus value of SHCC due to its larger grain size compared to that of silica sand. Based on the desire to maintain well-performing SHCC, a replacement ratio below 20% for fly ash or below 50% for recycled sand is deemed appropriate for creating sustainable SHCC, as concluded from this study.     

Morrocan composite cement with minor addition of fly ash

In order to lower the consumption of natural raw materials, to save fuel energy for clinker production and to make appreciable reduction of CO₂ emission, the Morrocan plants cements become to produce some composite cements types by adding some components. This paper describes the production of composite cements by intergrinding clinker, gypsum and limestone with a minor addition of fly ash up to 10%. Physical and mechanical properties are discussed, and the main result is that the addition of fly ash with low quantity acts as grinding agent by reducing the required time to obtain the same percentage of particles retained on an 80 μm sieve compared to the cement without addition of fly ash. From 28 days to 90 days, the compressive strength increases rapidly in the case of cement with a minor addition of fly ash.     

Analysis of the tensile behaviour of viscoelastically prestressed polymeric matrix composites

A novel composite material is reported, in which tension, applied to polymeric fibres, is released prior to moulding them into a matrix. Following matrix solidification, compressive stresses imparted by the viscoelastically strained fibres impede crack propagation. Previous Charpy impact studies had demonstrated that these viscoelastically prestressed composites could absorb typically 25–30% more energy than control (unstressed) counterparts and the current study focuses on their tensile behaviour as a function of fibre volume fraction, V_f. Tensile testing was performed on continuous unidirectional nylon 6,6 fibre–epoxy resin samples. Compared with control counterparts, the results showed that viscoelastic prestressing improved tensile properties, the effects being V_f-dependent. Increases in tensile strength, modulus and energy absorbed (to 0.25 strain) exceeded 15%, 30% and 40%, respectively, at an optimum V_f, this being 35–40%. Strain-to-failure was reduced by 10–20%, thereby lowering any improvement in tensile toughness (energy absorbed to fracture) to <10%. Mechanical properties of the fibres themselves were not significantly influenced by the treatment used for generating composite prestress, and we propose that the observed improvements to tensile properties may be attributed to: (i) direct contribution from compressive stress, (ii) attenuation of the dynamic overstress effect on fibre fracture and (iii) improved mechanical integrity through a more collective response from fibres to tensile loads.     

Analysis of the effect of mechanical recycling upon tensile strength of a short glass fibre reinforced polyamide 6,6

The effect of mechanical recycling upon tensile strength of an injection moulded polyamide 6,6 reinforced with 35% by weight of glass fibres has been experimentally investigated. Tensile tests have been conducted on specimens made of virgin material and containing different percentages of mechanically recycled material. Mechanical recycling consisted of regrinding of specimens and further injection moulding the granules into specimens of the same type. The main effect of this type of recycling is fibre breakage with consequent decrement of fibres contribution to composite strength. The results from the experimental tests have been compared with predictions obtained by applying a micro-mechanical model, which allowed taking into account the fibre length distribution and the properties of the phases of the composite. The model appeared to be a useful tool in the eco-design methodology, where the knowledge of property change of recycled material against those of the virgin one is necessary in the assessment of the environmental impacts of different recovery options.     

On the separation and recycling behaviour of textile reinforced concrete: an experimental study

This paper presents the results of recent experiments on the recyclability of the textile components in textile reinforced concrete (TRC). TRC as a multi-component system often contains organic ingredients such as carbon fibres and polymer impregnations. Consequently, the recycling of TRC is not trivial and has not yet been sufficiently clarified until now. In this study, an impregnated, bi-axially reinforced, and warp-knitted textiles made of carbon fibres was used in combination with a fine grained concrete. Flexural tests on TRC specimens containing recycled epoxy-impregnated carbon reinforcement were performed, whereby the recycling was simulated by a pre-treatment of the carbon fibre material in a jaw crusher. The results showed a pronounced decrease in flexural strength compared to untreated carbon reinforcement. Moreover, three different crushing methods were investigated with respect to their influence on the recovery of styrene-butadiene-rubber impregnated carbon textiles. Besides jaw crushing and impact milling, crushing with a hammer mill showed the best degree of purity but also caused the highest mechanical damage to the textile. The impact of material, structure of the composite and crushing methods on the separation behaviour could be deduced from the experiments.     

Mechanical analysis and toughening mechanisms of a multiphase recycled CFRP

The mechanical response of a recycled CFRP is investigated experimentally. A complex multiscale microstructure is revealed, with both dispersed fibres (with fractured-sections) and fibre-bundles. The specific properties of the recyclate compare favourably with those of aluminium and glass–fibre composites. Micromechanical studies show that tensile failure follows the pre-existing fractured-sections on the dispersed-fibres, while compressive failure occurs by shear-banding. Fracture toughness measurements coupled with SEM evidence how bundles considerably toughen the composite by complex failure mechanisms. This analysis can guide the optimisation of recycling processes and support the development of design methods for recycled CFRP; it also provides insight on the mechanical response of other multiphase short-fibre reinforced materials.     

Properties of cellulosic fibre reinforced plaster: influence of hemp or flax fibres on the properties of set gypsum

In the last few years, eco friendly materials have become an important part of the building materials market. Natural fibres are already used in various types of materials, like plastics, concrete and lime-based products. They demonstrate different attributes like the combination of good mechanical, thermal and acoustic properties that allow these types of materials to be used for different applications. The main drawback associated with plaster is its brittleness, especially under tensile stress. Therefore, it is interesting to investigate different methods that could potentially enhance the mechanical properties of plaster. Adding fibres to gypsum to obtain a composite material is one way to improve the behaviour of the product, especially after the failure of the matrix. The aim of this work was to the study the effects of adding natural fibres, namely hemp and flax fibres, on the setting time of plaster and the mechanical properties of the composite matrix. It was shown that hemp delayed the setting of plaster, unlike flax. The initial and final setting times almost doubled when hemp was added in a plaster matrix, whereas flax fibres did not drastically change them. Different chemical treatments of hemp were tested and the impact on the setting time was measured. The setting times of both composites made with hemp and flax were reduced once the fibres were treated (25–40% reduction), compared to the setting time of the calcium sulphate hemihydrate alone. The mechanical properties of the composite materials are also discussed. The behaviour of plaster was modified from brittle to a non-linear one when fibres were added, and even at small levels of addition, flax fibres allowed slightly higher values of flexural strength to be reached.     

再生钢纤维增韧超高性能混凝土的力学性能

采用来自于废旧轮胎的两种再生钢纤维制备含粗骨料的超高性能混凝土,并测定其抗压强度、劈裂抗拉强度、断裂能和静弹性模量等力学性能,空白组及普通钢纤维增韧超高性能混凝土作对比性能试验。结果显示,未附着橡胶颗粒的再生钢纤维使超高性能混凝土的抗压强度略微下降,降低幅度为3.91%,其余各类型钢纤维均有利于提高超高性能混凝土的力学性能;而附着橡胶颗粒的再生钢纤维显著提高了超高性能混凝土的断裂能,约为普通钢纤维增韧超高性能混凝土的4倍。此外,再生钢纤维对超高性能混凝土的劈裂抗拉强度和静弹性模量的提高效果均优于普通钢纤维。再生钢纤维,尤其是附着橡胶颗粒的再生钢纤维,可以作为一种增韧材料替代普通钢纤维应用到超高性能混凝土工程结构中。      

结构参数对树脂基纤维编织复合材料折叠夹芯结构力学性能的影响

折叠夹芯结构是一种新型的复合材料夹芯结构,其结构参数对力学性能有重要的影响。文中以碳纤维和Kevlar平纹编织预浸料为芯材原料,采用热压工艺,制备了复合材料折叠夹芯结构试样。通过压缩试验得到不同条件下折叠夹芯结构在静态压缩载荷作用下的力-位移变化曲线。构建了复合材料折叠夹芯结构有限元模型,对不同结构参数复合材料折叠夹芯的力学性能进行了数值模拟分析,并将模拟结果与实验结果进行对比验证了模型的可靠性。实验及数值模拟的分析结果表明,随着芯材厚度的增加,折叠夹芯层的压缩强度呈线性增加,其破坏形式由假塑性变形逐渐向脆性破坏转化;面板对夹芯层的约束作用能够极大地提高压缩模量和强度,而且上下面板对压缩性能曲线有着不同的影响;折叠夹芯单元的高度、长度、折叠夹角等参数对其力学性能具有不同程度的影响。     

INTERLAMINAR FRACTURE ENERGY OF UHMPE/EPOXY COMPOSITES BY DOUBLE CANTILEVER BEAM AND PEEL TESTS

Abstract— Interlaminar mechanical properties of composite materials such as shear strength and fracture toughness depend on the level of fibre-matrix adhesion which has to be optimised according to the end-use of the composite. Various surface treatments of fibres are used for this purpose. Double cantilever beam (DCB) tests are commonly used for estimating the interlaminar fracture toughness in Mode I. It is shown in this work that this parameter can be conveniently determined using a simpler technique involving a 90° flexible-to-rigid substrate peel test. The values of G_Ic determined by DCB and 90° peel tests are comparable within acceptable experimental error margins. These two alternative techniques are used for assessing the effectiveness of a novel surface engineering process for enhanced adhesion of ultra-high modulus polyethylene (UHMPE) fibres to an epoxy matrix.     

Static and dynamic testing of a quasi-isotropic composite with embedded optical fibres

An efficient use of composite materials in loaded structures requires NDT techniques that can reliably monitor the damage state of these materials in situ and continuously during service. A promising solution to this problem is the incorporation of optical fibres into the composite structure during manufacture. However, because optical fibres are always an order of magnitude bigger than material fibres, stress concentrations will inevitably be created which can lead to premature damage initiation and thus to a reduction in the mechanical properties. Therefore the first step in the development of a damage detection system based on optical fibre technology always has to be an investigation of the mechanical properties of the resulting structures. In this paper, optical fibres were incorporated in the different interfaces of a quasi-isotropic composite laminate. Both static and dynamic mechanical tests were carried out to determine the influence of the optical fibres on the mechanical properties of the resulting composite structures. Differences in behaviour between the different configurations were correlated with differences in damage propagation.     

Influence of the nanocellulose raw material characteristics on the electrochemical and mechanical properties of conductive paper electrodes

Paper-based conductive electrode materials of polypyrrole (PPy) and nanocellulose (NC) have received much attention lately for applications in non-metal-based energy storage devices, ion exchange, etc. The aim of this study was to study how the primary characteristics of NC raw materials impact and electrochemical properties of conductive NC–PPy composite sheets. Three NC raw materials were used: Cladophora cellulose (NC_UU) produced at Uppsala University, Cladophora cellulose (NC_FMC) produced at FMC Biopolymer, and microfibrillated cellulose (NC_INN) produced at Innventia AB. Composite paper sheets of PPy coated on the substrate NC material were produced. The NC raw materials and the composites were characterized with a battery of techniques to derive their degree of crystallinity, degree of polymerization, specific surface area, pore size distribution, porosity, electron conductivity, charge capacity and tensile properties. It was found that the pore size distribution and overall porosity increase upon coating of NC fibres for all the samples. The charge capacity of the composites was found to decrease with the porosity of the samples. It was further found that the mechanical strength of the pristine NC sheets was largely dependent on the overall porosity, with NC_INN having the highest mechanical strength and lowest porosity in the series. The mechanical properties of the composite NC–PPy sheets were significantly diminished as compared with pristine NC sheets because of the impaired H-bonding between fibres and PPy-coated nanofibres. It was concluded that to improve the mechanical properties of PPy–NC sheets, a fraction of additive bare NC fibres is beneficial. Future study may include the effect of both soluble and insoluble additives to improve the mechanical strength of PPy–NC sheets.     

Fractography of interlaminar fracture surfaces of CF/PI and CF/BMI composites

Interlaminar fracture is recognized as an important mode of failure of composite materials and structures. In order to characterize twobismaleinimide-matrix (BMI) composites and two polyimide-matrix (PI) composites regarding their delamination behaviour, interlaminar fracture tests in mode I, mode II and mixed-mode loading conditions were carried out. The aim of this study was to examine the fracture surfaces and to find relationships between features of the fracture surface and the corresponding mechanical data. The characteristic features of failure have been pointed out and the changes of the features with variations in matrix material, testing rate and loading mode have been shown. The results of the mechanical testing can be explained by means of SEM images.     

A study of the mechanical behaviour on fibre reinforced hollow microspheres hybrid composites

This paper presents the results of an investigation into the effects of hollow glass microsphere fillers and of the addition of short fibre reinforcements on the mechanical behaviour of epoxy binding matrix composites. Properties like flexural stiffness, compressive strength, fracture toughness and absorbed impact energy, were studied. The specimens were cut from plates produced by vacuum resin transfer moulding having a microsphere contents of up to 50% and with fibre reinforcement up to 1.2% by volume. The tests performed with unreinforced composites show that flexural and compressive stiffness, maximum compressive stresses, fracture toughness and impact absorbed energy decrease significantly with increasing filler content. However, in terms of specific values, both flexural and compressive stiffness and impact absorbed energy increase with microsphere content. The addition of glass fibre produces only a slight improvement in the flexure stiffness and fracture toughness, while increasing significantly the absorbed impact energy. In contrast, the addition of a small percentage of carbon fibres produces an important improvement in both fracture toughness and flexure stiffness, when hybrid composites with 0.9% carbon fibre are compared to unreinforced foam, but did not improved absorbed impact energy.     

The effects of temperature,pressure and water on the preparation of glass-mica composite material

A systematic investigation on the fabrication of glass-mica composite materials from recycled colourless soda-lime glass powders and phlogopite-type mica powders has been conducted. Mixtures of two specific compositions of the glass-mica system were used and the investigation was based on several chosen processing parameters. When compacted powder samples were sintered at temperatures in the range 780 to 900° C, samples of one composition formed a composite material having a cellular structure; whereas samples of the other composition formed a composite material having a highly-dense ceramic structure. Sample evaluations showed that both the sintering temperature and the quantity of water which is added to the glass-mica mixtures as wetting agent in the powder compaction process are sensitive processing parameters. They can control the physical, mechanical and thermal properties of the glass-mica composite material. It was found that when glass-mica dry mixtures were prepared with the addition of a quantity of water equivalent to about 10% of the sample weight and sintered at the temperature of 850° C, the resultant composite material exhibited optimum physical, mechanical and thermal properties. The compressive strength and thermal insulating value of the glass-mica composite material with the densified structure are found to be superior to those of several conventional building materials, such as masonary products, lightweight concrete and soda-lime glass components. The experimental findings suggest that the glass-mica composite material is a potential structural element for building construction applications as it may contribute to energy conservation.     

The effects of deforming knitted glass fabrics on the basic composite mechanical properties

The effects of deforming knitted fabrics on the tensile and compressive properties of their composites have been investigated for the weft-knit Milano rib fibre architecture. The properties have been studied for both the course and wale directions for composites with fabrics deformed in either of the two directions. It was found that any change in the mechanical properties of the deformed composites with respect to their undeformed counterpart is strongly related to the changes in the knit structure brought about by the induced deformation to the knitted fabric. Deformation in the knitted fabric also affects the tensile fracture mode whereby increased deformation, be it wale- or course-wise, transforms transverse fracture to shear fracture in either loading axis. On the contrary, the compressive fracture mode is insensitive to fabric deformation. Fractographic studies using stereo-optical and scanning electron microscopy have further revealed that tensile failure is caused by fibre breakages occurring at two locations of the knitted loops—one, at the leg components and, two, at fibre crossover points, whilst compression failure is controlled by Euler buckling of the looped fibres of the knitted composite. All these characteristics were revealed to be related to the microstructure of the knitted composite laminates.