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Weight reduction is often a key factor in modern mechanical design, therefore materials such as carbon fibre reinforced plastics (CFRP) are increasingly used and integrated within multimaterial structures using adhesive technologies, which require high effort and are difficult to disassemble. The capability of a blind rivet nut (BRN) to join different materials without these disadvantages has created a growing industrial interest in the fastener. However, installing a BRN in CFRP laminate induces a significant stress concentration in the plate, which potentially causes damage. Given that ‘damage free’ joints are demanded by the industry, the BRN is often not considered as a suitable joining technique. In the present research, an experimental campaign is performed to investigate the fatigue resistance of a BRN joint in CFRP. It is demonstrated that the resulting compressive stress after installing a BRN can enhance the fatigue resistance of the specimen. The results increase the potential of the BRN as a fastener for CFRP. ispartof: Key Engineering Materials vol:926 pages:1498-1504 status: Published online

Thermoplastic-based composites are becoming the preferred material for structural and non-structural components in the composite production industries. One of the most critical elements determining the characteristics and performance of fiber-reinforced composites is the fiber content. The fiber volume fraction (Vf) of carbon/polyamide410 (C-PA410) composite is evaluated using standard acid digestion, the unique two-stage thermogravimetry (TGA) method and 2D image processing method, with Vf values of 24.0%, 23.1%, and 18.3%, respectively. For statistical purposes, a substantial number of samples from each composite batch were tested and analyzed. The two-stage TGA approach used in this study, involves measuring the constituent content of carbon fiber thermoplastic composites in nitrogen first and subsequently in air. The standard acid digestion and the two-stage TGA method used in this study provided comparable results. The 2D image processing method, on the other hand, yields a lower volume fraction of fiber. The two-stage TGA approach has been proven to be more convenient in terms of accuracy, safety, and speed. This technique has been validated by testing a control specimen of a material (Toray Cetex® TC910 PA6) with a known weight fraction.

Expanded polystyrene (EPS) is a material that is widely used in energy absorbing applications, especially in helmets, despite its non-renewable origin. Cork and its derivatives, however, are proposed as a substitute for polystyrene foam (EPS) due to their renewable origin and their easy recyclability. In spite of the low-environmental footprint of cork and its derivatives, there is insufficient data on their mechanical behaviour. Consequently, under dynamic and quasi-static loads, four different-density EPS, a natural cork material and five different cork products with different grain sizes and heat treatments were tested. They were compared in terms of their stress–strain and specific stress–strain curve, their volumetric capability to absorb energy, their specific energy, average decelerations and peak deceleration. Finally, EPS foams cannot recover their initial shape upon deformation due to their low resilience capability. This is especially important in applications such as helmets, which are bound to be subjected to multiple impacts. However, cork and its products could have this capability for resilience and would therefore be more suitable for certain applications. ispartof: WOOD SCIENCE AND TECHNOLOGY vol:55 issue:2 pages:419-443 status: published

Objective Medical data has lead to the common understanding that bicycle helmets need to be improved to better protect against brain injuries resulting from rotational acceleration. Although many different technologies exist for reducing rotational acceleration during impacts, the lack of an official testing standard means that their evaluation is based on customized set-ups that may differ and not represent real accident conditions. Previously, the authors have shown that scalp tissue plays an important role during helmet testing by absorbing energy and creating a low friction interface between head and helmet, thus reducing rotational accelerations and velocities. However, no published study has yet examined the effectiveness of anti-rotational helmet technologies in the presence of a biofidelic scalp layer. The objective of this study is to address this gap. Methods Three different commercially available helmet models, each one equipped with a different technology, were tested in the presence of scalp tissue, in two different scenarios; with and without the technology present. The effectiveness of each of these technologies is already documented in other studies, but only in the absence of a biofidelic scalp layer. Tests were carried out using HIII headform with porcine scalp attached to the outmost layer. Motion tracking was used to compare the impact kinematics of each helmet model in both scenarios. Results Results showed that when a biofidelic scalp layer is present, there is no statistical difference between helmet models with and without the anti-rotational technology in terms of rotational acceleration, velocity, relative rotation, impact duration and injury risk. Conclusions Results suggest that the presence of the scalp can obscure the functionality of anti-rotational acceleration technologies. This could indicate that the effectiveness of technologies tested in previous studies, which have not tested anti-rotational acceleration technologies in the presence of a realistic scalp layer, may exaggerate the contribution of such technologies if compared with a more biofidelic set-up. The study supports the fact that headforms should be better designed by incorporating artificial skin layers that can better imitate scalp's behavior and, in addition, provides insights for the design of technologies against rotational acceleration.

The translaminar fracture toughness is a key property that governs the damage tolerance and notch sensitivity of fibre-reinforced composites. Compact tension tests were performed to investigate the translaminar fracture toughness of composites reinforced with three types of high-performance polymer fibres: polyarylate (PAR), polybenzobisoxazole (PBO) and aramid fibre. A carbon fibre composite was used as the reference system. The propagation translaminar fracture toughnesses of the PAR and PBO fibre composites were 492 kJ/m2 and 547 kJ/m2, respectively. These are among the highest translaminar fracture toughness values recorded in the literature. It was hypothesized that the fibrillation of the fibres upon failure was an important energy dissipating mechanism alongside pull-outs that were much longer than for carbon fibre. Replacing a small strip of a carbon fibre ply by a strip of PAR or PBO fibres successfully reinforced the material by locally arresting crack growth. By contrast, the performance of the aramid fibre composites and their hybrids with carbon fibre was lacklustre. The results presented in this work can be used to further improve the safety of composite parts by optimising the design for damage tolerance while also reducing the weight of the parts. ispartof: Composites Science And Technology vol:221 status: published

Oblique impact is the most common situation that cyclists experience during traffic accidents during which the human head undergoes both linear and rotational (angular) accelerations. Angular acceleration of the head is known to be linked to the majority of traumatic brain injuries. This paper proposes various solutions to mitigate angular accelerations of which an anisotropic column/matrix composite foam design is the most effective. This smart design allows tailor-made adjustment of shear and compressive resistance of the foam liner. Regarding helmet shells, tough fiber-reinforced composite materials such as self-reinforced polypropylene (PP) (Curv®) and silk/high-density polyethylene (HDPE) were benchmarked against conventional brittle polycarbonate (PC). Results demonstrate the superior performance of silk/HDPE composite compared to PC in resisting perforation in localized impact involving sharp objects. Regarding the helmet liner, two configurations were studied particularly, a multi-layered and column/matrix design. Their efficacy was benchmarked against single-layer homogenous expanded polystyrene (EPS) foam of equivalent weight and thickness in linear and oblique impact using experimental and finite element methods. The results showed the superior behavior of the column/matrix configuration. Such smart design could be combined with other smart systems such as multi-directional impact protection system (MIPS) technology for possible synergy and enhanced performance in head protection.

Natural fibre-based materials offer various advantages compared to synthetic fibres, however their applications are limited mainly due to their hygroscopic properties, which are affected by their chemical composition, microstructure and the porosity of the plant cells of which the fibre is composed. Therefore, this work investigates the hygroscopic behavior of natural fibres to obtain a better understanding of the relation of the chemical composition of the fibres, their crystallinity, and their equilibrium moisture content. The crystallinity index was determined to include amorphous cellulose into the developed models. Nine biomass samples were selected (flax, hemp, jute, spruce, bamboo, corn stalks, palm leaves, rice husk and wheat straw) to construct models via linear regression to predict the moisture sorption behavior of natural fibres. Thorough statistical (ANOVA, RMSE) analysis showed that the developed models are relevant and descriptive. From all major plant cell wall constituents (lignin, crystalline cellulose, amorphous cellulose and hemicellulose), it is hemicellulose's hygroscopicity that is largely responsible for the moisture uptake of the fibres, with (amorphous) cellulose and lignin playing a (much) smaller role. This study has improved the understanding of the hygroscopic behavior of natural fibres, and is important for optimal application of these fibres in composite materials.

The application of enzymes as alternative to dew retting of flax was studied in correlation to the characteristics of composites reinforced with these natural fibers. Fiber fineness and mechanical properties of biocomposites were evaluated. Furthermore, moisture absorption by biocomposites was studied and fracture surfaces were investigated using Scanning Electron Microscopy. Compared to dew retted fiber composites, improvements in mechanical performance can be observed for composites impregnated with fibers extracted after enzymatic treatments. All enzymatic treatments resulted in finer fibers than green fibers and led to biocomposites with a reduced equilibrium moisture content and lower diffusion coefficient. This study illustrates the high potential of enzymatic retting, in particular with polygalacturonase. Also, the manual extraction procedure used, produced fibers with an E-modulus up to 84 GPa and strength up to 800 MPa, likely due to reduced fiber damage, which illustrates the hidden potential of flax fibers ispartof: COMPOSITES PART A-APPLIED SCIENCE AND MANUFACTURING vol:123 pages:190-199 status: Published online

More than 1200 bamboo species are known; therefore, the question arises which species are suited for composites and which microstructural characteristics of the plant are decisive. Three different bamboo species are considered, each of two ages. Technical fibre tensile tests and impregnated fibre bundle tests were performed to characterize the fibres and maceration was applied to determine the length over diameter ratio of the elementary fibres. From the impregnated fibre bundle tests it is concluded that the stiffness of bamboo fibres from tropical species considered in this research is unaffected by the species nor the age of the plant. The strength is more affected by the region of growth, with the temperate bamboo species having lower strength as a result of a lower length over diameter ratio of the elementary fibre. Chemical analysis helped to investigate the age difference and it can be stated that the fibres are mature within one growing season.

© 2018 Elsevier Ltd The strain-rate sensitivity and stress relaxation of self-reinforced polypropylene (SRPP) and hybrid carbon fibre/SRPP composites were analysed under bending loads. For this purpose, three point bending static tests were performed at room temperature and displacement rates of 200, 20, 2, 0.2 and 0.02 mm/min. Tests of stress relaxation were also carried out, where a fixed strain was applied and the stress was recorded during the loading time. The study concluded that, for all materials, the bending stress and strain at the maximum bending stress were sensitive to the strain rate. The bending modulus, however, was found to be insensitive to the strain rate for hybrid composites, and sensitive (with a linear increase) for SRPP composites. From the stress relaxation tests and for both materials, the stress decreased with time and this decrease was more significant for higher strains. The results were fitted following the Kohlrausch-Williams-Watts model, evidencing good accuracy of the model for the stress relaxation time. ispartof: COMPOSITE STRUCTURES vol:209 pages:802-810 status: published

Despite the higher specific mechanical properties and the lower density of polymeric foams, these materials present cumulative damage behaviour that implies in the second and successive impacts, their mechanical properties decrease drastically. However, cork and cork agglomerates have the ability to absorb multiple impacts so they could be a more suitable material in some products, such as bumpers and helmets. This article is focused on the study of five different cork agglomerates and a natural cork under four different maximum deformations subjected to four consecutive compression loading cycles. Main diagrams, such as the stress–strain, energy density and efficiency, and the variation in diverse parameters, such as the absorbed energy density and maximum efficiency, were investigated and compared with an expanded polystyrene foam.

Cork and cork agglomerates could be suitable replacements for petroleum-based polymeric foams due to their similar internal structure of cells and grains. Additionally, cork products have a renewable origin and are recyclable. Despite these notable properties, few studies have analysed the mechanical properties, especially the specific properties, of these materials under compressive loads. Moreover, although efficiency, ideality, and energy-normalized stress diagrams are commonly used for polymeric foams and 3D-printed lattice structures, these types of diagrams are not yet applied to cork products. It must be highlighted that efficiency diagrams are plotted only against nonspecific properties so, this article proposes additionally the use of nonspecific properties to compare materials not only in terms of properties per unit volume instead but also in terms of properties per unit mass that is more suitable for certain applications in which the weight is crucial. The materials studied herein include three different white cork agglomerates, a brown cork agglomerate, a black cork agglomerate, natural cork, and expanded polystyrene foam, which are subjected to quasi-static compressive loads.

Expanded polystyrene (EPS) foam is a material, frequently used in a range of applications for its good energy absorption, thermal insulation, durability, and acoustic absorption. In this wide range of applications, the material undergoes a variety of loads that can extend from static to dynamic. When it comes to crush applications, these loading profiles create the need of well-defined compression and tensile properties for a range of densities and strain rates. Herein, static and dynamic tension tests are conducted on EPS foam dog-bone samples for material characterization. The target of the study is to obtain stress–strain curves for a range of densities spreading from 60 to 120 g L-1. For the low-strain rates, tensile testing machines are used, whereas for the high-strain rates, a modified drop tower set-up is used. The influence of the strain rate on the stress–strain behavior, the dynamic strength, Young''s modulus, and energy absorption capacities for different foam densities is measured. Furthermore, the experimental results for each density are compared with the predictions of Avalle''s model for crushable foams to evaluate the validity of the model. The quality of fit obtained between experimental and theoretical is between 70% and 97% depending on the strain rate.

Interlayer hybrid carbon fiber/self-reinforced polypropylene composites possess a rare combination of lightness, stiffness and ductility, but they do suffer from a catastrophic stress drop when the carbon fiber layer fractures. To promote a gradual failure of the hybrids, we introduced discontinuities in the carbon fiber layer by partially cutting it at multiple locations perpendicularly to the fiber direction. By altering characteristics of the discontinuities (their length and number over the specimen width), we were able to influence the failure mechanisms and tensile performance of these hybrid composites. When the cut length was increased to 15 mm, the carbon fiber layer fragmented and delaminated at the cut sites inducing a gradual failure development in the hybrid. The concept of fracture process zone was applied to explain the failure mechanisms in these hybrid composites.

This paper performs a systematic study of the effect of various unidirectional fibre patterns, ranging from different overlapping lengths of adjacent fibre bundles to a complete randomization of the position of individual fibre ends. The study is benchmarked with the mechanical behaviour of a fully continuous unidirectional bamboo fibre-epoxy composite and will show the feasibility of using UD discontinuous technical bamboo fibres in continuous preforms for high-end composites applications. The study shows that the tensile stiffness is hardly influenced by the discontinuity patterns, while the introduction of randomized fibre end discontinuities (individual fibres with segment length 50 mm) leads to a preservation of 85% of the longitudinal tensile strength in comparison with a UD continuous bamboo fibre composite.

The viscoelastic behaviour of self-reinforced polypropylene (SRPP) was investigated under bending loads. For this purpose, an overfed Twill 2/2 weave pattern was used, which in the hot compacted form is known under the trade name of Curv®. The areal density of the SRPP fabric is 130 g/cm3 and the density of this grade polypropylene is 0.92 g/cm3. Three point bending (3PB) static tests were performed with a span length of 20 mm, according to the EN ISO 178:2003 recommendations. The strain rate effects on the flexural properties were obtained by the 3PB static tests carried out at room temperature with a displacement rate of 200, 20, 2, 0.2 and 0.02 mm/min. Finally, tests of stress relaxation were also performed, where a fixed strain was applied and the stress was recorded during the loading time. It was possible to conclude that higher strain rates promote higher maximum bending stresses and bending modulus. For both cases, a linear model fits successfully the data and the strain-rate effect on the bending strain at a maximum bending stress showed that SRPP composites are strain rate sensitive. The stress relaxation tests evidence a decrease of the stress with time, and this tendency further persists with the increase of the strain values. Maxwell and Kohlrausch-Williams-Watts (KWW) equations were used to fit the data obtained from the stress relaxation tests and, while the Maxwell model was not good enough to predict the stress relaxation time, the KWW model could fit the data with good accuracy.

The paper investigates the split-disk test methodology for measuring mechanical properties of a filament wound composite. Hoop strain is measured by three-dimensional (3D) Digital Image Correlation (DIC) on the outer surface of ring specimens . A finite element model (FEM) is employed to investigate the strain distribution along the circumference and the thickness of the ring and the effect of friction between the ring and the disk. The FE simulation shows that the effect of friction between the ring and the disk is significant, but precise quantification is difficult because of irregularities of the ring/disk contact. Both DIC strain map and FE simulation show a low strain area at the level of the split, caused by local bending of the ring. In conclusion of this work, the authors recommend: (1) to measure the strain, which is used in the stiffness calculation in the area with a 10° offset from the disk split, where the influence of the local bending becomes insignificant, (2) to use 3D DIC for precise hoop strain measurement, (2) to use concurrent FEM to estimate errors related to the difference between the surface strains and the average strains over the entire ring thickness.

Polymeric foams are extensively used in applications such as packaging, sports goods and sandwich structures. Since in-service loading conditions are often multi-axial, characterisation of foams under multi-axial loading is essential. In this article, quasi-static combined shear-compression behaviour of isotropic expanded polystyrene foam and anisotropic polyethersulfone foam was studied. For this, a testing apparatus which can apply combined compression and transverse shear loads was developed. The results revealed that the shear and compression energy absorption, yield stress and stiffness of foams are dependent on deformation angle. The total energy absorption of the anisotropic polyethersulfone foam is shown to be direction dependent in contrast to isotropic expanded polystyrene. Furthermore, for similar relative density, polyethersulfone foam absorbs more energy than expanded polystyrene foam, regardless of deformation angle. This study highlights the importance of correct positioning of foam cells in anisotropic foams with respect to loading direction to maximise energy absorption capability.

A problem that arises when performing stereo digital image correlation in applications with large out-of-plane displacements is that the images may become unfocused. This unfocusing could result in correlation instabilities or inaccuracies. When performing DIC measurements and expecting large out-of-plane displacements researchers either trust on their experience or use the equations from photography to estimate the parameters affecting the depth of field (DOF) of the camera. A limitation of the latter approach is that the definition of sharpness is a human defined parameter and that it does not reflect the performance of the digital image correlation system. To get a more representative DOF value for DIC applications, a standardised testing method is presented here, making use of real camera and lens combinations as well as actual image correlation results. The method is based on experimental single camera DIC measurements of a backwards moving target. Correlation results from focused and unfocused images are compared and a threshold value defines whether or not the correlation results are acceptable even if the images are (slightly) unfocused. By following the proposed approach, the complete DOF of a specific camera/lens combination as function of the aperture setting and distance from the camera to the target can be defined. The comparison between the theoretical and the experimental DOF results shows that the achievable DOF for DIC applications is larger than what theoretical calculations predict. Practically this means that the cameras can be positioned closer to the target than what is expected from the theoretical approach. This leads to a gain in resolution and measurement accuracy. ispartof: Optics and Lasers in Engineering vol:91 pages:134-143 status: published

The application of natural fibers as reinforcement material in composites can make an important contribution toward a more sustainable bio-based economy. To use these natural fibers, they need to be released from the stem. Traditionally, this is realized by dew retting. However, there are some disadvantages to this retting procedure. Due to changing weather conditions, it is quite difficult to guarantee a good and consistent fiber quality. Replacing the traditional retting process by a biocatalytic process may resolve these issues through application of a combination of pectinase and hemicellulase activities. In this context, several strategic pectinase and hemicellulase activities were blended. Flax fiber fineness and mechanical properties of resulting unidirectional fiber–epoxy composites were studied after treatment of flax stems with these strategic enzyme combinations. Characterization of fiber fineness showed a higher percentage of fiber widths smaller than 100 µm after combined treatment with two enzymes (44.7%), and the percentage was even higher after some combinations with three enzymes (51.2%). Assessment of mechanical properties of composites showed no significant differences in tensile or transverse properties for combined enzymatic treatments. Fiber properties were back-calculated from unidirectional flax fiber–epoxy composites and a Young’s modulus of 88 GPa and a tensile strength of 865 MPa were observed. Enzymatic extraction resulted in the achievement of similar results as dew retting concerning mechanical composite performance and fiber fineness.

The influence of moisture on the mechanical properties of bamboo fibre reinforced composites was assessed both for static and hygroscopic cyclic conditioning. Static conditioning revealed that bamboo fibre reinforced composites show a smaller decrease in Young’s modulus with moisture content than other natural fibre reinforced composites such as for example flax based composites. The behaviour is explained by the chemical composition of the fibre and demonstrated by dynamic mechanical analysis that illustrates how composites made with fibres of high lignin content retain their glass transition temperature in moist conditions. The influence of hygroscopic cycling was also assessed, with emphasis on the influence of the remaining parenchyma on the fibre surface. Clean fibres, are beneficial for the long term behaviour, with less deterioration in properties. Porosity analysis was performed via X-ray computed tomography to provide insight into the material structure of the composite and the effects of hygroscopic cycling. ispartof: Composites Part A - Applied Science and Manufacturing vol:119 pages:48-60 status: published

Four Phyllostachys species grown in Europe were analysed with the aim of extracting long bamboo fibres for composite applications. The morphological characteristics of the plant were determined and compared to the species grown in their native region. Though the species tend to grow smaller, the Phyllostachys vivax showed similar Young’s modulus as the tropical Guadua angustifolia Kunth, i.e. 49 GPa versus 52 GPa. But the strength of all species was 16–44% lower. A chemical analysis of the fibres and an interface study of their composites was performed by analysing the wetting behaviour of the fibres and the chemical composition of their surface. All fibres showed a lignin rich surface, with minor differences in wetting behaviour; therefore difference in the fibre mechanical properties cannot be explained by interface differences. The chemical bulk composition of the fibres was determined as well, and the cellulose content of the fibres showed a linear correlation with the Young’s modulus and strength of the fibres. Seasonal investigation, performed on one selected species, revealed that harvest in autumn or winter lead to higher fibre extraction yields with higher mechanical properties. ispartof: Industrial Crops And Products vol:133 pages:304-316 status: published

Enzymatic treatment of flax is gaining more interest as a promising alternative for dew retting, which is known for its dependence on weather and climate. Therefore, the effect of enzymatic treatments of flax on the effectiveness of fiber separation from each other and chemical fiber composition was investigated in this study. Chemical composition was determined by a gravimetric method, while ease of separation (in the composites society, the process to obtain natural fibers from the plant is usually defined as extraction) was determined based on the amount of long fibers obtained as well as total time needed to release this fiber fraction, providing necessary insights in the extent to which fibers are loosened from the stem. Flax treatment with pectate lyase and polygalacturonase resulted in purified fibers with a cellulose content of 78 and 79% w/w and promising yield values of 24 and 17%, respectively. Besides these pectinases, xylanase activity also showed high potential for enzymatic retting. Hence, pectate lyase, polygalacturonase, and xylanase are promising enzymes to successfully replace the dew retting process.

© 2018 Elsevier Ltd In Part I, we performed quasi-static compression-shear tests on expanded polystyrene (EPS) foam. In the present study, dynamic combined compression-shear loading tests were performed on different densities of EPS foam by integrating the previously developed loading rig with a lab-built drop weight tower. Two impact loading wedge angles (45° and 60°) and three different drop heights (50, 130 and 175 cm) were tested in the experiments. It was found that both the compressive and shear stresses at yield of the EPS foam are density and strain-rate dependent. In addition, increasing the impact loading angle tends to decrease the compressive stress and increase the shear stress at yield of the EPS foam. ispartof: INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES vol:145 pages:9-23 status: published

A new type of fiber reinforced filament materials for Fused Deposition Modeling (FDM) applications has been developed. Polylactic acid (PLA), compounded with two types of plasticizer, is reinforced with bamboo and flax fibers. The fiber fractions are characterized by measuring their length (l) over diameter (d) before and after compounding so that the effect of the l/d on the final filament properties can be systematically studied. The pre-processing and processing activities, to produce filaments with a diameter of 3 mm, are described. The porosities and the orientation of the fibers in the filament are assessed in order to predict the theoretical stiffness of the filament via short fiber mechanics which is compared to the experimental results. Reinforcing the PLA filament with short bamboo fibers (l/d = 4.1-4.7) increases the modulus with 91-230%, whereas the dust-like fractions only show an increase up till 39%. An interfacial study is performed to study the effect of the plasticizer on the adhesion. Furthermore the glass transition temperature of the produced filaments is determined to investigate the suitability of the filaments and finally a demonstrator part was printed. ispartof: Polymer Composites vol:40 issue:5 pages:1-13 status: published

© 2018 Elsevier Ltd Expanded Polystyrene (EPS) foam material is widely used as an energy absorption engineering material. Its compression behaviour, both quasi-statically and dynamically, has been studied widely. However, its mechanical behaviour under combined compression-shear loading is poorly understood due to the difficulty of performing such tests. A novel test rig is presented to perform combined compression-shear loading tests in quasi-static and high strain rate dynamic conditions. Different densities of EPS foam were tested with this apparatus using a universal testing machine and a drop tower. The compressive and shear stress and strain curves were obtained and compared with the mechanical behaviour of EPS foam under pure compression. This showed that the shear deformation of EPS foam under combined compression-shear loading tends to lower the compressive stress at yield of the foam up to 40% compared with pure compression, depending on the density of the foam. A companion Part II paper will separately present and discuss the experimental results of the high strain rate dynamic tests of EPS foam under combined compression-shear loading. ispartof: INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES vol:144 pages:480-489 status: published

This paper describes a methodology to obtain and analyse data in composite reinforcement fabric compaction tests in dry and saturated conditions using a standard electromechanical test machine and an indirect thickness measurement method. The main goal is the identification of machine compliance sources and the understanding of its effects in typical compression tests and compression with relaxation stage tests. This methodology was used in the first international compressibility benchmark exercise promoted by NPL (National Physics Laboratory, UK) in the years 2017 and 2018. The methodology accounts for the test setup compliance and evaluates errors in fabric thickness measurements in the range of tens of microns especially during the relaxation phase. Overall, the setup compliance and data analysis strategy proved to have a significant effect on the data if not properly accounted for.

Oblique impact is the most common accident situation that occupants in traffic accidents or athletes in professional sports experience. During oblique impact, the human head is subjected to a combination of linear and rotational accelerations. Rotational movement is known to be responsible for traumatic brain injuries. In this article, composite foam with a column/matrix composite configuration is proposed for head protection applications to replace single-layer uniform foam, to better attenuate rotational movement of the head during oblique impacts. The ability of composite foam in the mitigation of rotational head movement is studied by performing finite element (FE) simulations of oblique impact on flat and helmet shape specimens. The performance of composite foam with respect to parameters such as compliance of the matrix foam and the number, size and cross-sectional shape of the foam columns is explored in detail, and subsequently an optimized structure is proposed. The simulation results show that using composite foam instead of single-layer foam, the rotational acceleration and velocity of the headform can be significantly reduced. The parametric study indicates that using a more compliant matrix foam and by increasing the number of columns in the composite foam configuration, the rotation can be further mitigated. This was confirmed by experimental results. The simulation results were also analyzed based on global head injury criteria such as head injury criterion, rotational injury criterion, brain injury criterion and generalized acceleration model for brain injury threshold which further confirmed the superior performance of composite foam versus single-layer homogeneous expanded polystyrene foam. The findings of simulations give invaluable information for design of protective helmets or, for instance, headliners for the automotive industry.

Anisotropy in foams generally originates from cell elongation in a certain direction. In this study, a composite concept is utilized to create anisotropy in foams at macro level. For this, layered composite foam is proposed by combining discrete layers of expanded polystyrene foam foam with different densities. The layers are positioned in parallel with the prime loading direction. The compression and biaxial combined shear-compression behavior of the composite foams are studied and compared with single-layer expanded polystyrene foam of equivalent density. The biaxial shear-compression test results demonstrate that the composite concept enables to decouple shear and compression properties of foam for a given overall density. In compression loading, the composite foam behavior is similar to that of single-layer foam of similar density, while in biaxial loading, the composite foam shows lower shear resistance than single-layer foam. Moreover, in biaxial loading, parameters such as the number of layers and the density difference between the high- and low-density layers affect the extent of decrease in shear resistance, while the compression stress component depends solely on the overall density of the composite foam. One of the potential applications of this behavior could be in protective helmets for mitigation of the head rotational acceleration. ispartof: Journal of Reinforced Plastics and Composites vol:37 issue:10 pages:657-667 status: published

© The Author(s) 2018. The mechanical properties of bamboo technical fibre, from the species Guadua angustifolia, have been studied showing values of strength up to 800 MPa and E-modulus up to 43 GPa, proving their adequate tensile properties that make this natural fibre suitable as reinforcement in composite materials. To fully explore the good mechanical properties and to make an adequate use of this new reinforcement, it is indispensable to comprehensively understand the fibre behaviour as a function of the microstructure. Microscopic observations have provided us with an extensive knowledge of the complex microstructure of this natural fibre from the macroscale down to the microscale level where different features like the distribution of the elementary fibres within the fibre bundle, dimensions and layering pattern of the elementary fibres and the main microfibrillar angles could be measured. The Young’s modulus of the elementary fibre is analysed based on the micromechanics of composite materials, commonly used for unidirectional short fibre composites, and the fibre microstructure. The predicted results are in reasonable agreement with experimental data, showing the appropriateness of the model for describing the elementary fibre stiffness. Also, the failure modes of single fibres after tensile testing are analysed by microscopic observations, to have an indication of the stress development in the elementary fibres and the different failure mechanisms. ispartof: JOURNAL OF REINFORCED PLASTICS AND COMPOSITES vol:37 issue:17 pages:1099-1113 status: published

© 2018 Author(s). The compression behavior of a material cannot be accurately determined if the machine compliance is not accounted prior to the measurements. This work discusses the machine compliance during a compressibility test with fiberglass fabrics. The thickness variation was measured during loading and unloading cycles with a relaxation stage of 30 minutes between them. The measurements were performed using an indirect technique based on the comparison between the displacement at a free compression cycle and the displacement with a sample. Relating to the free test, it has been noticed the nonexistence of machine relaxation during relaxation stage. Considering relaxation or not, the characteristic curves for a free compression cycle can be overlapped precisely in the majority of the points. For the compression test with sample, it was noticed a non-physical decrease of about 30 μm during the relaxation stage, what can be explained by the greater fabric relaxation in relation to the machine relaxation. Beyond the technique normally used, another technique was used which allows a constant thickness during relaxation. Within this second method, machine displacement with sample is simply subtracted to the machine displacement without sample being imposed as constant. If imposed as a constant it will remain constant during relaxation stage and it will suddenly decrease after relaxation. If constantly calculated it will decrease gradually during relaxation stage. Independently of the technique used the final result will remain unchanged. The uncertainty introduced by this imprecision is about ±15 μm. ispartof: PROCEEDINGS OF 21ST INTERNATIONAL ESAFORM CONFERENCE ON MATERIAL FORMING (ESAFORM 2018) vol:1960 ispartof: 21st International ESAFORM Conference on Material Forming (ESAFORM) location:ITALY, Univ Palermo, Palermo date:23 Apr - 25 Apr 2018 status: published

Numerical simulation tools for the thermoforming of unfilled thermoplastic polymers already exist for a while, but are seldom used to full extent in industry. When it is used, it is mostly only for comparative studies and prediction of relative wall thickness. One of the major reasons is the difficulty to correctly calibrate and integrate all necessary material and process parameters into the simulation software. This paper introduces and validates a methodology, in which digital image correlation (DIC) is used as the key enabling technology that improves the knowledge of the process parameters and optimizes simulation accuracy by taking away a number of uncertainties and assumptions. DIC in combination with infrared thermal measurements and pressure monitoring is used to track sheet sagging and bubble inflation of a HIPS sheet, the two main process steps in the thermoforming of positive (male) products or the only two steps in the case of free forming. The results of these in-situ measurements are used as a guideline for selecting the correct input parameters in the commercial thermoforming simulation software T-SIM®. A similar methodology can be further implemented for subsequent process steps such as forming and cooling or even to validate the material data used in the simulation software. ispartof: Express Polymer Letters vol:9 issue:2 pages:119-128 status: published

Bridging veins drain the venous blood from the cerebral cortex into the superior sagittal sinus (SSS) and doing so they bridge the subdural space. Despite their importance in head impact biomechanics, little is known about their properties with respect to histology, morphology and mechanical behaviour. Knowledge of these characteristics is essential for creating a biofidelic finite element model to study the biomechanics of head impact, ultimately leading to the improved design of protective devices by setting up tolerance criteria. This paper presents a comprehensive review of the state-of-the-art knowledge on bridging veins. Tolerance criteria to prevent head injury through impact have been set by a number of research groups, either directly through impact experiments or by means of finite element (FE) simulations. Current state-of-the-art FE head models still lack a biofidelic representation of the bridging veins. To achieve this, a thorough insight into their nature and behaviour is required. Therefore, an overview of the general morphology and histology is provided here, showing the clearly heterogeneous nature of the bridging vein complex, with its three different layers and distinct morphological and histological changes at the region of outflow into the superior sagittal sinus. Apart from a complex morphology, bridging veins also exhibit complex mechanical behaviour, being nonlinear, viscoelastic and prone to damage. Existing material models capable of capturing these properties, as well as methods for experimental characterisation, are discussed. Future work required in bridging vein research is firstly to achieve consensus on aspects regarding morphology and histology, especially in the outflow cuff segment. Secondly, the advised material models need to be populated with realistic parameters through biaxial mechanical experiments adapted to the dimensions of the bridging vein samples. Finally, updating the existing finite element head models with these parameters will render them truly biofidelic, allowing the establishment of accurate tolerance criteria and, ultimately, better head protection devices. publisher: Elsevier articletitle: Structural and mechanical characterisation of bridging veins: A review journaltitle: Journal of the Mechanical Behavior of Biomedical Materials articlelink: http://dx.doi.org/10.1016/j.jmbbm.2014.06.009 content_type: article copyright: Copyright © 2014 Elsevier Ltd. All rights reserved. ispartof: JOURNAL OF THE MECHANICAL BEHAVIOR OF BIOMEDICAL MATERIALS vol:41 pages:222-240 ispartof: location:Netherlands status: published

The scalp plays a crucial role in head impact biomechanics, being the first tissue involved in the impact and providing a sliding interface between the impactor and/or helmet and the skull. It is important to understand both the scalp-skull and the scalp-helmet sliding in order to determine the head response due to an impact. However, experimental data on the sliding properties of the scalp is lacking. The aim of this work was to identify the sliding properties of the scalp using cadaver heads, in terms of scalp-skull and scalp-liner (internal liner of the helmet) friction and to compare these values with that of widely used artificial headforms (HIII and magnesium EN960). The effect of the hair, the direction of sliding, the speed of the test and the normal load were considered. The experiments revealed that the sliding behaviour of the scalp under impact loading is characterised by three main phases: (1) the low friction sliding of the scalp over the skull (scalp-skull friction), (2) the tensioning effect of the scalp and (3) the sliding of the liner fabric over the scalp (scalp-liner friction). Results showed that the scalp-skull coefficient of friction (COF) is very low (0.06 ± 0.048), whereas the scalp-liner COF is 0.29 ± 0.07. The scalp-liner COF is statistically different from the value of the HIII-liner (0.75 ± 0.06) and the magnesium EN960-liner (0.16 ± 0.026). These data will lead to the improvement of current headforms for head impact standard tests, ultimately leading to more realistic head impact simulations and the optimization of helmet designs. ispartof: JOURNAL OF BIOMECHANICS vol:75 pages:28-34 ispartof: location:United States status: published

This study evaluates the efficiency of the extraction of enzymatically treated flax fibers for application in composite materials. Three extraction parameters were introduced. Fiber Efficiency (FE) represents the yield of long fibers extracted after enzymatic treatment. Time Efficiency (TE) is introduced in order to evaluate the ease of extraction. Taking into account both Fiber Efficiency and Time Efficiency results in the introduction of the overall Extraction Efficiency (EE). The Extraction Efficiency is determined for flax fibers treated with pectate lyase, pectin lyase, polygalacturonase, endoxylanase and Viscozyme L as reference while comparing with green and dew retted fibers. Moreover, fiber fineness and transverse properties of the resultant composite materials were investigated. Polygalacturonase treatment appears to be the most promising. Results demonstrate the potential of enzymatic extraction of flax fibers with an increased transverse bending strength of the composite reinforced with polygalacturonase treated fibers (26.4 ± 3.8 MPa) compared to green flax fiber composite (12.1 ± 1.4 MPa) and dew retted fiber composite (16.9 ± 2.2 MPa).

A recently developed mechanical method for extracting long bamboo fibres opens the possibility to exploit this new material as reinforcement in composite materials for high end uses. The strength distribution of the fibres was characterized in a novel approach to evaluate the effect of defects introduced by the extraction process as function of different scale variables: fibre length, fibre surface area and fibre volume. The modified Weibull distribution, a practical model requiring only three parameters, described accurately the fibre strength distribution of the fibres at different gauge lengths. The average fibre strength decreased with increasing gauge length, from 943 MPa at L = 1 mm to 733 MPa at L = 40 mm and it was nearly independent of the mean fibre volume. The Weibull shape parameter was found to be 7.6 for all tested fibres, showing low strength variability in comparison with other natural fibres and some synthetic fibres, indicating their high quality.

The specific stiffness of flax fibres is comparable to that of glass fibres. However, the application of flax fibres in composites still poses two important problems. The orientation of the fibres in the composite is not impeccable and the fibre–matrix adhesion is too weak. This research focuses on the former. To produce a continuous yarn for the textile industry, technical flax fibres are spun. The twisting of the fibres increases the yarn strength but the orientation of the fibres with regard to the fibre axis decreases. Experiments show that the slightly twisted yarns and the spun yarns possess the best processability but have lower mechanical properties than untwisted ribbons. The values fit very well to certain models which predict stiffness with regard to the twist angle.

Acute subdural hematoma (ASDH) is a type of intracranial haemorrhage following head impact, with high mortality rates. Bridging vein (BV) rupture is a major cause of ASDH, which is why a biofidelic representation of BVs in finite element (FE) head models is essential for the successful prediction of ASDH. We investigated the mechanical behavior of BVs in the KTH FE head model. First, a sensitivity study quantified the effect of loading conditions and mechanical properties on BV strain. It was found that the peak rotational velocity and acceleration and pulse duration have a pronounced effect on the BV strains. Both Young's modulus and diameter are also negatively correlated with the BV strains. A normalized multiple linear regression model using Young's modulus, outer diameter and peak rotational velocity to predict the BV strain yields an adjusted [Formula: see text]-value of 0.81. Secondly, cadaver head impact experiments were simulated with varying sets of mechanical properties, upon which the amount of successful BV rupture predictions was evaluated. The success rate fluctuated between 67 and 75%. To further increase the predictive capability of FE head models w.r.t. BV rupture, future work should be directed towards improvement of the BV representation, both geometrically and mechanically.