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.

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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.@en

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.@en

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.@en

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 @en

Rotational acceleration of the head is known to be the cause of traumatic brain injuries. It was hypothesized that by introducing anisotropy in a foam liner in head protection applications, for example, protective helmets, rotational acceleration transmitted to the head can be further mitigated. Therefore, composite foam with a cylinder/matrix configuration with anisotropy at “macro level” is proposed as a smart structural solution to replace single layer foam headliners of the same weight and thickness. In this paper, a parametric study on the cylinder/matrix configuration is performed and the results are compared with these of single layer expanded polystyrene foam. The structure is subsequently optimized for the best performance in mitigation of rotational acceleration and velocity. Oblique impact results show that the parameters such as the number of cylinders in a given structure, and the compliance of the matrix foam significantly affect the extent of rotational acceleration and velocity mitigation. Optimized composite foam configurations are subsequently proposed and they demonstrate a mitigation of rotational acceleration and velocity up to 44 and 19%, respectively. Moreover, relevant global head injury criteria such as HIC (Head Injury Criterion), RIC (Rotational Injury Criterion), HIPmax (Head Impact Power), GAMBIT (Generalised Acceleration Model for Brain Injury Threshold), and BrIC (Brain Injury Criterion) demonstrated reduction up to 27, 67, 31, 26, and 19%, respectively.@en

Although current standard bicycle helmets protect cyclists against linear acceleration, they still lack sufficient protection against rotational acceleration during oblique impact events. Rotational acceleration is correlated with serious traumatic brain injuries such as acute subdural haematoma and thus should be minimized. This study proposes using highly anisotropic polyethersulfone foam for bicycle helmet liners in order to limit the rotational acceleration. Helmet prototypes, made of polyethersulfone foam with cell anisotropy direction perpendicular to the head, have been produced and compared to a standard commercial helmet. Standard helmets consist of expanded polystyrene foam. Oblique impact tests were performed to measure both linear and rotational accelerations and impact pulse duration. Results demonstrate that the peak rotational acceleration of the polyethersulfone prototype helmet showed a decrease of around 40% compared to the reference expanded polystyrene helmet. Moreover, the peak linear acceleration showed an average decrease of about 37%. Upon impact, the polyethersulfone helmet showed improved head injury protection when analysed based on global biomechanical head injury criteria such as HIC15 and HICrot as well as generalized acceleration model for brain injury threshold, brain injury criterion and head impact power, with a predicted sixfold decrease in likelihood of concussion.@en

Rotational acceleration experienced by the head during oblique impacts is known to cause traumatic brain injuries. It is hypothesized that shear properties of a foam layer, used for head protection (e.g., protective helmet liners, headliners in cars) can be related to the extent of rotational acceleration transmitted to the head. Furthermore, it is hypothesized that by introducing anisotropy in a foam layer, rotational acceleration can be mitigated. In this study, an anisotropic composite foam concept is proposed to mitigate head rotational acceleration, hence reducing the risk of traumatic brain injuries. The composite foam concept introduces anisotropy in a foam at the “macro level”, combining different densities of foam in layered and quasi‐fiber/matrix configurations. The performance of expanded polystyrene (EPS) composite foams in quasi‐static compression and combined shear‐compression loading and also linear and oblique impact experiments, has been compared with the performance of single layer EPS foam of similar thickness and density. The results of oblique head impact have been analyzed by global head injury criteria such as HIC, HICrot, and HIP. The composite foam concept demonstrates a great potential to be utilized in applications such as protective helmets due to the significant mitigation of brain injury risk.@en

n this study, different configurations of layered composite foam liners for a protective helmet were prepared by arranging layers of EPS foams with different densities in a series configuration. The performance of the layered "composite foams" in terms of peak force/accelerations and time duration in linear impact were compared with single layer homogenous EPS foam. Linear impact tests were performed for two different initial energies of 40 and 66 J. Results demonstrate that in a liner with a density gradient through the thickness, positioning the higher density close to the head can reduce the peak accelerations transferred to the head. In addition, in his paper, the effect of using different materials as a helmet shell on the performance of a helmet in linear impact has been studied. For this purpose high energy absorbing composites such as Curv® and silk/HDPE have been benchmarked against conventional shell materials such as polycarbonate. Results demonstrated the superior performance of silk/HDPE composite compared to the other materials for more localized loads.@en

Rotational acceleration experienced by the head during oblique impacts is known to cause traumatic brain injuries. It is hypothesized that shear properties of a foam layer, used for head protection (e.g., protective helmet liners, headliners in cars) can be related to the extent of rotational acceleration transmitted to the head. @en

In this paper, a new anisotropic material concept namely composite foam is proposed as an alternative to next generation helmet liners which can potentially reduce head rotational accelerations. The Layered Composite foam concept comprises of foam layers with different densities which are glued to each other in a parallel or series configuration. Initial experiments such as static combined shear-compression and linear impact tests have been performed. The performance of composite foams has been compared with a commercial expanded polystyrene bead foam (EPS), with nominal density of 80 kg/m3, which is currently the most used material for bicycle helmets. In this study the two types of composite foam with parallel and series configuration have been produced. Combined shear compression results indicate the parallel configuration is more suitable in terms of shear-compressive properties as well as energy absorption. Moreover, linear impact results indicate that the composite foam concept can mitigate peak forces compared to a standard EPS foam.@en

yclists during bicycle traffic accidents, are prone to oblique impact which leads to rotational accelerations. Rotational acceleration is known to cause significant brain injuries, and should be minimized. Foam materials inside bicycle helmets undergo a combination of shear and compression loads during oblique impact. Therefore, developing an apparatus and a test method which can apply a combination of shear and compression loads to the foam at the same time is of great importance. This testing method has a broad application which is not only limited to the foams for bicycle helmet applications but has general relevance to sandwich core materials in structural composites. In this paper, the shear-compression behavior of different types of foams under different angles, particularly 15 ͦ, 45 ͦ and 60 ͦ is investigated and compared to the standard EPS foam used in bicycle helmets. Shear stresses in anisotropic PES foams under different angles in the combined shear-compression test were lower than for standard EPS, which is favourable because they lead to high rotational acceleration. The peak rotational and translational accelerations of the PES prototype helmets were measured by a rotational impact test set-up and showed a dramatic decrease of around 40% compared to the reference EPS helmet, however at increased pulse duration.@en

Polymer foams are extensively used in lightweight structures, often as energy absorbers. Since most of the loading modes in real life are complex, characterization of core materials under multi-axial loading is of great importance. In this work, a new testing apparatus for measuring combined shear-compression loading of sandwich core materials has been developed. The behavior of expanded polystyrene (EPS) foams has been studied in quasi-static mode. Three different densities of EPS foam have been tested under different angles of displacement (the resultant of shear and compression displacement), particularly 0° 15°, 45°, and 60°.The results reveal that the parameters such as energy absorption in (shear and compression) up to densification, yield stress and elastic modulus of the EPS foam have a strong dependency on the angle of displacement as well as the density of the foams.@en

lthough current standard bicycle helmets provide protection against so-called linear acceleration, they cannot deliver sufficient protection against rotational acceleration during oblique impact events. These events can cause severe head injuries and the rotational accelerations need to be minimized. We pursue this goal by developing anisotropic foams for bicycle helmets and define test methods to characterize foams under oblique loading. (1) (PDF) DEVELOPMENT OF ANISOTROPIC FOAMS AND CHARACTERIZATION METHODS FOR BICYCLE HELMETS. Available from: https://www.researchgate.net/publication/320110420_DEVELOPMENT_OF_ANISOTROPIC_FOAMS_AND_CHARACTERIZATION_METHODS_FOR_BICYCLE_HELMETS [accessed Apr 28 2020].@en