Silk fibers combine good stiffness and strength with a very high strain to failure and are as such highly promising to realize composites with high impact resistance. It is shown that to realize this potential it is quite beneficial to employ matrix materials of high strain to failure, particularly thermoplastic matrices. High impact resistance is thus achieved, well above the values for the pure matrices. Below the glass-transition temperature of the thermoplastic matrix, the impact energy absorption decreases. The adhesion between fiber and matrix also plays a significant role; lower adhesion typically increases the low-velocity penetration impact resistance, due to the spread of damage. Finally, the fiber architecture is pivotal; when a woven fabric is used which is unbalanced in strength, the impact resistance reduces in correspondence with the weakest material direction. A quasi-isotropic layup has a lower capacity for deformation than a balanced woven configuration which likely explains the observed lower penetration impact resistance.

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The combination of ductile Bombyx Mori silk fibre with ductile thermoplastic matrices, combined with the use of a woven fibre architecture, leads to ductile composites with quasi-isotropic strength. Bending and tensile properties were determined for (0,90)_s silk twill weave composites, with different thermoplastic polymers and as reference epoxy resin. Thermoplastic (0,90)_s woven silk fibre composites are isotropic in strength (about 135 MPa at 50% fibre volume fraction) due to the high elongation to failure of both the silk fibre and the thermoplastic matrix allowing extensive shearing of the silk fabric when loaded in the 45° direction. A good quality interface has a clear positive influence on the mechanical properties especially on the bending yield strength.

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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

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

Conventional composites are susceptible to impact loading due to their low toughness. Novel annealed steel fibres possess a unique combination of high stiffness and high strain to failure. A range of composite laminates incorporating various steel fibre architectures with thermoplastic and thermoset matrices were produced and the low-velocity penetration impact resistance was characterized by falling weight impact tests, showing the high potential toughness of steel fibre composites. The effect of different parameters was determined. High matrix strain to failure allows a better exploitation of the steel fibre ductility. The higher absorbed energy up to penetration, in case of laminates with lower levels of fibre/matrix adhesion is attributed to fibre debonding, potential for more plastic deformation, and pull-out mechanisms. Thinner fibres are more sensitive to premature failure due to inclusions in the steel fibres. Cross-ply laminates show higher energy absorption, probably due to increased delamination, whereas woven fibre structures lead to more localized damage. A preliminary study shows that layer by layer (‘macro’) hybridization of carbon and steel fabrics leads to penetration impact resistance close to the pure steel fibre composite, in case the steel plies are placed on the outside surfaces, showing a significant positive effect of hybridization.@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

This study investigates the impact-energy absorbing potential of steel fibre reinforced composites with different matrices. The composites were subjected to low-velocity impact using the drop-weight impact test. The interfacial adhesion strength between matrices and steel fiber was characterized by a transverse three point bending test. It was found that factors such as fibre architecture, matrix strain to failure, and the level of adhesion can play a crucial role in the absorbed energy for penetration. A cross-ply non-crimp fabric showed higher energy absorption than a plain woven fabric. Absorbed energy increased with polymer matrix strain to failure and lower fibre-matrix adhesion. (1) (PDF) On the potential of fine steel fibres to create stiff but tough polymer composites. Available from: https://www.researchgate.net/publication/293222932_On_the_potential_of_fine_steel_fibres_to_create_stiff_but_tough_polymer_composites [accessed Apr 28 2020].@en