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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

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

© 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

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.

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

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

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.

Highlights • Enzymatic treatment of flax offers a valuable alternative for dew retting. • Polygalacturonase combined with xylanase activity leads to a markedly chemical refining of flax fibers. • Enhanced fiber extraction efficiency reached by applying enzyme combinations. • Extraction efficiencies twice as high compared to green fibers by combining polygalacturonase and pectinmethylesterase., The effect of treatment of flax with strategic enzyme combinations on the ease of fiber extraction and the chemical fiber composition is reported in this study. To contribute to the increasing demand for bio-based and sustainable materials, it is of great importance to develop optimal enzyme formulations which can replace the yet poorly controlled traditional dew retting process. Regarding the chemical composition of the fiber, enzymatic treatments all resulted in similar improvements, with an enhanced cellulose content of 81 ± 1% after polygalacturonase + xylanase treatment (vs. 64 ± 2% for green fibers). Evaluation of extraction efficiency (EE) showed that several enzyme combinations significantly increased EE in comparison with green fibers. An EE of 23 ± 6% was found for fibers extracted after polygalacturonase + pectinmethylesterase treatment, in comparison with an EE of 11 ± 1% for green fibers. Combinations with three enzymes resulted in a higher reduction of the pectin content of the fibers. The combination of enzymes shows hence promising potential but further evaluation of mechanical performance of fiber reinforced composites is needed.

For industrial applications such as construction, sports or aviation the interest in novel 3D woven structures grows. Current production routes have typical downsides such as low production speed, need for dedicated looms for weaving one specific fabric type or time consuming modifications to the weaving loom. In contrast, the present research shows the production of different 3D woven fabrics in one run, at industrial speed on a weaving loom with minor adaptations. A wide range of 3D woven fabrics with integrated prismatic shaped cavities is produced. Moreover, switching from one fabric type to another is fast and feasible using the same loom. These novel 3D fabrics can be divided in three categories depending on the internal connection structure, i.e. hexagonal (type I), rectangular (type II) or X-shaped (type III). All novel weaving patterns are developed for high tenacity (HT) polyester and are characterised with tensile tests, showing promising results. In addition several structures based on aramid and glass yarns are produced, aiming towards lightweight composite materials. The high ductility of the HT-polyester fabrics is of interest for example in shock absorbing applications. Two applications, reinforced concrete beams and lightweight foam composite panels, are illustrated and indeed show excellent ductile properties. Keywords: 3D weaving, Multilayer fabrics, Composites, Ductility, Mechanical testing, HT-polyester

Introduction: Due to worldwide different health insurance policies, patients are often forced to reuse the catheters when performing Clean Intermittent Catheterisation (CIC). We have compared the physical qualities and the antimicrobial effects of two methods of reusing catheters: microwave heating and storage of the catheters in a 70% alcohol solution. The studies were performed during different lengths of time. Materials and methods: Three types of catheters (a standard polyvinylchloride catheter, a special polyvinylchloride catheter with flexible Ergothan ® tip and a prelubrified catheter), normally intended for single use, were submitted to the effect of a microwave oven (Multitech 215 High Grade and Whirlpool M220 750W and 1000W with rotating plate) or preservation in a 70% alcohol solution. To study the effects of microwave heating, a recipient of water was placed in the oven to spread the microwaves and to absorb the heat. The catheters were placed in a resealable plastic bag (Ziploc ® ). To study the effects of preservation in a 70% alcohol solution, the catheters were immerged in the solution for different lengths of time. Thereafter were the physical qualities of the catheters evaluated by using the technique of Differential Scanning Calorimetry (DSC). The antimicrobial effect of the method was evaluated after grafting the catheters with pathogenic E. coli , P. aeruginosa or S. aureus strains. Results: Microwave heating up to 12minutes at 750W caused only minimal changes in the physical qualities of all the catheters. However, there was only an antimicrobial effect of the microwave heating on E. coli and not on P. aeruginosa or S. aureus . If the catheter remained longer than 45minutes in a 70% alcohol solution, the physical qualities of the catheter changed either minimal in the special polyvinylchloride catheter with flexible Ergothan ® top but changed significantly in the prelubrified catheter). However, already after 5minutes of immersion in the 70% alcohol solution there was a complete antimicrobial effect on E. coli , P. aeruginosa and S. aureus in all catheters. Conclusions: It should be recommended to patients on CIC to use a sterile packed and not previously used catheter. In this study we have shown that immersing the catheters in a 70% alcohol solution during 5minutes can effectively disinfect the catheter without jeopardising the physical qualities. Thereafter, the catheters could be placed in a resealable (e.g. Ziploc ® ) bag without being rinsed under water, in order that the few drops of alcohol cause alcohol vapours within the closed plastic bag and maintain the antimicrobial effect.

In this work, natural fibres (sisal, kenaf, hemp, jute and coir) reinforced polypropylene composites were processed by compression moulding using a film stacking method. The mechanical properties of the different natural fibre composites were tested and compared. A further comparison was made with the corresponding properties of glass mat reinforced polypropylene composites from the open literature. Kenaf, hemp and sisal composites showed comparable tensile strength and modulus results but in impact properties hemp appears to out-perform kenaf. The tensile modulus, impact strength and the ultimate tensile stress of kenaf reinforced polypropylene composites were found to increase with increasing fibre weight fraction. Coir fibre composites displayed the lowest mechanical properties, but their impact strength was higher than that of jute and kenaf composites. In most cases the specific properties of the natural fibre composites were found to compare favourably with those of glass.

During the consecutive decortication stages of flax fibres (retting, scutching, hackling, …), the fibre properties change tremendously due to mechanical and chemical modifications. This study points out the influence of the flax processing parameters, and consequently of the fibre properties, on the mechanical properties of flax fibre reinforced epoxy composites. The better the retting degree of the fibres, the better the composite properties are. The use of long flax slivers does not necessarily result in better composite properties. The influence of certain chemical treatments, performed on the flax fibres, on the composite properties has been studied as well. Treatments with alkali, dilute epoxy, acetone and silane were carried out. A treatment consisting of a combination of alkali and dilute epoxy gives the highest improvement of the flexural properties. The longitudinal properties of the UD composites (both strength and modulus) enhanced with 40% or more, transverse strength increased with 250%, transverse modulus could improve up to 500%.

An overview of the mechanical properties of the core of woven sandwich-fabric panels is given. These materials provide a new type of sandwich structure with a high skin–core debonding resistance and the potential for cost-effective sandwich construction. The sandwich-fabric preforms are produced by a large-scale textile weaving process (velvet weaving). The basic mechanical properties of the sandwich core (compression and shear) were evaluated and compared with those of other core materials. There is a large variety in possible core layouts and thus mechanical performance for sandwich-fabric panels. Acceptable mechanical properties for cores of higher thickness (higher than 10 mm) can be obtained by weaving part of the pile fibres in the core under angles of ±45°, by creating networks of piles at lower degrees of stretching and sufficient pile density, or by filling the core with foam. There appears to be a strong synergistic effect between pile and foam properties in the sandwich core.

A finite-element preprocessing program was developed to predict the mechanical performance of the cores of woven sandwich-fabric panels. These materials, which are produced from velvet-weave sandwich-fabric preforms, provide a new type of sandwich structure with a high skin-core debonding resistance and the potential for cost-effective sandwich construction. There is a large variety in possible core layouts and thus mechanical performance. To model the core behaviour a detailed geometrical modelling of the core lay-out of unfoamed panels was carried out. A unit-cell of the sandwich-fabric panel under consideration is determined and the shape of the piles and pillars and the resin distribution inside the unit cell are modelled. As inputs, only weaving data and some panel production parameters are required. The predicted microstructure for the cases which were modelled had a good resemblance to reality. For the FE analysis a homogenization principle has been used. Linear static analyses for the basic core properties (flexural shear and flat compression) have been performed. The results for the shear modulus as a function of the pile shape were very good. Model predictions at higher resin contents and for the compression modulus were less accurate.

In this paper, the bending fatigue behaviour and damage development during fatigue of polyurethane (PUR)-epoxy 3D woven sandwich composites is investigated. 3D sandwich fabrics are produced by a velvet weaving technique. It allows relatively easy production of delamination-resistant sandwich panels, compared to more traditional sandwich structures with honeycomb or foam cores. In this paper, the 3D sandwich fabrics made of glass fibre are impregnated with epoxy resin while the empty core can be foamed up with PUR to improve shear resistance of the panels. Core properties and static bending strength of the panels were evaluated. Three-point bending (3PB) fatigue tests were performed on four different materials (two unfoamed epoxy panels and two PUR foamed epoxy panels) with varying static properties. After fatigue testing, the Wohler curves for each panel were determined. The relation between the observed damage development, the property degradation during bending fatigue and the static properties of the panels was investigated.

A three-dimensional geometric description for a large range of two-dimensional woven architectures is proposed which can serve as a basis for stiffness and strength modelling of fabric composites. We introduce a strict logical scheme for calculating the full geometry of a general yarn architecture. First, a library of 108 rectangular macro-cells has been put together to build complex material unit cells. This complexity is caused by the yarn interlacing style and the existence of different yarn sizes in the fabric composite. Next, each macro-cell is divided in a number of micro-cells. The geometric model, based on a lenticular yarn cross-section, predicts the weave cover factor, the global and local fibre volume fractions and the yarn orientations. Moreover, a logical and simple meshing for each woven structure is created. A Microsoft Excel® application (TEXCOMP) has been developed to transform the model into a useful design tool.

The bond between a fibre and the matrix is very important for the mechanical behaviour of CFRP. The quality of the fibre/matrix interface will influence the initiation and development of damage. This is certainly true for interlaminar fracture toughness. The present study is focused on the modelling of the influence of the interface properties on the fracture toughness of CFRP. Carbon fibres with four different wet oxidative treatment levels were investigated. The fibres were embedded in an epoxy resin and in a thermoplastic system. Mode I, mode II and mixed-mode tests were performed to determine critical strain-energy release rates. A micromechanical model was developed to explain how fracture toughness of the interface influences the initiation fracture toughness of the composite. A second model explains the R curve behaviour for mode I tests by analysing the strain in a single fibre bridging a crack.

The bond between a fibre and the matrix is very important for the mechanical behaviour of CFRP. The quality of the fibre/matrix interface will influence the initiation and development of damage. This is certainly true for interlaminar fracture toughness. The present study is focused on the influence of the interface properties on the fracture toughness of CFRP. Carbon fibres with four different wet oxidative surface treatment levels have been investigated. The fibres were embedded in a thermoset resin and in a thermoplastic system. Mode I, mode II and mixed-mode tests were performed to determine critical strain-energy release rates. Results show an important increase in the fracture toughness initiation value with increasing fibre surface treatment level. For tests dominated by mode I loading, however, the propagation value can reach a maximum at intermediate treatment levels as a consequence of fibre bridging effects.

With the introduction of laminates into primary loaded structures, it has become apparent that the delamination failure mode has the potential for being the major life-limiting failure mechanism. Delamination resistance has previously been increased using a number of techniques, including interleafing, rubber-toughened resin systems and stitching. However, all the methods proposed to date have attendant disadvantages, severely limiting their use in practical applications. This paper presents a novel solution—a 2.5-dimensional (2.5D) fabric—which has none of the aforementioned problems. The fabric is manufactured by cutting a simple three-dimensional weave, consisting of two two-dimensional (2D) fabrics connected by interwoven pile threads, to form a ‘hairy’ fabric. These 2.5D fabrics are impregnated with epoxy resin in the normal way, laminated and cured in an autoclave. Results are presented here for Mode I double cantilever beam and Mode II end load split tests performed on the plain 2D glass fabric and the 2.5D fabrics with glass piles. The fabrics were tested in different orientations (0°, i.e., parallel to the pile fibre weave direction, 90° and 45°), and a variety of pile lengths and pile densities were investigated. The presence of the short piles in the matrix-rich region between laminate plies is shown to increase the fracture toughness of the 2.5D composite over the conventional 2D fabric composite by virtue of the energy-absorbing effect produced by the piles in a similar manner to that produced during fibre bridging.

Carbon fibre surface treatments can affect the composite properties in the fibre direction. Two effects play an important role in the longitudinal tensile strength. First, the carbon fibre surface treatment has some influence on the individual fibre strength. Second, an improved interface strength leads to better load transfer in the composite. Using an elastic-slip model, it is shown that the ineffective length increases dramatically with decreasing degree of surface treatment. It is also shown that, as the debonded length of a broken fibre decreases, the strain in the matrix becomes very high, leading to matrix cracking and additional stress concentrations on the neighbouring fibres. This leads to a maximum strength at intermediate treatment levels. This conclusion is supported by UD tensile tests on [0]9 carbon/epoxy laminates.