Waviness Of Fibres Research Articles

Failure analysis of unidirectional CFRP composites with the coupled effects of initial fibre waviness and voids under longitudinal compression

This research delves into the failure mechanisms exhibited by unidirectional Carbon Fiber Reinforced Polymer (CFRP) composites under longitudinal compression, while considering the interplay of three types of manufacturing-induced defects: initial waviness of fibres, void volume fraction, and void size. The study employs 3D high-fidelity intact representative volume element (RVE) models, incorporating the initial waviness of fibres and voids based on Micro-CT imaging. A novel algorithm is proposed to generate more accurate 3D void shapes, departing from conventional circular or triangular approximations. The results highlight the substantial influence of the initial waviness angle on the reduction of predicted compressive stiffness and strength. The volume and size of voids play a significant role in determining damage initiation within the composites. The failure mechanisms of the composite under the coupled effects of initial waviness of fibres and voids are discussed, exhibiting reasonable agreement with experimental observations.

Buckling behaviour of laminated viscoelastic composites under axial loads

This study investigates the buckling response of viscoelastic composite laminates under compression with the objective of better understanding the ply wrinkling behaviour at the early stage of thermoset composite manufacturing. The composite laminate is modelled as a 3-D viscoelastic solid using two different approaches in Abaqus©. As the application of the built-in viscoelastic model of Abaqus© is limited to isotropic materials, a more versatile orthotropic viscoelastic constitutive model based on differential form (DF) of viscoelasticity implemented as a user material subroutine (UMAT) is employed to elucidate the effect of ply anisotropy on the buckling response of uncured/partially cured unidirectional laminates. Numerical results are compared with the experimental data available in the literature to evaluate the accuracy of the proposed approach. Using the numerical model, the influence of various parameters including loading rates, the instantaneous elastic modulus, and in particular the effects of the ply anisotropy on the buckling behaviour of composite laminates are analysed. It has been found that unlike cured composites, the compressive stiffness of uncured prepregs is mainly dominated by the resin modulus rather than the fibre modulus. Additionally, it is shown that the waviness of fibres (fibre-bed effect) which stiffens the prepreg's transverse and shear properties increases its compressive stiffness slightly while the waviness effect on the post buckling stiffness is relatively significant. Hence, conducting compressive tests on uncured and partially cured prepregs as well as partially cured resin films with different thicknesses is suggested to validate the proposed modelling approach more rigorously.

Internal structure and fatigue performance of quasi-unidirectional non-crimp fabric reinforced laminates

The aim of this work was to study the effect of stitch pattern and stitch tension on tension-tension fatigue performance of quasi-unidirectional non-crimp fabric reinforced glass-fibre/epoxy laminates. Internal structure variations of the laminates were characterised using cross-sectional microscopy imaging for the evaluation of fatigue test results. Reinforcements for the study were manufactured with three different stitch patterns and two different stitch tensions. Based on the results, the stitch pattern contributes to in-plane and out-of-plane waviness of fibres, as well as on the size and shape of the axial fibre bundles. The fatigue life and slope parameter of the fitted S-N curve were significantly dependent on the stitch pattern, while they were not dependent on the applied stitch tension.

Constitutive modelling of arteries considering fibre recruitment and three-dimensional fibre distribution.

Structurally motivated material models may provide increased insights into the underlying mechanics and physics of arteries under physiological loading conditions. We propose a multiscale model for arterial tissue capturing three different scales (i) a single collagen fibre; (ii) bundle of collagen fibres; and (iii) collagen network within the tissue. The waviness of collagen fibres is introduced by a probability density function for the recruitment stretch at which the fibre starts to bear load. The three-dimensional distribution of the collagen fibres is described by an orientation distribution function using the bivariate von Mises distribution, and fitted to experimental data. The strain energy for the tissue is decomposed additively into a part related to the matrix material and a part for the collagen fibres. Volume fractions account for the matrix/fibre constituents. The proposed model only uses two parameters namely a shear modulus of the matrix material and a (stiffness) parameter related to a single collagen fibre. A fit of the multiscale model to representative experimental data obtained from the individual layers of a human thoracic aorta shows that the proposed model is able to adequately capture the nonlinear and anisotropic behaviour of the aortic layers.

Influence of fibre architecture on the tensile, compressive and flexural behaviour of 3D woven composites

This paper presents a comprehensive study on the tensile, compressive, and flexural performance of six types of 3D woven carbon-fibre/epoxy composites which were manufactured using a traditional narrow fabric weaving loom and resin transfer moulding. Four orthogonal and two angle-interlock weaves were tested with the primary loading direction parallel to the warp direction. The mechanical performance was found to be affected by the distribution of resin rich regions and the waviness of the load-carrying fibres, which were determined by the fibre architectures. The binding points within the resin rich regions were found to be the damage initiation sites in all weave types under all loading conditions, which were confirmed with both visual observation and digital image correlation strain maps. Among all weave types, the angle interlock weave W-3 exhibited the highest properties under all loading conditions.

Influence of fibre misalignment and voids on composite laminate strength

This article studies the effects of manufacturing defects of laminate composite materials on the first ply failure load and ultimate strength of the laminate, which are calculated using finite element modelling with Zinoviev’s damage model. The following types of defects are considered for cross-ply and quasi-isotropic layups: in-plane misalignement and out-of-plane waviness of fibres, misorientation of the layers and void content. The list of laminates studied in this article includes cross-ply and quasi-isotropic layups, loaded in tension and compression in 0°, 90°, 45° and 5° directions. The limits of the first ply failure load and strength degradation for a realistic range of the defect intensities are estimated.

Influence of waviness and curliness of fibres on mechanical properties of composites

Polymeric composite materials reinforced by curved cylindrical inclusions of very high aspect ratio are studied for their elastic stiffness. The waviness and curliness of the fibres are described by sinusoidal and helical models with different amplitudes and spiral radii, respectively. Effects are investigated in detail by finite element-based homogenisation methods, analytical models and molecular dynamics simulations. Within the finite element models two types of discretisation are considered for the fibres, using continuum solid and beam elements, respectively. Periodic boundary conditions or a special set of mixed boundary conditions are applied for approximating the effective elastic properties. The analytical investigations use a mean-field approximation in which inhomogeneities are split into unconnected segments of appropriate orientation, the elasticity tensor being calculated with a Mori–Tanaka method. It is shown that both curved fibre geometries, sinusoidal and helical, significantly reduce the longitudinal elastic stiffness of the composite. Beam element-based fibre models and analytical solutions give low and high estimates, respectively, for the elastic constants. The continuum mechanical results are found to be in good agreement with the molecular dynamics predictions.

A structural constitutive model considering angular dispersion and waviness of collagen fibres of rabbit facial veins

BackgroundStructural constitutive models of vascular wall integrate information on composition and structural arrangements of tissue. In blood vessels, collagen fibres are arranged in coiled and wavy bundles and the individual collagen fibres have a deviation from their mean orientation. A complete structural constitutive model for vascular wall should incorporate both waviness and orientational distribution of fibres. We have previously developed a model, for passive properties of vascular wall, which considers the waviness of collagen fibres. However, to our knowledge there is no structural model of vascular wall which integrates both these features.MethodsIn this study, we have suggested a structural strain energy function that incorporates not only the waviness but also the angular dispersion of fibres. We studied the effect of parameters related to the orientational distribution on macro-mechanical behaviour of tissue during inflation-extension tests. The model was further applied on experimental data from rabbit facial veins.ResultsOur parametric study showed that the model is less sensitive to the orientational dispersion when fibres are mainly oriented circumferentially. The macro-mechanical response is less sensitive to changes in the mean orientation when fibres are more dispersed. The model accurately fitted the experimental data of veins, while not improving the quality of the fit compared to the model without dispersion. Our results showed that the orientational dispersion of collagen fibres could be compensated by a less abrupt and shifted to higher strain collagen engagement pattern. This should be considered when the model is fitted to experimental data and model parameters are used to study structural modifications of collagen fibre network in physiology and disease.ConclusionsThe presented model incorporates structural features related to waviness and orientational distribution of collagen fibres and thus offers possibilities to better understand the relation between structure and function in the vascular wall. Also, the model can be used to further study mechanically induced collagen remodelling in vascular tissue in health and disease.

The effect of flexion angle on the macro and microscopic appearance of the rupture surface of the ACL of rabbits

This study investigated how knee flexion angle affects the appearance of the rupture surface of the anterior cruciate ligament (ACL) using rabbit knees. Specimens were failed at flexion angles of 45° and 90° at displacement rates of 10 and 500 mm/min. Video recordings and scanning electron microscopy (SEM) studied the appearance of the rupture surfaces. At the displacement rate of 10 mm/min, the major mode of failure was fibre pullout for all flexion angles. At displacement rate of 500 mm/min, which more closely approximated loading conditions during trauma, we found that all modes of failure were exhibited. At 45° flexion, the majority of specimens tested failed by avulsion. The reverse was true for specimens tested at 90° where the majority of failures were fibre pullouts. At 45°, there were more pulled-out fibres on rupture surfaces of specimens tested at 10 mm/min than at 500 mm/min. At 90°, little difference was seen in the appearance of ruptures from both rates. SEM revealed that the waviness of collagen fibres was more pronounced at 10 mm/min. Therefore, the appearance of rupture surfaces of ACLs are affected by both flexion angle and displacement rate.

Modelling the effect of sarcomere length on collagen thermal shortening in cooked meat: consequence on meat toughness

Normal and contracted pieces of Semimembranosus and Longissimus Dorsi muscles from cull cows were cooked for 90 min at temperatures up to 80°C. For both muscles, at 50°C the normal samples have higher breaking stress than contracted samples. The breaking stress of normal samples decreases at 55°C. This decrease is not observed for contracted samples. The contracted samples become the tougher above 60°C. Drip and cooking losses are the highest in contracted samples. Sarcomere length decreases above 60°C whatever the raw sarcomere length. The amplitude of thermal shortening of perimysium collagen fibres in cooked meat has been calculated. This theoretical model takes into account the changes in the waviness of collagen fibres associated with changes in raw sarcomere length and the geometrical changes of fibre bundles due to drip, cooking losses and cooking shortening. The calculations lead to the conclusion that thermal shortening of collagen fibres at 60°C is lower in contracted samples than in normal samples. As the final modulus of collagen fibres decreases when their thermal shortening increases, this can explain part of the differences observed between the toughness of normal and contracted cooked meats. In particular, it can explain why contracted cooked meat becomes tougher than normal meat just above 60°C and why there is a decrease in normal meat toughness between 55 and 60°C. This work therefore emphasises the role of collagen in toughening associated with cold shortening.

Determining the power spectral density of the waviness of unidirectional glass fibres in polymer composites

A pilot study has been completed into the accurate measurement of 3D fibre waviness in high packing fraction, unidirectional, glass fibre reinforced polymer epoxy. It has been shown that the confocal laser scanning microscope (CLSM) can determine fibre waviness amplitudes,A⩽40 µm andsimultaneously fibre wavelengths, λ⩽4 mm. Knowing the fibre-centre coordinates in 3D with sub-micron precision, the fibre waviness may be characterised in terms of the power spectral densitiesS u andS w orthogonal to the fibre direction (taken to be in they direction) and also in terms of the power spectral densities of fibre slopes,S u′ andS w′. In future studies, these characterisation parameters will enable models linking random fibre waviness to compressive strength to be evaluated.

Deformation of collagenous, elastin and muscle fibres in raw meat in relation to anisotropy and length ratio

The influence of strain directions on the mechanical properties of raw meat samples in a compression test was analysed in relation to the contracted or stretched state and discussed in terms of connective network deformation. When the lateral deformation of a sample is in the direction of muscle fibres, the more the meat has been stretched ante-rigor, the smaller the strain range in which muscle fibres can be solicited alone and the lower the critical compression ratio at which collagenous fibres come under tension. Post-rigor contraction occurring in stretched muscles modifies the relations between muscle fibres and collagenous fibres and increases the critical compression ratio. The greatest post-rigor contractions have been observed in stretched slices of Semitendinosus and Lattissimus dorsi which are high elastin content muscles. This suggests that elastin has a major role in post-rigor contraction. In greatly contracted post-rigor samples, as muscle fibres are wavy, they are not involved at low strains, likewise collagenous fibres around muscle fibres cannot be stressed until the waviness of muscle fibres has been removed. Therefore, in greatly contracted post-rigor samples, the mechanical properties observed at low strains might belong mainly to elastin fibres. When the lateral deformation is perpendicular to the direction of muscle fibres these fibres are not involved and the more the meat has been stretched ante-rigor the higher the critical compression ratio.