Unidirectional Composites Research Articles

Thermo-mechanical behaviour of flax/green epoxy composites: Evaluation of thermal expansion coefficients and application to internal stress calculation

This work aims at evaluating the coefficients of linear thermal expansion (CLTE) of flax/green epoxy unidirectional composites and the CLTE of flax fibre. This required using high precision measuring instrument to experimentally evaluate the CLTE of unidirectional composites with various fibre contents. The flax fibre CLTE were first estimated using an inverse approach with two micromechanical models. From the longitudinal and transverse CLTE, the internal stresses of various symmetric and antisymmetric laminates, due to temperature variation, were then predicted by a 2D analytical model based on classical laminate theory. The inverse approach results showed that the transverse CLTE of flax fibre was positive and estimated at 75 ± 5 × 10−6 /K whereas the longitudinal CLTE was negative and equal to -1.2 ± 0.1 × 10−6 /K, highlighting the high anisotropy of flax fibres. The internal stress analysis in flax fibre laminates showed that the stacking sequence had a significant effect on the internal stresses, whatever the temperature variation. Regarding stacking the layers, choosing symmetric cross-ply laminate was more interesting than antisymmetric one for minimising the internal stresses. The normal stresses reached their maximum absolute values for the cross-ply laminates, whereas the maximum shear stress occurred in the [0/60]s and [0/60/0/60] stacking. This study highlighted the importance of choosing an optimised stacking sequence, such as the 0/30° oriented laminates, and a relevant curing cycle prior to the manufacturing process, in order to obtain flax fibre laminates with low internal stresses.

Spatial and temporal changes in physical properties of epoxy during curing and their effects on the residual stresses and properties of cured epoxy and composites

This study presents modeling and analyses of epoxy curing incorporating gradual changes in the thermal and mechanical properties of epoxy due to the cross-linking process. The model includes an exothermic process from the cross-linking, heat conduction across the specimen, and deformations of the specimen during curing. The study examines the influence of volumetric changes from the thermal expansion and shrinkage effects and gradual changes in the thermal and viscoelastic properties of epoxy on the deformation of the epoxy and the formation of stresses. The effects of curing temperatures and pressure on the formation of stress during curing and residual stress after complete curing are also investigated. The temporal and spatial changes in the physical properties of epoxy during curing affect the elastic modulus of the cured epoxy specimens. Furthermore, this study also investigates the existence of fiber reinforcements on the curing process of unidirectional fiber composites comprising carbon fibers in an epoxy matrix. A unit-cell model is generated and used to examine the formation of residual stresses in the composites and their influences on the mechanical properties of the composites.

Numerical Prediction of Failure in Unidirectional Fiber Reinforced Composite

Fiber-reinforced polymer composites exhibit orthotropic mechanical properties and particularly low strength in the out-of-plane direction. The use of classical failure criteria that consider transverse isotropy to evaluate these composite materials implies an overestimation of their out-of-plane strength, which could lead to a nonconservative and even catastrophic design. The Molker failure criteria developed for orthotropic materials consider the LaRC05 failure modes as a basis, with two additional failure modes for the out-of-plane direction of noncrimp fiber (NCF)-reinforced composites. Given the similarity in configuration and orthotropic behavior of unidirectional fiber fabric reinforced composites to NCF-reinforced composites, Molker failure criteria are implemented and applied in this research to determine the initiation of out-of-plane failure in unidirectional fiberglass fabric composites. The criteria are programmed in the form of a module coupled to a constitutive model available in a finite element method (FEM) package. Then, the mechanical properties and failure parameters of the unidirectional fiber-reinforced composite are determined. Model validation is accomplished by comparing numerical and experimental results of out-of-plane failure in a corrugated panel. In addition, several failure criteria used in unidirectional fiber-reinforced composite that consider transverse isotropy are evaluated. The results of critical load at the onset of transverse out-of-plane failure obtained by using the Molkerorthotropic criterion prove to be superior in accuracy compared to those obtained with the criteria commonly applied to this type of materials.

Characterization and modeling of the ratcheting behavior of unidirectional off-axis composites

The ratcheting behavior of unidirectional carbon/epoxy composites under cyclic off-axis loading with non-zero average stress was investigated, with a focus on the influence of peak stress (stress ratio R = 0) and fiber orientation. The experimental results show that nonlinear hysteresis and ratcheting behaviors heavily depend on fiber orientation and loading stress. Herein, the evolutionary features of the nonlinear hysteresis behavior and ratcheting deformation of the material are discussed. Furthermore, a creep damage-coupled viscoplastic cyclic constitutive model based on the nonlinear hysteresis behavior and cyclic creep is proposed to simulate the ratcheting behavior of carbon fiber composites under different stress levels and with various fiber orientations. The proposed model can adequately predict the nonlinear response during loading, hysteresis behavior during unloading and reloading, and ratcheting phenomena after a large number of cycles.

High through-plane thermal conductivity and light-weight of UHMWPE fibers/PDMS composites by a large-scale preparation method

The high through-plane thermal conductivity of thermal interface materials (TIMs) is a major challenge in the thermal management of electronic devices. In this work, we have developed a large-scale fabrication process for high through-plane thermal conductivity of ultrahigh molecular weight polyethylene (UHMWPE) fibers/PDMS unidirectional (UD) composites using aligned UHMWPE fibers as filler and PDMS as the matrix. Due to the high orientation (0.93) and high crystallinity (90%), the UHMWPE fibers provided an effective thermal conductive pathway along with the fiber's axis. The results showed the thermal conductivity of UHMWPE fibers/PDMS UD composites was up to 4.03 W/m·K, which is increasing by ca. 2140% compared with that of pure PDMS (0.22 W/m·K). Moreover, the composites have versatile properties such as low density (ca. 1.0 g/cm3), good compressibility, and excellent thermostability. It has great potential in industry-scale applications for TIMs.

Prediction of the Seebeck coefficient of thermoelectric unidirectional fibre-reinforced composites

In this work, new analytical equations are derived to precisely assess the longitudinal and transverse Seebeck coefficient values of a unidirectional (UD) carbon fibre reinforced polymer (CFRP) composite lamina, as a function of the properties of the constituents and the fibre volume fraction. Accurate Finite Element (FE) analyses were also carried out on fibre-matrix unit cells, and the results were compared to those obtained with the analytical methods showing a very satisfactory agreement.Experimental measurements were carried out on single carbon fibre (CF) filament level, as well as on a CF/epoxy UD lamina that corroborated the proposed equations with the experimental results.

Evolution of fibre deflection leading to kink-band formation in unidirectional glass fibre/epoxy composite under axial compression

We present the first experimental observation and quantification of the evolution of the three-dimensional (3D) deflection of fibres leading up to kink-band formation in notched unidirectional glass fibre-epoxy composites. This has been achieved by in situ synchrotron X-ray computed tomography (CT) quantified by advanced image analysis. The 3D trajectories of individual fibres were extracted to monitor the change in fibre profiles with increasing compressive load. Initially the fibre trajectory varies significantly from fibre to fibre, the waviness of which cannot be represented by a simple idealised profile. However, the change in the fibre shapes during axial compression is similar for all fibres, as the fibres deflect towards the final kinking direction. The precursor to kink-band nucleation has been identified as a local tilting of fibres in the region where the kink bands ultimately form, defining a micro-buckle band. Even at 99.9% of the failure load this micro-buckle band is much narrower (~150 μm (~12 fibre diameters) wide), and the angle of inclination of the band much shallower (~8°), compared to the final 300–400 μm (25–33 fibre diameters) wide, and 25–30° inclined, kink band. It is notable that kink-band boundaries are delineated by fibre fractures, many of which exhibit wedge-shaped multiple fractures.

The effect of fibers under compression on the simple shear response of a soft unidirectional fiber-reinforced composite

The aim of this work is to investigate the mechanical behavior of a soft matrix reinforced by a single family of parallel fibers subjected to simple shear. Specimens with fibers oriented at −45° with respect to the applied shear load were tested. The shear and normal stress components as well as the amount of shear were simultaneously measured. Due to the initial orientation, fibers were subjected to compressive loads, resulting in buckling. The positive Poynting effect was observed when soft materials were sheared. The off-axis stiffness theory and the Murphy model at small deformations were used to predict the mechanical behavior of the soft composites. In general, there is a reasonable correlation between model predictions and measured data by adjusting only one empirical parameter. However, the model based on the off-axis stiffness shows better prediction than the Murphy model for relatively higher fiber volume fractions. From universal relations for simple shear, the results indicate that the studied composites are of a coaxial type in a weak sense.

Tensile failure model of carbon fibre in unidirectionally reinforced epoxy composites with mean-field homogenisation

This paper presents an extension of the so-called incremental-secant mean-field homogenisation (MFH) formulation accounting for fibre bundle failure and matrix cracking in Unidirectional (UD) composites. First a model for fibre bundle failure is developed based on the strength failure probability of the carbon fibre described by a Weibull distribution. This fibre bundle failure model is then framed in a damage model of embedded bundles in a matrix by considering an exponential relation to describe the longitudinal stress build-up profile experimentally observed during failure of embedded fibre bundles. Cracking of the matrix in UD composites is accounted for through an anisotropic non-local damage model, which allows capturing the so-called 0° splits experimentally observed during the longitudinal tension of UD plies. A Mean Field Homogenisation (MFH) model is then extended to account for these damage models as component behaviours of the 2-phase composite material. A finite element multi-scale simulation of a notched laminate shows that the intra-laminar failure modes observed by an in situ experiment reported in the literature are well captured by the damage variables related to the matrix and fibre bundle failure processes. Inter-laminar failure is also captured by an extrinsic cohesive law introduced between the plies.

Mechanical analysis of fiber-reinforced plastics using an elastoplastic-damage constitutive equation considering asymmetric material behavior

Mechanical modeling of fiber-reinforced plastics to predict nonlinear mechanical behavior along the off-axis direction is very complex due to tension–compression asymmetry. This study aimed aims to develop a generalized constitutive model for different types of unidirectional and woven composites, considering tension–compression asymmetry in various deformation ranges from elastic to plastic, and finally to failure. To capture the asymmetry effectively, a new multivariable plastic model was proposed with additional dilatational components and different elastic moduli for tension and compression. Finally, asymmetric failure criteria and a damage evolution law were introduced for strength prediction. The proposed model was implemented in a finite element analysis software via a user material subroutine, and experimentally validated in terms of off-axis tension and compression tests in various loading directions. The experimental results were accurately predicted using a multi-parameter plastic model in this study compared to previous other models with few parameters.

On the rational choice of the properties of structural elements constituting unidirectional compositions

A technique is proposed that makes it possible to implement a rational choice of fibers constituting a unidirectional tensile composite. The choice is made in such a way as to avoid catastrophic failure. The tensile properties of the fibers are characterized by complete deformation diagrams with falling branches. It is shown that the fibers must have different properties since it is impossible to avoid a catastrophe (dynamic fracture of the composite) if the properties are identical.

Simulation on the mechanical performance and fracture behavior of unidirectional carbon fiber-reinforced composites

Unidirectional carbon fiber reinforced polyamide 6 (CF/PA6) laminates after tensile tests exhibited two different types of fracture modes, namely, splitting and step-like. In this study, the mechanical properties and fracture behavior of the unidirectional CF/PA6 laminates were simulated and analyzed by using finite element simulation of a tensile test. The simulations considered the actual deformation and damage mechanism (matrix plastic deformation, stochastic fiber fracture, and interfacial debonding between the fiber and the matrix) experimentally observed in previous research. Results of the numerical simulations were in good with the experimental stress–strain curves. Moreover, the above-mentioned two types of fracture mode were well simulated by changing the interfacial properties. The simulated results showed that the unidirectional laminates with weak interface properties exhibit the splitting fracture mode. Meanwhile, the unidirectional laminates with strong interface properties show the step-like fracture behavior.

Analysis of Mechanical Properties of Unidirectional Flax Roving and Sateen Weave Woven Fabric-Reinforced Composites

Abstract Natural fiber-reinforced composites are getting more attention from researchers and manufacturing companies to replace metals and synthetic materials that have dominated the manufacturing industries. In this study, the mechanical properties of unidirectional (UD) flax roving-reinforced composites and woven fabric-reinforced composites were investigated. Three different composites were prepared from flax rovings, which have the same linear density and epoxy resin matrix, with different reinforcement and composite preparation methods. The samples were subjected to experimental tests of flexural rigidity and tensile strength in a parallel and perpendicular direction to fiber orientation. The test results showed that flexural rigidity and tensile strength of flax fiber-reinforced composites are highly dependent on the direction of fiber orientation. The results also reveal that in a parallel direction to fiber orientation, UD composites have higher flexural rigidity and tensile strength than woven fabric-reinforced composite.

Fiber break model for tension-tension fatigue of unidirectional composites

Damage progression in unidirectional (UD) composites in tension-tension fatigue is vital to understanding fatigue in more complex laminates and loading conditions. We therefore developed the UD fatigue model that considers fiber breaks, fiber-matrix debond growth, fiber fatigue as well as interactions of these damage modes. The contribution of each mode of damage in fatigue is assessed and those that dominate fatigue damage are identified. The model accounts for Weibull statistics of fiber strength and stress redistribution around fiber breaks. The predictions for fatigue loading indicate a significantly higher density of fiber breaks and clusters compared to quasi-static loading at the same stress level. The model predicts S–N curves in pure uniaxial tension-tension fatigue loading and does not consider stress concentrations caused near the grips; the predictions therefore tend to overpredict fatigue life compared to experimental S–N curves available in the literature. The model has potential to be developed for different loading conditions.

Damage-entropy model for fatigue life evaluation of off-axis unidirectional composites

This research is devoted to studying the capability of the damage-entropy model for fatigue life prediction of the off-axis unidirectional composites. The damage process in composites was characterized by the micro-continuum damage mechanics (MCDM) and an available reformulated form of the Mori-Tanaka model was used. In the present study, the fracture fatigue entropy (FFE), which is the accumulated entropy generation up to final failure during the fatigue process, was considered as a fatigue failure criterion for the off-axis unidirectional composites. One of the outstanding characteristics of the damage-entropy model is considering the self-heating and temperature evolution during the fatigue life that is due to the damage evolution, irreversible work, and internal friction in composites. Moreover, the dissipated heat due to the viscoelastic intrinsic property of the polymeric matrix was considered. The proposed model was validated by investigating the fatigue behavior of unidirectional T800H/2500 and E-glass/epoxy composites under tension-tension cyclic loading. Results show that the fatigue life prediction of the damage-entropy model is in good agreement with the experimental data.

Multiscale modeling of unidirectional composites with interfacial debonding using molecular dynamics and micromechanics

In this study, a new multiscale micromechanical model based on the finite-volume direct averaging micromechanics (FVDAM) theory and molecular dynamics (MD) is constructed. It describes the interfacial debonding phenomena of unidirectional composites, whereby a solid fracture interface is incorporated with the FVDAM for the first time. To better elucidate the debonding behavior of the solid interface, a six-degrees-of-freedom (6-DoF) cohesive zone model (CZM) is proposed using MD simulation. The damage matrix Dc(k) in the kth subvolume of the interface is obtained using the 6-DoF CZM, and its explicit form is described herein to facilitate its implementation in commercial software packages. To verify the effectiveness of the model, both experimental data and numerical simulations were adopted for comparison. The experimental data of the unidirectional SiCf/Ti6Al4V composite were compared with the simulation results of FVDAM, and a good agreement was established. A finite element method-based unit cell model is constructed using ABAQUS for numerical validation, and homogenized responses and local fields are compared. The excellent correlations prove the effectiveness of the proposed model. Furthermore, the effects of thermal residual stress and fiber orientations are revealed, which may provide some useful information for the design and manufacture of composites.

A numerical study of progressive damage in unidirectional composite materials using a 2D lattice model

In this study, we explored the effects of different softening laws on the modelling of progressive damage in unidirectional composites materials using a 2D lattice model. The lattice model was based on axial springs that accounted for nearest-neighbour interactions. We considered linear and exponential softening laws and assessed their influence on the dynamic fracture response of unidirectional composite materials using two numerical examples. In order to overcome one of the drawbacks inherent to lattice models, i.e., the dependence on lattice cell size, we modified the material constitutive law by introducing a characteristic length parameter. The results obtained are also compared with experimental data reported by other researchers.

Prediction of the transverse elastic modulus of the unidirectional composites by an artificial neural network with fiber positions and volume fraction

This paper presents the development of artificial neural network (ANN) modeling for predicting the transverse elastic modulus of a unidirectional composite (E-glass/MY750) with fiber positions and volume fraction. For this prediction, random representative volume elements were generated according to the fiber volume fraction (). A training dataset consisting of input data ( and fiber locations) and output data (the effective elastic modulus), which were computed by the computational homogenization scheme (CHS), was used to train the ANN model to have proper weight and bias by back propagation. To demonstrate the performance of the proposed ANN model, prediction of the transverse elastic modulus of various test datasets whose transverse elastic moduli are known by CHS was conducted. The prediction accuracy was verified in terms of the mean squared error, correlation coefficient (R), and relative error. The prediction results showed excellent agreement with the test dataset and quickly predicted the transverse elastic modulus having random microstructures.

An efficient anisotropization technique to transform isotropic nonlinear materials into unidirectional and bidirectional composites

This work presents a procedure to transform an isotropic nonlinear and rate-dependent material model into orthotropic. This transformation relies on a fast computation modifying the material inelastic evolution along any desired direction. With this concept, the constitutive law of the pure isotropic, nonlinear and rate-dependent material is fully preserved. This paper shows a direct application to describe the nonlinear response of unidirectional (UD) or bidirectional (BD) fiber-reinforced polymer composites. Imposing one constraint leads to a UD composite, whilst imposing two orthogonal directions leads to a BD composite. The proposed methodology is implemented as user-defined material for Finite Element solvers. To prove the method, two different visco-plastic material models used to describe isotropic polymer resins are anisotropized to obtain their corresponding composite counterparts. The mechanical response of the simulations are compared qualitatively and quantitatively to experimental tests on a UD coupon under off-axis tensile test under different load directions. Consistency between the UD and BD anisotropization formulations is proved under tension, shear and different loading directions. Additionally, a complete study of computational performance is addressed to assess the anisotropization’s feasibility to be applied in modeling and design of new laminated and multilayer-based structures exhibiting nonlinear material response.

A Microscopic Cohesive Zone Model and Effects of Interface on the Transverse Mechanical Properties for Composites

The carbon fiber/epoxy matrix interface plays an important role in the behavior and damage onset of unidirectional fiber reinforced epoxy matrix composites and accurate modeling techniques are needed to study the effects of this complex region on the composite response. In this work, a microscopic cohesive zone model (MCZM) based on atomic potential energy is established for the interface. A multiscale analysis method is proposed for predicting the transverse mechanical properties of unidirectional fiber composites: (1) At the microscale, MCZM is established for the interface; (2) At the mesoscale, a unit cell model is established for the fiber, matrix and interface; and (3) At the macroscale, the homogenization method, failure criteria and damage degradation models are used for predicting the transverse mechanical properties. Subsequently, the transverse mechanical properties and the damage evolution process are simulated with the multiscale analysis method. A comparison between the simulations and experiments shows that: (1) The maximum errors of the predicted transverse modulus and transverse strength are −4.45 % and −1.28 %, respectively; and (2) MCZM can reflect the nonlinearity of epoxy matrix composite materials. Moreover, the effects of the interfacial strength on the macroscopic transverse mechanical properties and the damage onset are analysed. The simulation results show that: (1) The interfacial strength has a more significant effect on the transverse strength and ultimate strain than on the transverse modulus; (2) Decreasing the interfacial strength has a greater effect on the transverse modulus, strength and ultimate strain than increasing the interfacial strength; and (3) The interfacial strength can change the damage onset.