Unidirectional Fiber Composites Research Articles

Enhanced Estimation of Axial Compressive Strength for CFRP Based on Microscale Numerical Simulation and the Response Surface Method.

Compressive strength is one of the most important properties of carbon fiber reinforced plastics (CFRP). In this study, a new method for predicting the axial compressive strength of CFRP using the response surface method is developed. We focused on a microbuckling model to predict the compressive strength of unidirectional fiber composites. For the microbuckling model, axial shear properties are required. To obtain the compressive strength for various material properties, we perform individual shear tests and numerical simulations, but these require enormous computational costs and extended time. To address the issue of computational cost, in this study, we propose a new method to predict compressive strength using the response surface method. First, we perform shear simulation in a microscale fracture model for unidirectional CFRP with various parameters of the fiber and resin properties. Based on the results of the shear simulation, the response surface method is used to evaluate and develop prediction equations for the shear properties. This method allows for the study of the objective values of the parameters, without significant computational effort. By comparing both the results predicted from the response surface method (RSM) and the simulation results, we verify the reliability of the prediction equation. As a result, the coefficient of determination was higher than 94%, and the validity of the prediction method for the compressive strength of CFRP using the response surface method (RSM) developed in this study was confirmed. Additionally, we discuss the material properties that affect the compressive strength of composites comprised of fibers and resin. As a result, we rank the parameters as follows: fiber content, elastic modulus after resin yield, yield stress, and initial elastic modulus.

Effective magnetoelastic responses of hybrid fiber composites with viscoelastic matrices

This study examines the overall magnetoelastic responses of unidirectional fiber composites constructed by magnetostrictive fibers embedded in viscoelastic polymer matrices that had been reinforced by stiff fillers. In some polymer-based fiber composites, modifying polymer’s property by infiltrating particulate fillers is necessary. To design such hybrid composite systems with magnetoelastic coupling, it requires a detailed understanding of aggregate responses. In this study, we formulate a composite constitutive model based on a two-scale homogenization scheme for effective coupled viscoelastic and magnetoelastic responses for hybrid fiber composites. We first verify the validity of the model by comparing the predictions with experimental results available in literatures. We next investigate the performance of a hybrid composite in light of blocked stress and free strain which are two common factors of characterizing sensors and actuators. We finally elucidate how viscoelastic polymer matrices affect the overall magnetoelastic responses and ΔE-effect phenomenon in terms of some critical parameters such as phase concentrations, loading rates, prestresses and temperatures. The findings of this study serve as a preliminary guidance for the rational design of hybrid magnetostrictive composite materials. The proposed mathematics model can further integrate into a finite element framework for composite structure analysis.

Hair-Reinforced Elastomer Matrix Composites: Formulation, Mechanical Testing, and Advanced Microstructural Characterization.

Epoxy matrix composites reinforced with high-performance fibers, such as carbon, Kevlar, and glass, exhibit excellent specific stiffness and strength in many mechanical applications. However, these composites are disappointingly non-recyclable and are usually disposed of in landfill sites, with no realistic prospect for biodegradation in a reasonable time. In contrast, moldable composites with carbonized elastomeric matrices developed in the last decades possess attractive mechanical properties in final net-shape products and can also be incinerated or recycled. Many carbon and inorganic fillers have recently been evaluated to adjust the properties of carbonized elastomeric composites. Renewable organic fillers, such as human or animal hair, offer an attractive fibrous material with substantial potential for reinforcing composites with elastomeric matrices. Samples of unidirectional fiber composites (with hair volume fractions up to 7%) and quasi-isotropic short fiber composites (with hair volume fractions up to 20%) of human hair-reinforced nitrile butadiene rubbers (HH-NBRs) were produced in the peroxide-cured and carbonized states. The samples were characterized using scanning electron microscopy (SEM), Raman spectroscopy, and photoacoustic microscopy. Mechanical tests were performed under tension using a miniature universal testing machine. The expected effect of fiber reinforcement on the overall mechanical performance was demonstrated for both cured and carbonized composites. Considerable enhancement of the elastic modulus (up to ten times), ultimate tensile strength (up to three times), and damage tolerance was achieved. The evidence of satisfactory interfacial bonding between hair and rubber was confirmed via SEM imaging of fracture surfaces. The suitability of photoacoustic microscopy was assessed for 3D reconstructions of the fiber sub-system's spatial distribution and non-destructive testing.

Compressive strength improvements from noncircular carbon fibers: A numerical study

The benefits of high-performance unidirectional carbon fiber composites are limited in many cost-driven industries due to the high cost relative to alternative reinforcement fibers. Low-cost carbon fibers have been previously proposed, but the longitudinal compressive strength continues to be a limiting factor or studies are based on simplifications that warrant further analysis. A micromechanical model is used to (1) determine if the longitudinal compressive strength of composites can be improved with noncircular carbon fiber shapes and (2) characterize why some shapes are stronger than others in compression. In comparison to circular fibers, the results suggest that the strength can be increased by 10%–13% by using a specific six-lobe fiber shape and by 6%–9% for a three-lobe fiber shape. A slight increase is predicted in the compressive strength of the study two-lobe fiber but has the highest uncertainty and sensitivity to fiber orientation and misalignment direction. The underlying mechanism governing the compressive failure of the composites was linked to the unique stress fields created by the lobes, particularly the pressure stress in the matrix. This work provides mechanics-based evidence of strength improvements from noncircular fiber shapes and insight on how matrix yielding is altered with alternative fiber shapes.

Best Theory Diagrams for Laminated Composite Shells Based on Failure Indexes

This paper presents a novel approach to developing 2D structural theories for composite shells. The proposed approach uses the capabilities of the Carrera Unified Formulation (CUF) in conjunction with the Axiomatic/Asymptotic Method (AAM) to obtain the best theories for given structural layouts. Different structural cases are considered to analyze the influence of factors such as boundary conditions, lamination, and thickness on the accuracy of a model. The parameter chosen to evaluate a model’s performance is based on the Failure Indexes (FI) defined by the Hashin Failure criteria for unidirectional fiber composites. The outcome of this procedure is the Best Theory Diagram (BTD), containing the graphical representation of the highest accuracy as a function of the number of adopted unknowns. The results show the importance of higher-order terms to capture stress fields and the influence of thickness on the definition of the best theories.

A novel approach for prediction of transverse thermal conductivity of unidirectional random fiber composites

In fiber-reinforced composites, fibers are randomly positioned during the manufacturing process. Most of the theoretical studies assume regular arrangement of fibers in matrix that permits the use of representative volume element (RVE) for prediction of mechanical properties. Theoretical methods established in the literature for random fiber composites are complex and approximate. A simple approach is established in this work to accurately predict the transverse thermal conductivity of unidirectional random fiber-reinforced composites using the principle of electrical analogy (EA). In this study, circular fiber is transformed into an equivalent square fiber that facilitates the application of EA, and predicted the transverse thermal conductivity of random fiber composites. The conductivity of equivalent square fiber is evaluated by back-track approach using an equivalent thermal system. Python code is developed to generate the random location of fibers in the composite. The results obtained in this way are verified and found to be in good agreement with benchmark results over a wide range of fiber composites with varying conductivity ratios, fiber volume fraction, and randomness.

Finite element simulations of auxetic structure combined with honeycomb using unidirectional continuous carbon fiber composite properties

Metamaterials have been studied over the last few decades, as they may exhibit peculiar mechanical behavior. An example is the re-entrant auxetic structure, which can display negative Poisson ratio. Likewise, the honeycomb structure has also been widely used, mainly in sandwich-panels. Despite several studies including these geometries, the use of composite as raw material has not been reported in the literature, so this work aimed to perform finite element simulations of combined honeycomb and re-entrant auxetic structures using properties of continuous, unidirectional carbon fiber composite in epoxy matrix. For this, three types of modeling were used: beam, shell and solid and two sets of constraints were applied for each model. By analyzing the total deformation profiles obtained for these three models, it was possible to observe that both beam and shell modeling results were close to the three-dimensional modeling when applying a vertical compression displacement, in which beam modeling showed a better approximation. On the other hand, when applying the compression displacement horizontally, the beam modeling proved to be inadequate, while the shell modeling presented values close to the solid modeling. Therefore, it is concluded that it is possible to model similar structures using shell element instead of solid element.

UV accelerated aging of unidirectional flax composites: Comparative study between recycled and virgin polypropylene matrix

Previous studies on the photooxidation of natural fiber composites have focused on using short fibers and virgin matrices. In this work, unidirectional flax fiber composites with virgin and recycled polypropylene matrix were prepared by thermocompression to compare their accelerated aging. These composites were exposed to two accelerated xenon arc UV exposures where only the irradiance was modified. A multi-scale characterization was carried out to evaluate the consequences of the photooxidation on their mechanical and physicochemical properties with time. Results show that the photooxidation was limited to the surface. In addition, the photo-degradation of the surfaces of both composites was identified by the appearance of cracks, the variation of crystallinity, the decrease in weight average molar mass, and the appearance of new chemical products. These physicochemical variations are more evident with increasing irradiance. Despite these variations, the mechanical tensile properties of recycled matrix composites remain relatively indifferent to the two UV exposures compared to those with virgin polypropylene. It can be concluded that recycled polypropylene, if well selected, can replace virgin polypropylene in natural fiber composites for better mechanical resistance to photooxidation.

Compressive failure strain of unidirectional carbon fibre composites from bending tests

One of the major difficulties in establishing material properties for composites is to obtain reliable compressive strengths either from the current direct compressive testing methods due to the stress concentration or from the indirect flexural tests due to the localised stress concentration and strain gradient effect. In the present paper, unconventional flexural tests on carbon fibre/wood sandwich and monolithic composite beams have been developed and compressive failure strains of unidirectional carbon fibre composites have been obtained with high values and low variability. The compressive strain of 1.36% obtained for the IM7/8552 material is 14% higher than the highest published value found and the variability is only 3%. Strain gradient effects on the compressive failure strain from measurements with the same stressed volume have also been quantified for the first time using the proposed carbon fibre/wood sandwich beam with various wood core depths. The increase in failure strain due to the strain gradient effect reduced with the increase of total beam thickness and approached a constant value when the beam was thicker than 20 mm.

Machine learning-assisted modelling of stress concentration factor of unidirectional fiber composites for predicting their tensile strength

Significant variations in the tensile strength of unidirectional (UD) fiber-reinforced composites are frequently observed due to randomness in the fiber arrays. Herein, we propose a novel method for predicting tensile strength capable of quantifying uncertainty based on a new recurrence relation for fiber fracture propagation and a determination algorithm for the fracture sequence for random fiber arrays (RFAs). We performed finite element simulations, calculating the stress concentration factor (SCF) for UD composites with various RFAs. Then, we trained an artificial neural network with the obtained SCF data and used it to predict the SCF for composites with an arbitrary RFA. The tensile strength of UD composites was predicted over a range of values, demonstrating that accuracy was superior to conventional prediction methods.

Non-linear elastic longitudinal behaviour of continuous carbon fibres/epoxy matrix composite laminae: Material or geometrical feature?

Carbon fibres and unidirectional continuous carbon fibre composites exhibit a non-linear elastic behaviour. There has long been a debate on the physical origins of such a behaviour for these materials. Indeed, two main mechanisms have been proposed: either the reorientation of graphene sheets in the carbon fibre or that of the fibre itself due to initial fibre waviness arising from composite manufacturing. This paper addresses this issue by performing, on the one hand, specific mechanical tests to extract the non-linear elastic coefficients in a reliable way, and, on the other hand, using specific finite element analyses with a non-local model. It is shown first that the proposed experimental method is adequate. Besides, the simulations allow us to show that, while both contributions for non-linear elasticity indeed come into play for the non-linear elasticity of carbon fibres, the intrinsic behaviour of fibres can be considered as the main contributory factor. Composites with glass fibres stay fully linearly elastic, due to the limited stiffness of these fibres.

Computational Modeling of Polymer Matrix Based Textile Composites.

A simple approach to the multiscale analysis of a plain weave reinforced composite made of basalt fabrics bonded to a high performance epoxy resin L285 Havel is presented. This requires a thorough experimental program to be performed at the level of individual constituents as well as formulation of an efficient and reliable computational scheme. The rate-dependent behavior of the polymer matrix is examined first providing sufficient data needed in the calibration step of the generalized Leonov model, which in turn is adopted in numerical simulations. Missing elastic properties of basalt fibers are derived next using nanoindentation. A series of numerical tests is carried out at the level of yarns to promote the ability of a suitably modified Mori–Tanaka micromechanical model to accurately describe the nonlinear viscoelastic response of unidirectional fibrous composites. The efficiency of the Mori–Tanaka method is then exploited in the formulation of a coupled two scale computational scheme, while at the level of textile ply the finite element computational homogenization is assumed, the two-point averaging format of the Mori–Tanaka method is applied at the level of yarn to serve as a stress updater in place of another finite element model representing the yarn microstructure as typical of FE based multiscale approach. Several numerical simulations are presented to support the proposed modeling methodology.

Mechanical Response and Fracture of Pultruded Carbon Fiber/Epoxy in Various Modes of Loading

Pultrusion is a continuous process of forming constant cross-sections of unidirectional composites with a significant long length. This unique process is implemented widely in the composites industry due to its continuous, automated, and highly productive nature. The current research focused on mechanical response characterization at three modes of loading: tensile, compression, and shear loading of coupons made from a graphite/epoxy 1 mm sheet. In addition, the effects of holes and notches were examined in terms of mechanical properties. The mechanical behavior was assessed through stress–strain curves with careful attention on the curve profile, macroscopic fracture modes observations, and optical microscopic tracking with continuous video records. The mechanical tests follow standards with some critiques on the shear test. Finite element analysis (FEA) was used to accurately determine the shear modulus, and for other mechanical investigations. By nature, under tension, the unidirectional fiber composite at 0° orientation exhibits high strength (2800 MPa), with very low strength at 90° orientation (40 MPa). Both orientations display linear mechanical behavior. Under compression, 0° orientation exhibits low strength (1175 MPa), as compared to tension due to the kinking phenomena, which is the origin in the deviation from linear behavior. Under shear, both orientations exhibit approximately the same shear strength (45 MPa for 0° and 47 MPa for 90°), which is mainly related to the mechanical properties of the epoxy resin. In general, in the presence of holes, the remote fracture stress in the various modes of loading did not change significantly, as compared to uniform coupons; however, some localized delamination crack growth occurred at the vicinity of the holes, manifested by load drops up to the final fracture. This behavior is also attributed to the tension of notched coupons. FEA shows that the shear values were unaffected by manufacturing imperfections, coupon thickness, and by asymmetrical gripping up to 3 mm, with minor effect in the case of a small deviation from the load line. Selected experimental tests support the FEA tendencies.

Modeling the through‐thickness frontal polymerization of unidirectional carbon fiber thermoset composites: Effect of microstructures

AbstractThe frontal polymerization is a technique that creates a self‐sustaining “cure front” that propagates through the thermoset resin material. Such a technique is potentially capable of substantially reducing the curing time of thermoset resin fiber composites from several hours to only a few minutes or even seconds, which is promising for the additive manufacturing and repair of thermoset fiber composites. In this study, the effect of the microstructures (i.e., fiber volume fraction, fiber tow size, and fiber tow shape) of unidirectional fiber composites on the frontal polymerization initiated in the through‐thickness direction of the composites is investigated through computational modeling. The computational model is verified through comparisons with experimental data. The simulation results show that the frontal polymerization process is largely affected by the fiber volume fraction and the fiber tow shape and is insensitive to the fiber tow size. The average front velocity decreases significantly as the fiber volume fraction increases from 0% to 30% and then decreases mildly from 30% to 46%. Above 46%, the average front velocity plateaus. Moreover, the average front velocity decreases in an approximately linear fashion as the ratio between the major radius and minor radius of the elliptical cross section of fiber tow increases.

Influence of Test Specimen Geometry on Probability of Failure of Composites Based on Weibull Weakest Link Theory.

This paper presents an analytical model that quantifies the stress ratio between two test specimens for the same probability of failure based on the Weibull weakest link theory. The model takes into account the test specimen geometry, i.e., its shape and volume, and the related non-constant stress state along the specimen. The proposed model is a valuable tool for quantifying the effect of a change of specimen geometry on the probability of failure. This is essential to distinguish size scaling from the actual improvement in measured strength when specimen geometry is optimized, aiming for failure in the gauge section. For unidirectional carbon fibre composites with Weibull modulus m in the range 10–40, it can be calculated by the model that strength measured with a straight-sided specimen will be 1–2% lower than the strength measured with a specific waisted butterfly-shaped specimen solely due to the difference in test specimen shape and volume.

Interlaminar fracture property of Moso bamboo strips influenced by fiber distributions in bamboo internode and node

Bamboo is widely used as sustainable bio-composite material in the construction of houses, pavilions, furniture, and bridges, etc. As a unidirectional fibrous composite, it is usually damaged due to the interlaminar crack, which is directly related to the corresponding microstructure. In this study, the different fracture performances between bamboo internode and bamboo node are researched, as well as the fiber distributions in these zones. For the internode specimens, the results show that specimens with higher fiber contents tend to exhibit higher fracture energy than the case of lower fiber contents, so the fracture energy presents the functional gradient characteristics with fiber content. It is mainly because that more longitudinal vascular bundles make the fracture crack more tortuous and ultimately need more fracture energy. Furthermore, the node specimens have much-strengthened fracture performance than internode specimens with the same fiber contents, due to the radial fibers distributing in the bamboo node. The microscopic observation results indicate that the existence of radial vascular bundles causes the fracture process to include multiple damage modes such as interface delamination, vascular bundle pull-out, and crack deflection and finally play the effect of resisting crack propagation.

Influence of microstructure on rate-dependent response of unidirectional fibrous composites

This paper outlines prediction of the macroscopic response of unidirectional fibrous composites made either from basalt or carbon fibers impregnated by a polymeric epoxy matrix. The viscoelastic response of the matrix phase was represented by the Maxwell chain model. A series of creep tests performed at several stress levels served as a stepping stone for the model calibration. The macroscopic behavior of both composites was first examined via computational homogenization. Attention was accorded to computational cells with variable size extracted from large representative images. We observed that selecting the computational model as a sufficiently large test window should be approached with caution. Because initial designs often need a large parametric study to test various material and geometrical patterns, this study was then supported by computationally much more effective Mori-Tanaka averaging scheme, clearly showing its potential even if loading the composite beyond its elastic limit.

A micromechanics-driven model for compressive fatigue of fiber-reinforced composites

A micromechanics model to predict the fatigue life of fiber-reinforced polymer matrix composites (FRPC) is developed and used for estimating compressive fatigue failure of unidirectional carbon fiber composite. The micromechanics model utilizes constituent, fiber, and matrix material properties. An experimental procedure to obtain the matrix properties that are needed is also described. The model is implemented using the finite element method (FEM). The FEM simulations can predict the classic kink band failure seen in pre–preg-based and textile composites. In addition, an envelope fatigue loading approach is utilized to model the fatigue cycle. Using the combination of matrix fatigue degradation and the fatigue loading method, it is possible to numerically predict the S-N curve, based on the failure mechanics.

Influencing Factors on the Longitudinal Compression Properties of Pultruded Unidirectional Fiber Composites

Unidirectional fiber composites produced by pultrusion have good mechanical properties. We conducted compression experiments to investigate the factors that influenced the compression of pultruded unidirectional carbon fiber-reinforced plastics (CFRPs) under static axial loads. The CFRPS were composed of carbon fiber or carbon fiber cloth as reinforcement material, resin, ceramic, metal, cement, carbon or rubber as matrix. The modulus and compressive strength were measured. The experimental data were compared with the theoretical values obtained from the rule of mixtures. Moreover, the effects of the components, dispersion, interface states, and other factors on the compressive mechanical behaviors of the composites were studied. Scanning electron microscopy revealed the microstructure and fracture of the material. The results showed that the modulus and compressive strength of the CFRPs with a higher carbon fiber modulus decreased with the modulus and resin strength. The compressive modulus and compressive strength of CFRPs with the same resin increased with the modulus and strength of the carbon fibers. Upon increasing the modulus and strength of the carbon fibers, the compressive modulus and compressive strength of the CFRPs decreased with the modulus and strength of the resin. Moreover, glass fiber-reinforced plastics (GFRPs) were used to compare the difference between CFRPs, C/GFRP, and GFRP. The relationship between the the microstructure and promperties of these composites was systematically revealed. These research results are valuable for the design and synthesis of low-cost pultruded unidirectional fiber composites with excellent mechanical properties.

Characterizing fiber-matrix debond and fiber interaction mechanisms by full-field measurements

An experimental approach is developed and utilized to characterize the fiber-matrix interfacial debonding mechanism and its effect on matrix cracking in unidirectional (UD) fiber composites. Local deformation response at the fiber-matrix interface is first studied by analyzing the strain fields developed in the vicinity of macro fibers in single-fiber samples. A practical approach for the identification of normal cohesive behavior at the fiber-matrix interface is presented and implemented in a finite element model that replicates the experimental findings. Fiber-to-fiber interaction, debond formation, and failure mechanisms in multiple fiber systems are then studied by varying the distance and angle between adjacent fibers in double-fiber samples. The experimental results indicate that the spacing and angular orientation between adjacent fibers affect the interface debond initiation and propagation, as well as subsequent matrix failure mechanisms. It is also shown that compared with fiber spacing, angular distance has a more significant effect on matrix cracking in UD composites under transverse tension. Results presented in this work provide an experimental-based quantitative insight into the mechanics of fiber-matrix interface using in-situ full-field measurements.