Unidirectional Fiber-reinforced Composites Research Articles

Model prediction of unidirectional fiber-reinforced composites under finite deformation

The intuitive knowledge is that the mechanical modulus of unidirectional fiber-reinforced composites (UD-FRPs) decreases with higher fiber orientation angles. However, numerical results in this work and experimental results in previous literature indicate that the mechanical response of UD-FRPs has a U-shaped dependence on fiber orientation angle. To explain this phenomenon, we develop an anisotropic model to capture the mechanical behavior of UD-FRPs. The strain energy is decomposed into four components: matrix, fiber, fiber-matrix normal, and shear interactions. Each component can be determined by matching the mechanical responses of unit cells with 0°, 45°, and 90° off-axis. The results obtained from the presented model match well with the numerical response of unit cells with 15°, 30°, 60°, and 75° off-axis. With an increasing fiber orientation angle, the matrix part remains unchanged, the fiber component decreases, but the fiber-matrix normal component increases, and the fiber-matrix shear component increases and then decreases. The change in strain energy contributions explains the mechanism of the U-shaped dependence of the mechanical response on fiber orientation angle.

Energy transfer and release during the fracture of UD fiber-reinforced thermoplastic composites

This study delves into the complex dynamics of energy transfer and release during the fracture of unidirectional (UD) fiber-reinforced thermoplastic composites. Conducted through meticulous experimental observations and rigorous energy analysis, the research presents groundbreaking conclusions on the fracture toughness and energy conversion efficiency of UD carbon fiber composites under various loading conditions. A prominent focus is on how these mechanical properties are intricately linked to the sample’s fiber orientation angles and the external conditions applied during testing.The work employed tapered double cantilever beam (TDCB) as the far-field boundary to investigate the energy changes, transfer, and conversion processes during the crack propagation of unidirectional fiber reinforced thermoplastics (UDFRTPs) structures. It was found that throughout the entire propagation process, blunt extension occurs first followed by inertial extension. This is because in the initial stage of crack propagation, the stored elastic energy in the structure is relatively small, making the crack tip prone to blunting. This continuous blunting impedes the release of internal strain energy in the structure, with blunt extension accounting for a very small proportion of the entire fracture process. When the internal strain energy in the structure continues to increase to a certain extent, the blunting of the crack tip cannot prevent the release of a large amount of elastic energy inside the structure. At the same time, the excess released strain energy converts into kinetic energy and continually impacts the blunted region at the crack tip. At this point, the propagation transitions into the inertial extension stage, which occupies a significant portion of the entire fracture process.

Effect of Fiber Orientation and Volume Fraction on Young's Modulus for Unidirectional Carbon Fiber Reinforced Composites: A Numerical Investigation

The modulus of elasticity of unidirectional fiber-reinforced composites greatly depends on the fiber orientation and the fiber volume fraction. This paper investigates the effects of fiber orientation and fiber volume fraction using the commercially available Finite Element Analysis (FEA) software Abaqus. Carbon fiber is the reinforcing material, while epoxy is used as the matrix. Representative Volume Element (RVE) of the unidirectional carbon fiber reinforced composites are modeled in Abaqus, and the unidirectional tensile test is simulated to determine the modulus of elasticity for different fiber orientations and fiber volume fractions. The numerical results are verified by previously published results and by experimental results. It is found that the modulus of elasticity of the composite is maximum when the fiber inclination angle is 0°, i.e., the fibers are placed along the loading direction. As the fiber orientation angle increases, the modulus of elasticity also decreases and is almost constant after 45°. A linear increase in modulus of elasticity is observed for an increase in fiber volume fraction. This model will help the researcher to select the appropriate fiber orientation and volume fraction for a specific application.

Analysis of crack-tip stress field in unidirectional fiber-reinforced composites based on 3D micromechanical model

Accurately clarifying the influence of cracks on the mechanical behaviors of unidirectional fiber-reinforced composites (UD FRCs) is of great significance for the structural integrity evaluation of UD FRCs with defects. Although several experiments and numerical analyses have explored the impact of cracks on stress field, satisfactory quantitative characterizations of crack size, a/W and location, Δ, fiber content, Vf have not yet been obtained. In this paper, the accurate and detailed microscopic models containing crack were first established and thoroughly validated. Then, a parametric study was conducted using three-dimensional micromechanical analysis to investigate how the crack size and location, fiber content affect the von Mises, stress triaxiality, Lode angle and J-integral of UD FRCs. Results showed that homogeneous models, whether isotropic or anisotropic, cannot accurately describe the mechanical behavior of cracked UD FRCs. The von Mises stress and J-integral are greatly influenced by the crack and fiber content, while stress triaxiality and Lode angle are not sensitive to changes in these factors. Further, the von Mises and J-integral increase with the decrease of Vf and the increase of a/W, but show a more complex trend in the variation of Δ. These findings will help clarify the failure mechanism of cracked UD FRCs and can be used for reliability evaluation.

Synthesising realistic 2D microstructures of unidirectional fibre-reinforced composites with a generative adversarial network

The microstructure governs the behaviour of unidirectional fibre-reinforced composites. In this study, we developed a Deep Convolutional Generative Adversarial Network (DCGAN) to generate realistic 2D transverse microstructures of such composites. We evaluated the DCGAN-generated microstructures using three different methods: Fréchet inception distance, walking through the latent space, and feature matching. The results from these evaluations confirmed that the generated microstructures are distinct and not simply a replication of the training data. The generated microstructures were then compared to real microstructures, confirming that they match qualitatively and quantitatively with respect to detailed statistical characteristics, including fibre diameters, fibre volume fraction, fibre spatial distribution, and resin-rich pockets. We also found that microstructures created by traditional generators could not capture the real resin-rich pockets. This illustrates the capability and value of DCGAN to generate realistic transverse microstructures and provides insights for modelling methods based on real images.

Multiscale analysis method and experiments for the transverse mechanical property of unidirectional fiber-reinforced composites

Carbon fiber-reinforced epoxy matrix composite laminates have been widely used in the case of advanced aero-engines, due to their high specific mechanical properties and mature production technology. In order to predict accurately the transverse mechanical properties of composites, this paper developed a multiscale analysis method which consists of three scales: At the microscale, an interfacial cohesive zone model is established based on atomic potential energy to describe the interface. At the mesoscale, a unit cell model is established to represent the fiber, matrix, and interface. At the macroscale, the homogenization method, failure criteria, and damage degradation models are utilized to predict transverse mechanical properties. The results of predictions are compared with experimental tests, and it is found that the maximum errors are less than 3%. Furthermore, the predicted damage evolution path aligns with the experimental results, thus validating the accuracy of the multiscale analysis method. By employing the multiscale analysis method, the effects of interfacial strength on the macroscopic transverse mechanical properties are analyzed. The simulation results indicate that: The interfacial strength has a more pronounced influence on the transverse strength and ultimate strain compared with the transverse modulus. Reducing the interfacial strength has a greater influence on the transverse modulus, strength, and ultimate strain than increasing the interfacial strength. The interfacial cohesive zone model can reflect the nonlinearity of epoxy matrix composite materials. Overall, the multiscale analysis method proves to be effective in accurately predicting the transverse mechanical properties of composites, and provides insights into the interfacial strength’s role in these properties.

Numerical prediction of thermal conductivity and thermal expansion coefficient of glass fiber-reinforced polymer hybrid composites filled with hollow spheres

The optimal performance of composites enriched with hollow spheres has been reported in contemporary literature, whereas their thermal properties have received less attention. In this regard, a finite element method (FEM)-based micromechanical model has been developed systematically to investigate the role of intra-matrix embedding of hollow spheres on the thermal conductivity and coefficient of thermal expansion (CTE) of unidirectional fiber-reinforced hybrid composites. In so doing, the concept of representative volume element (RVE) considers microstructures comprising an epoxy matrix, E-glass fiber, and E-glass hollow spheres, assuming perfect bonding (ideal interface) between the components and modified approximate periodic boundary conditions. By computing the longitudinal and transverse temperature gradients generated due to the application of uniform heat flux as well as the geometrical variation in RVE owing to temperature enhancement, thermal conductivity and CTE have been respectively determined. Comprehensive evaluations have been conducted to examine the effects of microstructural-level features, including fiber volume content and orientation, plus volume content and thickness of hollow spheres, on the effective thermal conductivity and CTE of pseudo-porous ternary E-glass/epoxy composites.

Enhanced prediction of transverse mechanical behavior of unidirectional fiber-reinforced composites using new spatial descriptors based on deep neural networks

In this study, we developed a data-driven approach using a deep neural network (DNN) to predict the nonlinear stress-strain behavior of fiber-reinforced composites under transverse tensile loading. We explored the impact of different input features characterizing the composite microstructure on the prediction accuracy of the DNN model. Specifically, we introduced a spatial descriptor representing fiber arrangement and proposed new spatial descriptors based on fiber area to improve input feature quality. Incorporating widely-used spatial descriptors such as the second-order intensity function and radial distribution function improved the prediction performance compared to previous methods. Combining spatial descriptors for both fiber distribution and alignment further enhanced prediction accuracy. To address the limitations of conventional spatial descriptors, we developed new spatial descriptors using a continuous function. Our findings demonstrate the importance of selecting appropriate input features for improved DNN model performance, even with the same dataset. Moreover, the proposed fiber area-based spatial descriptors offer insights into the micromechanical behavior of composite materials.

A three-dimensional failure criterion model considering the effects of fiber misalignment on longitudinal tensile failure

This paper proposes a three-dimensional failure criterion model for unidirectional fiber-reinforced composites. In contrast to previous models that only accounted for the effect of fiber misalignment in the presence of longitudinal compressive stress, the proposed failure criterion model comprehensively considers the effect of localized misaligned regions on the failure under any stress state. Another key contribution of this study is the introduction of the effective misalignment angle. Considering effective misalignment angles, the proposed failure criterion model can reasonably reveal the effect of localized misaligned regions on the failure behavior under different stress states. The agreement between the predicted results and the experimental data proves that the proposed model has good applicability. Furthermore, the influence of initial misalignment angles on failure is analyzed under varying stress conditions. The results indicate that even under longitudinal tensile stress, the initial misalignment angle still plays an important role in the failure behavior of materials.

Modeling 3D transverse elasto-plastic damage of unidirectional fiber-reinforced polymer composites using a smeared crack approach

This work presents a novel formulation of a 3D smeared crack model for unidirectional fiber-reinforced polymer composites based on a stress invariant approach for transverse yielding and failure initiation, and on a continuum damage approach. This formulation is developed to facilitate the implementation in an implicit solver, increasing solution robustness and computational efficiency in quasi-static and long duration analyses. The performance of the model is evaluated using monotonic and non-monotonic damage evolution, verified with single element tests to demonstrate the consistency of the proposed formulation. Additional benchmark examples regarding off-axis tension and compression tests are simulated and compared with the experimental results, showing good agreement for the plastic response, failure load and failure strain.

A Numerical Model for the Effective Damping Properties of Unidirectional Fiber-Reinforced Composites

A Numerical Model for the Effective Damping Properties of Unidirectional Fiber-Reinforced Composites

Efficient generation of random fiber distribution by combining random sequential expansion and particle swarm optimization algorithms

Representative volume elements (RVEs) with random fiber distribution are widely used in micro-mechanics for determining the properties of unidirectional fiber-reinforced composites from their microstructure. In this paper, the random sequential expansion (RSE) and particle swarm optimization (PSO) algorithms are combined to develop an efficient methodology for generating such RVEs. This methodology can successfully eliminate the biased regions of dense fiber population and unreal matrix-rich corners that usually appear in created RVEs and resolve the complications in selection of the input parameters in the RSE algorithm. Statistical analysis of the generated microstructures and two- and three-dimensional finite element analyses are performed to validate the capability of the proposed algorithms.

Strong-anisotropy-induced instability and discussion on kink band under complex loads

Compressive instability, such as the occurrence of kink bands, is common in composite materials and other highly anisotropic layered materials, limiting their use. Most previous research has focused on the microstructure and heterogeneity in studying the compressive instability of composites. In this paper, to highlight and quantify the influence of anisotropy, we investigate the deformation instability of effective homogeneous solids with strong anisotropy. Firstly, we re-write the local instability criterion from an energy perspective and verify its applicability. Even when the heterogeneity of the composites is eliminated, the derived criterion can reasonably predict the kink band instability and can be reduced to the well-known Rosen’s estimate. The strong stiffness anisotropy plays a dominant role in kink band instability. For unidirectional fiber-reinforced composites, increasing the modulus of the hard phase contributes little to an increase in the compressive strength because, as the anisotropy increases, the critical strain decreases and becomes approximately inversely proportional to the anisotropy degree. The kink band instability is also investigated under complex loads. We find that lateral tension increases the critical kink band load, while lateral compression or shear decreases the critical kink band load. To estimate the instability, we develop a series of instability phase diagrams that can be used as a guideline for predicting the compressive instability of composite materials. We have found that reducing the anisotropy of the material is the main way to increase the critical load of kink band instability, which is important for material design and engineering applications.

Phase-field fracture modeling for unidirectional fiber-reinforced polymer composites

In the present work, a phase-field fracture model is developed to create an experimentally validated, physically motivated, and computationally tractable framework to predict the orientation-dependent complex fracture response of the unidirectional fiber-reinforced polymer matrix composites (PMC). The damage mechanism in PMC is many-fold, such as matrix micro-cracking, fiber breakage or pullout, fiber debonding, and delamination. The progressive damage and failure in the composite can be attributed to the competition of all these different damage mechanisms individually or in concert, depending on fiber volume fraction and orientation of the plies. Moreover, other important factors at the constituent level, such as viscoelasticity or plasticity of the matrix, can significantly influence the constitutive behavior of the composite. In this work, a homogenized, coupled thermo-mechanical constitutive model is developed considering the viscoelasticity of the polymer matrix for the fiber-reinforced PMCs. Subsequently, a phase-field fracture model is adopted to capture the essential damage mechanisms as matrix cracking and fiber pullout/breakage for unidirectional composite lamina. The phase-field variable considers an anisotropic tensorial term in its gradient to account for the orientation-dependent fracture of fiber-reinforced PMC. The model is numerically implemented by writing an ABAQUS user-element subroutine (UEL). The model can predict the direction-dependent damage propagation and the final load-deformation response at fracture in commercially acquired unidirectional glass-fiber-reinforced epoxy composites at different fiber orientations in reasonable agreement with the experiments. Based on the decoupling of the driving energies the model can also identify the dominant damage mechanism among the two such as fiber breakage or matrix cracking, for a given fiber orientation. Simulations are also conducted to validate the model predictions for carbon-fiber reinforced epoxy lamina with available experimental data in the literature.

Generation of unidirectional fiber random distribution structures based on an artificial fish swarm algorithm with random deletion after fiber filling

This study proposes a random distribution method for generating high-content volume fractions of fibers in their cross-sectional area. This approach is referred to as the artificial fish swarm algorithm (AFSA) with random deletion after fiber filling (RDAFF_AFSA). Initial fibers were first generated using AFSA, followed by fiber filling the matrix-rich region with a hard-core model. The desired representative volume element (RVE) was ultimately obtained by random deletion. The proposed method can generate RVEs with high-content volume fractions (up to 67%) of fibers. The generated RVEs were statistically analyzed and compared with the completely spatial random (CSR) pattern and experiment data. The results showed that RDAFF_AFSA exhibited a high degree of consistency with the CSR and experimental data. The elastic constants of the carbon fiber-reinforced plastic composites were predicted by finite element analysis. The predicted results are very reasonable compared to the experimental results. The proposed method can provide a highly valuable alternative for micromechanical and multiscale analyses of unidirectional fiber-reinforced composites.

Influence of interphase characteristics on the elastic modulus of unidirectional glass-reinforced epoxy composites: a computational micromechanics study

Abstract This article briefly discusses the role of interphase in the elastic moduli of unidirectional fiber-reinforced polymer composite materials. For this unidirectional glass fiber was chosen as reinforcement, and epoxy was selected as the matrix. A hexagonally packed representative volume element is used for the micromechanical analysis. Experimental validation was initially used to verify the accuracy of the established equations of the rule of mixing, Composite Cylinder Assemblage, Chamis, Halpin and Tsai, and Puck. The Chamis equation was found to be the most reasonable. Then the finite element approach in which interphase has been included was used to estimate the elastic moduli. The finite element model without interphase and the experimental result were taken as a reference. The influence of interface ratio and property of interphase on the homogenised elastic properties of the unidirectional fiber-reinforced polymer composites is analysed. A micromechanics plugin in Abaqus software was used to estimate the density and Young’s modulus of the unidirectional fiber-reinforced polymer composites. The interphase properties are varied, having 6.25%, 12.5%, 25% and 50% influence of the fiber phase and the remaining influence of the matrix phase with interface ratios of 0.1, 0.2 and 0.3. The interface ratio of 0.3, having 6.25% fiber phase influence, gave the most reasonable moduli values (with an error <10%) compared to the mean experimental moduli. The study showed interface ratio and interphase properties to critically influence the overall elastic property of the unidirectional fiber-reinforced polymer composites.

Effects of fiber surface modification on mechanical properties of short pineapple leaf fiber-carbon black reinforced natural rubber hybrid composites

At present, the world is focusing on the circular economy by turning waste into production resources again, reducing waste generation, and making it circulate in the system. This research focuses on using agricultural waste, which is pineapple leaves abundant in Thailand, to be processed into pineapple leaf fibers (PALF) and used to reinforce natural rubber. PALF has relatively high strength when compared to other natural fibers. On the other hand, the challenge of using PALF to reinforce natural rubber is the difference in the polarity of the fibers and rubber matrix. Thus, surface modification of PALF was studied to enhance the adhesion between the fiber surface and the matrix. Four systems were compared, i.e., untreated PALF, alkaline-treated, silane69-treated, and aminosilane-treated PALF. A unidirectional fiber-reinforced natural rubber composites were prepared with fixed fiber and carbon black contents of 10 and 30 phr, respectively. Mechanical properties of the composites were determined by tensile test and morphology was observed by scanning electron microscope (SEM). It was found that silane69-treated and aminosilane-treated PALF could reinforce the rubber matrix over the entire strain. However, silane69-treated and aminosilane-treated PALF had lesser reinforcement effect than alkaline-treated PALF at low strains. Despite a clear change in mechanical behavior, SEM could not tell the difference or changes that occur at the interface. Fiber surface modifications with silane69 and aminosilane have the potential to be further developed to improve properties of natural rubber composites and will, hopefully, increase the value of rubber products for Thailand industry.

Integrated digital image correlation for micro-mechanical parameter identification in multiscale experiments

Micromechanical constitutive parameters are important for many engineering materials, typically in microelectronic applications and material design. Their accurate identification poses a three-fold experimental challenge: (i) deformation of the microstructure is observable only at small scales, requiring SEM or other microscopy techniques; (ii) external loadings are applied at a (larger) engineering or device scale; and (iii) material parameters typically depend on the applied manufacturing process, necessitating measurements on material produced with the same process. In this paper, micromechanical parameter identification in heterogeneous solids is addressed through multiscale experiments combined with Integrated Digital Image Correlation (IDIC) in conjunction with various possible computational homogenization schemes. To this end, some basic concepts underlying multiscale approaches available in the literature are first reviewed, discussing their respective advantages and disadvantages from the computational as well as experimental point of view. A link is made with recently introduced uncoupled methods, which allow for identification of material parameter ratios at the microscale, still lacking a proper normalization. Two multiscale methods are analysed, allowing to bridge the gap between microstructural kinematics and macroscopically measured forces, providing the required normalization. It is shown that an integrated experimental--computational scheme provides relaxed requirements on scale separation. The accuracy and performance of the discussed techniques are analysed by means of virtual experimentation under plane strain and large strain assumptions for unidirectional fibre-reinforced composites. The robustness against image noise is also assessed.

A direct method for wave propagation in elastic fiber composites

A direct method is proposed to study wave dispersion in stiff unidirectional fiber-reinforced elastic random composites. The present model is featured by a modified form of classical elastodynamic equations supplied with a simple differential relation between the displacement field of the composite and the displacement field of the mass center of representative cell. Explicit formulas are derived for the attenuation coefficient and effective phase velocity of in-plane P-waves, SV-waves, and anti-plane SH-waves, respectively. The efficiency and accuracy of the model are demonstrated by comparing predicted results (on the frequency for the maximum attenuation and effective phase velocity at frequencies below the bandgap) with a large group of known data reported in past decades. The proposed model offers a novel general method for dynamics of stiff unidirectional fiber-reinforced elastic random composites.

Efficient generation of random fiber distributions in fiber reinforced composites

This paper proposes a new algorithm to generate representative volume elements (RVEs) with random fiber distribution in fiber reinforced composites (FRC). The proposed algorithm is straightforward and easy to implement based on judging the maximum and minimum distances between a new fiber and existing fibers. The generation results demonstrate that the maximum fiber volume fraction gradually increases and oscillates violently before reaching 78.54% as the fiber radius rises. Moreover, with the increase of RVE size, the maximum fiber volume fraction changes gently when the fiber radius does not exceed 6.5 μm, but it changes dramatically at other fiber radii. Then, the fiber distributions of the generated RVEs are evaluated using the nearest neighbor distance, Ripley’s K function, and pair distribution function. The evaluation results indicate that the fiber distributions present randomness. Lastly, the effective elastic properties of the Carbon/Epoxy unidirectional FRC are predicted using the RVEs generated by the proposed algorithm, the RVEs generated by regularization, and the Mori–Tanaka method. It is found that the prediction using the RVEs generated by the proposed algorithm is more accurate than the regularization, compared with the Mori–Tanaka and experiment results. The proposed algorithm contributes to microstructure modeling in computational micromechanics.