Unidirectional Fiber Composites Research Articles

Replacing stitching and weaving in natural fiber reinforcement manufacturing, part 1: mechanical behavior of unidirectional flax fiber composites

ABSTRACTThis paper presents a novel natural fiber reinforcement manufacturing approach, based on a combination of short flax fibers and unidirectional flax yarns inspired by a papermaking process. The short fibers are used as a binder, replacing weaving and stitching used in textile-based reinforcements. The tensile and compressive properties of three different composites, in both longitudinal and transverse directions, were studied in order to highlight the effect of the mat binder. The presence of short fibers in the composite resulted in a slight decrease of about 10% of the longitudinal tensile modulus and strength. However, the transverse tensile properties are almost doubled. A similar trend was obtained in compression loading, where the addition of short fibers decreases the compressive modulus and strength. Particularly, the compressive strength in the yarn direction was more influenced by the presence of short fibers and was reduced by 30% compared to a decrease of 10% in the case of tensile loading. Here again, the transverse modulus was slightly increased, even if no effect was observed on the transverse compressive strength. The presence of the short fibers also shows a significant positive effect on the fracture behavior in tensile loading. The fractured surface is more uniform and presents a reduction of the splitting and crack formation along the unidirectional yarn direction.

Strain gradient elasticity and size effects in the bending of fiber composite plates

The present work aims to examine whether strain gradient elastic theory is able to capture both positive and negative size effects appearing in the bending of unidirectional fiber composite plates. To this goal, the deflections of fiber composite plates subjected to a bending loading under plane strain conditions are numerically evaluated and compared to the corresponding ones provided by the bending of the same plates homogenized as gradient elastic materials. All the plates have the same dimensions and fiber volume fraction, while the radius of the embedded fibers gradually changes. Plates with up to 2352 periodically and randomly distributed cylindrical elastic fibers, embedded in a matrix medium, has been considered. The fibers are either harder or softer than matrix medium while the case of cylindrical cavities is also considered. Each of the considered problems is solved numerically through a Boundary Element method (BEM) supported by Adaptive Cross Approximation (ACA) techniques so that to be able to solve large-scale problems involved in the present study. The obtained results are demonstrated and commented.

Finite element analysis of the thermal residual stress distribution in the interphase of unidirectional fiber-reinforced resin matrix composites

By means of three-dimensional finite element method (FEM) which is based upon the micro-mechanical model of fiber-reinforced composites, this paper selects representative volume elements and studies the effect of the five factors, namely, cooling rate, matrix elasticity modulus, fiber elasticity modulus, interphase elasticity modulus and fiber volume fraction, on the interphase thermal residual stress and its distribution law in epoxy resin NPEF-170/unidirectional glass fiber composites. The results indicate that thermal residual stress is mainly distributed on the fiber and the matrix of neighboring interphase; the thermal residual stress on the fiber and the matrix declines as the distance to the interphase layer grows; and it tends to zero at the distance of 1.5 times the radius of the fiber away from the interphase. The increase in any of the four factors, namely, cooling rate, matrix elasticity modulus, fiber elasticity modulus, and fiber volume fraction would trigger the rise of thermal residual stress in epoxy resin NPEF-170/unidirectional glass fiber composites. The additional flexible interphase layer can eliminate and transfer thermal residual stress effectively, whose effectiveness mainly depends on the difference between interphase elasticity modulus and fiber elasticity modulus.

A fast algorithm for the elastic fields due to interacting fibre breaks in a periodic fibre composite

Monte Carlo simulations of the failure of unidirectional fibre composites typically require numerous evaluations of the stress-state in partially damaged composite patches. In a simulated composite patch comprised of N fibres, of which \(N_b\) fibres are broken in a common cross-sectional plane transverse to the fibre direction, the stress overloads in the intact fibres are given by the weighted superposition of the unit break solutions associated with each of the breaks. Determining the weights involves solving \(N_b\) linear equations, and determining overloads in the intact fibres requires matrix-vector multiplication. These operations require \(O(N_b^3)\), and \(O(N N_b)\) floating point operations, respectively. These costs become prohibitive for large N, and \(N_b\); they limit Monte Carlo failure simulations to composite patches of only a few thousand fibres. In the present work, a fast algorithm to determine the overloads in a partially damaged composite, requiring \(O( N_b^{1/3} N \log N)\) floating point operations, is proposed. This algorithm is based on the discrete Fourier transform. The efficiency of the proposed method derives from the computational simplicity of weighted superposition in Fourier space. Computations of the stress state ahead of large circular clusters of breaks in composite patches comprised of about one million fibres are used to demonstrate the efficiency of the proposed algorithm.

Improved interfacial properties of carbon fiber composites by building stress transition layer with carbon nanotubes

AbstractFor the purpose of improvement in interfacial property of carbon fiber composites through a facile process, the carboxyl‐functionalized multiwalled carbon nanotubes (MWCNTs‐COOH) were incorporated into the surface of JH‐T800 carbon fiber using sizing and deposition technology. The surface morphology of MWCNTs‐COOH‐modified carbon fibers and fracture surfaces of unidirectional carbon fiber composites were observed via scanning electron microscopy and atomic force microscopy. According to the results of composites fragmentation test, the adoption of MWCNTs‐COOH‐modified carbon fibers resulted in the improved carbon fiber/epoxy matrix interfacial bonding and interlaminer property by 32.1% and 31.9%, respectively. Through the combination of the above two kinds of pretreating process of carbon fibers, a stress transition layer with the gradient distribution of MWCNTs was built between carbon fiber and resin matrix, which was favorable for releasing stress concentration and hindering the propagation of crack, resulting in a higher reinforcing efficiency for the existing carbon fiber composites.

Effect of Chemical Treatments and Coupling Agents on the Properties of Unidirectional Jute Fiber Reinforced Polypropylene Composite

Due to different chemical nature of jute and polypropylene, their blended composite materials often show failures behavior when applied in load bearing purposes. The aim of the research is to improve the properties of unidirectional jute fiber (JF) reinforced polypropylene (PP) composite. Unidirectional fiber composite was produced by simple hot press molding method according to weight percentage of 30wt% filler (JF), 2wt% coupling agent (compatibilizer) degraded PP (DgPP) and maleated PP (MAPP) and 68 wt% matrix (PP). To improve the interfacial interactions between the fiber and PP, the fiber was chemically modified with 10w/v% NaOH, 6w/v% dicumylperoxide (DCPO), 0.05 w/v% K2Cr2O7 and 5 w/v% H2O2 solution. The water absorption, tensile and thermal properties of raw and surface treated fibers composites were compared in particular coupling agent. It has found that the fiber treatments and addition of coupling agents, composites showed better properties. Among the composites, 10 wt/v% NaOH treated jute reinforced PP composite by using MAPP compatibilizer was presented better water absorption and mechanical properties. The positive change of the treated fiber composites was also found in case of the thermal stability that investigated by the TG, DTA and DTG tests.

Damage initiation in unidirectional fiber composites with different degrees of nonuniform fiber distribution

This paper reports a study of the initiation of the first failure event in unidirectional composites subjected to transverse tension. Two energy based point failure criteria – critical dilatational energy density and critical distortional energy density – are considered. The manufacturing induced disorder in the fiber distribution in the composite cross section is described in terms of the degree of nonuniformity, which is quantified and for which an algorithm is developed. The nonuniformity is captured in a representative volume element (RVE) whose minimum size is determined based on statistics of nearest fiber distance distribution. Several realizations of the RVE for three fiber volume fractions and three degrees of nonuniformity are analyzed using a finite element model. A parametric study of the effect of matrix/fiber stiffness ratio on the damage initiation is also conducted. Significant effects of the fiber distribution nonuniformity on the strain to onset of damage are found.

EVALUATION OF EFFECTIVE PROPERTIES OF BASALT TEXTILE REINFORCED CERAMIC MATRIX COMPOSITES

The present paper is concerned with the analysis of a ceramic matrix composite, more specifically the plain weave textile fabric composite made of basalt fibers embedded into the pyrolyzed polysiloxane matrix. Attention is paid to the determination of effective elastic properties of the yarn via homogenization based on the Mori-Tanaka averaging scheme and the 1st order numerical homogenization method adopting a suitable representative computational model. The latter approach is then employed to simulate the response of the yarn when loaded beyond the elastic limits. The required mechanical properties of individual material phases are directly measured using nanoindentation with in-build scanning probe microscopy. Applicability of the proposed computational methodology is supported by the analysis of a unidirectional fibrous composite, representing the yarn, subjected to a macroscopically uniform strain.

Strength distribution of large unidirectional composite patches with realistic load sharing.

Monte Carlo simulations of the failure of unidirectional fiber composites in a plane transverse to the fiber direction are performed on much larger patches than in previous works, assuming a realistic load redistribution scheme from broken to intact fibers. Computational effort involved in these simulations is substantially reduced using an algorithm based on the quadtree data structure. The empirical strength distribution obtained from the simulations has a weak-link character, regardless of the variability in fiber strengths. The empirical strength distribution is well captured by a probabilistic model based on the growth of a tight cluster of fiber breaks. It is also well captured by regarding composite patch failure as the failure of the weakest equal load-sharing bundle of a certain size, following Curtin [Phys. Rev. Lett. 80, 1445 (1998)PRLTAO0031-900710.1103/PhysRevLett.80.1445]. The approximate coincidence of these two predictions identifies the dominant failure mechanism underlying Curtin's empirical scaling relationship.

Modified thermal resistance networks model for transverse thermal conductivity of unidirectional fiber composite

Abstract In this paper, a facile method based on thermal-electrical analogy technique was developed to solve the heat flow transfer behavior in unidirectional fiber reinforced polymer composite. A modulating thermal resistance R md , correcting the heat flux, was introduced in the thermal resistance network to analyze two-dimensional square arrayed square fiber model of composite for its effective transverse thermal conductivity. The result of present method showed a better agreement with that of finite element method (FEM) than existed parallel or series thermal resistance network. Around this consideration, the composite reinforced by fiber with circular-cross section was also modeled to evaluate the transverse thermal conductivity, which still exhibit well consistent with experimental data. It indicates that the proposed method is enough accurate and effective for evaluating the thermal conductivity of unidirectional fiber composite and provides a facile approach to understand the complicated heat flow transfer behavior in composites.

Growth and Interaction of Debonds in Local Clusters of Fibers in Unidirectional Composites during Transverse Loading

Fiber/matrix debonding in transverse tensile loading of a unidirectional composite is analyzed calculating energy release rate (ERR) for interface crack propagation. Non-uniform fiber distribution (local hexagonal fiber clustering) is assumed in the model. The matrix region containing the central fiber with the debond and the 6 surrounding fibers is embedded in a large block of homogenized composite which has the same fiber content as the region analyzed explicitly. Some of the fibers surrounding the central fiber may also have a debond. The effect of the local clustering and of the presence of other debonds on magnification of the ERR is analyzed.

The Thermal Conductivities of Periodic Fibrous Composites as Defined by a Mathematical Model

In this paper, a geometric body-centered model to simulate the periodic structure of unidirectional fibrous composites is presented. To this end, three prescribed configurations are introduced to predict in a deterministic manner the arrangement of internal and neighboring fibers inside the matrix. Thus, three different representative volume elements (RVEs) are established. Furthermore, the concept of the interphase has been taken into account, stating that each individual fiber is encircled by a thin layer of variable thermomechanical properties. Next, these three unit cells are transformed in a unified manner to a coaxial multilayer cylinder model. This advanced model includes the influence of fiber contiguity in parallel with the interphase concept on the thermomechanical properties of the overall material. Then, by the use of this model, the authors propose explicit expressions to evaluate the longitudinal and transverse thermal conductivity of this type of composite. The theoretical predictions were compared with experimental results, as well as with theoretical values yielded by some reliable formulae derived from other workers, and a reasonable agreement was found.

Measurements and predictions of the viscoelastic properties of a composite lamina and their sensitivity to temperature and frequency

We perform finite element analysis of the mechanical response of random RVEs representing the microstructure of a unidirectional (UD) fibre composite, predicting its anisotropic stiffness and damping properties and their sensitivity to temperature and frequency, using as inputs only the measured response of the constituents. The simulations are validated by DMTA measurements on a UD composite; then, the numerical predictions are compared to those of previously published theoretical models. New equations are proposed to predict the viscoelastic constants, providing better accuracy than existing models. The accuracy of these new equations is tested, over wide ranges of fibre volume fractions and stiffness ratios of the constituents, against the numerical predictions.

Analysis of stress rupture data on fiber composites. Part 2. Determining uncertainty and removing bias in estimates

Stress rupture is a failure mode for unidirectional continuous fiber composites that is of increasing concern in composite overwrapped pressure vessels (COPVs). Stress rupture is a catastrophic failure mode with a large variance in failure times due to inherent randomness. Prediction of a composite structure's resistance to stress rupture is typically based on extensive testing at higher loads than used in service. The resulting datasets are then analyzed within the framework of a statistical model to determine an estimate of the probability that a composite structure will survive for a given lifetime under a particular load profile. For instance, in life safety applications the interest would be in ensuring a very small corresponding failure probability, such as one in a million. The statistical model parameters can be estimated in many ways, one of which is to use a maximum likelihood approach, but these estimates are of questionable value in the absence of a measure of their uncertainty. This paper presents a procedure for determining the uncertainty in such estimates, as well as removing any inherent bias. Details of the procedure are fully illustrated using data generated on model carbon/epoxy COPVs tested at the NASA White Sands Test Facility. This procedure is based in Monte-Carlo simulation of ‘typical’ datasets, which are then analyzed using the same method as the original dataset, thus giving a distribution of estimates. This distribution allows for quantification of uncertainty and bias. The Monte-Carlo procedure can also be used to evaluate experimental test design to determine the expected amount of uncertainty for a given test setup.

An approach on capturing the influence of the stochasticity of fibre distributions for modelling the variability of cutting forces in composite materials

Abstract The paper presents a novel model to evaluate the variation of orthogonal cutting forces for randomly distributed unidirectional fibres when cutting composite structures. The distribution analysis of fibre locations is first introduced into this topic, and has been experimentally validated, resulting in a Gaussian distribution based random method to simulate the stochastic fibre locations within composites. The dedicated cutting force model divides the resultant forces into fibre and matrix cutting forces, and for the first time is able to predict the force variations corresponding to the stochastic fibre distributions. The orthogonal cutting of unidirectional fibre composite is classified into three cases based on the relation between the tool feed direction and the fibre orientation, all these cases are taken into account to validate the proposed model. The validation shows that both cutting forces and fibre locations follow the Gaussian distribution with a standard error lower than one percent, and the simulated fibre locations show a satisfactory agreement with the real composite internal structure during distribution analysis. Furthermore, the predicted cutting forces coincide with the experimental data, especially their variations evaluated by probability density function and cumulative distribution function demonstrate the high degree of consistency between them. This work also indicates the proposed modelling framework could be of high relevance to other relevant applications, such as energy consumption and tool wear estimation, where the understanding of the dynamic cutting forces is crucial.

Experimental investigation and numerical simulation of unidirectional carbon fiber composite under multi-axial loadings

Abstract Unidirectional carbon fiber composites were tested under multi-axial loading conditions including tensile/compression/shear loadings along and perpendicular to fiber directions. Compression dominated tests showed brittle fracture mode like local kicking/buckling, while tension dominated tests showed fracture mode like delamination and fiber breakage. Pure shear tests with displacement control showed material hardening and softening before total failure. Tsai-Wu failure criterion with Mises-Hencky interaction term was used as the yield criterion, and an associated flow rule (AFR) was used to describe the plastic flow of this material. The modified Tsai-Wu failure criterion was able to predict the failure of the composites under different loading conditions. A new material post-failure softening rule was proposed, where a new parameter ψ was introduced to represent different loading conditions of the composite. This model was implemented to ABAQUS/Explicit as a user material subroutine (VUMAT). Numerical simulations using finite element method well duplicated the elasticity and plasticity of this material. Failure features like delamination was simulated using cohesive surface feature. This model showed good capacity in predicting both failure initiations and fracture modes for the unidirectional fiber composites. This model has good application potentials to large-scale engineering problems.

A microstructure modeling scheme for unidirectional composites using signed distance function based boundary smoothing and element trimming

A simple and accurate scheme for modeling microstructures is proposed with the help of element trimming combined with signed distance function based boundary smoothing. To accommodate randomly distributed fibers in unidirectional composites, digital image processing is used. The interfaces of multi-materials are identified by introducing a signed distance function, and then, square background elements crossing the interfaces are simply trimmed and divided to represent a single material behavior by a single element. After element trimming, the elements that are polygon-shaped in the two-dimensional domain are split into conventional three-node triangle elements (six-node prism elements in the three-dimensional domain) available in many commercial software packages. The present modeling scheme was verified through benchmark examples in terms of the accuracy and efficiency and then applied to the modeling of unidirectional composites based on real microscopic images to evaluate the equivalent elastic properties.

Analysis of stress rupture data on fiber composites: Part 1- A unified maximum likelihood method

Stress rupture is a failure mode of increasing concern in composite overwrapped pressure vessels (COPVs) in particular and continuous unidirectional fiber composites in general. Stress rupture is catastrophic and occurs randomly with little or no advance warning. Prediction of a composite structure's resistance to stress rupture is typically based on extensive test data from samples, measuring both strength and lifetime at multiple load levels. Lifetime tests are often censored due to practical limitations on available test time, thus complicating the statistical prediction process. Furthermore extensive extrapolation within the framework of a robust statistical model is required as the desired lifetimes are typically much longer than available time for testing, the service loads are much lower and the desired reliabilities are much higher than the data alone can support. This paper presents a maximum likelihood based prediction method for stress rupture using the classic power law model in a Weibull framework. Details of the method are fully illustrated using data generated on model carbon/epoxy COPVs tested at the NASA White Sands Test Facility. This method treats the strength and lifetime data all at once in one unified procedure, as compared to current sequential industry standard approaches that typically treat strength data first and then lifetime data separately using strength estimates. Compared to such approaches when applied to typical real world data sets, this unified procedure provides much tighter estimates of failure probability at a given desired lifetime and load level, and with lower amounts of bias.

Prediction of the tensile strength of unidirectional carbon fiber composites considering the interfacial shear strength

The tensile strength of unidirectional (UD) carbon fiber composites was predicted considering the interfacial shear strength (IFSS). First, the effect of the IFSS on the load transfer to the surrounding fibers when a fiber was broken was analyzed using finite element method and then the stress concentration factor (SCF) of each surrounding fiber was determined. The multiple fracture number was calculated using the SCF and a statistical approach that can calculate the probability of fiber breakage propagation due to the stress concentration when the broken fibers exist in neighbor. Finally, the tensile strength of UD carbon fiber composites was predicted using the multiple fracture number and was compared with experimental results, demonstrating the validity of the current method. An optimal IFSS that can ensure the maximum tensile strength of UD carbon fiber composites is discussed based on the calculation results.

Enhancement of Mechanical Properties of Bio-Resin Epoxy/Flax Fiber Composites using Acetic Anhydride

Chemically treated acetic anhydride (AA) flax fiber mats were investigated. Bio-based epoxy resin and conventional epoxy resin unidirectional fiber composites were manufactured using a vacuum bagging technique. Flax fibers in the bio-resin composites were chemically treated with 1, 2, 3 and 4% (AA), while the fibers used with the conventional resin were not treated. The composites with the conventional resin were compared with the bio-resin in an untreated condition. A 2% AA treatment improved the bio-resin composite tensile strength, stiffness and bond shear strength by 55%, 58% and 7%, respectively. These three properties were evaluated and the results statistically analyzed using ANOVA. The AA reduced moisture absorption intake and improved adhesion of the fiber/matrix interface. The composites treated with 1–2% AA were most successful with a 65% moisture resistance. The scanning electron microscope was used to observe the fiber surface and fractured surfaces of the untreated and treated flax fibers. A chemical pre-treatment has improved the composite mechanical and moisture resistance over the non-treated fiber composites.