Multi-fiber Composites Research Articles

Questioning the Representativeness of Damage Mechanisms in Single-Fiber Composites via In Situ Synchrotron X-Ray Holo-Tomography.

In fiber-reinforced polymer composites, the fiber-matrix interface controls stress transfer mechanisms, thereby affecting mechanical performance. Interfacial properties are often extracted via single-fiber composite tests. In these tests, the load is transferred from the polymer to the fiber through interfacial shear stresses, necessitating the evaluation of interfacial shear properties. To adopt these properties in the design of industrially relevant composites, one must assume that the damage mechanisms in single-fiber composites are representative of those in multi-fiber composites, consisting of highly aligned, unidirectional plies with high fiber volume fractions. That assumption, however, has never been validated. In this paper, the real-time damage development is monitored in single-fiber and multi-fiber composites using in situ X-ray holo-tomography at 150-nm pixel size. The technique enables the first-ever 3D detection of longitudinal interfacial debonding in carbon and glass single-fiber composites. This mechanism is not detected in multi-fiber composite specimens, suggesting that single-fiber composites are intrinsically unrepresentative of realistic composite behavior.

Preparation of graphene based natural fiber (Jute)-synthetic fiber (Glass) composite and evaluation of its multifunctional properties

In traditional composite, either natural fiber or synthetic fiber has been used independently as reinforcing fiber. Natural fiber-reinforced composites have poor mechanical and interfacial characteristics because of the random fiber orientation and weak fiber-matrix interface. The addition of natural fiber (jute) to synthetic fiber (glass fiber) composites can significantly improve the characteristics; mechanical qualities are more affected by layer succession. In this study, both jute and glass fiber was employed to prepare graphene-based composite for the improvement of mechanical qualities as well as electrical and thermal conductivity to overcome the disadvantages of natural fiber composite. In comparison to the composite(without rGO), the composite's(prepared with rGO) tensile strength was increased by 146%, flexural strength by 122%, impact energy by 144%, thermal conductivity by 108.33%, and electrical conductivity by 127.21%. As evidenced by good physico-electrical properties, the prepared composite could be used as the structural support where both strength (maximum tensile strength 118.20 MPa), electrical conductivity (maximum gettable current 16.7μA), and thermal conductivity are needed. In this study going beyond the traditional way of composite making, and for multifunctional properties instead of just having specific properties, the composite prepared with both natural(jute) and synthetic fiber (glass) together with rGO could lead to the development of next-generation smart, strong, and environmentally friendly multi fiber composites for high-performance engineering applications.

Simulation analysis of thermal stress/strain of single fiber and multi-fiber composites caused by elevated environment temperature

To study thermal expansion problem caused by environment temperature changes during service life, single and multi-fiber finite element models were established to investigate thermal stress/strain of composite. Single fiber model was shown that longitudinal thermal stress/strain distribution of fiber or matrix is uniform, but stress/strain distribution of transverse direction was more complex than longitudinal direction. Furthermore, longitudinal and transverse thermal stress at composite free end zone is higher than that of in inner zone. Then the influence of different fiber arrangement on thermal stress was discussed by finite element model of multi-fiber composite. It indicated that damage initiation would emerge at somewhere randomly in fiber regular arrangement model. But in fiber random arrangement model, initial damage will concentrate on interface of fiber-rich zone. In addition, thermal expansion model in this paper may provide simple and practical method to calculate thermal stress/strain, so as to prevent matrix cracking or interface debonding due to thermal stress concentration.

Relation between dark-field images derived using X-Ray Talbot–Lau interferometer and carbon fiber distribution in CFRP

This study proposes a model that relates the carbon fiber arrangement and orientation in CFRP to the dark-field images captured by X-ray Talbot-Lau interferometer (TLI). X-ray TLI visualizes X-ray phase contrast, which is more sensitive to light elements such as carbon, than conventional X-ray absorption contrast. By using TLI, novel non-destructive inspection (NDI) technique for CFRP reinforced by distributed discontinuous carbon fibers, which precisely evaluates local orientation and distribution of fibers in large area, can be developed. The model proposed in this study forms the basis of such NDI technique. In this paper, the model is proposed first, and material parameters needed for the model are then identified from the TLI dark-field images of the multi-fiber composite (MFC) specimens. The proposed model is validated by comparing the dark-field images simulated by the model to those experimentally measured by TLI.

Finite element-based phase-field simulation of interfacial damage in unidirectional composite under transverse tension

A finite element-based phase field method for modeling the complex failure mechanism of a unidirectional fiber composite under transverse tensile loading was developed. An interface layer with a finite width was incorporated between a fiber and the matrix to implement interfacial failure. Different processes such as onset of interfacial cracking and kinking, matrix cracking, and their interactions were modeled within a unified framework. The sensitivity of the onset of interfacial cracking to the fracture mode was assessed by incorporating a phenomenological model of the interface fracture toughness in the computational model. It was found that the onset of interfacial cracking involved a combination of cracking modes I and II over a finite incremental length. The single-fiber composite model was verified by comparing its predictions of the stress distribution along the interface and the critical stress at the onset of interfacial cracking with those from the analytical solutions. The effects of the stiffness and fracture properties of the constituents on the critical stress at the onset of interfacial cracking were also evaluated, as well as the effects of the interface width and the fiber radius. Finally, the validated model was applied to the fracture analysis of a multi-fiber composite. The complex failure mechanisms were found to agree well with experimental observations.

On the suitability of a Discrete Element Method to simulate cracks initiation and propagation in heterogeneous media

The present paper investigates the suitability of a Discrete Element Method (DEM) to simulate cracks initiation and propagation in heterogeneous media. We focus our studies on the cohesive hybrid-particulate model in which the continuous medium is modeled using a cohesive beam model. Previous works exhibited the ability of such an approach to simulate the mechanical behavior of continuous materials under various solicitations. The DEM is actually well-suited to take into account local heterogeneities and complex fracture phenomena occurring at very fine scales. In a first step, several tests are performed in the context of a 2D homogeneous medium in order to better quantify micro-macro scale transition effects related to the DEM on cracks initiation and propagation. Then, in a second step, unidirectional fibre-reinforced composites are modeled using 2D models taking into account a brittle matrix failure and interfacial debonding effects. Numerical tensile tests are set up for two main configurations: a single-fiber composite constituted of a single fiber embedded in an alumina matrix and the case of multi-fiber composites constituted of parallel fibers also embedded in an alumina matrix. The results exhibit the suitability of the DEM to yield suitable stress fields and crack patterns in the investigated heterogeneous media. Scale transition effects are noticeable in terms of stress fields but turn out to be relatively limited in the mechanism leading to cracks initiation and propagation. Furthermore, the competition between debonding and failure is also well captured whatever the fiber arrangement.

Bimorph piezoelectric energy harvester structurally integrated on a trapezoidal plate

A bimorph piezoelectric energy harvester is developed for harvesting energy under the vortex induced vibration and it is integrated to a host structure of a trapezoidal plate without changing its passive dynamic properties. It is aimed to select trapezoidal plate as similar to a vertical fin-like structure which could be a part of an air vehicle. The designed energy harvester consists of an aluminum beam and two identical multi fiber composite (MFC) piezoelectric patches. In order to understand the dynamic characteristic of the trapezoidal plate, finite element analysis is performed and it is validated through an experimental study. The bimorph piezoelectric energy harvester is then integrated to the trapezoidal plate at the most convenient location with minimal structural displacement. The finite element model is constructed for the new combined structure in ANSYS Workbench 14.0 and the analyses performed on this particular model are then validated via experimental techniques. Finally, the energy harvesting performance of the bimorph piezoelectric energy harvester attached to the trapezoidal plate is also investigated through wind tunnel tests under the air load and the obtained results indicate that the system is a viable one for harvesting reasonable amount of energy.

Resin matrix shear band and its special effect on stress concentration of fiber composites

A constitutive model is constructed to consider the resin matrix post-yield softening and progressive hardening behaviors. A user-defined material mechanical behavior (UMAT) subroutine is created, then the non-linear three-dimensional finite element analysis on the tensile processes of multi-fiber composites is conducted. The approximate 45° shear bands emanating from the matrix crack tip are found, being coincided with the experimental observations. The shear stress on the adjacent intact fiber/matrix interface is strongly influenced by the shear band and thus the stress concentration factor (SCF) changes obviously in the adjacent fibers. The distinct stress redistribution in the adjacent intact fibers implies the significant effect of the shear bands on the progressive fiber fracture initiation. As the inter-fiber spacing increases, the peak value of the SCF in the adjacent intact fiber decreases, whereas the overload zone becomes wider. The research has provided a helpful tool to evaluate the failure of fiber composites and optimize the composite performance through the proper selection of resin matrix properties and fiber volume fraction.

Modeling of fiber-reinforced cement composites: Discrete representation of fiber pullout

The discrete modeling of individual fibers in cement-based materials provides several advantages, including the ability to simulate the effects of fiber dispersion on pre- and post-cracking composite performance. Recent efforts in this direction have sought a balance between accurate representation of fiber behavior and computational expense. This paper describes a computationally efficient approach to representing individual fibers, and their composite behavior, within lattice models of cement-based materials. Distinguishing features of this semi-discrete approach include: (1) fibers can be positioned freely in the computational domain, irrespective of the background lattice representing the matrix phase; (2) the pre- and post-cracking actions of the fibers are simulated with little computational expense, since the number of system degrees of freedom is independent of fiber count. Simulated pullouts of single fibers are compared with theory and test results for the cases of perfectly-plastic and slip-hardening behavior of the fiber–matrix interface. To achieve objective results with respect to discretization of the matrix, pullout forces are distributed along the embedded lengths of fibers that bridge a developing crack. This is in contrast to models that lump the pullout force at the crack surfaces, which can lead to spurious break-off of matrix particles as the discretization of the matrix is refined. With respect to fracture in multi-fiber composites, the proposed model matches theoretical predictions of post-cracking strength and pullout displacement corresponding to the load-free condition. The work presented herein is a significant step toward the modeling of strain-hardening composites that exhibit multiple cracking.

Mechanical Behavior and Failure Mode of Unidirectional Fiber Composites at Low Strain Rate Level

The rate responses of unidirectional fiber composites at low strain rate level (range from 2E-5 to 2E-2 s-1), including tensile mechanical and stress relaxation behaviors, were investigated in this article. The results obtained showed that the ultimate tensile stress increases as strain rate increases, and the stress relaxation time decreases as pre-tensile strain rate increases. And besides, the macro-failure modes of fiber composites changed from ductile fracture feature to brittle fracture feature as strain rate increasing. Furthermore, the micro-failure modes of multi-fiber composite were investigated originally, with the results indicating that fiber-fiber interactions decreased while increasing tensile strain rate.

Propagation of axial shear magneto–electro-elastic waves in piezoelectric–piezomagnetic composites with randomly distributed cylindrical inhomogeneities

The integral equations of the scattering problem for piezoelectric–piezomagnetic composites with an inhomogeneity are derived. In the long-wave limit, the solutions of these integral equations for the composites containing a single inhomogeneous fiber are solved in close forms. The total scattering cross-section for the one-fiber composites is also obtained. By the so-called effective field method, the multi-fiber scattering problem is simplified to the one-fiber scattering problem, and the analytical expressions of magneto–electro-elastic fields for the multi-fiber composites are obtained in the long-wave limit. These solved magneto–electro-elastic fields are then used to solve the expressions of the static effective moduli, effective wave velocity and attenuation factor of piezoelectric–piezomagnetic composites with randomly distributed cylindrical inhomogeneities. Through numerical examples, it concludes that, if the random set of fiber cross-sections is homogeneous and isotropic, the effective field method is coincident with the Mori–Tanaka mean field method when the static effective moduli of piezoelectric–piezomagnetic composites are looked for. Moreover, the rules of the effective wave velocity versus the volume fraction of fibers are investigated for specific materials.

Evaluation of Failure Behavior of Transversely Loaded Unidirectional Model Composites

A leading reason for the limited use of laminated composite materials in primary structural applications is that the failure initiates in the ply oriented transverse to the direction of the applied load at a much lower strain level than that which would cause the ultimate failure of the laminate. Previous studies indicate that transverse failure is manifested as either cavitation-induced failure of the matrix system or fiber-matrix debonding. The mechanism causing the failure initiation event is not decidedly known and depends on the local stress field of the constrained matrix that is a function of fiber spacing. In the present study a model composite system using a transparent matrix is employed in a cruciform-shaped specimen to evaluate the viability of several transverse failure theories. The cruciform-shaped specimen utilizes a low strain-to-failure 828/D230 RT cured epoxy and stainless steel wires arranged such that a fiber is placed at the intersection of face diagonals of four remaining fibers located at corners of a square. The transverse failure mechanism is observed in-situ via the reflected light method and recorded utilizing high resolution, high magnification microscope cameras. A parametric study is conducted using three dimensional finite element models to analyze the stress state in the cruciform specimen as a function of fiber spacing. The result of the 3-D FE models in conjunction with experimental observations are used to evaluate the transverse failure theories suggested in the literature. In addition this data will be used to develop a comprehensive failure criterion for transversely loaded multi-fiber composites that encompasses the dependence on fiber spacing.

A time dependent micro-mechanical fiber composite model for inelastic zone growth in viscoelastic matrices

In this work, a fiber composite model is developed to predict the time dependent stress transfer behavior due to fiber fractures, as driven by the viscoelastic behavior of the polymer matrix, and the initiation and propagation of inelastic zones. We validate this model using in situ, room temperature, micro-Raman spectroscopy fiber strain measurements. Multifiber composites were placed under constant load creep tests and the fiber strains were evaluated with time after one fiber break occurred. These composite specimens ranged in fiber volume fraction and strain level. Comparison between prediction and MRS measurements allows us to characterize key in situ material parameters, the critical matrix shear strain for inelastic zones and interfacial frictional slip shear stress. We find that the inelastic zone is predominately either shear yielding or interfacial slipping, and the type depends on the local fiber spacing.

Elastoplastic shear-lag analysis of single-fiber composites and strength prediction of unidirectional multi-fiber composites

A new procedure is proposed to predict the strength of multi-fiber composite based on the single fiber composite test. First, the flaw distribution in an embedded fiber is estimated with the statistical simulation. The stress distribution in the simulation is obtained by the elastoplastic shear-lag analysis considering the linear strain hardening effect of matrix. The simulated results are found to fit well with the experimental data, which shows the validity of the present simulation to estimate the statistical strength parameters for the embedded fiber. Then, the multi-fiber composite strength is predicted based on the obtained statistical fiber strength parameters. The stress profile in the multi-fiber composite is calculated with the elastoplastic three-dimensional (3D) shear-lag-analysis. The predicted strength via the weakest size scaling technique has a good agreement with our previous experimental data.

In situ monitoring of residual strain development during composite cure

AbstractInternal (residual) stresses build up in a thermosetting composite as the matrix shrinks during cure, and again as the composite is cooled to ambient from its elevated processing temperature. These stresses can be significant enough to distort the dimensions and shape of a cured part as well as initiate damage in off‐axis plies, either during fabrication or under the application of relatively low mechanical loads. The magnitude of these stresses depends on a number of factors including constituent anisotropy, volume fraction and thermal expansion, ply orientation, process cycle, and matrix cure chemistry. In this study, embedded strain gauges were employed to follow, in situ, the buildup of residual strains in carbon fiber‐reinforced laminates during cure. The data were compared to those from volumetric dilatometer studies to ascertain the fraction of resin shrinkage that contributed to residual stress buildup during cure. Based on earlier studies with single‐fiber model composites, the process cycle in each case was then varied to determine if the cycles optimized to minimize residual stresses for isolated fibers in an infinite matrix were applicable to the reduction of residual stresses in conventional multifiber composites. The results of these studies are reported here.

Statistics of fragmentation in a single-fiber composite under matrix yielding and debonding with application to the strength of multi-fiber composites

We study the statistics of fiber fragmentation in a single-fiber composite (SFC) when matrix yielding followed by interface debonding and fiber slip occur near fiber breaks as the applied strain is increased. Fiber strength is governed by flaws that follow Weibull/Poisson statistics. The model assumes that, under increasing applied strain on the SFC, fiber breaks occur randomly along the fiber from which matrix yield zones grow. At a critical strain, debond zones also initiate and grow from these breaks, resulting in fiber sliding with friction at the interface. This results in a two-step, shear traction profile producing bi-linear fiber stress recovery from breaks up to a plateau stress. These zones of yielding and debonding form growing exclusion zones around breaks within which further fiber fractures cannot occur. Eventually such zones cover the fiber, a condition called saturation. We model the fragmentation process by using a newly developed statistical theory that embodies the exclusion zone concept. Key parameters are the debond stress and the friction-yield stress ratio. These parameters are varied to determine the effects of debonding on the average fiber fragmentation length versus applied strain. We also obtain accurate estimates of the mean and variance in the stress along a fiber versus applied strain, and mention complications involved in obtaining the true values. These quantities are used to study the strength distribution and toughness of a long multi-fiber composite undergoing global load-sharing among fibers. Finally, we suggest how to estimate the model parameters from SFC test data.

Acoustic emission characteristics of micro-failure processes in polymer blends and composites

Acoustic emission (AE) characteristics of micro-failure processes in HDPE/PP blends with and without compatibilizer, single-fibre composites (glass/epoxy, carbon/epoxy, glass/polycarbonate) and unidirectionally reinforced multi-fibre composites (glass/polypropylene) were studied. For blends, the number and the elastic fracture energy release of micro-failure processes are theoretically approximated and correlated with the number of AE signals and the AE energy. A qualitative correlation of the mechanical energy released from fibre/matrix debonding and fibre-fracture processes in single-fibre pull-out experiments with the measured AE energy is demonstrated. For the single-fibre fragmentation of glass fibres and carbon fibres, a quantitative approximation of the AE amplitudes at locations of the fragmentation sources is achieved. A new method for the selection of single transient acoustic emissions and the classification of failure mechanisms in composites is introduced. Selected emissions are classified into matrix cracking, fibre breakage and interface processes (fibre/matrix debonding or fibre pull-out) from their total power in defined frequency intervals of the spectral power density. A fracture-mechanics investigation of the delamination behaviour of unidirectional composites accompanied by AE examinations is discussed. The extension of the damage zone around the crack tip is quantified by the location of AE events and compared with the theoretically approximated dimensions. The size of the damage zone is used for theoretical calculations of the mechanical energy release from micro-failure processes. A correlation of the AE energy-release rates with the mechanical energy-release rates from participated failure mechanisms like matrix cacking, fibre/matrix debonding and fibre breakage is presented.

Analyses of the micromechanics of stress transfer in single- and multi-fiber pull-out tests

Analyses have been carried out on the micromechanics of elastic stress transfer taking place across the fiber/matrix interface in both single- and multi-fiber pull-out tests. A two-cylinder model for the single-fiber pull-out test and a three-cylinder model for the multi-fiber pull-out test were employed in order to study the fiber pull-out problems. The difference in the stress transfer between the two models is clearly shown. Moreover, since real multi-fiber composites are inhomogeneous, the local fiber volume fraction would undoubtedly influence the stress transfer between the pulled-out fiber and the neighbouring matrix. The effect of the local fiber volume fraction on the stress transfer is discussed. When the fibers in the composite medium cylinder of the three-cylinder model are short, a fiber length factor for the composite modulus is introduced and its effect on the stress transfer is discussed. Furthermore, special attention is given to how the neighboring and remote fibers affect stress transfer. The results show that the neighboring fibers play a major role in determining the stress transfer while the influence of the remote fibers on the stress transfer is negligible. In addition, the present model for the single-fiber pull-out test is compared with other existing theories.

A new method of evaluating the interfacial properties of composites by means of the gradual multi-fiber fragmentation test

A new method for evaluating the interfacial properties of fibrous composites based on a fragmentation technique has been proposed by use of the gradual multi-fiber composite, in which the inter-fiber spacing is gradually changed. The results showed that as the inter-fiber distance increased, the aspect ratio of broken fibers decreased while the fibre/matrix interfacial shear strength increased. When the reciprocal of the inter-fiber distance was taken for the above relationships, both the aspect ratio and interfacial shear strength were found to show a saturated value. This means that the gradual multi-fiber composite indicates an upper bound in aspect ratio and a lower bound in interfacial shear strength, while the single-fiber composite shows a lower bound in aspect ratio and an upper bound in interfacial shear strength. It was concluded that this fragmentation test could be a new method for composite evaluation, since reducing the difference between these two bounds is effective for composite strengthening. In addition an elasto-plastic finite-element analysis was carried out to relate the above results with the fiber stress distribution around fiber breaks. It is proved that the bound obtained in the gradual multi-fiber composite test is closely concerned with stress concentrations caused by a group of multi-fiber breaks.

Comparison of the stress transfer in single- and multi-fiber composite pull-out tests

Analyses of the elastic stress transfer taking place across the fiber-matrix interface are presented for single- and multi-fiber composite pull-out tests. The multi-fiber composite is treated as a three-cylinder assemblage consisting of a central fiber, a matrix annulus, and a composite medium. The forms of the fiber axial stress and the interface shear stress distributions along the embedded fiber length are determined for single- and multi-fiber composite pull-out tests and their dependences on the fiber volume fraction, the dimensions of the specimen, the fiber-to-matrix modulus ratio, and the embedded fiber aspect ratio are displayed. The stress transfer for a perfectly bonded interface for the two pull-out tests is compared and the difference is clearly shown. In addition, for the single-fiber composite pull-out test, the present theory is compared with some existing theories.