Unidirectional Fiber-reinforced Polymer Composites Research Articles

Micromechanical Modelling of Elliptic Vibration-Assisted Cutting of Unidirectional FRP Composites

Fibre-reinforced polymer (FRP) composites have been widely used in industry. However, the machining of FRP products is difficult, because of very different properties of the fibres and matrix. This paper discusses the development and implementation of a microstructure-based three-dimensional finite element model for the elliptic vibration-assisted (EVA) cutting of unidirectional FRP composites. The results showed that the EVA cutting has a good potential to the machining of FRP composites, featured a much reduced cutting force, better surface integrity and controllable chip size.

The effects of fiber’s surface roughness on the mechanical properties of fiber-reinforced polymer composites

An improved shear-lag model is developed in this paper to study the effects of interface roughness on the mechanical properties of unidirectional fiber-reinforced polymer composites with a staggered structure, in which the roughness is incorporated by establishing equilibrium equations for the fiber platelets with varying thickness along its axial direction. The stress transfer and effective Young’s modulus of composites are mainly investigated due to the influence of fiber’s surface roughness. Since the polymer matrix can be chosen as thermoplastic or thermosetting materials, a uniformly interfacial shear stress distribution due to the frictional transfer along fiber/matrix interfaces and a non-uniformly one due to the elastic transfer are analyzed, respectively. It is found that when the surface roughness becomes larger, fibers in the former will carry more tensile loads, while the tensile loads keep almost invariant in fibers and the shear stress reduces in matrix in the latter. Moreover, the effective Young’s modulus of composites will be enhanced due to increasing fiber’s surface roughness. However, the enhancing effect will gradually reduce with an increasing aspect ratio of fibers. The results should be very useful for the design of novel fiber-reinforced polymer composites, especially for those that needed interfacial modifications in order to improve the interfacial adhesion, for example, carbon-fiber reinforced polymer composites.

Microscopic failure mechanisms of fiber-reinforced polymer composites under transverse tension and compression

Abstract The mechanical behavior of unidirectional fiber-reinforced polymer composites subjected to tension and compression perpendicular to the fibers is studied using computational micromechanics. The representative volume element of the composite microstructure with random fiber distribution is generated, and the two dominant damage mechanisms experimentally observed – matrix plastic deformation and interfacial debonding – are included in the simulation by the extended Drucker–Prager model and cohesive zone model respectively. Progressive failure procedure for both the matrix and interface is incorporated in the simulation, and ductile criterion is used to predict the damage initiation of the matrix taking into account its sensitivity to triaxial stress state. The simulation results clearly reveal the damage process of the composites and the interactions of different damage mechanisms. It can be concluded that the tension fracture initiates as interfacial debonding and evolves as a result of interactions between interfacial debonding and matrix plastic deformation, while the compression failure is dominated by matrix plastic damage. And then the effects of interfacial properties on the damage behavior of the composites are assessed. It is found that the interfacial stiffness and fracture energy have relatively smaller influence on the mechanical behavior of composites, while the influence of interfacial strength is significant.

The effect of temperature on fatigue damage of FRP composites

The present study intends to investigate the effect of temperature on cumulative fatigue damage (D) of laminated fibre-reinforced polymer (FRP) composites. The effect of temperature on fatigue damage is formulated based on Ramkrishnan–Jayaraman and Varvani-Farahani–Shirazi residual stiffness fatigue damage models. The models are further developed to assess the fatigue damage of FRP composites at various temperatures (T). This task is fulfilled by formulating the temperature dependency of Young’s modulus (E) and ultimate tensile strength (σult) as the inputs of the models. Temperature-dependant parameters of Young’s modulus and ultimate tensile strength are found to be in good agreement with the experimentally obtained data when used for unidirectional, cross-ply and quasi-isotropic FRP laminates. The proposed fatigue damage model is evaluated using six sets of fatigue damage data. The proposed temperature-dependent model was also found promising to predict the fatigue damage of unidirectional (UD) and orthogonal woven FRP composites at different temperatures.

The effect of temperature on fatigue strength and cumulative fatigue damage of FRP composites

Polymer-matrix composites are viscoelastic materials and their mechanical properties are significantly influenced by temperature. The present study intends to investigate the effect of temperature on cumulative fatigue damage (D) of laminated fibre-reinforced polymer (FRP) composites. The fatigue strength (S-N relation) of FRP composite laminates was formulated with temperaturedependent parameters. The effect of temperature was further included in the fatigue damage approach formulated earlier by Varvani and Shirazi. The analysis was developed to assess the fatigue damage of FRP composites at various temperatures (T). Inputs of the proposed analysis included temperature-dependent parameters including Young’s modulus (E), ultimate tensile strength (σult), and fatigue life (Nf). The proposed temperature-dependent terms and equations were evaluated with the experimental data available in the literature. Temperature dependant-parameters of Young’s modulus, ultimate tensile strength, and S-N relation were found to be responsive when used for unidirectional, cross-ply, and quasi-isotropic FRP laminates. The proposed fatigue damage model was examined using six sets of fatigue damage data and was found promising to predict the fatigue damage of unidirectional (UD) and orthogonal woven FRP composites at different temperatures.

Stress-Corrosion Cracking in Unidirectional GFRP Composites

A micromechanical theory of macroscopic stress-corrosion cracking in unidirectional glass fiber-reinforced polymer composites is proposed. It is based on the premise that under tensile loading, the time-dependent failure of the composites is controlled by the initiation and growth of a crack from a pre-existing inherent surface flaw in a glass fiber. A physical model is constructed and an equation is derived for the macroscopic crack growth rate as a function of the apparent crack tip stress intensity factor for mode I. Emphasis is placed on the significance of the size of inherent surface flaw and the existence of matrix crack bridging in the crack wake. There exists a threshold value of the stress intensity factor below which matrix cracking does not occur. For the limiting case, where the glass fiber is free of inherent surface flaws and matrix crack bridging is negligible, the relationship between the macroscopic crack growth rate and the apparent crack tip stress intensity factor is given by a simple power law to the power of two.

Modelling of chip separation in machining unidirectional FRP composites by stiffness degradation concept

Progressive failure of unidirectional glass fiber-reinforced polymer composites (FRP) was studied using finite element analysis in orthogonal machining. Chip formation process and damage modes such as matrix cracking, fiber–matrix debonding and fiber breaking were modelled by degrading the material properties. Damage analysis was carried out using Hashin, Maximum stress and Hoffman failure criteria. After damage was detected, selective stiffness degradation was applied to the workpiece material. The objective of this study is to better understand the chip formation process and to analyse the cutting-induced damage from initiation stage until complete chip formation. The effect of the fiber orientation on cutting forces and sub-surface damage was investigated with different failure criteria. The results were addressed in terms of cutting forces evolution and damage progression in the composite structure during machining. It was demonstrated that the use of the stiffness degradation concept with the appropriate failure criterion responds potentially in a predictable fashion to changes in chip formation process for machining of FRPs.

Finite Element Simulation of the Fiber– Matrix Debonding in Polymer Composites Produced by a Sliding Indentor: Part II – Parallel and Anti-Parallel Fiber Orientation

A debonding simulation technique, based on a series of nonlinear FE evaluations, has been developed to study the contact behavior and the debonding process of unidirectional fiber-reinforced polymer composites subjected to a sliding diamond indentor. In Part I, the normal fiber orientation was studied, while in Part II, the parallel and anti-parallel fiber orientations relative to the sliding direction are analyzed. The technique developed can consider both the shear and the tension type debonding events, and also the effect of friction causing limited debonding. The results show a dominant limited shear type debonding under compression for both fiber orientations.

Anisotropic Strength Approach for Wear Analysis of Unidirectional Continuous FRP Composites

Based on the anisotropic strength failure criteria, a model is introduced to predict the wear behavior of unidirectional continuous fiber reinforced polymer composites (FRP) for arbitrary fiber orientations. This approach, which includes the influence of the friction on the failure strength, is initially validated by comparison to experimental data for T300/Epoxy. The benefits of the model and trends for variation of wear with fiber orientations are subsequently ascertained and discussed.

Two-Dimensional Anisotropic Contact Behavior of Unidirectional Continuous FRP Composites

Using an explicit expression for the Barnett-Lothe tensors, an analytical method is introduced for determining the contact characteristics of anisotropic composite materials. The influence of arbitrary fiber orientations on the contact pressure distribution for unidirectional continuous fiber-reinforced polymer (FRP) composites is investigated. To demonstrate the proposed analytical method, the frictional sliding contact between a unidirectional FRP composites and a rigid parabolic cylinder is analyzed. Based on the theoretical results for different fiber orientation angles, the relationships between the contact characteristics of fiber-reinforced plastics and their tribological performance are analyzed and discussed.

Analysis of wave attenuation in unidirectional viscoelastic composites by a differential scheme

Wave attenuation characteristics of unidirectional fiber-reinforced polymer composites are analyzed theoretically by a micromechanical differential (incremental) scheme for their macroscopic acoustic properties. In this approach, the effect of neighboring fibers on the wave scattering by a single fiber is accounted for in an approximate and averaged sense. The analysis also takes into account the viscoelastic nature of the matrix, so it yields the attenuation of the composite due to both wave scattering loss and the viscoelastic absorption loss. As an example, frequency-dependent attenuation coefficients of the longitudinal and transverse waves are computed for unidirectional carbon/epoxy composites. The computed results are favorably compared to the experimental data regarding their dependence on the frequency and the fiber volume fraction. The analysis shows that the attenuation in these composites is principally governed by the wave absorption of the epoxy matrix for a practical frequency range.

Dynamic compressive behavior of unidirectional E-glass/vinylester composites

Results from an experimental investigation on the mechanical behavior of unidirectional fiber reinforced polymer composites (E-glass/vinylester) with 30%, 50% fiber volume fraction under dynamic uniaxial compression are presented. Specimens are loaded in the fiber direction using a servo-hydraulic material testing system for low strain rates and a Kolsky (split Hopkinson) pressure bar for high strain rates, up to 3000/s. The results indicate that the compressive strength of the composite increases with increasing strain rate. Post-test scanning electron microscopy is used to identify the failure modes. In uniaxial compression the specimens are split axially (followed by fiber kink band formation). Based on the experimental results and observations, an energy-based analytic model for studying axial splitting phenomenon in unidirectional fiber reinforced composites is extended to predict the compressive strength of these composites under dynamic uniaxial loading condition.

A unified fatigue failure criterion for unidirectional laminates

Abstract A unified fatigue failure criterion using micromechanics related to the fracture plane has been developed to predict fatigue lives of unidirectional fibre reinforced polymer composites subjected to cyclic off-axis tension–tension loading. Since the failure criterion incorporates both stresses and strains it may be characterized as energy based. Accounting for the fibre load angle as well as the stress ratio is the novelty of this fatigue failure criterion. The criterion only requires the stress ratio to be known from the experimental procedure. The relation between the applied load and the micro-stress and micro-strain field can be determined from a numerical method. The fatigue failure criterion has been verified by applying it to different sets of experimental data. Several fibre load angles, different fibre/matrix combinations as well as stress ratios are covered. The predicted fatigue lives are in good agreement with the experimental results for both different fibre load angles and stress ratios.

Orthogonal cutting of fiber-reinforced composites: A finite element analysis

Orthogonal cutting of unidirectional fiber-reinforced polymer composites was analyzed using the finite element method. A dual fracture process was used to simulate chip formation incorporating both the maximum stress and Tsai—Hill failure criteria. All aspects of the cutting tool geometry are considered in the model including the tool rake and clearance angles, nose radius and wear land, as well as friction between the tool and work material. Predictions for the cutting forces from numerical simulations are verified with experimental measurements for orthogonal trimming of unidirectional graphite/epoxy. The principal cutting force predictions agree very well with those obtained from experiments. The influence of fiber orientation and tool geometry on the fracture stress are highlighted and their effects on the material removal process in orthogonal trimming of reinforced polymers are discussed.

Influence of Interfacial Debonding and Free Edges upon Compressive Strength of Unidirectional FRP Composites

The uniaxial compressive behavior of fiber-reinforced polymer (FRP) composites is investigated in this paper. In the present study, the analytical model developed by the authors in previous studies is applied to investigate the effect of free edges and interfacial debonding upon the compressive strength of unidirectional composites. Results show that interfacial debonding has a great effect on the compressive strength, with fiber microbuckling strength falling to zero for the extreme case of complete fiber/matrix debonding with a frictionless interface. It is also found that free edges significantly affect the microbuckling behavior of unidirectional FRP composites by lowering the compressive strength of fibers located along a free edge as compared to that of inner fibers. Furthermore, the model indicates that the compressive strength of surface fibers can be strengthened by increasing the thickness of the outer matrix coating to the extent of theoretically being twice as high as the inner fibers. This finding may have significant practical implications for the strengthening of composite structures under compression by suppressing kink band initiation at free edges of the structure.

Wear of Unidirectional Polymer Matrix Composites with Fiber Orientation in the Plane of Contact

In a previous paper, a simple model for failure and wear of unidirectional fiber-reinforced polymer composites was proposed. The model relates fiber stresses and sliding contact conditions to bulk wear rates for fiber orientation parallel to the sliding direction. In this work, the model has been extended to examine the case where the fibers are oriented transverse to the sliding direction, while still parallel to the plane of contact. When simulating the counterface contact with an ideal hemispherical asperity, the model overpredicts wear rates for the transverse fiber orientation. However, when the counter-face contact is simulated according to observed composite material transfer and corresponding surface topographical changes, indicating deviation from an ideal hemispherical asperity to a more general elongated or ellipsoidal form, the model correlation compares more favorably with existing friction and wear test data in the literature. Presented at the 50th Annual Meeting in Chicago, Illinois May 14–19, 1995

Influence of Unidirectional Composite Compression Specimen Thickness and Loading Method

A combined numerical and experimental study was performed to investigate the effect of both specimen thickness and loading method on the compressive strength of a unidirectional fiber-reinforced polymer composite. A three-dimensional finite element analysis was used to study the stress distribution within a tabbed coupon specimen. The compressive load can be applied directly on the ends of the specimen, or be generated by longitudinal shear forces on the tab surfaces. The end loading method was found to be superior to the shear loading method, yielding a less severe stress concentration at the tab termination points and a similar stress concentration at the outer ends of the specimen. For thick specimens this benefit is more obvious and therefore a higher compressive strength should be obtained for a thick specimen subjected to end loading. Hercules AS4/3501-6 unidirectional carbon/epoxy composite specimens of various thicknesses were tested using both ASTM D3410, Procedure B (shear loading) and the Wyoming End-Loaded Side-Supported Test Method (end loading). The experimental data were found to correlate very well with the numerical predictions.

A model for the abrasive wear of fiber-reinforced polymer composites

A model for the abrasive wear of unidirectional fiber-reinforced polymer composites is proposed. Unlike steady state wear in which the different constituents wear at the same rate, the proposed model is cyclic (or quasi-steady state) in nature in which the fiber and the matrix do not always wear at the same rate. Two mechanisms, each representing the two extremes of cyclic wear behavior, are described. From these, modified rules of mixtures for the abrasive wear of unidirectional composites, incorporating the composite modulus, are derived and compared with existing data. The proposed model provides closer bounds to the abrasive wear resistance of polymer composites than predictions from the simple linear and inverse rules of mixtures that one obtains from the steady-state model.

Damping characterization of unidirectional fibre reinforced polymer composites

The main concern of this paper is the modelling of material damping in composite structures. A method is derived to predict the homogenized damping properties analytically. The procedure is based on the micromechanical theory of a representative fibre-matrix cell. Since the dissipative phenomena cannot generally be defined by a unique mathematical model, an equivalent viscous damping model is suggested. On the structural level damping can be analysed by the finite element method. Modal damping is illustrated by numerical applications.

On the Wear Behavior of Longitudinally (Parallel) Oriented Unidirectional Fiber-Reinforced Polymer Composites

A simple model for failure and wear of continuous unidirectional fiber-reinforced polymer composites with fiber orientation longitudinal (parallel) to the contact plane and sliding direction is presented. Failure or weakening of the immediate surface region, caused by fiber cracking, with subsequent fiber-matrix separation and wear, is the basis for the proposed model. The composite is modeled as an anisotropic (transversely isotropic) half-space whose effective elastic properties are estimated from composite micro-mechanical considerations. The counterface is modeled as an ideal rigid hemispherical indenter sliding against the composite. In this contact configuration, indentation by a hemispherical asperity produces a generally elliptical contact region whose geometry depends on the elastic constants of the composite. The individual fiber is modeled as an infinite beam on an elastic foundation, with the foundation stiffness approximated from the results of the anisotropic contact simulation. From these r...