Unidirectional Polymer Composites Research Articles

Fracture Criteria for Matrix and Fibers in Unidirectional Polymeric Composites at Static Loadings

When analyzing the strength of structures made of layered fibrous polymer composite materials; the criteria for failure of a monolayer – a unidirectional reinforced composite – are used. A criterion of strength according to the conditions of matrix fracture is formulated, corresponding to conical limiting surfaces and the lowest destructive loads. The criterion of strength according to the condition of fiber failure, which does not allow the paradox of increasing strength in the region of transition from fiber destruction to matrix failure, is given. Experimental verification of the criteria for volumetric, planar, and one-dimensional loads is carried out. Their better correspondence to empirical data is shown and their advantages in comparison with known criteria are marked. A small number of easily detectable parameters of these criteria contribute to their reliability and stability in strength calculations of composite structural elements.

Strength Criterion Variants for Unidirectional Polymer Composites by the Fracture Condition of the Inter-fibre when there is Compression Perpendicular to the Fibers

Strength Criterion Variants for Unidirectional Polymer Composites by the Fracture Condition of the Inter-fibre when there is Compression Perpendicular to the Fibers

An invariant-based elasto-visco-plastic model for unidirectional polymer composites at finite strains

An invariant-based constitutive model for unidirectional composites, including viscous effects in the elastic and plastic regimes at finite strains, is proposed. The model is based on the multiplicative decomposition of the deformation gradient, using an isoclinic configuration to avoid the non-uniqueness of intermediate configurations. The visco-elastic behaviour is represented by the generalised Maxwell model, combined with a transversely isotropic yield function and a non-associative plastic potential. Visco-plastic effects are introduced through the Perzyna overstress function. The theoretical basis of the material model and the computational treatment within the finite element solution procedure are presented in detail. Two algorithms are proposed for an implicit integration of the evolution equations: (i) a semi-implicit stress update, and (ii) a fixed-point iterative fully implicit stress update. A comparison of their performance reveals that the former becomes faster, even though the latter guarantees the quadratic convergence rate in the Newton–Raphson scheme. The predicted stress–strain response for different fibre orientation angles is in close agreement with experimental data obtained under quasi-static and dynamic loading. The model can also capture the important re-orientation of the fibres due to external loading.

Ultimate Strength Prediction of Fibrous Composites only upon Independently Measured Constituent Properties

This study presents an accurate and easy-to-use micromechanical model to predict the ultimate strength of unidirectional polymer composites under an arbitrary load condition only upon independently measured constituent properties. In this model, the micromechanical method based on generalized model of cells (GMC), which effectively predict the nonlinear deformation of unidirectional composites, is used to analyze the repeating unit cells of composites. At the same time, a unified plastic theory (i.e. modified Ramaswamy-Stouffer model) is incorporated into the GMC's analytical framework to describe viscoplastic behaviors of matrix phase. Additionally, because of the stress concentration effect which causes the difference between the matrix in-situ and original strength behaviors, a stress concentration factor is introduced in order to utilize the measured constituent properties directly. The prediction results with SCF match better with the experimental results than the prediction results without SCF. In addition, the prediction results show that the presence of thermal residual stress and material plastic effects generally has important influence on the strength prediction of a composite.

Size effects in hybrid unidirectional polymer composites under longitudinal tension: A micromechanical investigation

While fibre hybrid composites result in hybrid effects and improved ductility compared to non-hybrid laminates, it is unclear how material size influences hybrid properties (i.e. the so-called ‘size effect’). This paper addresses this phenomenon in a carbon–carbon hybrid composite by means of micromechanical modelling using a progressive failure model. Results demonstrate that increasing either the length or the cross-section of the virtual Representative Volume Element (RVE) causes an earlier failure, slightly decreases ductility and significantly improves the hybrid effect. In addition, increasing the RVE volume results in a higher maximum cluster size, while the presence of ductility does not alter the magnitude of the size effects. Further, the presence of a larger hybrid effect does not necessarily translate into greater ductility.

Thermal Expansion and Shrinkage of Unidirectional Composites at Elevated Temperatures

The model of multiphase media analyzes the effectiveness of describing thermal expansion and shrinkage of unidirectional polymer composites at their uniform heating by linear law. It is shown that the nature of change in thermal deformation as a function of the composite temperature is similar for an unreinforced matrix, and the maximum absolute value of chemical shrinkage is slightly lower than that of a matrix. The calculated deformations describing thermal expansion are compared with experimental deformations obtained in the process of thermal-oxidative destruction.

Failure of hybrid composites under longitudinal tension: Influence of dynamic effects and thermal residual stresses

A progressive failure model including dynamic effects and thermal residual stresses able to simulate the failure and damage development of hybrid unidirectional polymer composites under fibre tensile loading is presented. The model is used to study the influence dynamic effects and thermal residual stresses have on the development of clusters of fibre breaks and the failure process of different hybrid composites. The results obtained show that while the dynamic effects change cluster formation, nonetheless they do not significantly alter the final failure of the material. Overall, the influence is greater for the more brittle materials. Although the thermal residual stresses do not affect the formation of clusters, they can delay damage initiation and final failure by inducing compressive stresses into the fibres.

A mechanistic criterion for the mixed-mode fracture of unidirectional polymer matrix composites

A mechanistic fracture criterion has been developed to predict the mixed mode through thickness (translaminar) fracture behaviour of unidirectional (UD) polymer composites. This criterion is energy based and is able to predict the toughness of UD composites. The prediction requires only UD composites constituents and interfacial properties. In order to develop the criterion, the through-thickness fracture behaviour of bulk polymer (epoxy) and UD composite (glass fibre epoxy) along with their fracture surface have been investigated. The fracture testing of glass fibre epoxy and neat epoxy under various in-plane loading is performed using compact tension shear (CTS) fixture. The loading ranges from pure mode I to different mixed mode I/II ratios and pure mode II. To consider the energy absorbed during the plastic deformation of reinforced matrix, the fracture surface morphologies of glass epoxy and neat epoxy were investigated which show an increase by the increase in the shear mode. This effect was included in the developed criterion as well. Consequently, the predictions from the criterion show very good match with the CTS experimental results.

An assessment of micromechanical models to predict the elastic constants of unidirectional polymer composites

The accuracy of existent closed-form micromechanical models to predict the five independent elastic constants of unidirectional fiber reinforced composites is assessed. A finite element model is developed to serve as a reference, properly benchmarked against other numerical models and published experimental data, and then used as a baseline for comparison. The detailed numerical analysis of the existent micromechanical models using a formal statistical metric for error quantification allows a quantitative assessment of these models, providing recommendations for the most suited model(s) for each material constant.

Evaluating unidirectional composite thermal conductivities through engineered interphase

ABSTRACTEffect of fibre/matrix interphase parameters, including thickness and material properties on the equivalent thermal conductivities of unidirectional fibre-reinforced polymer composites. A unit cell-based micromechanical method is proposed to evaluate the thermal conductivities of unidirectional multi-phase composites. The longitudinal thermal conductivity of unidirectional fibre-reinforced polymer matrix composites is seen to be independent of interphase region. When the thermal conductivity of interphase is higher than that of matrix, the increase of interphase thickness leads to an improvement in transverse thermal conductivity of fibre-reinforced polymer composites. The influences of fibre volume fraction, orientation angle and shape of cross-section as well as temperature on the thermal conducting behaviour are widely examined. The model predictions are in good agreement with the experimental data reported in the literature.

An Eulerian Orthogonal Cutting Model for Unidirectional Fiber-Reinforced Polymers

An Eulerian model is described that simulates orthogonal cutting of unidirectional fiber-reinforced polymer (FRP) composites. The continuous finite element method (FEM) and the discontinuous Galerkin (DG) method are combined to solve the governing equations. A progressive damage model is implemented to predict subsurface damage in the composite. A correction factor that accounts for fiber curvature is included in the model that incorporates the influence of fiber bending. It was found that fiber orientation has a dominant influence on both the cutting forces and subsurface damage. Good agreement was found between predicted cutting forces and subsurface damage and published experimental observations.

Micromechanical analysis of unidirectional polymeric composites material with triangular fibers

The purpose of this study is to investigate the mechanism of load transfer in unidirectional polymeric composites material with triangular fibers in the presence of broken fibers. It is assumed that all fibers with triangle-cross section lie in a same direction while loaded by a load at infinity. In previous studies, the behavior of polymeric matrix is assumed elastic, while the behavior of most polymeric matrix is elastic-perfect plastic. For this purpose, equilibrium equations by use of shear-lag theory have been derived and by proper using boundary and boundness conditions, displacement field and stress distribution were computed. Finally, the results of stress concentrations and di shear stress, at neighbor of broken fibers for carbon-epoxy composite material were extracted and compared with composite materials with circular fibers. The analytical results are compared to the detailed finite element values. A close match is observed between the two methods.

Micro-scale progressive damage development in polymer composites under longitudinal loading

A detailed investigation of micro-scale damage development in uni-directional polymer composites under longitudinal loading (loading along the fiber-direction) has been presented in the current work. Three major damage modes have been considered, namely, matrix damage, fiber–matrix interface debonding and multiple fiber fragmentation. The development and progression of these damage modes and their effect on the overall stress–strain response of the uni-directional fiber reinforced plastic (UD-FRP) is of interest. From the study, it has been demonstrated that the dimensions of the three dimensional repeating unit cell (3D-RUC) needed to generate repeatable results are inherently dependent on the fiber volume fractions. The length of 3D-RUC is longer for a higher fiber volume fractions. In addition, the physics of fiber fragmentation is inherently linked to the proximity of other fibers triggering failure mechanics different from the single fiber fragmentation behavior. Stochastic fiber fragmentation characteristics have also been studied by quantifying the effect of Weibull parameters on overall response of the micro-structure.

Assessment of Elasticity Characteristics and Strength Parameters of Unidirectional Polymer Composites at High Temperatures

Within the framework of transversally isotropic body, the possibilities of describing the elasticity characteristics and load-bearing capacity of unidirectional carbon-reinforced polymer composites at high temperatures are analyzed. The dependences of the elasticity characteristics and strength parameters of the composite on the heating temperature and rate are established. The models for calculation of strength characteristics of ablating composite in cases of tensile or compressive loads codirected with the reinforcement axis are developed and discussed.

Fatigue Life Prediction of a Unidirectional Carbon Fiber Composite Under Off-Axis Spectrum Loads Using 3D Constant Life Diagram

Abstract In this study, a novel concept of 3D constant life diagram (CLD) for fatigue life prediction of a unidirectional (UD) polymer composite under spectrum load is proposed. Further, it is constructed and used to predict the fatigue life of a UD carbon fiber composite (CFC) subjected to a standard spectrum load sequence at an arbitrary off-axis angle. First, UD IMA/M21 CFC laminates were fabricated by an autoclave process. Static mechanical tests were conducted to determine the tensile and compressive strength at various off-axis angles ranging from 0° to 90°. Then the constant amplitude (CA) fatigue tests at three different stress ratios, R = σmin/σmax of 0.1 (tension-tension), −1.0 (tension-compression), and 10.0 (compression-compression) and at various off-axis angles between 0° and 90° at each of these stress ratios were performed to determine stress-life curves. Using the static and CA fatigue data generated, 3D CLD for UD CFC was constructed. Further, fatigue life of UD CFC subjected to a standard mini-FALSTAFF spectrum load sequence at an arbitrary off-axis angle of 20° was predicted following an empirical method using 3D CLD. Fatigue tests under the mini-FALSTAFF spectrum load sequence at an off-axis angle of 20° were also conducted at various reference stresses and compared with the predictions. A reasonably good correlation was observed between the predicted and experimental fatigue lives under off-axis spectrum loads.

Hygrothermal Ageing Modeling and Numerical Testing of Unidirectional Polymeric Composites

Moisture severely affects aerospace structures and offshore platforms during their service life span. It deteriorates the mechanical performance of polymeric composites through long term and cyclic hygrothermal ageing. This reduction in mechanical performance must be considered during product design to ensure long-term structure endurance. In order to decide the long-term moisture effects on composite components, they are exposed to a rapid moisture conditioning. Presently the available qualified methodologies allowing only simple geometry and an assumption that diffusivity rates are independent of the flow path or direction. Therefore a more advanced finite element method is required. In this research work the finite-element analysis was performed to study the moisture diffusion in unidirectional composites. The final goal for this study was to determine the exposure time for rapid moisture conditioning that produces the most accurate moisture distribution in composite laminates.

Nonlinear shear behavior and interlaminar shear strength of unidirectional polymer matrix composites: A numerical study

Detailed finite element implementation is presented for a recently developed technique (He et al., 2012) to characterize nonlinear shear stress–strain response and interlaminar shear strength based on short-beam shear test of unidirectional polymeric composites. The material characterization couples iterative three-dimensional finite element modeling for stress calculation with digital image correlation for strain evaluation. Extensive numerical experiments were conducted to examine the dependence of the measured shear behavior on specimen and test configurations. The numerical results demonstrate that consistent results can be achieved for specimens with various span-to-thickness ratios, supporting the accurate material properties for the carbon/epoxy composite under study.

A novel numerical–experimental approach for predicting delamination in high temperature polymer matrix composites

In this paper, a novel numerical–experimental methodology is outlined to determine cohesive stress and damage evolution parameters for pristine as well as isothermally aged (in air) polymer matrix composites. A rate-dependent viscoelastic cohesive layer model was implemented in an in-house test-bed finite element analysis (FEA) code to simulate the delamination initiation and propagation in unidirectional polymer composites before and after aging. To determine the model parameters, double cantilever beam (DCB) experiments were conducted on both pristine and isothermally aged IM-7/bismaleimide (BMI) composite specimens. The J-integral approach was adapted to extract cohesive stresses near the crack tip. A principal-stretch dependent internal damage state variable defines the damage in the cohesive layer. Within the cohesive layer, pristine and cohesive stresses were compared to estimate the damage parameters.Once the damage parameters had been characterized, the test-bed FEA code employed a micromechanics based viscoelastic cohesive layer model to simulate interlaminar delamination. The present cohesive-layer based FEA model was able to accurately predict not only the macro-level load–displacement curve, but also the micro-level crack growth history in IM-7/BMI laminate before and after thermal aging.

Modelling of strain rate effects on matrix dominated elastic and failure properties of unidirectional fibre-reinforced polymer–matrix composites

A phenomenological-based, strain rate dependent failure theory, which is suitable for the numerical modelling of unidirectional (UD) carbon fibre reinforced polymer composites (CFRPs), is presented. A phenomenological-based approach is also proposed for the three-dimensional (3D) modelling of strain rate induced material hardening in UD polymer composites. The proposed theory and approach are implemented in the Finite element (FE) code ABAQUS/Explicit for one integration point solid elements. Validation is presented against experimental data from dynamic compressive tests using results available in the published literature.Conclusions indicate that the proposed method can be applied for predicting the elastic and failure properties of UD carbon fibre polymer composites for generic, 3D, quasi-static (QS) and high-rate loading conditions with very good accuracy. In particular, it is shown that the phenomenological approach to modelling here proposed allows prediction of all matrix dominated properties, i.e. moduli of elasticity and strength, including parallel-to-the fibres compressive strength, with the knowledge of one strain rate dependent parameter, which is characterised using dynamic strength data for one specimen configuration.

3-D FEM Modeling of fiber/matrix interface debonding in UD composites including surface effects

Fiber/matrix interface debond growth is one of the main mechanisms of damage evolution in unidirectional (UD) polymer composites. Because for polymer composites the fiber strain to failure is smaller than for the matrix multiple fiber breaks occur at random positions when high mechanical stress is applied to the composite. The energy released due to each fiber break is usually larger than necessary for the creation of a fiber break therefore a partial debonding of fiber/matrix interface is typically observed. Thus the stiffness reduction of UD composite is contributed both from the fiber breaks and from the interface debonds. The aim of this paper is to analyze the debond growth in carbon fiber/epoxy and glass fiber/epoxy UD composites using fracture mechanics principles by calculation of energy release rate GII. A 3-D FEM model is developed for calculation of energy release rate for fiber/matrix interface debonds at different locations in the composite including the composite surface region where the stress state differs from the one in the bulk composite. In the model individual partially debonded fiber is surrounded by matrix region and embedded in a homogenized composite.