Compressive Strength Of Unidirectional Composites Research Articles

Influence of Fiber Distribution and Interfacial Strength on Transverse Compressive Strength of Unidirectional Composites

The representative volume element(RVE) of the computational micromechanics is established with random fiber distribution being generated by random sequential expansion algorithm. The plasticity of matrix and interfacial decohesion are simulated by using Drucker-Prager model and cohesive zone model respectively. The effects of the random fiber distribution and interfacial strength on the transverse compressive strength of unidirectional composites are analyzed. The results show that the random fiber distribution is a factor to cause the instability of the transverse compressive strength. Meanwhile, the matrix plastic shear damage and non interfacial damage is dominated in compression failure. Therefore, the RVE model without interface element adopted can clearly predict the compressive strength and the damage process of unidirectional composites, which contributes to simplify the modeling without considering the value of interfacial parameters.

Failure strength predictions for closed-cell polyvinyl chloride foams

This paper describes an effort to model mechanical strength of closed-cell polyvinyl chloride foams under static loading. The study presented here is a continuation of an earlier study to model elastic stiffness of closed-cell polyvinyl chloride foams as effective transversely isotropic materials. An engineering approach is used in the study and governing equations are developed for predicting the strength of polyvinyl chloride foams. To account for foam microstructure and cell-shape anisotropy on foam strength, a unit cell representation of the polyvinyl chloride foam microstructure is used to derive equations to assess tensile and shear strengths of polyvinyl chloride foams. The differential stretching of polyvinyl chloride foam cell walls (in the rise direction and in the in-plane directions) on the strength of the foam-matrix polymer is also taken into account in modeling the mechanical strength of polyvinyl chloride closed-cell foams. The behavior of closed-cell polyvinyl chloride foams under compression is different from that under tension. In the paper, the equations for predicting compressive strength of closed-cell polyvinyl chloride foams are based on an approximate theory developed in an earlier study of compressive strength of unidirectional composites. The validity of the foam strength predictive equations, derived in the paper, is first demonstrated through comparison of the predictions with the results on Divinycell H (DIAB) foams obtained from a systematic in-house test program. A comparison is also carried out between the strength predictions and the test results published by two polyvinyl chloride foam manufacturers for different density polyvinyl chloride foams. Good agreements are found for all the different density foams studied.

Semianalytical compressive strength criteria for unidirectional composites

The compressive strength of unidirectional composites is strongly influenced by the elastic and strength properties of the fiber and matrix phases, as well as by the local geometrical properties, such as fiber volume fraction, misalignment, and waviness. In the present investigation, two microbuckling criteria are proposed and examined against a large volume of measured data of unidirectional composites taken from the literature. The first criterion is based on the compressive strength formulation using the buckling of Timoshenko’s beam. It contains a single parameter that can be determined according to the best fit to experimental data for various types of polymeric matrix composites. The second criterion is based on buckling-wave propagation analogy using the solution of an eigenvalue problem. Both criteria provide closed-form expressions for the compressive strength of unidirectional composites. We propose modifications of the two criteria by a fitting approach, for a wide range of fiber volume fractions, applied to four classes of unidirectional composite systems. Furthermore, a normalized form of the two models is presented after calibration in order to compare their prediction against experimental data for each of the material systems. The new modified criteria are shown to give a good match to a wide range of unidirectional composite systems. They can be employed as practical compression failure criteria in the analysis and design of laminated structures.

Critical compressive stress for continuous fiber unidirectional composites

Dow and Rosen’s work in 1965 formed an intellectual framework for compressive strength of unidirectional composites. Compressive strength was explained in terms of micro-buckling, in which filaments are beams on an elastic foundation. While groundbreaking, the discrepancy between model predictions and observed compressive strength is well known. This study builds on Dow and Rosen’s model specifically with respect to the dominant shear mode instability. A new method that accounts for matrix thermal residual strain, matrix compressive and shear stresses, with associated reductions in tangent modulus due to matrix non-linearity, is proposed. The method is validated using a specific test case, which is conducted to precisely account for microscopic fiber alignment and matrix non-linearity. Accordingly, a method of measuring and accounting for a continuum of fiber misalignment is developed. Predictions are compared to literature values, with variations in fiber modulus, matrix modulus, and volume fraction. Good agreement is shown, both in terms of trend and magnitude. The approach successfully preserves initial in-plane shear stiffness, while showing that in-plane shear stiffness decreases significantly as compressive stress increases.

On the longitudinal compressive strength prediction of unidirectional laminated composites based on an improved model

AbstractThe compressive failure of unidirectional (UD) composite laminates is investigated using an improved model. Most of the existing models to predict the compressive strength of UD composites are either based on microbuckling or kinking of fibers. However, the failure mechanism analysis based on the SEM microscopy observations of failed specimens shows that the two mechanisms are responsible for the failure simultaneously when kinking occurs following the microbuckling of fibers. Therefore, the fiber microbuckling and fiber kinking models are combined to predict the compressive strength of UD T300/QY8911 composite laminate. Furthermore, this study proposes a new model for the stiffness of the foundation determination, making the predicted microbuckling strength closer to its real value. Also the effects of parameters presented in the prediction formula on the compressive strength are discussed, and it is found that the Young's modulus of matrix is the most important parameter. The predicted compressive strength using the improved model with combined modes shows a very good agreement with the experiment measurement. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers

Inverse Micromechanical Models for Compressive Strength of Unidirectional Composites

Compressive strength of composites is an important engineering requirement. Experimental determination of compressive strength of composites is very much time consuming and error prone. At present, there are some micromechanical models available in the literature to predict the compressive strength of unidirectional composites. But these models require fiber and resin mechanical properties as input. Resin mechanical properties can be obtained experimentally. But there are no standard test techniques for the evaluation of mechanical properties of fibers. In this study, an analytical method is presented for the evaluation of elastic properties of transversely isotropic fibers. Starting with experimentally obtained compressive strengths of typical unidirectional composites and inverse micromechanical models, elastic properties of the transversely isotropic fiber are obtained. This approach gives current elastic properties of the fibers, and not the virgin properties. Further, starting with the current fiber and resin elastic properties, compressive strength of unidirectional composite is determined using micromechanical models for different fiber volume fractions.

Determination of compressive strength of unidirectional composites by three-point bending tests

A simple way for obtaining compressive strength of unidirectional composites by three-point bending tests is proposed in the present work. The interpretation of test results includes the stress singularity induced by the applied concentrated load and the determination of the contact zone between loading roller and specimen. Unidirectional carbon/epoxy composite T6T/F593 from Hexcel Composites has been tested by three-point bending with different thicknesses and spans. When failure occurs by compressive stresses, experimental results agree with the strength obtained by compressive tests. The best agreement is obtained in the case of 7 mm nominal thickness and span of 120 mm.

Effects of fiber nonlinear properties on the compressive strength prediction of unidirectional carbon–fiber composites

Compressive strength of unidirectional carbon–fiber composites is analyzed with emphasis on the nonlinear elasticity of carbon fiber. The deformation and stress state in a kink band is solved numerically using an incremental form of the governing equations, and compressive strength is determined for a given kink band angle and initial fiber misalignment. Constitutive equations are based on one-parameter plasticity model and nonlinear elasticity, which is attributed to matrix plasticity and fiber nonlinearity, respectively. Effects of fiber nonlinear properties on the predicted compressive strength are investigated under various conditions of initial fiber misalignments, kink band angles, and fiber properties. Nonlinear fiber properties turn out to reduce the predicted compressive strength of unidirectional composites in the cases of small fiber misalignments.

Compressive strength of unidirectional composites: evaluation and comparison of prediction models

The analytical models to study the behaviour of unidirectional (UD) composites under compressive loading along the fibre direction are either based on microbuckling or kinking of fibres. In reality both of these mechanisms are together responsible for the failure with kinking following the microbuckling of fibres. The present work evaluates and compares some of the existing models proposed for predicting the compressive strength of UD composites along the fibre direction. It is an attempt to evaluate analytical models that can be used for the design purposes with a greater degree of confidence, and are easy to use and computationally simple.

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.

Analysis of Geometric Imperfections Affecting the Fibers in Unidirectional Composites

The compressive strength of unidirectional composites is partly limited by the presence of geometric imperfections, mainly including undulation and general misalignment of the fibers. By examining a number of regularly spaced cut sections under an optical microscope, the fiber paths within the material can be reconstructed using image processing techniques to determine the coordinates of the fiber centers in each cut. For each fiber, these coordinates are projected in main planes in inertia to determine the overall misalignment angle and smooth the undulation using a sinusoidal waveform. Four carbon fiber composites were examined in this way. The complete analysis of the parameters relative to misalignment and undulation shows the magnitude of the imperfections affecting each composite. Main component analysis is performed to identify the interdependence between several of these factors. In addition, correlating the positions of the different fibers show that they interact only at very short distances. These results call into question the approximation generally applied up to now, which was to consider unidirectional composites as a series of mutual parallel fibers undulating in phase. This method therefore opens the way to more realistic modeling of geometric imperfections as is required both for numerical and finite element analyses of the compression behavior of such materials.

Splitting-related kinking failure mode in unidirectional composites under compressive loading

The longitudinal compressive strength of unidirectional composites is empirically found to depend on many factors including the composite fabrication and testing techniques [1,2], the strength and modulus of the matrix or fibre [3, 4], the shear properties of the composite [5], poor alignment of the fibres [6] and composite defects [7]. However, an analytical understanding of what factors affect the compressive strength of fibre composites has been slow to develop. In reality, a typical compression failure mode in carbon fibre-reinforced plastics (FRP) and organic FRP is so-called kinking, which is the formation of reoriented material bands on a plane at some angle to the direction of loading [8, 9]. The kinking was observed in unidirectional glass and boron FRP [8, 10], and in multidirectional fibrous [11] and cloth [29] laminates under compression or bending [12]. There are a variety of opinions about the kinking failure mode. Some scientists have presented an idea that kinking was initiated by microbuckling of fibres in small volumes of a composite near defects [7], free edges [11, 13] or voids [14]. Budiansky [15] treated kinking as one of the specific forms of elastic fibre buckling when the direction of buckling was not perpendicular to the reinforcement axis. The compressive strength, cry-, in this model is given by [15]

Compressive Strength of Unidirectional Composites

A simple equation which can describe the compressive behavior of uni directional composites has been derived. With this equation, the effects of resin stiffness, fiber anisotropy, fiber misalignment and waviness, fiber/matrix interface, and void content on composite compressive strength can be assessed quantitatively and qualitatively. Fur thermore, the equation can help determine the failure modes of composites under com pression.

Effect of Interphase on Compressive Strength of Unidirectional Composites

Effect of Interphase on Compressive Strength of Unidirectional Composites

An improved engineering test method for measurement of the compressive strength of unidirectional carbon fibre composites

A modified test coupon has been developed for measurement of the compressive strength of unidirectional composite materials. The coupon proposed has a machined gauge length 8 mm long with integrally moulded end tabs of 0°/+/−45° construction. Strengths were measured on the new coupon and found to lead to improvements of over 50% compared with the CRAG standard method. In addition, all failures occurred within the gauge lengths instead of at the end tabs. Failure stresses were in keeping with the stresses achieved in the 0° layers of laminates loaded to compressive failure. It is therefore strongly recommended that laboratories make use of the waisted coupon proposed in this work to generate unidirectional compressive strength design data for composite materials. In addition, it is recommended that the standards bodies seriously consider modifying their approved test method for unidirectional compressive strength and consider adopting the coupon design proposed in this work.