Strength Of Unidirectional Composites Research Articles

Crystal structures of polyamide 6 at the interphase and around carbon fiber and mechanical properties of their composites

This study reports the effect of the polyamide 6 (PA6) crystal structure at the interphase and around the carbon fiber (CF) on the mechanical properties of CF-reinforced PA6 composites. The transverse tensile properties of the unidirectional composites and the four-point bending properties of the quasi-isotropic composites were investigated using three types of CFs. Raman spectra of PA6 were measured to characterize the crystal structures of PA6 at the interphase and around the CFs. The results showed that higher α-crystal (the most thermodynamically stable crystal) content of PA6 at the interphase compared to β-crystal (mesophase) and γ-crystal (thermodynamically metastable) contributed to greater transverse tensile strength of unidirectional composites. Furthermore, it was demonstrated that the flexural strength of the quasi-isotropic composites increased in the case of small variations in the distribution of each crystal structure (α-, β-, and γ-crystal structures) around the CFs. This study provides a framework for the PA6 crystals around CFs to reflect the superior properties of CFs in PA6/CF composites.

An efficient micromechanics-based finite element model for failure analysis of laminated composites

Accurately predicting ultimate strength of composite laminate is still a great challenge, especially only based on the constituent properties. In this paper, the micromechanics Bridging Model and systematic failure criteria for the constituent materials with emphasis on matrix failures subjected to three dimensional (3D) loads are compiled into a UMAT subroutine for ABAQUS to analyze failures of a laminated composite structure. The main purpose of this research is to greatly reduce calculation costs for loading conditions of in-plane tension or compression by introducing a simplified 3D finite element model based on the Pipes-Pagano theory. In all calculation cases, only the constituent properties together with the transverse tensile strength of unidirectional composite are needed as input data. Later in this paper, sufficient illustrations of 81 different composite laminates composed of 14 material systems are analyzed to confirm the practicability of this model.

Development of the Optimal Shape and Reinforcement Structure of the Specimen for Adequate Determination of the Tensile Strength of Unidirectional Composites

Development of the Optimal Shape and Reinforcement Structure of the Specimen for Adequate Determination of the Tensile Strength of Unidirectional Composites

Stochastic modelling of continuous glass-fibre reinforced plastics–considering material uncertainty in microscale simulations

This paper presents a probabilistic micromechanics-based approach to simulate the influence of scatter sources in composite materials as an alternative to deterministic approaches. Focus is given to the effect of microscopic and macroscopic voids, material inhomogeneity induced by manufacturing processes and stochastic fibre patterns on the mechanical properties of continuous glass-fibre reinforced polymer components. Various periodic unit cells of neat resin and embedded fibre clusters are generated with random distributions of the abovementioned scatter sources, while the voids are represented by degrading locally the pristine properties in an element-wise manner. Subsequently, the models are mechanically loaded under transverse tension as an exemplary case and the resulting responses are correlated with the stochastic inputs. In particular, the relative influence of pore size, porosity and fibre/resin interface strength on the transverse tension modulus and strength of unidirectional composites are numerically investigated. The present approach is suggested as a computational efficient but reliable alternative to geometrical representations of imperfection in composite materials.

A new phenomenological creep residual strength model for the life prediction of the laminated composites

AbstractFiber‐reinforced plastics have extensive applications in advanced industries in which the material is subjected to long time sustained loadings that stimulates time‐dependent damage development in laminated composites. In the present article, a new phenomenological residual strength model is proposed for unidirectional composites under sustained loading. A normalization technique is suggested, which accumulates the spread residual strength data, and a power‐law model is developed to describe the normalized residual strength data under arbitrary states of stress and sustained times. Accelerated creep experiments were performed on T300/L20 carbon/epoxy unidirectional laminates. The residual strength data of tested T300/L20 unidirectional laminates and available data in the literature for Kevlar/epoxy and S‐glass/epoxy strands are used to examine the model predictions. It is shown that the presented normalization procedure is an effective way for modeling the residual strength of unidirectional composites, and the model can discriminate the rate of damage growth under various stress states.

Effect of graphene oxide surface treatment on the interfacial adhesion and the tensile performance of flax epoxy composites

The high stiffness and damping properties of flax fibres promote the integration of biocomposites in structural applications. However, the strength of flax/epoxy composites is still limited compared to glass/epoxy composites. Graphene oxide (GO) has proved to be a promising building block for nanocomposites due to its high toughness, stiffness and tunable interfacial interactions with polymers. This study aims to understand the potential of GO-based surface treatment of flax fibres to modify the interfacial adhesion and tensile performance of flax fibre/epoxy composites. GO-modification improves the interfacial shear strength of elementary flax fibre/epoxy by 43%. The interfacial improvement is also established by the 40% higher transverse bending strength compared to untreated flax/epoxy composites. The tensile moduli of GO-modified flax/epoxy composites are on average 2 GPa higher than for untreated flax fibre/epoxy composites in all strain ranges. The quasi-static longitudinal tensile strength of unidirectional composites is not affected by GO-modification.

Computational evaluation of the effects of void on the transverse tensile strengths of unidirectional composites considering thermal residual stress

To analyze the effects of void on transverse tensile properties of composites considering thermal residual stress (TRS), finite element models with different distribution patterns and shapes for voids are established. Two different void modeling methods, with voids explicitly established and with voids modeled within elements, are compared first. For the method with voids explicitly established, circular, elliptical and arbitrary voids are considered. To reveal the effects of TRS, two different TRS analysis methods are evaluated. Results show that for the models without considering voids and TRS, the interface debonding is the dominant failure mechanism. After considering the voids, the effects of matrix damage on the crack initiation become more important. There are more variations in tensile strengths of models with voids explicitly established than those from the models with voids modeled within elements. It is also found that higher TRS values are obtained when the change of matrix properties with temperature is considered. After considering the TRS, the matrix damage contributes more to the crack initiation and the interface debonding is inhibited. Besides, the TRS results in thermal residual matrix plasticity around the void, which contributes to the premature initiation of crack around the void.

Calculation of Tensile Strength of Unidirectional Composites Based on Probability Theory and Monte Carlo Method

In this paper, Monte Carlo method combined with the random critical-core probability model is proposed to calculate the longitudinal tensile strength of unidirectional composites. This method considers two-dimensional distribution of fibers on the cross-section, while the theoretical analysis method only takes the linear distribution of fibers into account. Using the weakest link principle, the failure probability and average strength of the unidirectional composites are obtained. The results show that the calculated values of longitudinal tensile strength of T300/5208 composite and unidirectional C/C composites agree well with the experimental results.

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.

Effect of Splitting of Polymer Fibers on the Strength of Unidirectional Composites

With the example of Aramid fibers, it is shown that the model of acumulation of fiber breaks in composites cannot explain their observed tensile strength exceeding that of unimpregnated Aramid fibers. The model of longitudinal splitting of fibers proposed by Wagner, with account of fiber-matrix interaction, is based on the idea that the break of fibers begins at great strains owing to their interaction with the matrix. This model better describes the failure of composites with anisotropic fibers having an internal fibrillar structure. The results obtained can also be used for other types of fibers with an internal microstructure.

On micromechanics approach to stiffness and strength of unidirectional composites

The purposes of this paper are sixfold. First, any micromechanics model for predicting elastic property (stiffness) of a composite is applicable to a reasonable prediction of a composite strength, provided that the homogenized internal stresses in the matrix are converted into true values. The conversion for all of the stress components free of biaxially transverse loads is presented in the paper. Second, the predictability of 12 well-known micromechanics models for stiffness and strength of a unidirectional composite is assessed against the measured data of all of the nine unidirectional composites used in three world-wide failure exercises. An accuracy ranking is made accordingly. Third, it is demonstrated that the smallest fiber volume in a representative volume element for a finite element approach plays a more dominant role than other issues such as a random fiber array to achieve the highest simulation accuracy. This feature has been largely ignored in the current literature. Fourth, a consistency of a micromechanics model in calculating the internal stresses in the fiber and matrix of the composite is an issue that should be taken into account. Among the 12 models considered, only Bridging Model is consistent. A non-consistency implies that a full three-dimensional approach should be used to predict an effective property of the composite even though it is subjected to a uniaxial load. Fifth, an effective method to detect an interface debonding between the fiber and matrix subjected to an arbitrary load is presented in the paper. Only a transverse tensile strength of the composite, in addition to the original fiber and matrix properties, is needed. Whereas essentially any load can cause the interface to debond earlier before a composite failure, only a transverse tensile load carrying capacity of the composite is influenced significantly by the debonding. Specifically, an interface debonding has insignificant effect on a shear strength of the composite. Sixth, a fiber misalignment-induced kink band failure is analyzed by virtue of the original fiber and matrix properties, and a longitudinal compressive strength is predicted micromechanically.

Experimental and micromechanical investigation of T300/7901 unidirectional composite strength

We experimentally find mechanical properties in uniaxial tension/compression and torsional deformations of the 7901 epoxy resin used as a matrix to fabricate T300/7901 unidirectional (UD) fiber‐reinforced composite. A series of off‐axis tensile tests on the composite were conducted, and micrographs of the fiber–matrix interface at different load levels, using a Scanning Electron Microscope, taken to ascertain their ultimate tensile strengths. Values of the elasto‐plastic parameters for the epoxy determined from the tests, and the handbook values of the fiber parameters were used in a micromechanics‐based bridging model to predict ultimate tensile strengths of off‐axis loaded composite by considering stress concentration effects due to debonding at the fiber/matrix interface. For the 30° off‐axis loaded laminate, the predicted ultimate strength is found to agree well with that determined experimentally, and the interface debonding occurs before the ultimate failure. The UD composite is assumed to fail when either the fiber or the matrix failure criterion is satisfied. POLYM. COMPOS., 40:2639–2652, 2019. © 2018 Society of Plastics Engineers

Mathematical Prediction of Strength of Ablating Polymer Unidirectional Composites in Transverse Direction and in Shear Along and Across Fibers

The scope of the investigations of the paper includes the efficiency in the prediction of strength characteristics of the ablating polymer unidirectional carbon-fiber-reinforced plastics at elevated temperatures based on the properties of the unreinforced matrix and a bundle of fibers in the process of thermal oxidative breakdown at different types of stress state. The characteristics of elasticity and parameters of strength of the transverse-isotropic carbon-fiber-reinforced plastic, as well as of its components (epoxy matrix and carbon fibers of T700 type), have been determined in compliance with the regulatory documents of Ukraine at low heating rates with the subsequent exposure of the specimens at the fixed temperature, when the inertia effects can be neglected. It is illustrated that the variation in densities of the ablating unreinforced matrix and the bundle of fibers at elevated temperatures is rather well described via the models of multiphase media employing the modified integral exponential function. The authors analyze the possibility to evaluate the critical stresses of the transverse-isotropic composite at elevated temperatures based on the mechanics of multiphase media considering the thermal and mechanical characteristics of the ablating polymer matrix and the bundle of carbon fibers. It is assumed that the task of heat and mass transfer can be solved separately from the coupled tasks of thermal mechanics of the ablating materials since mechanical stresses have no effect on its parameters. It is found that the calculation of critical stresses using the hypothesis for the total hermetic sealing between the phases or in the assumption on the low pressure in pores determines the lower and upper lines of variation of the composite strength at different types of stress state. In particular, the calculated strength of the unidirectional composite under tension in the transverse direction and in shear along and across the fibers is in good correlation with the experimental data obtained by the authors and other researches.

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.

Effect of Fiber Length on the Tensile Strength of Unidirectional Composites

Effect of Fiber Length on the Tensile Strength of Unidirectional Composites

Analytical model for predicting the tensile strength of unidirectional composites based on the density of fiber breaks

Abstract While analytical fiber fragmentation models following the global load-sharing (GLS) assumption efficiently reproduce the stress strain curves of unidirectional composites loaded in the direction of the reinforcement when the number of breaks is moderate, they completely fail to predict tensile strength. In this paper, we propose that failure takes place when a critical density of breaks, which depends entirely on the constituent properties, is reached. Therefore, we rewrite classic GLS fragmentation models in terms of the linear density of breaks. The critical number of breaks for a set of glass and carbon reinforced polymer composites is extracted from published experimental data and fitted to an empirical law, with good predictive capability. Our approach complements GLS fragmentation models because it identifies the ultimate stress, from which the stress-strain curve given by the model becomes unrealistic.

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.

Compressive failure analysis of unidirectional carbon/epoxy composite based on micro-mechanical models

A 2D micro-mechanical model was proposed to study the compressive failure of UniDirectional (UD) carbon/epoxy composite. Considering the initial imperfection and strength distribution of the fiber, the plasticity and ductile damage of the matrix, the failure of T300/914 UD composite under longitudinal compression and in-plane combined loads was simulated by this model. Simulation results show that the longitudinal compressive failure of the UD composite is caused by the plastic yielding of the matrix in kink band, and the fiber initial imperfection is the main reason for it. Under in-plane combined loads, the stress state of the matrix in kink band is changed, which affects the longitudinal compressive failure modes and strength of UD composite. The failure envelope of σ1–τ12 and σ1–σ2 are obtained by the micro-mechanical model. Meanwhile, the compressive failure mechanism of the UD composite is analyzed. Numerical results agree well with the experimental data, which verifies the validity of the micro-mechanical model.

Computational micromechanics evaluation of the effect of fibre shape on the transverse strength of unidirectional composites: An approach to virtual materials design

Computational micromechanics of composites is an emerging tool required for virtual materials design (VMD) to address the effect of different variables involved before materials are manufactured. This strategy will avoid unnecessary costs, reducing trial-and-error campaigns leading to fast material developments for tailored properties. In this work, the effect of the fibre cross section on the transverse behaviour of unidirectional fibre composites has been evaluated by means of computational micromechanics. To this end, periodic representative volume elements containing uniform and random dispersions of 50% of parallel non-circular fibres with lobular, polygonal and elliptical shapes were generated. Fibre/matrix interface failure as well as matrix plasticity/damage were considered as the fundamental failure mechanisms operating at the microscale under transverse loading. Circular fibres showed the best averaged behaviour although lobular fibres exhibited superior performance in transverse compression mainly due to the higher tensile thermal residual stresses generated during cooling at the fibre/matrix interface.

Influence of thermally induced microstress and microstructural randomness on transverse strength of unidirectional composites

An investigation is performed into the interaction between thermally and mechanically induced stresses and their relation to failure in a unidirectional composite under transverse tension and longitudinal shear. Multiple realizations of periodic unit cells with randomly positioned fibers are used. It is found that for microstructures with close fiber spacing, thermally induced stresses from cooling after cure tend to relieve the critical mechanically induced stress in the matrix for transverse tensile load, while they tend to worsen the critical stress for microstructures with space between fibers. Matrix strength properties are obtained through the solution of an inverse problem and are then used to perform progressive failure analyses at various temperatures. These analyses indicate that for random microstructures with closely spaced fibers, cooling after cure tends to increase the transverse tensile strength of the unidirectional composite, while cooling tends to weaken models using a hexagonal microstructure.