Unidirectional Composite Materials Research Articles

Reducing stress concentrations in static and fatigue tensile tests on unidirectional composite materials: A review

The longitudinal tensile strength is an important basic property of unidirectional (UD) composites or plies that often governs failure in multidirectional laminates. The standard tensile testing methods almost always result in sample failure near the grip. Finite element analyses revealed the presence of multiaxial stress concentrations, including longitudinal, transverse, and shear stresses in the tabbed section of the standards’ recommended design. There are still unknowns and uncertainties about the causes of these stress concentrations and ways to eliminate them. A major challenge is obtaining acceptable failure within the gauge section and yielding the highest tensile strength of UD composites. This paper reviews the different methods for performing quasi-static and fatigue tensile tests on UD composites. The primary sources of stress concentrations and the parameters that affect them are reviewed using the available experimental and modeling investigations. We survey the effects of the different specimen and end tab designs as well as test setups on quasi-static and fatigue loading. Specific proposals are made for each of the discussed parameters for more reliable results.

ELASTIC AND STRENGTH PARAMETERS DETERMINATION OF POLYMER COMPOSITE MATERIALS AND CYLINDRICAL SHELLS

The article presents a method for layout optimization in polymer composite materials that allows to assess their strength. In order to assess the stress-strain state of each layer, an orthotropic elasticity model is used assuming that the fibers in the material are parallel. The behavior model of polymer composite materials is based on the Hashin criterion, which makes it possible to separate the destruction mechanisms of matrix and fiber. The strength assessment was carried out in accordance with the structural approach involving studying the strength of each layer separately. The parameters of the deformation model and the Hashin strength criterion were determined based on the results of testing. Tests were carried out on unidirectional polymer composite materials samples cut at angles of 0, 90 and 45° to the direction of reinforcement. The verification was based on the results of bending tests of polymer composite materials samples. Verification showed high proximity between the calculation results and the experimental data. A method for selecting the layout angles of each layer of polymer composite materials has been developed making it possible to calculate the greatest strength of the cylindrical shell under various external loads. The method involves determining the stresses in each of the polymer composite materials layers, as well as Hashin indicators corresponding to the destruction of the fiber or matrix under tension or compression. Based on the indicator values, the optimal angle and layout pattern can be selected, providing the greatest margin of safety for the shell. This method allows for a preliminary assessment of the safety factor of cylindrical shells made of polymer composite materials based on the experimentally determined elastic parameters of the material and the parameters of the Hashin strength criterion. The method takes into account the angle and layout of the prepreg under the influence of internal pressure, axial compressive force and torque.

Determination of elastic characteristics for a package of monolayers thin-walled plates from composite fibrous materials

An analytical method for calculating the equivalent elastic characteristics of anisotropic multilayer plates made of composite fibrous materials is laid out. The main assumptions taken into consideration when calculating elastic moduli and Pousson's coefficients are that fibers are elastic materials with orthotropic mechanical characteristics that deform together when multilayer plates are loaded. Analytical methods for calculating effective modulus of elasticity most common in practical applications of applied mechanics are laid out in the list of cited publications. These include the rules of the mixture, the model proposed by Hill and Khashin, the model of Kilchinsky, the methods of Vanin and L.P. Khoroshun. Numerical methods for determining the elastic mechanical properties of reinforced unidirectional and layered composite materials are based on information technologies of finite-element modeling of representative volumes of composite materials and solving a number of boundary value problems for them. For the constructions of thin-walled plates with composite fibrous materials, traditional calculation schemes are used, for which the plane stress state is typical. The stress-strain relationship for a monolayer of plates loaded at an arbitrary angle is presented in the form of Hooke's law for aniotropic materials. Deformations of a package of monolayers with composite fibrous materials in a plane elastic-deformed state are determined, as for a monolayer, by four independent elastic constants. With the use of a universal calculation model based on the equations of applied mechanics, the results of the calculations of elastic moduli and Poisson's coefficients were obtained for a package of monolayers of thin-walled plates with composite fibrous materials made of carbon fiber and carbon fiber. Research results are presented in an analytical and graphic form. The influence of the construction structure of composite fibrous materials of thin-walled plates on its mechanical properties and their dependence on the angle of the force load vector is presented. The research results can be used to determine the rational mechanical properties of multilayer composite plates, taking into account their structural and technological purpose in various industries.

A micromechanical mean‐field homogenization surrogate for the stochastic multiscale analysis of composite materials failure

SummaryThis paper presents the construction of a mean‐field homogenization (MFH) surrogate for nonlinear stochastic multiscale analyses of two‐phase composites that allows the material response to be studied up to its failure. The homogenized stochastic behavior of the studied unidirectional composite material is first characterized through full‐field simulations on stochastic volume elements (SVEs) of the material microstructure, permitting to capture the effect of the microstructural geometric uncertainties on the material response. Then, in order to conduct the stochastic nonlinear multiscale simulations, the microscale problem is substituted by a pressure‐dependent MFH reduced order micromechanical model, that is, a MF‐ROM, whose properties are identified by an inverse process from the full‐field SVE realizations. Homogenized stress‐strain curves can be used for the identification process of the nonlinear range, however, a loss of size objectivity is encountered once the strain softening onset is reached. This work addresses this problematic introducing a calibration of the energy release rate obtained with a nonlocal MFH micromechanical model, allowing to scale the variability found on each SVE failure characteristics to the macroscale. The obtained random effective properties are then used as input of a data‐driven stochastic model to generate the complete random fields used to feed the stochastic MF‐ROM. To show the consistency of the methodology, two MF‐ROM constructed from SVEs of two different sizes are successively considered. The performance of the MF‐ROM is then verified against an experimental transverse‐compression test and against full‐field simulations through nonlocal Stochastic Finite Element Method (SFEM) simulations. The implementation of the energy release rate calibration allows to conduct stochastic studies on the failure characteristics of material samples without the need for costly experimental campaigns, paving the way for more complete and affordable virtual testing.

A Method for Plotting Failure Envelopes of Unidirectional Polymer Composite Materials under Different Strain Rates

This article emphasizes the significance of investigating the nonlinear behavior and strength characteristics of polymer composite materials under various strain rates. The study utilizes test results of a unidirectional (UD) composite material subjected to compression at different angles relative to the reinforcement direction, using quasi-static, static, and dynamic strain rates. The analysis focused on a UD layer experiencing compressive stresses perpendicular to the fiber reinforcement and in-plane shear stresses. A novel model is presented, enabling the calculation and prediction of the strength of a UD composite under uniaxial loading at different angles to the fiber direction, considering various strain rates. The developed model facilitates the derivation of equations for the failure envelopes of UD Carbon Fiber-Reinforced Polymers (CFRPs) under quasi-static, static, and dynamic loading conditions. To construct the failure envelopes of CFRPs, it is necessary to acquire experimentally determined values of tensile and compressive strength in the direction perpendicular to the reinforcement, as well as the ultimate strength in uniaxial compression of a specimen with reinforcement at a 45° angle to the loading axis. The failure envelopes generated using the proposed model exhibit excellent agreement with experimental data, with coefficients of determination ranging from 0.864 to 0.957, depending on the deformation rate. Consequently, the developed model holds promise for predicting the strength of other UD polymer composite materials.

Influence of fibre cross-section profile on the multi-physical properties of uni-directional composites

The present work comprehensively examines the influence of fibre-matrix interface perimeter on the multi-physical properties of uni-directional composite materials. Three-dimensional microstructures (containing fibres of triangular, elliptical, rectangular, C-shape and two-lobe cross-section shape) are analysed to evaluate effective thermal conduction, thermo-elastic and piezo-electric properties. Each of these properties is normalised with the respective property of RVE with circular fibre cross-section; These normalised properties are, objectively, compared with the normalised fibre cross-section perimeter (shape factor). For all the considered properties and fibre cross-sectional shapes, a novel observation is that the property experiences a drop (or rise) in the initial range of the shape factor but rises (or drops) monotonically afterwards. This is in contrast with the existing literature observations, where properties are understood to have only a monotonic increase/decrease with the shape factor. Further, it is observed that the magnitude of the initial drop (or rise) is sensitive to the fibre volume fraction and the fibre-matrix property contrast. In accordance with the literature, a strong correlation is observed among the variation of in-plane shear moduli, transverse thermal conduction and transverse dielectric constants.

Super-Resolution Processing of Synchrotron CT Images for Automated Fibre Break Analysis of Unidirectional Composites.

Fibre breaks govern the strength of unidirectional composite materials under tension. The progressive development of fibre breaks is studied using in situ X-ray computed tomography, especially with synchrotron radiation. However, even with synchrotron radiation, the resolution of the time-resolved in situ images is not sufficient for a fully automated analysis of continuous mechanical deformations. We therefore investigate the possibility of increasing the quality of low-resolution in situ scans by means of super-resolution (SR) using 3D deep learning techniques, thus facilitating the subsequent fibre break identification. We trained generative neural networks (GAN) on datasets of high-(0.3 μm) and low-resolution (1.6 μm) statically acquired images. These networks were then applied to a low-resolution (1.1 μm) noisy image of a continuously loaded specimen. The statistical parameters of the fibre breaks used for the comparison are the number of individual breaks and the number of 2-plets and 3-plets per specimen volume. The fully automated process achieves an average accuracy of 82% of manually identified fibre breaks, while the semi-automated one reaches 92%. The developed approach allows the use of faster, low-resolution in situ tomography without losing the quality of the identified physical parameters.

Influence of interphase characteristics on the elastic modulus of unidirectional glass-reinforced epoxy composites: a computational micromechanics study

Abstract This article briefly discusses the role of interphase in the elastic moduli of unidirectional fiber-reinforced polymer composite materials. For this unidirectional glass fiber was chosen as reinforcement, and epoxy was selected as the matrix. A hexagonally packed representative volume element is used for the micromechanical analysis. Experimental validation was initially used to verify the accuracy of the established equations of the rule of mixing, Composite Cylinder Assemblage, Chamis, Halpin and Tsai, and Puck. The Chamis equation was found to be the most reasonable. Then the finite element approach in which interphase has been included was used to estimate the elastic moduli. The finite element model without interphase and the experimental result were taken as a reference. The influence of interface ratio and property of interphase on the homogenised elastic properties of the unidirectional fiber-reinforced polymer composites is analysed. A micromechanics plugin in Abaqus software was used to estimate the density and Young’s modulus of the unidirectional fiber-reinforced polymer composites. The interphase properties are varied, having 6.25%, 12.5%, 25% and 50% influence of the fiber phase and the remaining influence of the matrix phase with interface ratios of 0.1, 0.2 and 0.3. The interface ratio of 0.3, having 6.25% fiber phase influence, gave the most reasonable moduli values (with an error <10%) compared to the mean experimental moduli. The study showed interface ratio and interphase properties to critically influence the overall elastic property of the unidirectional fiber-reinforced polymer composites.

Closed-form asymptotic micromechanics model of fiber reinforced polymer and metal matrix composites

This work presents an analytical asymptotically-correct micromechanics model that helps to predict the effective material properties of a unidirectional composite material. The conventional and numerical approaches estimate the homogenized material properties of composites for their defined component volume fractions, their constituent properties and configurational geometry. Presently these approaches are based on kinematic assumptions such as having displacement or stress components vary through the cross section for beam like structures or through the thickness for plate like structures according to certain predefined functions that doesn’t always logically follow the 3D analysis. In the present formulation, the micromechanics model is developed by accommodating all possible deformations without assuming the displacement function or stress components. These are derived by minimizing the potential energy in terms of generalized strain measures. In the present formulation, Berdichevsky’s Variational Asymptotic Method (VAM) is employed as a mathematical tool to accomplish the homogenization procedure. The Hashin-Rosen model popularly referred to as the Concentric Cylinder Model (CCM) serves as the framework to estimate all the relevant homogenized elastic moduli and coupling coefficients. The derived quantities of interest are obtained as closed form expressions which are functions of the properties of the reinforcement material, the matrix material, their volumes fraction and the geometry of their relative arrangement. These expressions are arrived following the 3D elasticity governing rules by satisfying the interfacial displacement continuity and transverse stress equilibria conditions at the reinforcement and matrix materials interface. The developed expressions for the elastic moduli, shear moduli and Poisson’s ratios of few typical polymer and metal matrix composite materials are validated with some of the relevant results available in the literature.

Study of the laminas’ stress-strain state for cross-ply composites with the [± 45°]2s lay-up: tensile tests

In this paper we have analysed the relations that allows one to determine the components of strains and stresses in the orthotropic axes of layered fiber reinforced composite materials in tensile and compression experiments for test specimens with the [±45°]2s lay-up. In particular, such relations were compiled on the assumption that in a cross-ply fiber composite two adjacent layers with the [±φ] lay-ups can be considered as one symmetrically reinforced layer with orthotropic properties. To establish the degree of accuracy of these relationships and assumptions, numerical experiments were carried out to determine the parameters of the stress-strain state of specimens consisting of two and four monolayers of a unidirectional fibrous composite material with the [±45°]2s lay-up. The analysis has been performed in the ANSYS finite element analysis program system in a linear formulation of two- and three-dimensional problems. It is shown that in the central zone of the specimen, the theoretical-experimental methodology based on the above mentioned relations has a sufficient degree of accuracy. Based on the analysis of results of three-dimensional problems, the formation of a linear torque in the specimen, which causes the twisting of some zones of the specimen along its length, has been revealed.

IN MECHANICS OF DEFORMABLE SOLIDS PRIMARY PLASTIC ON PANEL DURING POWER UP DEVELOPMENT OF DEFORMATIONS

One of the most important issues in solid mechanics is the development and application of optimal methods for estimating the resistance of dispersion materials. Currently, the theory of propagation of cracks in solid bodies is widespread due to the wide use of high-strength materials and large-sized structures and devices in various fields of technology. The article discusses the methods of studying the problems of the interaction of two factors - strengthening the panel with stiffening ribs made of unidirectional composite material and weakening it with a number of circular holes and cracks protruding from its contour. An infinite isotropic elastic riveted panel weakened by a periodic system of circular holes and two cracks along the abscissa axis segments near each hole is considered. The contours of the holes and the edges of the cracks are free from external forces. Transverse stiffness ribs made of metal composite material are riveted to the plate. In the calculation scheme, the effect of riveted reinforcing ribs is replaced by the concentrated forces applied at the points of location of the rivets. The values of the concentrated forces are found depending on the geometrical and physical parameters of the problem. Stress intensity factors and ultimate loadings were calculated depending on the geometrical and physical parameters of the problem. It has been shown that there is a stable phase of crack development under certain specified conditions. The development of initial plastic deformations (embryonic cracks) emerging from the contour of circular holes in a plate reinforced with stiffness ribs is studied. The material of the plate is elastic-perfect-plastic satisfying the Tresca-Saint-Venant condition. It is assumed that the plastic deformations are concentrated along some slip lines emanating from the contour of the hole. By satisfying the boundary conditions, the solution of the problem leads to a singular integral equation. Then the singular equation of the problem is brought to a system of algebraic equations without the intermediate step of bringing it to the Fredholm equation. The resulting system was solved by the Gaussian method with the selection of principal elements for different values of n (n is the number of Chebyshev nodes in the division of the interval). The values of concentrated forces are found depending on the geometrical and physical parameters of the problem. The dependences of the length of the plasticity bands on the applied load, the geometrical parameters of the problem and the yield point of the material were found. Using the local dissipation condition (KPT-criteria), a ratio was obtained to determine the destructive edge loading.

Finite element modeling of integral viscoelastic properties of textile composites

The problem of modeling the effective integral viscoelastic properties of unidirectional composite materials is considered. To calculate the integral properties of viscoelasticity, the Fourier transform and the inverse Fourier transform are used, as well as the method of asymptotic averaging for composites with steady polyharmonic vibrations, and a finite element algorithm for solving local problems of the viscoelasticity theory on the periodicity cell of the composite. To obtain the material constants, a method of approximation of the Fourier images of the relaxation and creep kernels is proposed, which makes it possible to avoid the numerical error of the inverse Fourier transform.

Random field modeling of local strength of a randomly arranged unidirectional fiber-reinforced composite material under transverse tensile loading

This study examines the random field modeling of local strength, which is the apparent strength of each window extracted from an SEM image of a cross-section of a randomly arranged unidirectional fiber-reinforced composite material specimen. In particular, computation of the spatially varying random apparent fracture strength based on the microstructure simulation is performed. To analyze the apparent fracture strength, single-scale analyses assuming plane-stress and plane-strain conditions, as well as multiscale stress analysis based on the homogenization theory, are attempted. In addition, the moving window method is employed to compute the respective random fields and their autocorrelation functions. First, the influence of the analysis method on the identified random field is investigated. Furthermore, the influence of the window and image size is discussed. The numerical results revealed that a smaller window leads to a more accurate evaluation of the apparent strength, and thus, is preferable for the identification of the respective random field.

Güçlendirilmiş Polyester Reçine Matrisli Kompozit Malzemelerin Tek Yönlü Karbon Fiberler Üzerinde Katmanların Etkisi

In this study, composite materials are produced, placing unidirectional carbon fiber reinforcements with 12 filament numbers in a polyester matrix to form one, two, and three layers at room temperature through the hand lay-up method. The samples are prepared from composite plates with the help of a mold in line with the fiber direction angle by the standards. The tensile, three-point bending, and falling weight low-speed impact strength values of the samples were measured and were investigated the mechanical performance of the specimens. As a result of the tensile test, the highest tensile strength value among layers is obtained in the two-layer unidirectional fiber-reinforced composite materials. In bending tests, the highest elasticity values of unidirectional carbon fiber reinforcements are observed in three-layer composite materials with a fiber direction angle of 00. 00 unidirectional fiber-reinforced one-layer composite materials are observed in composite materials with the highest deformation values in the falling weight low-velocity impact tests.

Formability Experiments for Unidirectional Thermoplastic Composites

Reliable composite forming experiments are required to characterize composite formability, to aid material development, and to validate process simulations models. Due to practical reasons, however, typically a limited amount of forming configurations is studied. The objective of this study is, therefore, to develop a methodology for obtaining controlled forming results in a wide range of configurations. Press forming experiments using a dome geometry were used to explore the formability of two commercial unidirectional thermoplastic composite materials. A variety of forming configurations was employed by changing the blank dimensions and layup. The observation of wrinkling defects was simplified by leaving an additional 3 mm tool gap. Blank width and layup had the most influence on the wrinkling severity, followed by blank thickness and length. Quasi-isotropic layups were found to produce wrinkles in nearly all cases, confirming a difficulty in general to form double curved parts. The size and number of wrinkles in these layups were found to change with the stacking sequence. Cross-ply layups showed better formability, but significant wrinkles were still observed depending on the orientation of the blank relative to the layup. The formability experiments using a dome geometry provided a reliable methodology for controlled forming results in many configurations using a generic toolset. Additionally, a comprehensive comparison of formability for two commercial thermoplastic UD materials in a variety of scenarios was provided.

Evaluation of modal frequencies obtained with the impact hammer technique on an epoxy matrix composite material reinforced with glass fibers

obtain the modal frequencies of polymeric composite materials reinforced with unidirectional fibers (glass fiber and epoxy resin) for possible aeronautical applications. The objective of this study is to compare the behavior of an isotropic material with an orthotropic one. This comparison is to observe the influence of the material properties on its performance under dynamic conditions, where the modal frequencies of a material can directly affect the performance of each element of a structure. The first case describes the numerical and experimental identification of the modal frequencies of an isotropic material (6065 T5 aluminum). The second case study is presented to show how this methodology is adapted to the composite material. The experimental results are obtained by applying the impact hammer testing method. The comparison provides new insights into the modal behavior of vibrations in composite materials. A significant finding of this work is to provide a detailed analysis of the behavior of a unidirectional composite material in terms of the fiber’s orientation. Then, this work would be established the fundamentals of the composite material performance for rotative elements applications.

High velocity impact on generic CFRP blade specimen: baseline free method for impact localisation and damage assessment on complex structures

Nowadays components made of unidirectional composite materials are largely diffused in many engineering fields, such as automotive, railways, marine and aerospace. Main drawback of this class of materials lies in their low out-of-plane properties making them very sensible to impulsive loads such as impact events. After a collision with an external object, composites structures could be affected by damage, sometimes not visible from visual inspections (barely visible damage) hence with detrimental consequences on structure resistance and strength. Therefore, it is fundamental in terms of safety to continuously assess the healthy state of structures during their life and determine whether an impact event has occurred and if it caused damage or not. This work proposes a baseline free methodology to determine the coordinates of very high velocity impact on complex structures and evaluate if damage has occurred during the impact by only acquiring signal during the impact event. The technique overcomes the common limitations of previous technique presented in literature, i.e. a priori knowledge of mechanical properties, vibration response analysis, wave propagation direction dependency, sensor locations. The routine developed is based first on the estimation of the power of the acoustic emission generated by impact events, at sensors location, then the power information through the entire structure is reconstructed exploiting radial basis function network. The actual impact estimation is finally obtained using a weighted method. Furthermore, damage assessment is conducted with a novel method based on Hilbert–Huang transform and mode decomposition. Experimental tests were performed on a generic carbon fibre reinforced polymers blade specimen with a complex stacking sequence and embedded sensors. Two test configurations at different velocities were considered: one at 90 m s−1 and one at 190 m s−1. Before and afterwards the actual impact tests, the blade was excited as well with a modal hammer (pre and post impact). The results from the impact analysis highlighted the validity and reliability of the proposed method, with a high level of accuracy in terms of impact localisation estimation, and qualitative integrity state was effectively evaluated.

Interval-Based Computation of the Uncertainty in the Mechanical Properties and the Failure Analysis of Unidirectional Composite Materials

An interval-based method is presented to evaluate the uncertainty in the computed mechanical properties and the failure assessment of composite unidirectional (UD) laminates. The method was applied to two composite laminates: a carbon/epoxy and a glass/epoxy. The mechanical properties of the UD lamina were derived using simplified micromechanical equations. An uncertainty level of ±5% was assumed for the input properties of the constituents. The global minimum and maximum values of the properties were computed using an interval branch-and-bound algorithm. Interval arithmetic operations were used to evaluate the uncertainty in the Hashin-type failure criteria in a closed form. Using the closed-form uncertainties of intervals and sets of stresses obtained by finite element analysis, the uncertainty in the failure assessment was quantified for the two composite laminates. For the assumed uncertainty level of ±5%, the computed uncertainty for the mechanical properties ranges from 6.64% to 10.63% for the carbon/epoxy material and from 6.72% to 12.28% for the glass/epoxy material. For evaluating the uncertainty effect on the efficiency of failure criteria, a probability of failure function, which employs interval boundaries, was defined and proved capable of evaluating the whole spectrum of stresses.

On the relationship between moisture uptake and mechanical property changes in a carbon fibre/epoxy composite

Polymer composite materials are widely used in marine applications where an understanding of the long-term performance is essential for economic, safety and durability requirements. Although moisture absorption in composites has been studied for many years, the relationship between the mechanisms of moisture absorption and consequent changes in material behaviour has received much less attention. Understanding degradation is necessary for developing lifetime assessments. In this work, long-term exposure of a unidirectional carbon fibre epoxy composite material in water has been investigated in relation to mechanical property changes and moisture uptake. Using diffusion modelling and microscopy of the fracture surfaces, it is possible to correlate water absorption to experimental data showing how the position of water in the material causes changes to flexural properties (modulus decreased by 14% and strength by 20%) and in the glass transition temperature, Tg reduced by 18%. Flexural modulus has been shown to be most affected by water interaction with the fibre interface; Tg by water interaction within the resin; and flexural strength equally by water interaction with both fibre interface and resin.

Design and Analysis of CFRP Drilling by Electrical Discharge Machining.

The novelty of EDM machining as a drilling operation for composite materials means that there is no consensus on the influence of the parameters that optimise the final quality. For these reasons, a characterisation of the EDM drilling process of a unidirectional composite material has been developed. The influence of several cutting parameters has been related to the quality of the final hole obtained. Thus, macrogeometric aspects in terms of conicity and final diameter and microgeometric aspects in terms of surface quality have been evaluated. In addition, the final state of the material and the wear of the electrode have been evaluated by visual inspection, allowing the range of cutting parameters that offer the best performance to be established. Finally, a series of contour diagrams based on predictive models have been obtained to establish a direct relationship between input and output variables.