Unidirectional Composite Laminates Research Articles

In-plane shear strength analysis of Basalt Fiber-Reinforced Epoxy Laminates with Biowaste Catalyst Free Carbon

This study aims to analyse the effects of carbon nanospheres (CNSs) on the in-plane shear properties of basalt fiber-reinforced epoxy composite laminate (BFR). The CNSs were obtained from an economical fibrous residue attained from the sago palm tree, which is known as biowaste sago bark. Hand lay-up method was used to fabricate the unidirectional basalt fiber-reinforced epoxy composite laminates. The epoxy resin was mixed with carbon nanosphere particles (i.e., 0.6 wt% - 1 wt%). In-plane shear tests have been conducted as per ASTM standards. In addition, emphasis on the microstructural investigation using Scanning Electron Microscopy (SEM) is given, in order to study the fracture surfaces of the composite laminates. The results demonstrated significant improvement in in-plane shear strength when carbon nanosphere particles were included in the basalt fiber-reinforced epoxy composite laminate. The best result was obtained at 1.0 wt% CNSs. It displayed an increment of 37.1% in in-plane shear strength, and 36.4% increment in modulus of rigidity, respectively, in comparison to neat basalt fiber-reinforced epoxy composite laminate. The improved accomplishment of CNSs/basalt fiber-reinforced epoxy composite laminate is due to good distribution of CNSs particles in the epoxy matrix.

Micro-mechanical damage model accounting for composite material nonlinearity due to matrix-cracking of unidirectional composite laminates

A new micromechanical damage model for predicting the effect of matrix-cracking on the mechanical behavior of the composite material is proposed. The model is based on the volumetric change that occurred due to the presence of cracks in a composite lamina due to uniaxial off-axis loading. It determines the volumetric crack-density (VCD) by combining the macro-mechanical and micro-mechanical principles. A representative volume-element is proposed that determines the material mechanical properties (E1, E2, G12 and ν12) in terms of crack-density, fiber and matrix properties and initial volume-fraction of fibers. The rule-of-mixture in combination with Halpin-Tsai model is used to determine the mechanical properties of a cracked composite lamina. It has been shown that, matrix-cracking is the main cause for composite-material nonlinearity. Moreover, the model has been shown to give a reliable and reasonable predictions of the VCD and the tangential damage-factor (TDF) for various fiber/matrix systems using the corresponding available data from literature. An alternative secant damage-factor is being proposed, which has a linear relationship with the VCD. In order to validate the model, two composite materials; Boron/Epoxy (Narmco-5505) and Graphite/Epoxy (4617/Modmor-II), have been considered using laminates at different fiber-orientation angles. The maximum volume-crack-density (MVCD) and maximum secant damage-factor (MSDF) are obtained using equations that depend on the fiber-orientation angle and the initial material mechanical properties.

Response of carbon-basalt hybrid fiber reinforcement polymer under flexural load

A study on the flexural performance of 6-ply unidirectional hybrid composite laminates reinforced by carbon and basalt fibers is presented in this paper. The hybrid composites were fabricated following the resin transfer molding (RTM) process in an inter-ply configuration with varying plies of basalt fibers added to the surface of carbon laminate. Three-point bending test was carried out according to ASTM D790-10 at a span to depth ratio of 32. The laminates were examined by optical and scanning electron microscopy(SEM), which enabled a better comprehension of the failure modes. The results showed that various failure modes occurred through almost all of the layers and mainly concluded fiber buckling on the compressive side, shear failure on the middle layer, and fiber pull-out on the tensile side. Flexural properties of hybrid composites were strongly dependent on the basalt fibers added to the laminate. When adding two layers of basalt fibers to the laminates, flexural strength of hybrid composites increase most, and it can be seen that the flexural modulus decreased with increasing percentage of basalt fibers. And the experiments showed a positive hybridization effect by substituting carbon fibers with basalt fibers. Here, hybridization of carbon and basalt fibers can improve the balance of the mechanical properties of carbon fiber reinforced polymer(CFRP), thereby extending its application scope and reducing the cost.

Interlaminar fracture toughness and electromagnetic interference shielding of hybrid-stitched carbon fiber composites

In the current study, the production of multifunctional hybrid-stitched composites with improved interlaminar fracture toughness and electromagnetic interference shielding effectiveness is reported. Unidirectional carbon fiber-epoxy composite laminates stitched with Kevlar, nylon, hybrid stitched with both Kevlar and nylon and unstitched were prepared using resin infusion process. Representative specimens from unstitched and stitched composites were tested using rectangular waveguide and Mode I double cantilever beam tests. The Mode I experimental results showed that composite stitched with Kevlar exhibited the highest crack initiation interlaminar fracture toughness (GIC-initiation), whereas composite stitched with nylon exhibited the highest maximum crack propagation interlaminar fracture toughness (GIC-maximum). The four-hybrid stitching patterns exhibited higher GIC-initiation than the unstitched and stitched with nylon composites and lower than stitched with Kevlar composite, whereas they had higher GIC-maximum than the unstitched and stitched with Kevlar composites, although lower than stitched with nylon composite. The electromagnetic shielding effectiveness experimental results showed that stitched composites exhibited improved shielding effectiveness compared to unstitched composites. For example, composite stitched with nylon had highest shielding effectiveness value of 52.17 dB compared by the composite stitched with Kevlar which had 40.6 dB. The four hybrid-stitched composites exhibited similar shielding effectiveness with an average value of 32.75 dB compared to the unstitched composite shielding effectiveness of 22.84 dB. The experimental results comply with the initial goal of this study to manufacture multifunctional hybrid stitching composites with combined properties between Kevlar and nylon-stitched composites.

Quantitative analysis of QSI and LVI damage in GFRP unidirectional composite laminates by a new ultrasonic approach

Abstract Our work is focused on a new experimental approach for the comparison between Quasi Static Indentation (QSI) damage and Low-Velocity Impact (LVI) damage in polymer composites starting from the results of ultrasonic goniometric immersion tests. In particular, the comparison is performed through the analysis of the additional anisotropy induced by the damage in unidirectional Glass Fiber-Reinforced Polymer (GFRP) composites due to QSI and LVI damage tests performed with a low level of the employed energy. To this aim, we ultrasonically reconstruct the acoustic curves (velocity curves and slowness curves) before and after the damage. Ultrasonic experiments are performed by using a goniometric ultrasonic immersion device designed and built at our laboratory, aimed at the mechanical characterization of anisotropic materials. We highlight differences and similarities between QSI and LVI damage starting from the analysis of the variations of the acoustic behavior and by using a suitable anisotropic damage model developed in the framework of the Continuum Damage Mechanics theory. The proposed experimental approach can be suitably developed for in situ investigations on low-velocity impact damage in polymer composite components.

Heat partition in dry orthogonal cutting of unidirectional CFRP composite laminates

Carbon fiber reinforced polymer (CFRP) components can be generally prepared near-net-shape, however, they still need machining after manufacturing to meet the geometrical accuracy with excellent surface quality required for assembly. Due to the cutting temperature is prone to exceed the glass-transition temperature of the resin, the irreversible chemical and mechanical degradation are therefore difficult to avoid. It becomes a crucial challenge to eliminate the particular thermal effect on the composite machining process. In comparison with the temperature measurement, the analytical model not only can reveal the physical essence of thermal effect, but also can predict the temperature field distribution to provide the reasonable cutting parameters. In particular, a key parameter for calculating the cutting temperatures is the heat partition ratio. However, the research work on this issue was rarely found. In this paper, a fiber orientation-based analytical model was developed to predict the heat partition ratio based on the classical Hertz contact theory. The finite element model was also built with the validation of the experimental measurement from the thermal imaging tests. The results suggest the heat partition ratio is mainly determined by the cutting parameters. Moreover, the fiber orientations have a remarkable impact on this ratio. Due to the heat partition ratio is considerably larger in a CFRP workpiece than the cutting tool, more heat energy was transferred to the CFRP during machining. Therefore, a small depth of cut leads to a reduced tendency for thermal effect on the CFRP composites.

Micromechanical analysis of composites with fibers distributed randomly over the transverse cross-section

A new method to generate the random distribution of fibers in the transverse cross-section of fiber reinforced composites with high fiber volume fraction is presented in this paper. Based on the microscopy observation of the transverse cross-sections of unidirectional composite laminates, hexagon arrangement is set as the initial arrangement status, and the initial velocity of each fiber is arbitrary at an arbitrary direction, the micro-scale representative volume element (RVE) is established by simulating perfectly elastic collision. Combined with the proposed periodic boundary conditions which are suitable for multi-axial loading, the effective elastic properties of composite materials can be predicted. The predicted properties show reasonable agreement with experimental results. By comparing the stress field of RVE with fibers distributed randomly and RVE with fibers distributed periodically, the predicted elastic modulus of RVE with fibers distributed randomly is greater than RVE with fibers distributed periodically.

Matrix toughness transfer and fibre bridging laws in acrylic resin based CF composites

The interlaminar fracture toughness, GIc, of unidirectional carbon fibre composite laminates produced by infusion moulding with two different thermoplastic acrylic matrices, one plain and one rubber toughened, was studied. For the composite with the plain matrix fracture was dominated by failure at the fibre-matrix interface. With the toughened matrix, fracture occurred within the matrix-rich layer, although the matrix toughness could not be fully exploited due to the interaction of the fibres with the development of the process zone. A fibre bridging model based on experimentally derived cohesive laws was developed and it confirmed that fracture occurs within the matrix.

Modelling and simulation methodology for unidirectional composite laminates in a Virtual Test Lab framework

A reliable virtual testing framework for unidirectionally laminated composites is presented that allows the prediction of failure loads and modes of general in-plane coupons with great realism. This is a toolset based on finite element analysis that relies on a cohesive-frictional constitutive formulation coupled with the kinematics of penalty-based contact surfaces, on sophisticated three-dimensional continuum damage models, and overall on a modelling approach based on mesh structuring and crack-band erosion to capture the appropriate crack paths in unidirectional fibre reinforced plies. An extensive and rigorous validation of the overall approach is presented, demonstrating that the virtual testing laboratory is robust and can be reliably used in for composite materials screening, design and certification.

Numerical and experimental approach for improving quasi-static and fatigue testing of a unidirectional CFRP composite laminate

Mechanical response of CFRP unidirectional prepreg was investigated under tensile quasi-static and fatigue loading. Firstly, the findings of an initial investigation on the effects of the end tabs and gripping condition are presented. A dialogue between numerical simulations and experimental tests led to avoid premature splitting failures in the clamps. Proper tensile failure is assessed in the gage length, at enhanced strength levels for the material considered. Consequently, the unidirectional material was successfully tested under tension-tension fatigue, at high load levels. Fatigue damage mechanisms arise at high stress amplitude, revealing that also a UD laminate can suffer from fatigue degradation along the fibres direction.

Parametric study of composite bolted joint under compressive loading

Use of composite structures in various sectors is rapidly increasing. The composite materials are becoming very popular due to high strength to weight ratio, high stiffness to weight ratio, low coefficient of friction, improved appearance etc. Extensive literature is available for the behavior of composite bolted joints subjected to tensile loading for the aerospace structures. The work reported in this paper is an attempt to extend the analytical model for studying the behaviour of composite bolted joint under compressive loading. The model is capable of conducting the parametric study on the joint stiffness variation with respect to parameters such as plate width, bolt diameter and stacking sequence for compressive loading. The model is verified experimentally. The experiments were designed to study the effect of variation in the parameters on the joint stiffness. Unidirectional and bidirectional carbon fibre epoxy composite laminates were tested as per ASTM D5961. The parameters viz. stacking sequence, bolt diameter, plate width of the joint stiffness are identified as significant.

Process monitoring in milling unidirectional composite laminates through wavelet analysis of force signals

The extensive application of composite materials in the aerospace and automobile industry and significant machining challenges warrant the need of process monitoring techniques. These techniques help in detecting dynamical instabilities, monitoring machining induced damage and surface quality within a given tolerance. This study discusses the application of continuous wavelet transform analysis in trimming of unidirectional graphite epoxy laminates. The suggested approach is used to obtain time-frequency scalograms and windowed Shannon entropy to characterize the process for different machining conditions and fiber orientation angles. The relative wavelet entropy (RWE) proved to be an efficient classifier in characterizing and developing optimum values at different experimental conditions. A deviation from these optimum values indicated severe damage. In addition, a positive correlation was found between RWE and surface quality.

A multi-parameter model for stiffness prediction of composite laminates with out-of-plane ply waviness

In this paper, an analytical multi-parameter model based on the classical lamination theory (CLT) is developed and the study of the stiffness response for unidirectional composite laminates with out-of-plane ply waviness is followed. A range of parameters associated with the waviness defect, including the waviness geometry, the fiber off-axis angle, the extent of wavy region and the waviness pattern, are incorporated into the analytical model. The fiber volume fraction and the type of reinforcement are also taken into account in the parametric analysis. The three-dimensional stiffness parameters of thick wavy laminates are investigated systematically for different cases. An observation of the negative in-plane Poisson’s ratios for wavy laminated composites reinforced with carbon fibers is presented under the given assumptions of CLT. This phenomenon provides a possible, new technique to design the auxetic materials. The modeling approach provides an effective procedure to quantify the effects of ply waviness on the stiffness of composite laminates.

A validation of a modified Paris relation for fatigue delamination growth in unidirectional composite laminates

Abstract The aim of this paper is to provide evidence on the validation of a modified Paris relation in fatigue crack growth with fibre bridging. Energy principles were first applied to investigate energy release in fatigue delamination growth. A comparison between the Paris and the modified Paris relations was completed subsequently. The application of the Paris relation can artificially cause different resistance curves of fatigue delamination with fibre bridging, violating the hypothesis of similitude and the energy dissipation results. However, the use of the modified Paris relation can result in a master resistance curve, which is physically consistent with the law of similitude and the energy release regulations. This derives from using an appropriate similitude parameter in data reduction. It is therefore reasonable to use the strain energy release rate (SERR) actually applied on the crack front, rather than the total applied SERR, as similitude to correlate fatigue crack growth. The master resistance curve was finally employed to predict fatigue results from literature. Acceptable agreement between predictions and experiments was achieved, validating the effectiveness of the modified Paris relation in determining fatigue crack growth with fibre bridging.

Delamination Modeling of Double Cantilever Beam of Unidirectional Composite Laminates

Delamination crack growth in a double cantilever beam laminated composites is modeled by using simple stress analysis beam theory combined with simple linear elastic fracture mechanics and consideration of the theory of elastic failure in mechanics of material. Furthermore, advanced finite element (FE) model is built up. The FE approach employs surface cohesive zone model that is used to simulate the debonding and crack propagation. The analytical modeling, moreover, cracks growth and strain measurements, which are obtained from FE models, are compared with the available published experimental work. The predicted results give good agreement with interlaminar fracture toughness and maximum load which correspond to crack initiation point. The FE models results agree well with the available experimental data for both crack initiation and propagation.

Semi-continuous strategy for the modelling of damage mechanisms in unidirectional composites under low velocity impacts

Abstract This article deals with the development of a finite element model for the prediction of low velocity impact damage within unidirectional composite laminates. This model is based on analysis of the impact damage observed experimentally. The modelling scale is that of the bundle of fibers of the unidirectional ply. These bundles are represented with 1D rod elements. The matrix is modeled with 2D damageable shell elements. The laminate is the built using cohesive elements. The strategy is validated by a comparison with low velocity drop weight impact tests. Several experimental parameters are varied: the materials (T700/M21 and HTA7/913), the thickness ( 1.44 m m and 2.4 m m ), the stacking sequence and the impact velocity ( 2 m . s − 1 and 3 m . s − 1 ). The calculated load-displacement curves and the damage extent correlate well with experimental results.

Adhesive joints with improved mechanical properties for aerospace applications

Adhesion is attracting increasing interest in the aerospace field since composite materials have become, together with aluminium alloys, the main structural materials for aircraft primary structures. Nano-graphite was demonstrated to improve the mechanical performance of several polymers used as composite matrices. In this work Single Lap Joints (SLJs) of unidirectional composite laminates were manufactured, tested and simulated: two families of specimens were investigated and compared, one joined using conventional epoxy resin, the other joined with an adhesive obtained mixing the same epoxy resin with nano-graphite particles. The dispersion of expanded and sonicated graphene stacks (EGS, 3% wt) in the epoxy matrix was obtained by the swelling method, dispersing first the filler in acetone and then mixing it with the epoxy oligomers. Finally the solvent was evaporated and the filler-epoxy mixture was degassed under vacuum before adding an amine curing agent in a stoichiometric quantity. The research demonstrates the superior mechanical properties of the adhesive with the addition of nano-graphite through experimental characterization of its behaviour in terms of strength and energy absorption. Finite element numerical simulations have been carried out using the Cohesive Zone Model (CZM) element, obtaining as parameters the maximum shear stress and the critical fracture energy for the two adhesives. A good correlation between numerical and experimental results has been achieved and the criteria for developing reliable and accurate non-linear models of the adhesive failure have been established.

Simulation of Low Velocity Impact Induced Inter- and Intra-Laminar Damage of Composite Beams Based on XFEM

In this paper, the Inter-Fiber Fracture (IFF) criterion of Puck failure theory based on the eXtended Finite Element Method (XFEM) was implemented in ABAQUS code to predict the intra-laminar crack initiation of unidirectional (UD) composite laminate. The transverse crack path in the matrix can be simulated accurately by the presented method. After the crack initiation, the propagation of the crack is simulated by Cohesive Zoom Model (CZM), in which the displacement discontinuities and stress concentration caused by matrix crack is introduced into the finite element (FE) model. Combined with the usage of the enriched element interface, which can be used to simulate the inter-laminar delamination crack, the Low Velocity Impact (LVI) induced damage of UD composite laminate beam with a typical stacking of composite laminates [05/903]S is studied. A complete crack initiation and propagation process was simulated and the numerical results obtained by the XFEM are consistent with the experimental results.

Effects of cutting edge radius and fiber cutting angle on the cutting-induced surface damage in machining of unidirectional CFRP composite laminates

Carbon fiber reinforced polymer composite laminates are anisotropic, inhomogeneous, and mostly prepared in laminate form before undergoing the finishing operations. The edge trimming process is considered as one of the most common finishing operations in the industrial applications. However, the laminate surface is especially prone to damage in the chip formation process, and the most common damage mode is burrs. Burrs may increase cost and production time because of additional machining; they can also damage the surface integrity. Many studies have been done to address this problem, and techniques for reducing burr size in material removal process has been the focus of the research. Nonetheless, the combined effects of the cutting edge radius and the fiber cutting angle on the burr formation have seldom been conducted, which in turn restricts to find out the mechanism of burr formation. The purpose of the present paper is to study the particular mechanism that leads to burr formation in edge trimming of CFRP laminates and investigate the effects of fiber cutting angle and cutting edge radius on burr formation. The results indicate that the burrs are prone to form in the fiber cutting angle range of 0° < χ < 90° when a large cutting edge radius of the tool is used for both milling and drilling of CFRP composites.

Machining analysis of unidirectional and bi-directional flax-epoxy composite laminates

Natural fibers, and more particularly, flax fibers, have a considerable potential as replacements for synthetic fibers. These fibers are of significant economic and environmental interest because they are natural products, are biodegradable, and unlike synthetic fibers, are entirely recyclable. They are also less expensive than synthetic fibers, less abrasive for machining, and their specific properties (strength-to-weight ratio) are comparable to those of glass fibers. Consequently, they thus provide economic and environmental benefits for companies. Unfortunately, machining knowledge with respect to this kind of material is low, and research in this domain has barely begun. The objective of this study is to describe the machinability of unidirectional and bidirectional flax/epoxy composites and to analyze the influence of cutting parameters and fiber orientation on cutting forces and surface finish. Milling tests were performed on unidirectional composite laminates with two different tools. The results show that the surface finish and cutting forces depend largely on the feed rate, and to a lesser extent, on the cutting speed. The PCD cutting tool, with a zero helix angle, showed the best performances as compared to the CVD cutting tool, which had a different geometry. The former provided a better surface finish, a lower delamination factor, and lower cutting forces. The material was found to be easy to machine and low abrasive, since no tool wear was observed following the cutting tests. Finally, it was found that an intermediate feed rate value and a high cutting speed were the best of all parameters tested for achieving a low cutting force level, low surface roughness, and high throughput.