Carbon-epoxy Laminates Research Articles

Integration of inter-ply electrical percolation phenomena in the multiphysics modelling of laminated composite materials

PurposeThis study aims to introduce a predictive homogenization model incorporating electrical percolation considerations to forecast the electrical characteristics of unidirectional carbon-epoxy laminate composites.Design/methodology/approachThis study presents a method for calculating the electrical conductivity tensor for various ply arrangement patterns to elucidate phenomena occurring around the interfaces between plies. These interface models are then integrated into a three-dimensional (3D) magneto-thermal model using the finite element method. A comparative study is conducted between different approaches, emphasizing the advantages of the new model through experimental measurements.FindingsThis research facilitates the innovative integration of electrical percolation considerations, resulting in substantial improvement in the prediction of electrical properties of composites. The validity of this improvement is established through comprehensive validation against existing approaches and experimentation.Research limitations/implicationsThe study primarily focuses on unidirectional carbon-epoxy laminate composites. Further research is needed to extend the model's applicability to other composite materials and configurations.Originality/valueThe proposed model offers a significant improvement in predicting the electrical properties of composite materials by incorporating electrical percolation considerations at inter-ply interfaces, which have not been addressed in previous studies. This research provides valuable information to improve the accuracy of predictions of the electrical properties of composites and offers a methodology for accounting for these properties in 3D magneto-thermal simulations.

Characterization of multidirectional carbon-nanotube-yarn/bismaleimide laminates under tensile loading

Unidirectional, cross-ply, and quasi-isotropic composite laminates are made from continuous carbon nanotube (CNT) yarns with bismaleimide (BMI) resin. The laminates are highly graphitic and have low resin content. Elastic modulus and strength of CNT/BMI laminates and IM7/8552 carbon-epoxy laminates are measured using a scaled-down tensile test method. For CNT/BMI laminates, the variation in the measured tensile modulus is high and the laminates fail in a more gradual manner than IM7/8552 laminates. Microscopy of the failed specimens indicates that intra-yarn splitting is a common feature in all CNT/BMI laminates tested. The results of this investigation will inform the development of CNT yarn reinforced composites for structural applications.

Experimental Study on the Optimization of the Autoclave Curing Cycle for the Enhancement of the Mechanical Properties of Prepreg Carbon-Epoxy Laminates.

In this study, the influence of the technological parameters of autoclave curing on the resulting mechanical properties of laminates was investigated. The main criterion for optimizing the curing was to extend the processing window with a lower prepreg viscosity. At the same time, the issue of setting the pressure level before the heat ramp to the final cure temperature was also addressed. An experimental method of measuring the indentation viscosity of the prepreg was used to determine the viscosity profile. Despite the experimental nature of the method, the reliability of this method for rapid approximate identification of the processing window of the prepreg was verified by the results of the study. Several laminates with the same ply orientation were produced using the selected cure cycles, from which test specimens were cut with a water jet and inspected by confocal microscopy. The mechanical properties of tension and flexure were measured within the individual curing cycles using tests according to ISO standards. The data reported demonstrate that the experimental method of optimizing the curing parameters has successfully increased the selected mechanical properties. The resulting mechanical properties of the laminates were enhanced by up to 20% compared to the non-optimized cure cycle. The influence of the type of cure cycle on the resulting thickness of the cured laminate was evaluated in this study.

Carbon-graphene-epoxy sandwich dielectric for improved dielectric response to humidity

A sandwich structure of carbon-epoxy laminate with graphene nanoplatelets is used for increasing the AC conductivity, dielectric constant and sensitivity towards moisture. The Quasi Fickian distribution of moisture absorption of epoxy is retained with the inclusion of carbon fibers and graphene nanoplatelets of 1 weight percent. The effect of relative humidity (RH) over a range of 30–90% on AC conductivity and capacitance has been investigated. Use of carbon fibers with graphene nanoplatelets significantly enhances the electrical conductivity and dielectric constant as compared to carbon-epoxy laminate without graphene nanoplatelets. The two parameters reveal linear dependence on humidity in the low frequency range of 100–1000 Hz. The AC conductivity increases at the rate of 2 nSm−1/% RH in laminate with carbon fiber and it increases to 50 nSm−1/%RH with the incorporation of graphene nanoplatelets. The increase in AC conductivity from 100 Hz to 1000 Hz is in the range of 0.1 to 0.2 μS/m with carbon fibers and it increases from 1 to 4 μS/m with the inclusion of graphenenanoplatelets. Increase is AC conductivity from 1 kHz to 10 MHz is by five orders of magnitude, with a steep increase around 5 MHz in both laminates. The incorporation of graphenenanoplatelets helps to increase the dielectric constant by 2.4–2.8 times as compared to epoxy with carbon fiber. The swelling in the laminates estimated after 96 h of exposure to 90% RH is observed to be less than 0.05%. The use of graphenenanoplatelets improves moisture sensitivity of the carbon-epoxy laminate.

Effect of flax fiber orientation in carbon-flax fiber composite on tensile and visco elastic behavior

This paper deals with the effect of Flax fiber orientations on the tensile and viscoelastic response of Carbon/Flax hybrid composite. The composite laminates were prepared by varying flax fiber orientations of [0°4F/0°1C]1, [+30°2F/0°1C/−30°2F]1, [+45°2F/0°1C/−45°2F]1, [+60°2F/0°1C/−60°2F]1, and [+75°2F/0°1C/−75°2F]1 using the Hot press compression molding technique. The tensile and DMA (Dynamic Mechanical Analyzer) tests have been done for evaluating strength, strain to failure, toughness and storage and loss modulus, and also the damping factor of the laminates. The maximum tensile strength of 262 MPa was observed for 0-Degree oriented Flax fiber Carbon epoxy laminates. However, a decrease in tensile strength was noticed as the Flax fiber orientation increased from 0-Degree to 75-Degree. The maximum strain of 7.5% was induced in the 30-Degree Flax fiber carbon composite. Additionally, it was noticed that the rotation of fibers during load applied is the main cause for inducing additional strain to failure of Flax fibers. Changes in the storage modulus, loss modulus, and, damping factor were also observed for different flax fiber orientations.

Post‐curing and fiber hybridization effects on mode‐II interlaminar fracture toughness of glass/carbon/epoxy composites

AbstractFailure by delamination is one of the primary concerns in laminated composites. The development and progression of the delamination results in the degradation of composite stiffness affecting its structural integrity and may ultimately result in composite structure failure. Therefore, knowledge of the evolution of delamination resistance and the factors which affect the delamination in composites is of utmost importance for the selection of materials. This study investigates the effect of fiber hybridization and post‐curing on mode‐II interlaminar fracture toughness (GIIC) of Glass/Carbon/Epoxy (GCE) composites. Carbon/Epoxy (CE), Glass/Epoxy (GE), and GCE composite laminates were molded employing the process of hand layup technique and post‐cured at different temperatures. GE laminates exhibited a superior load for delamination and deformation compared to CE and GCE laminates. The hybrid GCE laminates exhibited higher interlaminar fracture toughness compared to non‐hybrid GE and CE laminates, which can be attributed to the synergetic effect of Carbon and Glass fibers. The laminates post‐cured at 180°C exhibited higher fracture toughness in their corresponding groups. The delaminated surfaces of CE, GE, and GCE laminates were further examined under Scanning electron microscopy (SEM) to understand the effect of post‐curing and fiber hybridization on mode‐II interlaminar fracture toughness.

High velocity impact response of carbon/epoxy composite laminates at cryogenic temperatures

Composite laminates are subjected to cryogenic temperatures in exposed aircraft structures during flight or in cryogenic tanks. The combination of cryogenic temperatures and high velocity impacts represents a threat to their integrity. This work investigates the behaviour of carbon-epoxy laminates under high velocity impacts (from 70 to 500 m/s) at room and cryogenic (-150 °C) temperatures and under two different plate orientations with respect to the projectile direction. The damage pattern of impacted specimens at low temperature, revealed by C-scan and X-ray tomography, exhibits a higher density of fiber breaks and shear matrix cracks which do not translate to a larger projected damaged area. The experimental analysis through interlaminar shear strength tests and the calculation of ply thermal stresses exclude the association of this particular pattern to damage mechanisms induced during the temperature decrease.

Identification of temperature-dependent elastic and damping parameters of carbon–epoxy composite plates based on experimental modal data

High-strength composite materials are receiving increased attention within the aerospace and transportation industries. These materials, although light in weight, still impart high stiffness when compared to conventional structural materials, e.g., aluminum and steel. Fibers and matrix are the basic constituents of composite materials. During high-speed maneuvering of aircraft and high-speed trains, the composite materials are subjected to dynamic loads in changing temperatures. The dynamic behavior of composites strongly depends on the ambient thermal environment. Furthermore, during the in-situ operation, the quantification of real-time dynamic loads is a challenging task. Therefore, the experimental investigation of the dynamic behavior of several carbon-fiber epoxy laminated composite plates at different temperatures, namely 0°C, 25°C, 50°C, 75°C, 100°C, and 125°C, is carried out using operational modal analysis, to identify the modal characteristics of the structure, and the temperature-dependent modal data of the tested composite plates is given in the supplementary data. The temperature-dependent elastic and damping parameters of the carbon–epoxy laminate are estimated using a genetic algorithm-based parameter identification scheme for different sets of modal contribution. A combined experimental and numerical simulation procedure is implemented to estimate deterministic material parameters at different temperatures. To obtain the in-situ material parameters for a given operating frequency range, the modal contribution is selected such that the operating frequency of interest lies within the considered resonance modes. As an example of matrix-dominated elastic parameter, the shear modulus, has been found to degrade significantly with increasing temperature, and shown a strong correlation with temperature.

Load Eccentricity of Compressed Composite Z-Columns in Non-Linear State.

The study investigated short, thin-walled Z-shaped carbon-epoxy laminate columns. Z-columns were compressed while considering the eccentric force realized from the center of gravity of the column section. The study involved performing a nonlinear analysis of the structures with implemented geometric imperfections reflecting the first buckling modes. The nonlinear analysis was performed by using the Tsai-Wu criterion to determine the effort of the composite material. The computations were run until the critical parameter was reached in the Tsai-Wu criterion, allowing for a description of the failure initiation mechanism in the composite material. The first signs of damage to the composite material were determined by using the acoustic emission method. Based on the results, postcritical equilibrium paths of the numerical models were determined. The equilibrium paths were then compared with the experimental characteristics of real structures. The numerical results and experimental findings show a satisfactory agreement. The results confirmed that the numerical models were adequate for estimating the performance of composite structures in the postcritical range, depending on the amplitude of compressive load eccentricity. The research topic undertaken is important because the thin-walled structure design relates to actual loads which, in most cases, differ from the idealized theoretical load conditions.

WEAR RESISTANCE OF SELECTED ANTI-WEAR COATINGS USED IN MULTI-MATERIAL COMPOSITE HYDRAULIC CYLINDERS

The paper presents tribological tests of selected materials used as liners in a composite hydraulic cylinder. Three coating materials were used for the analysis: Sika F180 polyurethane, Huntsman Araldite LY1564 with Aradur 3487 amine hardener and Bezlona 1111 Super Metal. In addition, uncoated carbon epoxy laminate (CFRP) samples were used as reference material. The instantaneous course of the friction force, its maximum and average value, were the analysed parameters. Ball-on-disk tests were performed on a specialised tester that allows, among other things, to control the operating conditions (clamping force, operating temperature, speed, friction path) and measurement of the instantaneous value of the friction force. After these tests, the friction paths were observed using a Leica DCM8 contactless profilometer with an EPI 20x lens. The amount of energy dissipated due to friction was also determined. The conducted tests and comparative analyses allowed to conclude that the F180 material is the most desirable in terms of tribological properties.

The effect of load eccentricity on the compressed CFRP Z-shaped columns in the weak post-critical state

The study investigated short, thin-walled Z-shaped carbon-epoxy laminate columns. Two multi-layer composite lay-up configurations were tested [0/45/-45/90]s and [90/-45/45/0]s, each consisting of eight symmetric laminate layers. The Z-columns were loaded in compression, and the loading mode included the scenario of force application eccentric to the center of gravity of the column’s cross-section. The modeled boundary conditions corresponded to the ball-jointed support at the end sections of the column. The study set out to determine the effect of loading eccentricity on the buckling mode, the values of the buckling load (the perfect structure), and the critical force (the structure with initial imperfections). The critical force of the real structure was established using the approximation method, based on experimentally determined post-critical equilibrium paths of the structure. For the numerical finite element analysis (FEM), Abaqus software was employed. In the first step, the FEM solved the eigenvalue problem, which lead to establishing the mode of stability loss and the determination of the corresponding bifurcation load for the Z-column subjected to axial and eccentric loading. The second stage of analysis focused on the nonlinear behavior of the structure with initial imperfections. The critical force was derived from the results of approximation based on the numerical post-critical equilibrium paths. The experimental data then served as the base for the validation of the developed numerical models. The findings enabled determining the effect of the compressive load eccentricity on the buckling mode and the critical force values. The project/research was financed in the framework of the project Lublin University of Technology-Regional Excellence Initiative, funded by the Polish Ministry of Science and Higher Education (contract no. 030/RID/2018/19).

An experimental and numerical study of industrially representative wrinkles in carbon fibre composite laminates

Wrinkles (out-of-plane waviness) are a common carbon fibre composite manufacturing defect that causes significant reductions in the components’ mechanical properties. The substantial cost implications of empirical testing has led to industrial demands for reliable and representative wrinkle models. This paper presents a novel modelling method for laminates with wrinkle geometries measured from non-destructive testing and/or sample cross-sections. The resulting finite element modelling pre-processor offers a self-contained system of mesh generation, local material orientation calculation, material assignment, boundary condition selection and solver specification. The approach was validated by comparison with pre-existing analytical models and experimental data. Through the resulting rapid model generation, the influence of various physical wrinkle parameters were assessed at an unprecedented rate and sensitivity. For example, this highlighted the dominant dependence of wrinkle angle on the laminate’s compressive strength. Additionally, increases in wrinkle extent and reductions in amplitude decay rate were shown to be detrimental to mechanical properties.The model was then validated against an experimental study on industrially relevant carbon-epoxy laminate samples to understand the effect out-of-plane waviness on compressive properties. The model results showed good agreement to the experimental values (<15% error) and at least a 32% reduction in compressive strength.

Computational model for predicting the low-velocity impact resistance and tolerance of composite laminates

We present a numerical model for predicting the low velocity impact resistance and tolerance of multidirectional carbon fiber-reinforced composite laminates made of non-crimp fabric. A finite element model is developed wherein the heterogeneous plies are replaced by equivalent homogeneous orthotropic plies. The low-velocity impact of a carbon–epoxy laminate is investigated using an explicit finite element model. Intralaminar failure and damage is evaluated using the 3D Hashin failure criteria and a surface-based cohesive behavior is implemented to capture the delamination between the plies. Following the low velocity impact, the finite element model is subjected to axial compression to investigate the compressive residual strength after impact, which is a measure of damage tolerance. The computational model is used to investigate the impact resistance and tolerance of a 24-ply multidirectional symmetric laminate reinforced with carbon fiber non-crimp fabric. The low velocity impact response and the compressive residual strength after impact are validated with experimental data for different levels of impact energy. It is found that the computational model can predict the impact resistance and damage tolerance for impact energies up to 50 J.

Design considerations in the strengthening of composite lap joints using metal z-pins

Through-thickness reinforcements are an effective technique to improve the structural properties of bonded fibre-polymer composite lap joints. However, there is a lack of understanding of the effectiveness of this technique for internal step-lap joints. This study examines the strengthening effects of z-pins on different types of composite step-lap joints. Co-cured single, quadruple and multi-step lap joints made of carbon-epoxy laminate were reinforced with steel z-pins. The results reveal that z-pins were highly effective at increasing the ultimate strength (+20%) and elongation limit (+250%) of the single lap joint under tensile loading by suppressing catastrophic failure through plastic shear deformation following fracture of the co-bonded interface. However, the z-pins were less effective at improving the strength (+7.5%) and maximum strain (+18%) for the quadruple lap joint and did not provide any strengthening for the multi-step lap joint. The mutually exclusive relationship between the number of bonded interfaces and the z-pin strengthening efficacy is discussed.

Damage mechanisms in the CAI failure of thin z-pinned composite laminates

The results of an experimental investigation into the compression after impact (CAI) performance of thin z-pinned carbon-epoxy laminates are reported in this paper. Unpinned and z-pinned [02/902]S and [0/±45/90]S samples were impacted at energies varying between 2 J and 35 J and then subjected to compression loading until failure. The amount and nature of the damage induced by impact and the damage mechanisms leading to CAI failure were characterized by X-radiography and through visual observation of the sample surfaces. The study shows that the effect of z-pins may be beneficial or detrimental to the post-impact strength of the laminates depending on the energy of the impact. The sequence of the main damage mechanisms observed on the laminates during the compression tests is illustrated and discussed to clarify the role of z-pins in controlling the residual strength of the impacted laminates over the full range of examined impact energies.

Experimental analysis of the effects of wrinkles in the radius of curvature of L-shaped carbon-epoxy specimens on unfolding failure

An experimental study has been carried out to describe the evolution of the failure load relative to the maximum misalignment angle of curved carbon–epoxy laminates with out-of-plane wrinkle defects for a large range of misalignments. L-angle specimens with different levels of fibre wrinkling were manufactured and tested in four-point bending tests. Digital Image Correlation (DIC) and infrared thermography were used to monitor the tests. They allowed the stress concentrations induced by the wrinkles to be identified, together with their influences on the failure load of the sample. In most cases, increases in wrinkling levels led to decreases in failure loads.

Composite-to-metal joining using interference fit micropins

Interference fit micropinning is a novel joining method that involves inserting thin pins with a size interference into an array of ultra-thin holes to create high strength structural joints. An experimental and finite element (FE) modelling study demonstrates the high structural properties of composite-to-metal joints using interference fit micropins. The joint studied consisted of plates of carbon-epoxy laminate and aluminium alloy joined using an array of steel micropins (505 μm diameter) with a close interference (1%) to the holes. The interference fit micropins are effective at creating composite-to-metal joints with high specific ultimate strength and ductility. The capacity of micropins to provide composite-to-metal joints with high structural properties under lap shear and peel load conditions is demonstrated. The effect of the spacing and number of micropins on the joint properties is evaluated.

Effects of inter-ply mismatch angle on interlaminar properties and their influence in numerical simulations

In this study, the interlaminar shear strength and mode II fracture toughness of carbon-epoxy laminates with respect to different inter-ply mismatch angles are determined and shown to generally decrease as inter-ply mismatch angle increases. Despite this dependency, most numerical simulations of multi-directional laminates involving delamination adopt interlaminar properties measured from unidirectional laminates only. Using the derived angle-dependent shear strength and fracture toughness, computational simulations of short-beam shear and out-of-plane loading of multi-directional laminate plates were performed and compared with physical experiments. It is shown that delamination tends to form intensively at interfaces with larger mismatch angles where delamination resistance is lower. This characteristic is captured by assigning interlaminar properties corresponding to the inter-ply mismatch angles in the numerical simulation. The practice of applying interlaminar properties obtained from tests on unidirectional samples may lead to erroneous predictions of varying severity for the simulation of multi-directional laminates.

Cyclic Relaxation, Impact Properties and Fracture Toughness of Carbon and Glass Fiber Reinforced Composite Laminates.

In this paper, the mechanical properties of fiber-reinforced epoxy laminates are experimentally tested. The relaxation behavior of carbon and glass fiber composite laminates is investigated at room temperature. In addition, the impact strength under drop-weight loading is measured. The hand lay-up technique is used to fabricate composite laminates with woven 8-ply carbon and glass fiber reinforced epoxy. Tensile tests, cyclic relaxation tests and drop weight impacts are carried out on the carbon and glass fiber-reinforced epoxy laminates. The surface release energy GIC and the related fracture toughness KIC are important characteristic properties and are therefore measured experimentally using a standard test on centre-cracked specimens. The results show that carbon fiber-reinforced epoxy laminates with high tensile strength give high cyclic relaxation performance, better than the specimens with glass fiber composite laminates. This is due to the higher strength and stiffness of carbon fiber-reinforced epoxy with 600 MPa compared to glass fiber-reinforced epoxy with 200 MPa. While glass fibers show better impact behavior than carbon fibers at impact energies between 1.9 and 2.7 J, this is due to the large amount of epoxy resin in the case of glass fiber composite laminates, while the impact behavior is different at impact energies between 2.7 and 3.4 J. The fracture toughness KIC is measured to be 192 and 31 MPa √m and the surface energy GIC is measured to be 540.6 and 31.1 kJ/m2 for carbon and glass fiber-reinforced epoxy laminates, respectively.

Influence of Hole Localization on Local and Global Dynamic Response of Thin-Walled Laminated Cantilever Beam.

In this study, we discuss the effects of the diameter and position of a hole on the dynamic response of a thin-walled cantilever beam made of carbon-epoxy laminate. Eigen-frequencies and corresponding global and local eigen-modes were considered, where deformations of the beam wall were dominant, without significant deformation of the beam axis. The study was focused on the circumferentially uniform stiffness (CUS) beam configuration. The laminate layers were arranged as [90/15(3)/90/15(3)/90]T. The finite element method was employed for numerical tests, using the Abaqus software package. Moreover, a few numerical results of the structure’s behaviour, with and without a hole, were verified experimentally. The experimental eigen-frequencies and the corresponding modes were obtained using an experimental modal analysis, comprising the LMS system with modal hammer. We found that the size and location of the hole affected the eigen-frequencies and corresponding modes. Furthermore, even a small hole in a beam could significantly change the shape of its local modes. The numerical and experimental results were observed to have high qualitative compliance.