Unidirectional CFRP Research Articles

Estimation of Contact Resistivity in Lightning Protection Equipotential Bonding Joints of Wind Turbine Blades

Modern lightning protection systems for wind turbine blades with conducting structural elements, e.g., carbon fiber reinforced polymer (CFRP) spar caps, contain equipotential bonding joints to prevent sparking during strikes. Significant current levels are experienced through the joints and the characterization of the electrical contact at the bonding regions is essential for reliable protection. Therefore, this article aims to characterize the contact resistivity of several equipotential bonding joints. The proposed methodology first measures the total resistance of the samples, and then the bulk resistance of the conductive elements is computed using the finite-element method. The latter is required to predict the spreading effects in CFRP components due to the strong anisotropic nature of such materials. After that, the contact resistance is calculated by subtracting the predicted bulk resistances from the measured total resistances. The developed procedure was applied to three typical equipotential bonding materials: expanded copper foil (ECF), biaxial (BIAX) CFRP, and unidirectional (UD) CFRP. Both ECF and BIAX CFRP showed superior contact quality than the UD CFRP, with one to two orders of magnitude smaller contact resistivity.

Microscale viscoplastic analysis of unidirectional CFRP composites under the influence of curing process

Cure-induced residual stresses have a great impact on the mechanical behavior of carbon fiber reinforced polymer (CFRP) composites. This paper develops an integrated analytical framework to study the effect of the cure-induced residual stresses on the mechanical behavior of unidirectional IM7/8552 composites. During the curing process, evolution of the material properties of the resin matrix is captured using the cure hardening instantaneous linear elastic (CHILE) model. In addition to the mechanical strain, thermal and chemical shrinkage strains of the resin are taken into account in the determination of the residual stresses. By introduction of the predicted residual stresses into the representative volume element (RVE), the quasi-static and dynamic compression analyses are performed to demonstrate the local viscoplastic deformation of the matrix and the homogenized stress–strain response of the composites. Simulation results are in good agreement with the experimental data. The comparative analysis indicates that the cure-induced residual stresses lead to an earlier nonlinearity and lower compressive strength. Furthermore, to investigate the effect of the fiber distribution on the residual stresses and compressive response of the composites, a parametric analysis is performed, in which two parameters are considered, i.e., minimal inter-fiber distance and fiber volume fraction.

An integrated modeling of the curing process and transverse tensile damage of unidirectional CFRP composites

In this work, an integrated numerical approach for predicting the damage mechanism of unidirectional CFRP composites at the micro-scale level has been developed using a multi-scale modeling strategy, where the curing residual stresses induced in autoclave curing process is taken into account. This research frames of the curing residual stresses development and the transverse tensile damage development through finite element computation. The curing residual stresses of CFRP composites induced in the curing process are characterized by coupling thermal-chemical analysis at macro-scale level and thermal–mechanical at micro-scale level. Then, the obtained residual stresses are combined with the micro-scale representative volume element (RVE) to compute the transverse tensile damage of CFRP composites. The predicted homogenized stress–strain curve at micro-scale fits the experimental results well. The microscopic damage distribution caused by the introduction of curing residual stresses reveals the intrinsic mechanisms of CFRP composites from fabrication to service. In addition, the influence of curing process on the strength performance of CFRP composites are further studied using the proposed approach.

The influence of cut direction and process parameters in wire electrical discharge machining of carbon fibre\u2013reinforced plastic composites

The paper details a comprehensive experimental investigation on the influence of operating parameters and cut direction (parallel and perpendicular to fibre orientation) when wire electrical discharge machining (WEDM) unidirectional CFRP composites using zinc-coated brass wire (0.25-mm diameter). A Taguchi L18 fractional factorial orthogonal array considering four variable parameters including open voltage, ignition current and pulse-on time as well as pulse-off time was carried out for each cut direction. Results showed that a ~ 16% increase in maximum material removal rate was achieved when machining parallel to fibre direction (2.41 mm3/min) compared to cutting perpendicular to the fibres (~ 2.08 mm3/min), which was attributed to the higher electrical conductivity of the workpiece along the fibre length leading to greater discharge energies. This however resulted in relatively larger average kerf widths and poorer workpiece surface roughness (Sa) caused by the Joule heating effect. Workpieces machined parallel to fibre direction were generally free of any major edge defects, in contrast to severe delamination observed on both the top and bottom surfaces of specimens cut perpendicular to fibre orientation. High-magnification scanning electron microscopy of the machined surfaces revealed the presence of adhered resin material, fibre cracking, cavities/gaps in the matrix phase and interlayer cracks on most of the samples analysed, with damage severity dependent on the operating parameters and cut direction.

Linear-Nonlinear Stiffness Responses of Carbon Fiber-Reinforced Polymer Composite Materials and Structures: A Numerical Study.

The stiffness response or load-deformation/displacement behavior is the most important mechanical behavior that frequently being utilized for validation of the mathematical-physical models representing the mechanical behavior of solid objects in numerical method, compared to actual experimental data. This numerical study aims to investigate the linear-nonlinear stiffness behavior of carbon fiber-reinforced polymer (CFRP) composites at material and structural levels, and its dependency to the sets of individual/group elastic and damage model parameters. In this regard, a validated constitutive damage model, elastic-damage properties as reference data, and simulation process, that account for elastic, yielding, and damage evolution, are considered in the finite element model development process. The linear-nonlinear stiffness responses of four cases are examined, including a unidirectional CFRP composite laminate (material level) under tensile load, and also three multidirectional composite structures under flexural loads. The result indicated a direct dependency of the stiffness response at the material level to the elastic properties. However, the stiffness behavior of the composite structures depends both on the structural configuration, geometry, lay-ups as well as the mechanical properties of the CFRP composite. The value of maximum reaction force and displacement of the composite structures, as well as the nonlinear response of the structures are highly dependent not only to the mechanical properties, but also to the geometry and the configuration of the structures.

Delamination in carbon fiber epoxy DCB laminates with different stacking sequences: R-curve behavior and bridging traction-separation relation

R-curve and traction-separation relation are commonly used to characterize the fiber bridging, which is significant toughening mechanism in composite laminates. Current investigations are mainly focused on unidirectional laminates. However, the dependence of the R-curve and traction-separation relation on the stacking sequence is still lack of comprehensive studies. Here, the effect of stacking sequence on the R-curve and traction-separation relation in unidirectional and multidirectional CFRP DCB laminates with designed generic +θ°/−θ° and 0°/θ° interfaces were systematically investigated, and interpreted combining with the fractographical analysis of failed fracture surface. Experimental results showed the fiber bridging length, steady-state fracture toughness and maximum bridging stress were strongly influenced by the stacking sequence. However, there was no clear relationship between them and the stacking sequence. In addition, the maximum bridging stress increased with the flexural modulus while a reverse trend presented in the relation between the maximum COD at the end of the fiber bridging zone and the flexural modulus. The obtained traction-separation relations were finally integrated into a tri-linear cohesive zone model. Numerical results from this model agreed well with the test results, which illustrated that the proposed cohesive zone model was applicable for delamination modelling in composite laminates with the effect of fiber bridging.

Characterization of the cutting forces and friction behavior in machining UD-CFRP using slot milling test

The friction behavior in machining carbon fiber reinforced polymers (CFRPs) is associated with many responses relating to cutting forces and temperatures, such as machining quality and cutting tool longevity. In milling operation where the fiber cutting angle is constantly alternating with the rotation of the tool, it is quite difficult to establish the coefficient of friction compared to the case of homogeneous materials. By using slot-milling as an open tribosystem, this paper investigates the behavior of cutting forces in machining unidirectional CFRP using uncoated single flute milling tools with different rake angles (− 15, 0 and 15°) and estimates the coefficients of friction based on cutting force decomposition using a simplified two-region cutting model. The friction coefficient in slot milling of UD-CFRP was found to depend mainly on fiber cutting angle and mode of cutting, i.e., up milling vs. down milling. The peak friction coefficient occurred at fiber cutting angle of 115°. The effect of rake angle on the cutting forces was most profound in the shearing region of the cutting model, but has no significant effect on the frictional forces.

Micro-mechanics modeling of compressive strength and elastic modulus enhancements in unidirectional CFRP with aramid pulp micro/nano-fiber interlays

Compressive strength and elastic modulus of unidirectional carbon fiber reinforced polymer (UD-CFRP) have been enhanced experimentally by using soft micro-length Aramid pulp (AP) micro/nano-fiber interlays. This study presents a micro-mechanics model to prove theoretically such enhancements in the compressive strength and elastic modulus of UD-CFRP are possible. The micro-mechanics model recognizes the importance of in-situ formed graded interfacial region between carbon fiber plies generated from the random distribution and out-of-plane movements of micro-length AP micro/nano-fibers. The micro-mechanics model shows that even an areal density of only ~4 g/m2 of AP micro/nano-fibers (with the interlay thickness < 30 μm) is sufficient to generate the enhancements in both compressive strength and elastic modulus along the carbon fiber direction of UD-CFRP. The new micro-buckling or shear failure mechanism has been identified to be responsible for the compressive strength enhancement, instead of local delamination in plain UD-CFRP. The theoretical and experimental findings show interfacial microstructural designs at the carbon fiber ply interface can be critical to the bulk properties of CFRP.

Influence of extremely cold environmental conditions on interfacial fracture phenomenon of aerospace grade unidirectional composites

An investigation of interfacial fracture modelling of carbon fibre reinforced plastic (CFRP) laminates at the atmosphere similar to the cruise condition of an aircraft has been carried out in this article. The Mode I & Mode II fracture tests are carried out using specimen of aerospace grade composite (AS4/914) at−550C. Compliance based methods for estimation of crack growth have been adopted for monitoring during the fracture tests. A new set of thermally influenced interfacial fracture properties are evaluated for AS4/914 laminates. A modified traction – separation law has been derived for Mode-I loadings considering the influence of fibre bridging under influence of extremely cold environmental conditions. This law has been evaluated from the superposition of traditional bilinear law and derived bridging law. A significant reduction in interlaminar fracture toughness has been observed due to the fragile nature of the matrix under cold environment. Experimentally obtained interfacial fracture properties are implemented into the numerical formulation. Extended Isogeometric Analysis augmented with Cohesive Zone Modelling (XIGA-CZM) for unidirectional CFRP laminate has been enhanced and enriched with thermally influenced fracture toughness properties for further investigation. Results obtained from the present XIGA-CZM formulation shows a good agreement with experimentally obtained results. Scanning Electron Microscope (SEM) observation of fractured surfaces are carried out to investigate the nature of crack propagation in CFRP laminate at−550C.

Mechanical Deburring of Drilling-Induced Exit Burrs in Carbon Fibre Reinforced Polymer Composites

Carbon fibre reinforced polymer (CFRP) composites have excellent specific mechanical properties, which have contributed to the replacement of metallic structural components in high-tech sectors. However, the anisotropic and inhomogeneous properties of CFRPs render them difficult to cut. Burr is one of the main machining-induced macro-geometrical defects in CFRPs. Even though burr does not weaken the resultant strength of the composites (unlike delamination), its removal is time-consuming and costly. The main aim of the present paper is to investigate the efficiency of the mechanical deburring method. Deburring experiments were carried out on unidirectional CFRP, based on a full factorial experimental design using a special solid carbide cutting tool. The effects of feed and cutting speed were analysed using digital image processing and visual evaluation of high-resolution images. The experimental results show that the examined factors seem to have no significant effect on the results over the applied parameter range, because the exit burrs were successfully removed at each parameter setting. Furthermore, during the deburring process, the formation of a significant amount of chamfers was observed. Since the size of the chamfers depends on the size of delamination-induced material deformation and process control, it should be either compensated for or monitored in the future to develop a more reliable deburring process.

Geometric empirical modelling of forces influenced by the cutting edge microgeometry in orthogonal cutting of unidirectional CFRP

In optimising the cutting of carbon fibre reinforced plastics (CFRP), increasing emphasis is placed on the cutting edge microgeometry. A significant challenge lies in understanding the engagement conditions between tool and workpiece, influencing tool wear mechanisms and performance. This paper aims to provide a more in-depth insight into the influence of cutting parameters on process forces when applying ideally rounded cutting edges in orthogonal cutting. A geometric empirical process model is developed for calculating the cutting force and the passive force, considering the cutting edge radius, the uncut chip thickness, and the fibre cutting angle. As a result, the minimum uncut chip thickness was determined, and process forces were calculated.

Experimental and numerical failure analysis of horizontal axis water turbine carbon fiber-reinforced composite blade

High-performance composites are used in many applications due to their design flexibility, corrosion resistance, high strength-to-weight ratio, and many other excellent mechanical properties. In this study, the location and magnitude of failure initiation in horizontal axis water turbine carbon fiber reinforced polymer (CFRP) blades with different lay-up orientations were investigated. Unidirectional [0°]4 and cross-ply [0°/90°]S layups were selected to study the effect of the buildup direction on the failure of the composite water turbine blades. A finite element analysis (FEA) model was generated to examine the stresses along blade span under both flexural and hydrodynamic loads. Flexural destructive tests were conducted to validate the results obtained from the numerical simulations. In addition, a blade element momentum theory model was created to calculate the hydrodynamic forces acting along the span to determine the maximum loading radial location, which was used for the fixture design and FEA simulation input. Both unidirectional and cross-ply composite blades were tested for failure. There was a general agreement between the experiments and simulations, which validated the results. Moreover, FEA simulations were performed to apply the load to the samples with different pitch angles (−10°, −5°, 0°, 5°, and 10°). Even though the unidirectional CFRP composite blades showed higher strength at 0° pitch angle than the cross-ply blades, the strength of the unidirectional blade dropped significantly when the load was applied with different pitch angles other than 0°, while the strength of the cross-ply blades was less responsive to this pitch angle variation.

An Experimental Comparative Analysis of Twist Drilling, Helical Milling and Pilot Hole Machining for Large Diameters in CFRPs

Carbon fiber-reinforced polymer (CFRP) large diameter hole drilling is certifiably not a broadly examined theme on account of their non-homogeneity and anisotropic highlights. In the scope of aerospace industrial uses of this material, thousands of holes have to be machined for purposes of assembly. The quality of this machined hole is influenced adversely by matrix grid cratering, thermal damage, spalling, surface delamination, and material debasement or fiber pullout. Among these different deformities, delamination is considered the most severe for CFRP composite. The main objective of this work is to compare twist drilling, helical milling and pilot hole machining technologies concerning unidirectional CFRP composite. In addition, the force modules were also discussed. In the scope of this work, numerous machining experiments were conducted in unidirectional CFRP composite: herein the impact of the type of cutting tool and of process parameters on the quality of machined holes are analysed and discussed (diameter of holes, circularity error and characteristics of uncut fibres). In other to achieve a better hole quality, the Scanning electron microscope (SEM) analysis and evaluation of acquired data were conducted. Experimental results show holes machined using twist drilling method present the lowest-quality, with damages present at the tool entry and exit as compared to the other studied methods. This study intended to help in assisting technicians working on assembly large structural parts in aircraft or automobile industries in choosing the best hole machining procedure when this three are in consideration.

Investigations on tool clogging and surface integrity in ultrasonic vibration-assisted slot grinding of unidirectional CFRP

Carbon fibre-reinforced plastic (CFRP) composites have been widely used in the field of aerospace and transportation because of their excellent mechanical properties. However, excessive tool clogging commonly occurs because of low chip storage space, thereby leading to workpiece failure. The slot grinding experiments with and without ultrasonic vibration were performed by using abrasive tools with defined grain pattern to improve tool life. The mathematical model of the fibre cutting angle in ultrasonic vibration-assisted slot grinding (USG) was established. The influences of ultrasonic vibration on the grinding force, tool clogging and surface integrity of unidirectional CFPR were discussed quantitatively. Results show that the USG featured lower grinding force, shorter fibre chip and better surface integrity than the conventional slot grinding (CSG). In addition, the lowest and highest reduction rates of grinding forces are obtained by using the fibre orientation of 90° and 135°, respectively. The maximum fibre chip lengths produced in USG are approximately 78% and 66% lower than those in CSG with fibre orientations of 0° and 90°, respectively. Several adhesive chips are observed on the USG tool surface. The chip storage spaces in CSG are filled with adhesive chips. The USG has a positive influence on surface roughness and damage because of the changes in fibre cutting angle. Compared with CSG, the USG produces similar fibre fracture patterns, except at the fibre orientation of 0°. Furthermore, the surface defects in USG are lower than those in CSG.

Effect of strain rate on strength of unidirectional CFRP

An analysis of the anisotropy of failure stress envelopes for unidirectional carbon fiber reinforced plastic at different strain rates, including dynamic loading, is carried out. Based on the study of transverse compressive and share strengths, a peak of the failure stress envelopes is observed, which is described by using the parabolic function approximation. It is shown that with an increase in the strain rate, not only an increase in strength but also a shift of the peaks to the area of higher transverse compressive stresses are observed.

Numerical analysis of RC wall with opening and strengthened with CFRP

Abstract The present work simulates and analyzes numerically two one-way walls (OW) with a central unforeseen cut-out-opening, which were previously tested experimentally. The models are based in the Finite Element Method and consider nonlinear behavior of the concrete material, such as hardening/softening and fracture. The first wall model was analyzed without any type of strengthening whereas the second one was strengthened using externally bonded CFRP along the opening edges. The strengthening modeling process was discussed through three different approaches suitable for unidirectional CFRP wrap. Load results were presented with load-displacement and load-strain curves that evidenced the flexural behavior of these structures. The crack pattern and failure mode observed showed good agreement with reported literature. The CFRP strengthening presented a small influence in the load capacity for big opening size.

Fiber break evolution and fatigue life prediction of CFRP with random fiber distribution

A new method on fatigue life prediction of unidirectional (UD) carbon fiber reinforced plastic (CFRP) is proposed based on fatigue characteristic of single fiber and expansion of broken fiber clusters. Both of statistical fatigue strength of single fiber and non-uniform fiber distribution are considered to assess the fiber break evolution characteristic and the fatigue life of CFRP. A stress redistribution coefficient (SRC) model is proposed to calculate the magnitude of stress overload in intact fibres after nearby fibre break. The SRC depends on not only the distance between broken and intact fibers but also the number of fibers adjacent to the broken fiber. A flowchart is established to predict fatigue life of UD CFRP based on the proposed SRC model. The proposed SRC model predicts clear defection point on the curve of fiber breaking number and stiffness degradation as increase of loading cycles, which agrees well with experimental phenomena in available literatures.

Stochastic nonlinear analysis of unidirectional fiber composites using image-based microstructural uncertainty quantification

We present a data-driven nonlinear uncertainty quantification and propagation framework to study the microstructure-induced stochastic performance of unidirectional (UD) carbon fiber reinforced polymer (CFRP) composites. The proposed approach integrates (1) microscopic image characterization, (2) stochastic microstructure reconstruction, and (3) efficient multiscale finite element simulations enabled by self-consistent clustering (SCA) analysis. To model the complex microstructural variability, the proposed UQ methods take the non-Gaussian uncertainty sources into account through a distribution-free sampling approach leveraging nonparametric and asymptotic statistical tools. A hierarchical conditional sampling strategy enables the simultaneous sampling of multiple sources of uncertainties. Our approach provides insights into the impact of microstructural variabilities, which are shown to have an increasing impact on the nonlinear responses of UD CFRP parts under progressive compression loading and ultimately on the failure rate over time. We discover that before CFRP parts start to fail, a characteristic time period emerges with distinctive uncertainty distributions specific to the microstructure variability and the probability of failure. Identifying the failure time period is crucial to the reliability prediction, which is an essential component of CFRP design.

Residual strength prediction for unidirectional CFRP using a nonlinear viscoelastic constitutive equation considering entropy damage

We propose a viscoelastic/plastic constitutive equation for viscoelastic media based on irreversible thermodynamics and the viscoelastic theory. Uniaxial tensile tests on a TriA-X polyimide resin are performed at different temperatures and strain rates to determine the material parameters for the constitutive equation. In the numerical analysis, the FORTRAN programming language is used to apply the proposed equation to the matrix resin. Transverse tensile failure analysis is performed on the unidirectional carbon fiber reinforced plastic (CFRP) in constant strain rate condition after holding constant strain. In this condition, the failure strength is lower than the static tensile strength. The same tendency is observed in the three-point bending fracture test of the unidirectional CFRP. The proposed model expresses the accumulation of resin damage while holding a constant strain and predicts the residual strength and lifetime of the unidirectional CFRP.

An integrated computational materials engineering framework to analyze the failure behaviors of carbon fiber reinforced polymer composites for lightweight vehicle applications

A bottom-up multi-scale modeling approach is used to develop an Integrated Computational Materials Engineering (ICME) framework for carbon fiber reinforced polymer (CFRP) composites, which has the potential to reduce development to deployment lead time for structural applications in lightweight vehicles. In this work, we develop and integrate computational models comprising of four size scales to fully describe and characterize three types of CFRP composites. In detail, the properties of the interphase region are determined by an analytical gradient model and molecular dynamics analysis at the nano-scale, which is then incorporated into micro-scale unidirectional (UD) representative volume element (RVE) models to characterize the failure strengths and envelopes of UD CFRP composites. Then, the results are leveraged to propose an elasto-plastic-damage constitutive law for UD composites to study the fiber tows of woven composites as well as the chips of sheet molding compound (SMC) composites. Subsequently, the failure mechanisms and failure strengths of woven and SMC composites are predicted by the meso-scale RVE models. Finally, building upon the models and results from lower scales, we show that a homogenized macro-scale model can capture the mechanical performance of a hat-section-shaped part under four-point bending. Along with the model integration, we will also demonstrate that the computational results are in good agreement with experiments conducted at different scales. The present study illustrates the potential and significance of integrated multi-scale computational modeling tools that can virtually evaluate the performance of CFRP composites and provide design guidance for CFRP composites used in structural applications.