Multidirectional CFRP Research Articles

Fatigue life determination based on infrared thermographic data for MultiDirectional (MD) CFRP composite laminates

Infrared thermography is a useful tool for the rapid determination of fatigue properties. A previously proposed fatigue life model, which combines the stiffness degradation k(N) with thermographic data ΔTstab, has been successfully applied to UD and ±45° CFRP laminates. In view of more complex damage mechanisms in multidirectional laminates, the model is modified in this work to overcome the conservative predictions. The S-N curves determined by the modified model are compared with the experimental results of traditional fatigue tests on Quasi-Isotropic (QI) and cross-ply CFRP laminates. Very good consistency was observed. In particular, the S-N curve corresponding to fatigue life of Nfl=106 cycles is conservative compared to that from the traditional fatigue test results, but the maximum safety factors are no more than 1.15 and 1.08 for QI and cross-ply laminates, respectively. Both S-N curves corresponding to Nfl=107 cycles for two tested laminates lie within the 95% confidence interval of traditional experimental results. Therefore, based on infrared thermographic data, the modified model proposed in this study is more general. It allows to determine the S-N curves of QI and cross-ply CFRP laminates just in about ten hours of testing machine time instead of one month and a half using traditional fatigue tests, even though more experimental data (other types of MD stacking sequences, composite materials, under other fatigue modes, etc.) are necessary to demonstrate the applicability of modified model to all types of MD laminates.

A new method for determination of fatigue limit of composite laminates based on thermographic data

In this paper, a new fatigue limit determination method based on the thermographic data is proposed to overcome the limit of Risitano and Luong’s method. The maximum stress corresponding to peak value of the angle change normalized by its amplitude is defined as the fatigue limit. The method was evaluated by the experimental data of Multidirectional (MD) CFRP laminates in this study, and by the data found in the literature for different materials. The analyze of the results from different methods demonstrated that this method can effectively apply to determine the fatigue limit of composite laminates whatever their stacking sequence is.

Tool wear prediction in edge trimming of carbon fiber reinforced polymer using machine learning with instantaneous parameters

Rapid tool wear occurs during the edge trimming of CFRP components due to the high strength and abrasive nature of the carbon fibers, leading to potential surface damage and delamination. This paper presents a hybrid approach by combining the machine learning technique and the physics-based kinematics model for tool wear prediction in edge trimming of CFRP. For edge trimming of unidirectional (UD) CFRP, the feature parameters of the machine learning models consist of the instantaneous fiber cutting angle, uncut chip thickness, and instantaneous radial force instead of the average force of a certain cutting period. The training data from UD CFRP is used to predict the overall radial force in edge trimming of multidirectional (MD) CFRP considering the nonuniform wear length distribution along the tool edge due to different fiber orientation angles of the UD layers. The machined surface, tool edge profile, and chip morphology were examined to demonstrate the nonuniform tool wear length and its effect on surface quality in edge trimming of MD CFRP. The tool wear monitoring in edge trimming of MD CFRPs with various lay-up configurations is achieved without the need for data training experiments on MD CFRP. The predictions from the machine learning methods are compared with experimental results in edge trimming of UD and MD CFRPs. As a result, the proposed method is able to reduce the experimental cost, considering the increased dimensionality if experimental data training of various fiber orientations of UD CFRP and various fiber lay-up configurations of MD CFRP is required.

3D photothermal reconstruction of subsurface defects in notched multidirectional CFRP laminates due to tensile fatigue loading

This paper presents results from 3D photothermal reconstruction of real subsurface defects in carbon fiber-reinforced polymers using the virtual wave concept. We apply pulsed thermography during and after interrupted fatigue loading in tensile-tensile configuration for notched multidirectional laminates. In addition, we discuss the influence of the ambient conditions from in-situ photothermal testing on the virtual wave signal. The results obtained from 3D photothermal reconstruction are directly compared with those obtained from laser ultrasound testing. To summarize, we demonstrate a practicable and fast 3D photothermal evaluation of subsurface defect evolution utilizing the virtual wave concept.

The Machinability Characteristics of Multidirectional CFRP Composites Using High-Performance Wire EDM Electrodes

Due to the abrasive nature of the material, the conventional machining of CFRP composites is typically characterised by high mechanical forces and poor tool life, which can have a detrimental effect on workpiece surface quality, mechanical properties, dimensional accuracy, and, ultimately, functional performance. The present paper details an experimental investigation to assess the feasibility of wire electrical discharge machining (WEDM) as an alternative for cutting multidirectional CFRP composite laminates using high-performance wire electrodes. A full factorial experimental array comprising a total of 8 tests was employed to evaluate the effect of varying ignition current (3 and 5 A), pulse-off time (8 and 10 µs), and wire type (Topas Plus D and Compeed) on material removal rate (MRR), kerf width, workpiece surface roughness, and surface damage. The Compeed wire achieved a lower MRR of up to ~40% compared with the Topas wire when operating at comparable cutting parameters, despite having a higher electrical conductivity. Statistical investigation involving analysis of variance (ANOVA) showed that the pulse-off time was the only significant factor impacting the material removal rate, with a percentage contribution ratio of 67.76%. In terms of cut accuracy and surface quality, machining with the Compeed wire resulted in marginally wider kerfs (~8%) and a higher workpiece surface roughness (~11%) compared to the Topas wire, with maximum recorded values of 374.38 µm and 27.53 µm Sa, respectively. Micrographs from scanning electron microscopy revealed the presence of considerable fibre fragments, voids, and adhered re-solidified matrix material on the machined surfaces, which was likely due to the thermal nature of the WEDM process. The research demonstrated the viability of WEDM for cutting relatively thick (9 mm) multidirectional CFRP laminates without the need for employing conductive assistive electrodes. The advanced coated wire electrodes used in combination with higher ignition current and lower pulse-off time levels resulted in an increased MRR of up to ~15 mm3/min.

Heterogeneous ultrasonic time-of-flight distribution in multidirectional CFRP corner and its implementation into total focusing method imaging

A precise time-of-flight (TOF) of phased array ultrasonic is crucial to the high-quality total focusing method (TFM) imaging in the corner of multidirectional carbon fiber reinforced plastics, but quite challenging owing to the complex material and geometric factors. With a comprehensive consideration of the curved shape, laminated structure, and elastic anisotropy, we propose a high-fidelity finite element model of the corner based on the stiffness matrix analysis. A strategy in virtue of the maximum energy of A-scan signals, is further implemented to determine the TOF and compared with the exiting minimum time method. The resulted heterogeneous TOF distributions are input into TFM imaging, respectively. Results show that of an isotropic approximation presents a larger positioning error and a higher noise level of defects. In contrast, the anisotropic algorithms, especially for the maximum energy method, obviously improve the defect echo amplitude and suppress the sub-surface noise, demonstrating a superior performance in CFRP characterization.

Eddy-current array-probe technique for imaging near-surface and deep-lying delaminations in multidirectional carbon fiber composites

An eddy-current array-probe method was proposed to visualize delamination occurring within multidirectional carbon fiber-reinforced plastics. First, samples with a near-surface delamination were scanned by a coil array probe comprising a single driver coil and multiple pickup coils, and the detectability of the delamination was evaluated by the signal-to-noise ratio. An appropriate selection of the image phase and the distance between the driver and pickup coils enabled detection of the delamination suppressing the signal variation in the non-defective region. Furthermore, the experimental results suggested that the distance between the driver coil and the most sensitive pickup coil correlated with the delamination size. Second, the array-probe technique was used to detect a delamination located at various depths. The detectability of the delamination was found to be strongly affected by the rotation angle of the array probe. The optimum probe rotation angle depended on the depth of the delamination, indicating that a delamination occurring in deeper regions can be visualized by setting an appropriate probe rotation angle.

Rake angle effects on ultrasonic-assisted edge trimming of multidirectional CFRP laminates

Machining of CFRP composites is usually described as a challenging process from tool life and surface quality perspectives. To achieve a flawless machined surface at reasonable cutting tool life/cost, an optimum combination of cutting tool material and geometry is the key. The cutting tool angles are of importance as these dictate the cutting mechanism as well as the cutting edge robustness and the introduction of vibration to the machining process opens up new horizons of improvement and stirs further research questions. This article investigates the effect of tools with different geometry possessing different rake angles when used in ultrasonic-assisted edge trimming operation dealing with multidirectional CFRP laminates. A full-factorial experimental design was adopted to analyze the effect of process parameters typically cutting speed, feed rate, rake angle, amplitude, and their interactions. Machining performance indicators were captured which were the cutting forces, tool wear, chip temperature, and surface roughness. The results showed that UAM mode contributed to an increase in the cutting forces, tool wear, and chip temperature compared to the conventional mode. On the other hand, UAM mode improved the quality of the machined surface. Additionally, the ultrasonic mode enhanced the material removal mechanism using a tool with a negative rake angle.

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.

An Experimental and Numerical Investigation to Characterize an Aerospace Composite Material with Open-Hole Using Non-Destructive Techniques.

In this study, the open-hole quasi-static tensile and fatigue loading behavior of a multidirectional CFRP thick laminate, representative of laminates used in the aerospace industry, is studied. Non-destructive techniques such as infrared thermographic (IRT) and digital image correlation (DIC) are used to analyze the behavior of this material. We aim at characterizing the influence of the manufacturing defects and the stress concentrator through the temperature variation and strain distribution during fatigue and quasi-static tests. On the one hand, the fatigue specimens were tested in two main perpendicular directions of the laminate. The results revealed that manufacturing defects such as fiber waviness can have a major impact than open-hole stress concentrator on raising the material temperature and causing fracture. In addition, the number of plies with fibers oriented in the load direction can drastically reduce the temperature increment in the laminate. On the other hand, the quasi-static tensile tests showed that the strain distribution around the hole is able to predict the crack initiation and progression in the external plies. Finally, the experimental quasi-static tests were numerically simulated using the finite element method showing good agreement between the numerical and experimental results.

Heat partition in edge trimming of fiber reinforced polymer composites

Thermal loading of fiber reinforced composites during traditional machining is inevitable. This is due to the fact that most of the mechanical energy utilized in material removal is converted into heat, which is subsequently dissipated into the workpiece and the cutter, and is carried away by the chips. Heat conduction into the workpiece during machining might cause thermal damage due to matrix softening and decomposition if the generated temperatures exceeded the glass transition temperature of the epoxy resin. In this work, the amount of heat flux applied to the machined edge and the temperature distribution in multidirectional CFRP and GFRP composite laminates was determined using an iterative inverse heat conduction method. The transient heat conduction problems in the laminate and cutter were simulated independently using the finite element method and the amount of heat flux applied to each was determined. It was also found that the heat flux conducted to the workpiece represented only a small fraction of the total heat and is more influenced by the feed speed than the spindle speed. The temperature of the machined surface was estimated and correlations with the resulting machined surface texture were made.

A New Cutting Device Design to Study the Orthogonal Cutting of CFRP Laminates at Different Cutting Speeds.

Carbon Fiber-reinforced plastics (CFRPs) are widely used in the aerospace industry due to their highly mechanical properties and low density. Most of these materials are used in high-risk structures, where the damage caused by machining must be controlled and minimized. The optimization of these processes is still a challenge in the industry. In this work, a special cutting device, which allows for orthogonal cutting tests, with a linear displacement at a wide range of constant cutting speeds, has been developed by the authors. This paper describes the developed cutting device and its application to analyze the influence of tool geometry and cutting parameters on the material damage caused by the orthogonal cutting of a thick multidirectional CFRP laminate. The results show that a more robust geometry (higher cutting edge radius and lower rake angle) and higher feed cause an increase in the thrust force of a cutting tool, causing burrs and delamination damage. By reducing the cutting speed, the components with a higher machining force were also observed to have less surface integrity control.

Study of Interaction of Eddy Current Probes and Delamination in Multidirectional CFRP

This paper describes the physics of the interaction of the eddy current probe and the delamination in multidirectional carbon fiber reinforced polymer (CFDR). Currently, there is an opinion that eddy current signals obtained due to delamination in multidirectional CFRP are caused by the redistribution of the vertical eddy current arising from the excitation of eddy currents in the sample by a rectangular coil whose surface is orthogonal to the surface of the sample. The analysis performed by us shows that the possibility of creation and deformation of the vertical eddy current is very questionable, and that the observed eddy current signals associated with the delamination are most likely caused by carbon fiber plies misalignment towards the sample surface. In this paper was compare eddy-current signals from the delamination in CFRP in samples where the existence of vertical eddy current is possible in theory and in samples where this is not possible. The obtained signals had similar measurements, and this indirectly indicates the doubtfulness of the hypothesis of the delamination detection in CFRP samples due to vertical eddy current.

Dijkstra’s algorithm-based ray tracing method for total focusing method imaging of CFRP laminates

Accurate determination of ultrasonic travel time from a start point to an end point with known positions is fundamental to the application of total focusing method (TFM) to ultrasonic array imaging. However, this is particularly challenging for CFRP laminates whose wave propagation is complex due to material anisotropy and refractive ply interfaces. In this paper, a ray tracing method using Dijkstra’s path-finding algorithm is presented and used to calculate time delays of TFM. This modified TFM, termed ray tracing TFM, was demonstrated by post-processing full matrix capture (FMC) data from a 5.79-mm-thick multidirectional CFRP laminate containing three 1.5 mm diameter side-drilled holes (SDHs) at 3 mm depth. It shows that the ray tracing TFM enables the FMC signals to be accurately delayed and coherently summed to synthesize a focus at every image point. Consequently, peak amplitudes of the SDHs were increased by up to 49.74 dB and 2.69 dB with respect to those from paintbrush acquisition and the isotropic TFM with a constant velocity, respectively. Ray tracing results have also confirmed that use of a coupling layer is a simple way to minimize the anisotropic effect of CFRP and to reduce the structural noise from reflections at ply interfaces.

An approach to predict the mechanical properties of CFRP based on cross-scale simulation

The excellent mechanical properties of Carbon Fiber Reinforced Plastics (CFRP) have great influence on the deformation and failure of the components. The accurate prediction of the mechanical properties of CFRP is always a necessary but tough task, especially the transverse elastic modulus and shear modulus. This paper proposed an approach to predict the mechanical properties of CFRP based on cross-scale simulation. Firstly, the structural representative volume element (RVE) of unidirectional CFRP (UD-CFRP) is established in ABAQUS software at microscopic level considering periodic boundary conditions. The macroscopic mechanical properties of UD-CFRP are predicted by applying the different loading conditions for RVE in each direction. Secondly, based on the homogenization theory, the mesoscopic simplified RVE of multidirectional CFRP (MD-CFRP) is established according to the dimension of structural RVE. The mechanical properties of CFRP with the change of the angle and the different stacking sequence of UD-CFRP are also predicted through the mesoscopic simplified RVE. Finally, a series of experiments have been carried out, which showed that the results of the prediction approach and the experiments are in good agreements.

Drill-exit temperature characteristics in drilling of UD and MD CFRP composites based on infrared thermography

This paper presents a comprehensive study on the complex drill-exit temperature characteristics in the drilling of unidirectional (UD) and multidirectional (MD) CFRPs using a state-of-art microscopy infrared imaging system. For the first time, temperature variation and distribution at drill exit have been revealed in full detail, associated with the CFRP material properties and drilling conditions. Results suggest that the actual drill/CFRP interactions have critical but similar effects on the drill-exit temperatures for UD and MD CFRPs. Specifically, three distinct cutting regions with varying temperature characteristics are evident when the main cutting edge is acting on the drill exit material. In all cases, the temperature distribution features elliptical shape, of which the eccentricity depends on the lay-up sequence and the drilling depth. In addition, the real-time temperature profiles and 2D/3D maximum temperature distribution maps are created with high visualization. With the aid of those findings, the relationships between drilling temperature maxima, their locations and drilling depths have been discovered and temperature effects on drill-exit damages have been elucidated for the first time. MD CFRP is proven more difficult to achieve high drilling qualities at certain fiber cutting angles than UD CFRP due to the associated temperature effects. Such important knowledge enables the identification of the heat affected zones and subsequently informs strategies for reducing the negative temperature effects.

Open hole fatigue testing of laser machined MD-CFRPs

The effect of fiber laser machining (FLM) on multi-directional carbon fiber reinforced plastics (MD-CFRP) with [0/90/45/−45]s lay-up was studied under tension-tension fatigue with stress ratio R = 0.1 for an open hole fatigue (OHF) test configuration. The FLM parameters were chosen to yield a heat affected zone (HAZ) corresponding to a damage factor of approximately 1.4. The growth of this initial damage under tension-tension fatigue was investigated using digital image correlation (DIC), and damage growth characterization was carried out using scanning acoustic microscopy (SAM) and micro computed tomography (micro-CT). The endurance limit of the MD-CFRPs was evaluated by examining the fatigue life at 70, 50 and 30% of its ultimate tensile strength. Results show a strong correlation of damage mode on the applied load levels like conventionally machined samples.

Total focusing method imaging of multidirectional CFRP laminate with model-based time delay correction

Precise knowledge of ultrasonic travel time in carbon fiber reinforced plastics (CFRP) is difficult to obtain due to the layered anisotropic nature. This paper proposes a model-based method to calculate the travel time of qP wave in CFRP laminates. The aim is to correct the time delays of total focusing method (TFM) applied to post-process the full matrix capture data for enhanced ultrasonic imaging. Experiments show that a 4.84 mm delamination is accurately sized by corrected TFM. The associated detection sensitivity is 46.0 dB and 3.6 dB above those of paintbrush acquisition and basic TFM, respectively.

Cutting force model for machining of CFRP laminatewith diamond abrasive cutter

The article presents a cutting force model for trimming operations of CFRP laminate with diamond abrasive cutters. Those tools are more and more encountered on industrial applications of CFRP trimming, due to their abrasion resistance and their low cost. Contrary to endmills, they consist of a large number of cutting grits, randomly distributed around the tool. To tackle the issue, a continuous model of tool engagement is proposed. Validity of the approach is verified. A mechanical model of cutting forces, adapted to CFRP laminate, is then presented. The evolution of specific cutting coefficient in relation to fibres orientation is investigated through a piecewise constant model. It leads to the proposal of a sine model for the specific cutting coefficients. The simulated forces are in good agreement with the experimental results of cutting tests, carried out in multidirectional CFRP laminate for different fibres orientation and widths of cut. Cutting mechanisms are finally discussed depending on fibres orientation.

Effect of Cutting Fluid Delivery Method on Ultrasonic Assisted Edge Trimming of Multidirectional CFRP Composites at Different Machining Conditions

Edge trimming of CFRP composite laminates, using abrasive tools, is needed as a finishing operation for better quality and dimensional accuracy. Fiber delamination and resin burning are the major difficulties associated with traditional machining processes of CFRP. Although dry cutting is preferable for cutting CFRP which not absorb cutting fluid and eliminate the need for subsequent cleaning, increasing cutting temperature leads to degradation, smearing, and burning of epoxy resin. This paper investigates the effect of different fluid delivery methods (MQL/flood) on ultrasonic assisted machining (UAM) of multidirectional CFRP laminates using diamond abrasive tools at different cutting conditions. The process parameters studied were cutting speed, feed rate, radial depth of cut, and cooling modes. Cutting forces, surface integrity, and bouncing-back effect were the output responses of this study. Statistical analysis was conducted to study the effect of each factor on the performance characteristics. The results showed that flood coolant provide better results than MQL. However, MQL responses still comparable with flood coolant. The conditions that gave the best responses of MQL were determined which can be utilized for near dry cutting demands.