Carbon Fiber Reinforced Polymer Structures Research Articles

Advances in joining technologies for the innovation of 21st century lightweight aluminium-CFRP hybrid structures

In the twenty-first century, the application of carbon fiber reinforced polymer (CFRP) materials in the vehicle industry are growing rapidly due to lightweight, high specific strength, and elasticity. In the automobile and aerospace industries, CFRP needs to be joined with metals to build complete structures. The demand for hybrid structures has prompted research into the combination of CFRP and metals in manufacturing. Aluminium and CFRP structures combine the mechanical properties of aluminium with the superior physical and chemical properties of CFRP. However, joining dissimilar materials is often challenging to achieve. Various joining technologies are developed to produce hybrid joints of CFRP, and aluminium alloys include conventional adhesives, mechanical and thermal joining technologies. In this review article, an extensive review was carried out on the thermal joining technologies include laser welding, friction-based welding technologies, ultrasonic welding, and induction welding processes. The article primarily focused on the current knowledge and process development of these technologies in fabricating dissimilar aluminium and CFRP structures. Besides, according to Industry 4.0 requirements, additive manufacturing-based techniques to fabricate hybrid structures are presented. Finally, this article also addressed the various improvements for the future development of these joining technologies. Ultrasonic welding yields the maximum shear strength among the various hybrid joining technologies due to lower heat input. On the other hand, laser welding produces higher heat input, which deteriorates the mechanical performance of the hybrid joints. Surface pretreatments on material surfaces prior to joining showed a significant effect on joint shear strength. Surface modification using anodizing is considered an optimal method to improve wettability, increasing mechanical interlocking phenomena.

© ПНИПУ Numerical and experimental research of the evaluation of the mechanical behavior of carbon fiber reinforced polymerspecimens with an embedded Smart-layer

Currently, scientists have paid special attention to the possibility of creating so-called Smart-structures that are capable of real-time self-diagnostics and/or functional changes. Within the framework of this work, the possibility of creating structures capable of diagnosing the deformation state in real-time is considered. When creating such Smart-structures, fiber-optic, piezo-, and strain-gauge sensors are usually used as control sensors. The use of such sensors is due to a number of their advantages, such as small size, the ability to integrate into a single measuring network, ease of use, a wide range of measured values, the ability to transmit data over long distances. However, when such sensors are embedded into the structure, several difficulties arise in installation/dismantling or implementation due to the fragility of the fiber, the difficulty of locating the sensitive element, ensuring the integrity of the fiber-optic sensors at the exit from the polymer structure. The team of the Scientific and Educational Center of Aviation Composite Technologies, PNRPU, has developed a prototype of a Smart-layer based on 3D-printing technology, capable of solving the problems described above. However, the influence of such an embedding on the structure of structures made of carbon fiber reinforced polymer (CFRP) remains open. Thus, this work aims to carry out computational and experimental studies to assess the mechanical behavior of CFRP samples with an embedded Smart-layer. Within the framework of this work, the technology and scheme for manufacturing a Smart-layer with embedded fiber-optic sensors, as well as a scheme for laying a Smart-layer into CFRP samples, are described. Since the properties of molded plastic specimens differ from those of printed specimens, mechanical tensile tests of printed specimens were carried out. The obtained values of the strength limit and the modulus of elasticity were used to identify the mathematical model. Within the framework of the experimental part, mechanical tests for tension, compression, and interlayer shear of CFRP samples with an embedded Smart-layer were carried out. When analyzing the obtained experimental results, it was found that the maximum percentage deviation of the physical and mechanical characteristics (PMC) is no more than 15 %, which lies within the spread of the PMC of carbon fiber in a prepreg roll. As a result, we can conclude that the embedded of such Smart-layers does not significantly affect the PMC of the final CFRP structure. According to the results of mathematical modeling of the tensile strength of a CFRP sample with an embedded Smart-layer, it was found that the maximum value of normal stresses in the sample reaches 540,28 MPa, which is 1,46 % higher than the maximum value for the statistical ultimate strength of the sample. The maximum values of the Mises stress for the Smart-layer do not exceed the ultimate strength, while for the epoxy binder there is a significant excess of the ultimate strength, on the basis of which one can make an assumption about its destruction during deformation.

A Route to Sustainable Aviation: A Roadmap for the Realization of Aircraft Components With Electrical and Structural Multifunctionality

Increased electrification of aircraft power systems has been widely presented as a route toward meeting environmental and emissions targets for aircraft performance, via more-electric aircraft and future hybrid-electric aircraft concepts. In parallel, the superior mechanical performance of carbon fiber reinforced polymer (CFRP) has resulted in its increasing use for aircraft structures. The relatively low electrical conductivity of CFRP compared to traditional aluminum structures and copper conductors limits the use of structural CFRP structures as electrical elements, so separate systems are needed. This adds structural mass and volume to a system, negating some of the benefits of using CFRP. Closer integration of the composite structure and electrical power system (EPS), with an ultimate goal of achieving components with multifunctionality (combined thermal, electrical, and structural functionality), offers a route toward the light-weighting of these systems, thus supporting improvements in aircraft performance. This article presents a roadmap to achieve this multifunctionality, supported by the combination of introducing definitions for different levels of multifunctionality, associated design thresholds, and trades between the EPS and CFRP materials/structures. Existing multifunctional (MF) electrical–thermal–structural CFRP-based solutions are contextualized within this roadmap. This enables the realization of viable routes for developing MF systems for the strategic focus of future research efforts.

An experimental study of impulse-current-induced mechanical effects on laminated carbon fibre-reinforced polymer composites

Lightning strike damage has become a primary issue limiting the extensive application of carbon fibre-reinforced polymer (CFRP) composites in the aerospace field. A detailed understanding of the physical interactions involved in the lightning strike process is still lacking, especially regarding the lightning-induced mechanical effects that potentially contribute to fracture and ply delamination in CFRP structures. This paper presents research on shock wave pressure induced by impulse currents on CFRP laminates. A novel method that exploits a polyvinylidene fluoride (PVDF) piezoelectric sensor was developed to directly measure the pressure induced by arc channel expansion. The influences of discharge polarity, strike distance and surface condition on the shock wave pressure were investigated. The results showed that the shock wave pressure produced by negative discharge was greater than that produced by positive discharge, the difference between which was reasonably explained based on the polarity effect concept. Increases in current amplitude and strike distance both contributed to larger measured pressures. A woven copper mesh protection could diminish the shock wave pressure to some extent, while an additional insulating epoxy resin layer enhanced it. Analysis results demonstrated that the pressure imposed on the laminated samples was related to the energy injected into the arc channel. In addition, the energy consumed by vaporized materials at the interface between the arc root and sample was defined as another source of shock wave pressure. The presence of an insulating layer confined the diffusion of the plasma channel at the arc root, as confirmed by high-speed imaging observations.

Global and local area inspection methods in damage detection of carbon fiber composite structures

The paper presents multiple impact damage detections and characterization in a multi-layered carbon fiber reinforced polymer structure using a multi-step combination of the global area and local area damage detection methods. A global area analysis using laser Doppler vibrometer (LDV) with piezoelectric lead zirconate titanate (PZT) actuator-based guided waves (GW) excitation was used to detect and locate the damage. The GW full wavefield (FWF) mode conversion and root mean square (RMS) based LDV studies helped to visualize the impact crack damage (ICD) when tested at varying excitation frequencies. The experimental results were cross-verified numerically using the spectral element method (SEM). The implemented structural health monitoring (SHM) based sectorial elliptical code (SEC) reduced the overall calculation time in damage localization and accurately identified the damage from experimental and numerical data. An infrared thermography (IRT)-based crack identification algorithm was developed in pinpointing the ICD shape. The automatic method removes the influence of high heat sources and highlights the damage zones easily. The accuracy of the localization strategy was successfully verified with non-contact active IRT analysis. The crack size and damage severity were quantified using the ultraviolet radiation (UVR) method with a solution mixture based organic dye. The global area method allows detecting possible damage, validated and quantified with the non-destructive testing (NDT) techniques.

Direct-Write Piezoelectric Transducers on Carbon-Fiber-Reinforced Polymer Structures for Exciting and Receiving Guided Ultrasonic Waves.

Advancements in the structural health monitoring (SHM) technology of composite materials are of paramount importance for early detection of critical damage. In this work, direct-write transducers (DWTs) were designed for the excitation and reception of selective ultrasonic guided waves and fabricated by spraying 25- [Formula: see text]-thick piezoelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TRFE)] coating with a comb-shaped electrode on carbon fiber-reinforced polymer (CFRP) plates. The characteristics and performance of the ultrasonic DWTs were benchmarked with the state-of-the-art devices, discrete lead zirconate titanate (PZT) ceramic transducers surface-mounted on the same CFRP plates. The DWTs exhibited improved Lamb wave mode excitation (A0 or S0 mode) relative to the discrete PZT transducers. Moreover, high signal-to-noise ratio was obtained by effectively canceling other modes and enhancing the directivity with the periodic comb-shaped electrode design of the DWTs, despite the smaller signal amplitudes. The enhanced directivity overcompensates for lower amplitude attenuation, making DWT a good candidate for locally monitoring critical stress hot spot regions in the CFRP structure prone to early damage initiation. It is shown that pairing a DWT sensor with a discrete PZT actuator could further achieve balanced performance in both wave mode selection and signal amplitudes, making this combination really attractive for ultrasonic SHM.

The Mechanical Investigation of Filament-Wound CFRP Structures Subjected to Different Cooling Rates in Terms of Compressive Loading and Residual Stresses-An Experimental Approach.

Although cooling at ambient temperature is widely used and is said to be safe and convenient, faster cooling may have an influence not only on the time of the manufacturing process but also on the mechanical response, especially the residual stress. The study aimed to investigate the influence of the cooling rate after curing on the mechanical response of filament-wound thick-walled carbon fiber reinforced polymer (CFRP) rings. Three cooling rates were taking into consideration: cooling with the oven, at room temperature, and in the water at 20 °C. The splitting method was used to examine the residual strains. In the radial compression test, the mechanical response was investigated between the rings with different cooling regimes. The FEM analysis of the compression test in elastic range was also performed. Both the splitting method and the radial compression test showed no significant difference in the mechanical response of the CFRP rings. The presented results showed that the fast-cooling rate slightly decreases the mechanical performance of the filament-wound rings.

Numerical Investigation of Residual Stresses in Welded Thermoplastic CFRP Structures

Using thermoplastics as the matrix in carbon fiber-reinforced polymers (CFRP) offers the possibility to make use of welded joints, which results in weight savings compared to conventional joining methods using mechanical fasteners. In this paper, the resulting temperature distribution in the material due to resistance welding is investigated by transient finite element (FE) simulations. To examine the effects on the component structure, a numerical modeling approach is created, which allows determining the residual stresses caused by the welding process. It is shown that the area of the structure, especially near the joining zone, is highly affected by the process, especially in terms of residual stresses. In particular, the stresses perpendicular to the fiber direction show failure relevant values up to a maximum of 221 MPa, which might lead to the formation of microcracks in the matrix. In turn, that is assumed to be critical in terms of the fatigue of welded composite structures. Thus, the suggested modeling approach provides residual stresses that can be used to determine their effects on the strength, structural stability, and fatigue of such composite structures. In a subsequent step, these findings could play an important role in the design process of thermoplastic composite structures.

Multiparameter Approach for Damage Propagation Analysis in Fiber-Reinforced Polymer Composites

Assessing the damage evolution in carbon-fiber-reinforced polymer (CFRP) composites is an intricate task due to their complex mechanical responses. The acoustic emission technique (AE) is a non-destructive evaluation tool that is based on the recording of sound waves generated inside the material as a consequence of the presence of active defects. Proper analysis of the recorded waves can be used for monitoring the damage evolution in many materials, including composites. The acoustic track associated with the entire loading history of the sample or the structures is usually followed by using some descriptors, such as the amplitude of the sound waves and the number of counts. In this study, the acoustic emission in CFRP single-lap shear joints was monitored by using a multiparameter approach based on the contemporary analysis of multiple features, such as the absolute signal level (ASL), initiation frequency, and reverberation frequency, to understand whether a proper combination of them can be adopted for a more robust description of the damage propagation in CFRP structures. For selecting the best features, principal component analysis (PCA) was used. The selected features were classified into different clusters using fuzzy c-means (FCM) data clustering for analyzing the damage modes.

Health Monitoring of RAM-Coated Structures by Active Microwave Thermography

In this article, the application of active microwave thermography (AMT) is investigated as an efficient nondestructive technique (NDT) for health monitoring of structures coated with radio frequency absorbing materials (RAMs). The uniqueness of AMT for this particular application is the use of microwave excitation, which acts as a highly efficient thermal source due to the inherent absorptive electromagnetic properties of RAM-coated structures. In addition, the microwave-induced heating takes place subsurface within the absorbing material of an RAM-coated structure, as opposed to the sole surface heating of conventional thermography. Specifically, this work focuses on detection of delamination, a common defect in carbon fiber reinforced polymer (CFRP) structures, via AMT. To this end, a formulation and analysis method based on electromagnetic/thermal modeling is presented, which enables estimation of the output of an AMT inspection of an RAM-coated CFRP structure suffering from delamination. The effect of the structure’s constitutive properties, the microwave excitation parameters, and the delamination size and depth on its detectability is investigated. In addition, several simulated and experimental results are provided, which verifies the accuracy of the proposed approach and shows the high efficiency of AMT for detection of delamination in such structures.

Ultrasonic imaging of wrinkles in carbon-fiber-reinforce-polymer composites

<sec> Carbon-fiber-reinforced-polymer (CFRP) composites are widely used in aerospace, automobile and other industries because of their excellent performances. However, wrinkles will be formed during the manufacturing and service life process of composites, which is detrimental to the fatigue limit and tensile strength. Due to the fiber waviness, the presence of such defects leads to the acoustic energy difference in different beam directions during ultrasonic nondestructive testing. The directly obtained imaging amplitude cannot characterize the wrinkle properly. In order to solve this problem, the total focusing method (TFM) imaging combing the acoustic beam directivity function correction is presented. </sec><sec> Firstly, the longitudinal wave signal is excited by an ultrasonic phased array, from which the echo data is collected by full matrix capture (FMC) in both the healthy and defected samples prepared. Then, considering the difference in acoustic energy from different beam directions of each ultrasonic transmitting/receiving array element, a correction TFM is proposed. Finally, the echo signals are post processed while the results are compared with the optical images. </sec><sec> No wrinkle information can be indicated from the direct TFM imaging. In contrast, the wrinkles and layer-up information can be recovered from the corrected TFM imaging with an improved signal-to-noise ratio and resolution. Furthermore, owing to the directivity function, the near-surface noise can be effectively reduced and the near-surface fibrous layer information is restored. This method paves the way for the accurate, quantitative and rapid characterizing of wrinkles in real CFRP structures. </sec>

Modeling the thermo-mechanical deformations of machine tool structures in CFRP material adopting data-driven prediction schemes

The thermo-mechanical effects in machine tools (MTs) are represented by complex models since they may produce non-linear distortions overtime, impacting significantly on the machining accuracy. This paper aims to model the deformation of CFRP (Carbon-Fiber-Reinforced-Polymers) structures using data-driven schemes to predict and compensate the structural thermo-mechanical behavior. A novel study is presented to investigate the thermally-induced distortions of CFPR structural materials, selecting and positioning sensors, simulating and validating models to compensate the error in real-time. Anisotropic materials are becoming an effective solution to reduce structure mass and increase damping of a MT, nevertheless their physical complexity and the different thermal-coefficients at the interface with conventional materials may generate undesired effects, limiting the obtained advantages. The proposed strategy is based on the evaluation of a set of data-driven models simultaneously, identifying the most suitable solution and comparing finite element simulations with machine learning approach. The study is developed on a vertical axis frame made of CFRP material. The experimental validation is executed on a commercial 5-axis machine tool by varying the temperature conditions and evaluating the structural thermo-mechanical deformation effect on the Tool-Tip-Point (TTP) displacement. The thermo-mechanical behavior is measured by fiber Bragg grating (FBG) sensing technology embedded in the CFRP structure. Data-driven lab tests are evaluated in operational conditions during 36 h, considering: i) training-deployment periods (875 min interval), ii) typical machining stresses and iii) environmental perturbations. The final selected data-driven model is able to reduce the detected error lower than 10 μm range. In particular, the achieved results indicate a congruence between the TTP displacement measured and predicted with a residual error lower than 7.0 μm (Y-direction) using the ANN- multilayer perceptron algorithm.

Experimental and numerical study of hat shaped CFRP structures under quasi-static axial crushing

Carbon fiber reinforced polymers (CFRP) has been increasingly applied in automobile industry for vehicle body lightweight and safety performance improvement. However, design of CFRP components especially for crushing structures is still highly ambiguous. The present study aims to study the deformation behaviour and energy absorption of the hat shaped CFRP structures and optimize the section shape. Two types of hat shaped CFRP structures with various thicknesses and ply orientation were tested under axial quasi-static crushing. The results show that the Type II hat shaped structure presents a stable progressive crushing mode and better energy absorbing ability as compared with the Type I hat shaped structure. Then, a finite element model was developed using the multi-layer shell element method, and was validated by the axial crushing test results. Finally, the section shape of the Type II CFRP structure was optimized through the surrogate model of radial basis function and global response surface method, and the influences of the section shape on crushing behaviours and energy absorbing abilities were analysed.

Defect Depth Determination in Laser Infrared Thermography Based on LSTM-RNN

Carbon fiber reinforced polymer (CFRP) has been increasingly used in aviation industry since it significantly enhances the performance of aircraft. However, imperfections inside the CFRP structures pose a threat to aviation safety. Apart from the defect shape and size, flaw depth is crucial to assess the defect severity. In this work, we utilize a laser infrared thermography (LIT) system to inspect an aviation CFRP sheet and adopt a long-short term memory recurrent neural network (LSTM-RNN) to determine the defect depth. Thermographic sequences obtained by LIT are processed using thermographic signal reconstructions (TSR) method. Raw data and TSR processed data are separately used to train and test the LSTM-RNN. Results show that background noises in the original thermal signals can be effectively reduced by the TSR method, which is helpful for the models to learn the signal characteristics. Compared with two traditional methods, recurrent neural network (RNN) and convolutional neural network (CNN), we find the LSTM-RNN outperforms.

Sensor based online quality monitoring system for detection of milling defects on CFRP structures

In the aviation industry, increasing research and applications of carbon fibres have led to a higher demand of carbon fibre reinforced polymer (CFRP) material. The moulding process of CFRP material is not precise enough to be directly moulded into its end form and an additional machining process is required. The machining of CFRP materials can result into generation of defects such as fibre overhang, cracking and delamination. Normally, the visual quality checks are carried out after machining is completed and hence, the damage at the end of the process is significantly more which can result into rework or rejection of the product. To solve this problem, an online quality monitoring system was developed in which the defects were measured, processed and identified online during the milling process. In order to achieve this, a line sensor was mounted behind the tool, which followed the tool and captured the data of the edge while milling. In the end, defects such as fibre overhang and delamination were detected simultaneously during the milling process.

Fabrication of the silver modified carbon nanotube film/carbon fiber reinforced polymer composite for the lightning strike protection application

Carbon fiber reinforced polymer (CFRP) composites with low density, corrosion resistance and excellent mechanical properties are widely applied in the aircraft industry, instead of the traditional metallic material. Their poor electrical conductivity leads to the vulnerability to the lightning strike (LS). In this paper, highly conductive silver modified carbon nanotube film (SMCNF) was developed via the electrophoretic deposition (EPD) method for protecting CFRP structures and components of the aircraft. Lightning strike protection (LSP) efficiency of the SMCNF/CFRP composite was characterized from the micro-scale to the macro-scale, and its possible protective mechanism was discussed. The compressive strength of the SMCNF/CFRP composite after the simulated LS test maintains 91.05% and is higher than that of the Cu mesh/CFRP composite (83.28%). Compared to commercial copper mesh LSP material, it can reduce the weight by 27.4% with the better residual compressive strength ratio. Therefore, highly conductive and lightweight SMCNF as a LSP layer can effectively reduce the damage of the CFRP matrix caused by the LS.

An application of non-ordinary state-based peridynamics theory in cutting process modelling of unidirectional carbon fiber reinforced polymer material

The usage of carbon-fiber-reinforced polymer (CFRP) composite keeps increasing in modern industries. However, because of the complexity of CFRP structure, the analysis for CFRP failure process, especially for machining process, is quite difficult. This paper presents a non-ordinary state-based peridynamic approach that can be applied to CFRP machining analysis. In order to describe the composite’s anisotropy, fiber bond and matrix bond is first introduced in this approach. For describing the failure characteristic of composite, the Maximum stress failure criteria and Hashin failure criteria is then incorporated into the state-based peridynamic approach. Finally, the contact state between tool and workpiece is detected by utilizing the penalty method in modeling the machining process. To demonstrate the effectiveness of the present method, tension and cutting experiments are conducted as well.

Research on Optimization Design of Shipborne Carbon Fiber Reinforced Polymer Frame Structure

Display consoles are the core frame structure of the command and control system on the military combat unit, which can be called the “brain” of the equipment. With the great development of new research on materials and applications, using Carbon Fiber Reinforced Polymer (CFRP) instead of aluminum alloys to fabricate the display console frame structure has become an international hot point. However, whether the components obtained by CFRP can meet the requirements of complex working conditions, and whether the structural design is reasonable, are still existing problems. This study analyzed the stress distribution of the bottom frame structure of CFRP display console under impact and vibration conditions. The frame structure was optimized based on the stress distribution and topology. The results show that the maximum stress value of the frame structure under complex conditions was 16.97MPa, the maximum stress after optimization was 22.94MPa. The reduction of weight of this complex structure was 13%. The prototypes have passed the actual working condition test. This study shows that the route method based on impact stress to optimize the wall thickness of CFRP structures is feasible and effective.

High-frequency viscoelasticities of CFRP structures by ultrasonic dynamic mechanical analysis

In this study, ultrasonic dynamic mechanical analysis (U-DMA) was proposed to directly measure high-frequency viscoelastic properties of carbon fiber-reinforced polymer (CFRP) structures. The representative CFRP structures (unidirectional laminates, cross ply, hybrid laminates, and sandwiches) for engineering applications were fabricated and their viscoelastic properties along the fiber and transverse directions were investigated. It is confirmed that U-DMA method enabled measurements of both basic mechanical properties and anisotropic viscoelasticity of practical CFRP structures in the MHz region. The mechanical properties in different directions of CFRP structures could be distinguished. The experiments were compared with the traditional DMA, tensile tests, and the rule of mixture, and reasonable results are confirmed. Furthermore, the temperature dependence of the high frequency viscoelasticity in the sandwich structure was investigated. Overall, the U-DMA could be used to directly measure basic mechanical properties and anisotropic viscoelasticities of various materials in high-frequency region of MHz.

Thermomechanical Coupling Damage and Analysis of Buckling Load of Laser Irradiated CFRP Cylinder Shell under Axial Load

This study aims to analyze the effect of thermomechanical coupling damage caused by laser irradiation on a Carbon Fiber Reinforced Polymer (CFRP) cylindrical shell with axial compression using finite element method (FEM). The quasi-static thermomechanical coupling damage process of a typical CFRP structure was studied intensively. First, a thermomechanical coupling model was established, then the influence of the size of the laser spot, the thickness of the shell and the different exposure location of the laser spot on the critical buckling load (CBL) were investigated. The results show that the value of CBL decreases near quadratically as the shell thickness decreases. The value of CBL decreases nonlinearly with increase of the spot size. The CBL decreases and then increases until the laser reaches the midpoint of cylinder axis as the laser moves along the axial direction of the cylinder; the CBL decreases dramatically as the laser deviates from the axis. DOI: http://dx.doi.org/10.5755/j01.mech.25.2.22340