Carbon Fiber Reinforced Plastics Structures Research Articles

Investigation of a crash concept for CFRP transport aircraft based on tension absorption

Transport aircraft made of carbon fibre reinforced plastics (CFRP) have to provide an equivalent crashworthiness compared to today's aluminium aircraft designs. However, CFRP structures typically show brittle failure behaviour under complex loading conditions and little energy absorption, whereas aluminium structures provide comparably high energy absorption due to their ductile failure characteristics. Improved crashworthiness for CFRP fuselages can be obtained by the installation of special crash devices, which are designed for energy absorption by progressive failure in compression, tension or bending. The realisation of crashworthy CFRP fuselage designs with the focus on compression or bending absorbers is often associated with significant mass penalty compared to the purely static sizing of the corresponding fuselage structure. In this context, an alternative crash kinematics was developed and numerically investigated in which most of the kinetic energy is dissipated by tension absorption in the sub-cargo area of a fuselage structure. The numerical study was performed on the basis of a purely vertical impact with a two-bay fuselage section using the explicit finite element (FE) solver Abaqus/Explicit. The simulation results show for the developed crash kinematics several advantages, e.g. reduced mass penalty, with the tension absorption concept compared to crash concepts that use energy absorption by progressive crushing in the sub-cargo area.

Statistical prediction of tensile creep failure time for unidirectional CFRP

The tensile strength along the longitudinal direction of unidirectional carbon fiber reinforced plastics (CFRP) is one of the important data for the reliable design of CFRP structures. This paper is concerned with the statistical prediction of creep failure time under the tension loading along the longitudinal direction of unidirectional CFRP based on the viscoelasticity of matrix resin. It was cleared in this study that the statistical creep failure time under the tension loading along the longitudinal direction of unidirectional CFRP can be predicted by using the statistical static tensile strengths of carbon mono-filament and unidirectional CFRP, and the viscoelasticity of matrix resin based on Christensen’s model of viscoelastic crack kinetics.

Validation of flash thermography using computed tomography for characterizing inhomogeneities and defects in CFRP structures

Active thermography is an efficient non-destructive testing method for investigating the internal structure of larger carbon fiber reinforced plastic (CFRP) components as well as smaller CFRP components in mass customization. The method can be applied contactless and automated. This study contains systematic investigations of CFRP structures with typical defects and inhomogeneities occurring during production by means of flash thermography in reflection and transmission configuration and by computed tomography (CT). The latter one was used as a reference method, since also very small defects at larger depth can be visualized with high spatial resolution. The CFRP structures consist of plates which contain metallic and non-metallic inclusions, contaminations with glue or wax rests, areas with inhomogeneous re-injection of dry parts, fiber misalignments, and fiber damages. Further on, two specimens have been glued together with different artificial inhomogeneities of the four glue beads. The results of the applied methods are compared and the advantages and disadvantages of each configuration are discussed based on the detectability of the inhomogeneities. It is shown that although CT has led to best contrasts and spatial resolutions in displaying the inhomogeneities and inclusions, flash thermography is very well suited to detect most of these structures. Considering that flash thermography can be applied on-site and has a high potential for automation and for a fast and efficient testing, it can be highly recommended for quality assurance during and after production of CFRP structures.

Experiments on Active Control of Counter-Rotating Open Rotor Interior Noise

The efficiency of future aircraft has to be increased because of the CO2 restrictions layed down by the European Union. Two key technologies to reach this ambitious goal are a consequent light-weight design of future aircraft and new engine concepts like the Counter-Rotating open rotors (CROR). However, the combination of lightweight materials like carbon-fibre-reinforced plastics (CFRP) with CROR is acoustically demanding because of the very high sound pressures emitted by this type of engine and the poor transmission loss of CFRP structures in the lower frequency range. Therefore, this work conducts a preliminary study to improve the transmission loss of a CFRP panel excited by a synthesised CROR pressure field in lower frequency range. As a first step, a typical aircraft fuselage panel mounted in a sound transmission loss facility is equipped with actuators and sensors to implement multiple-input multiple-output (MIMO) feedforward control of flexural vibration. The CFRP panel is excited via a CROR pressure field synthesised by a 112-channel loud speaker array. The active vibration control (AVC) system is realised by accelerometers and inertial exciters. A considerable vibration reduction of the flexural vibration on the accelerometers is achieved. The local attenuation around the accelerometers leads to a new controlled vibration pattern that radiates sound in a different way than the uncontrolled one. The difficulties in reducing the radiated sound power through the AVC system are due to low observability, the “pinning” effect, and the restructured vibration patterns. All of these effects are studied in detail through surface vibration scans and sound intensity measurements. Additionally, the radiation resistance matrix is used to analyse the controlled vibration patterns.

Monitoring of compression-after-impact testing of CFRP laminates subjected to simulated lightning damage by acoustic emission

The progress of damage during compression-after-impact (CAI) tests of carbon-fibre reinforced plastics (CFRP) laminates made of nanoparticles-coated carbon fibres and subjected to a simulated lightning strike was monitored by acoustic emission (AE). The effects of nanoparticles coating and the degree of damage induced by the simulated lightning strikes on AE activities were examined, with the relationship between the compressive residual strength and AE behaviour being evaluated in terms of AE event counts and the onset of AE activities during the compressive loading. AE monitoring appeared to be useful to assess the degree of impact-damage hence the structural integrity of laminated CFRP structures damaged by lightning strikes.

Characterization of an Eigenfrequency Adaptable Machine Tool Carriage

Current machine tools feature fixed eigenfrequencies which are often excited by high performance cutting operations leading to unfavorable process conditions or poor workpiece quality, for example, due to chatter. Approaches to resolve this problem either constitute in changes of the process conditions, adaptable auxiliary mass dampers or vast and expensive changes within the machine tool structure. All of these approaches are either expensive, lead to a lower cutting ability or do not address the underlying problem.A novel approach to avoid chatter is described in this paper. It deals with a frequency adaptable machine tool carriage made out of hollow carbon fiber reinforced plastic (CFRP) profiles. This light structure enables chambers to be filled separately. This allows a tuning of the eigenfrequencies by pumping fluid into the chambers of the carriage reducing the eigenfrequencies. The CFRP structure has a high “added mass to component mass” ratio due to the high stiffness in relation to the density of the CFRP. Thus shifting the eigenfrequencies on a larger scale in comparison to the carriages of the same kind but made out of steel or cast iron.Within this paper, the approach as well as measurement results of the prototypical realization will be presented. The paper will address measurements of sloshing, the influence of added fluid mass regarding different filling levels and the influence of the control behavior of the feed axis. The paper shall conclude with a comparison of the measurements to a similar machine tool carriage made out of steel. In the end, an outlook on upcoming research topics and activities shall be given.

Laser Cutting of Carbon Fiber Reinforced Plastics – Investigation of Hazardous Process Emissions

Abstract Carbon fiber reinforced plastics (CFRP) show high potential for use in lightweight applications not only in aircraft design, but also in the automotive or wind energy industry. However, processing of CFRP is complex and expensive due to their outstanding mechanical properties. One possibility to manufacture CFRP structures flexibly at acceptable process speeds is high-power laser cutting. Though showing various advantages such as contactless energy transfer, this process is connected to potentially hazardous emission of respirable dust and organic gases. Moreover, the emitted particles may be fibrous, thus requiring particular attention. Here, a systematic analysis of the hazardous substances emitted during laser cutting of CFRP with thermoplastic and thermosetting matrix is presented. The objective is to evaluate emission rates for the total particulate and gaseous fractions as well as for different organic key components. Furthermore, the influence of the laser process conditions shall be assessed, and first proposals to handle the emissions adequately are made.

Experimental Study on Impact Damage in CFRPs

With the weight-reduction requirement for transportation tools, more and more carbon fiber reinforced plastics (CFRPs) will be applied. It is known that the plastics will have a brittle fracture under impact loads, and therefore, their resistance capability on impact damage is a crucial factor which restricts the application of CFRPs. The present paper studies the impact damage and residual stiffness of the CFRPs through experiments. The identical specimens were impacted repeatedly by pendulum with a specified constant energy, and then, their residual bending stiffness and strength were measured by three-point-bending tests. Finally, the relationship between the residual stiffness and impact numbers were established. The results will be used in damage-based stiffness predition for CFRPs material and structures.

Development of the Damage Detection Method for CFRP Structures Using Distributed BOCDA Optical Fiber Sensor

The Brillouin optical correlation domain analysis (BOCDA) technology is one of the distributed optical sensing technologies utilizing the Brillouin scattering phenomena. The authors are developing, verifying and validating this technology. They are also developing the installation technique of optical fiber sensors, evaluating the durability of the BOCDA system and manufacturing the new device compatible to avionics bays for size. Furthermore, the authors are also improving the monitoring ability for composite damages. This paper reports the developmental status of the BOCDA monitoring ability for composite damages such as debonding at adhesive joints and micro-damages at bolted joints. First, debonding detection tests were conducted using carbon fiber reinforced plastics (CFRP) stiffened or repaired panels. In this test, test plates with a sheet stringer or a repair patch were applied tension load, and expanding of debonding areas was monitored by measuring strain distribution changes near these areas. Second, bearing damage detection tests were conducted using CFRP bolted joint specimens. In this test, multi-fastener single-lap CFRP specimens were applied tension load, and occurrences of micro-damages near bolt holes was detected. Micro-damages were detected by monitoring shapes changes of the Brillouin Gain Spectrum, which is one of the BOCDA measurement results.

135 CFRPの雷撃損傷数値シミュレーション

Recently, the application of composite materials such as carbon fiber reinforced plastic (CFRP) is widely adopted to aircraft structures. Because of lower electrical and thermal conductivity of CFRP compared to metal materials, CFRP structure can be severely damaged by lightning without any protection systems. In this study, understanding the principal lightning damage mechanisms of CFRP, artificial lightning test with coupon specimen was conducted and the damage process was visualized with high-speed camera observation. The obtained highspeed-imaging revealed that ply-lift of surface layer along fiber direction was occurred subsequent to gasification reaction of epoxy resin expanded towards orthogonal to the surface fiber direction. In order to simulate the damage behavior of CFRP laminate, coupled thermal-electrical analysis was conducted. In order to simulate the early phase gasification reaction of epoxy resin, the analysis model considering electrical property change due to insulation breakdown of CFRP is newly applied. The analysis result with newly proposed analysis model can predict the temperature rising orthogonal to the surface fiber direction; the area is well correlated with resin deterioration area observed in the experiments. It is confirmed that the major factor of early phase gasification reaction is electrical property change associated with insulation break down of CFRP in thickness direction.

Fast Curing of Adhesives in the Field of CFRP

The interest in carbon fiber reinforced plastic (CFRP) structures does not only increase in the area of aircraft construction, but also in the automotive industry. Due to the two-dimensional load transfer adhesive technology offers a particularly high potential of joining structures in lightweight construction. The adhesive systems applied for joining relevant CFRP structures show an insufficient initial strength thus necessitating additional fixing of the components or an accelerated curing of the adhesive. Induction technology has proved to be the most adequate heating technique for fast curing of metal bonds since it achieves high heating rates and enables a drastic reduction of the curing times. Due to their electrical conductivity, CFRP can also be heated by induction. Well-founded studies have been published explaining the physical mechanism behind the generation of heat within CFRP by electromagnetic induction. However, the relation between material and process parameters and the resulting heat generation and temperature distribution in CFRP parts still need to be investigated since, contradictory statements can be found in the literature. A profound knowledge of these relations, though, is elementary in order to predict the heat generation which is necessary to design a safe, fast curing process and to avoid damages to the material. Thus, the fundamental relations existing between the material/process parameters, the heat generation, and the resulting joining characteristics will be verified and discussed.

Current Distribution Characteristics of CFRP Panels

CFRP (Carbon Fiber Reinforced Plastic) is widely used in the structures of aircraft, automobiles, wind turbines, and rockets because of its qualities of high mechanical strength, low weight, fatigue resistance, and dimensional stability. However, these structures are often at risk of being struck by lightning. When lightning strikes such structures and lightning current flows through the CFRP, it may be structurally damaged because of the impact of the lightning strike or by ignition between layers. If there are electronic systems near the CFRP, they may break down or malfunction because of the resulting electromagnetic disturbance. In fact, the generation mechanisms of these breakdowns and malfunctions depend on the current distribution in the CFRP. Hence, it is critical to clarify the current distribution in various kinds of CFRPs. In this study, two kinds of CFRP panels, one composed of quasi-isotropic lamination layers and the other composed of 0°/90° lamination layers of unidirectional CFRP prepregs, are used to investigate the dependence of the current distribution on the nature of the lamination layers. The current distribution measurements and simulations for CFRP panels are compared with those for a same-sized aluminum plate. The knowledge of these current distribution characteristics would be very useful for designing the CFRP structures of aircraft, automobiles, wing turbines, rockets, etc. in the future. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(6): 32–40, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11457

1809 引張荷重を受ける擬似等方積層CFRP単純重ね合せ接着継手の応力解析と強度(G04-2 複合材とサンドイッチ材,G04 材料力学部門)

Recently, carbon fiber reinforced plastic (CFRP) material has been used in structural element of transport vehicles. Besides, adhesive bonding is known as better method for joining the CFRP structures The purpose of our work was to examine the characteristics of strength for CFRP composite structures. We modeled single-lap adhesive joints composed of quasi-isotropic CFRP laminates having various lay-up pattern subjected to static tensile load to carry out finite element method analysis, and compared the analysis results of stress distribution at adhesive layer. In addition, failure load of various adhesive joint specimens were measured. As a result, it was found that the stress distributions on adhesive layer and failure load for joints were affected by each lay-up patterns of CFRP adherends.

Development of Composite Structures for Launch Vehicles

Ultimate weight reduction and low manufacturing cost are strongly required in the development of next-generation launch vehicles. It is one of effective methods for these purposes to change structural materials from conventional aluminum alloys to composites, such as CFRP (Carbon Fiber Reinforced Plastics). Under these situations, we developed a launch vehicle structure by using CFRP/honeycomb sandwich panels in order to achieve both weight and cost reduction. FEM (Finite Element Method) simulation was utilized to effectively carry out the structural design. Metallic structures are applied for the flanges (joining interfaces between the other structures) in general; however, we used CFRP structures also for the interface flanges for the weight reduction and manufacturing simplicity. The designed CFRP structure was fabricated and a static load test was conducted considering flight loads during actual launch. The strain distribution was obtained by FBG (Fiber Bragg Grating) sensors and three-dimensional displacement was measured by laser tracker as well as normal strain gauges in the limit load test. These measured data were compared with the FEM simulation results and they show good agreement. We will apply these manufacturing, testing, simulation and sensing techniques for the future launch vehicle developments.

Industrializing metrology—Machine vision integration in composites production

The production of carbon fibre-reinforced plastics (CFRP) is still characterized by manual work. This paper presents how metrology enables the capable automation through the integration of an in-process machine vision system (MVS). Based on Structure Tensor Method the anisotropic CFRP structure parameters, e.g. fibre orientation, are measured and provided as control variables for automation and quality assurance in closed control loops during the layer-wise CFRP build-up. A Bayesian decision theoretical model assures that the MVS integration achieves capable processes at lower costs than current productions. The MVS and its integration model are evaluated for the automatic braiding of CFRP structures.

J044034 電気抵抗変化法によるCFRP損傷検知に及ぼす温度変化の影響([J04403]知的材料・構造システム(3))

Carbon Fiber Reinforced Plastic (CFRP) laminates are weak in durability against impact load, and it yields internal damage like delamination cracks. The electrical resistance change method (ERCM) is useful for CFRP structures, because electrical conductivity of carbon fiber can be used as a sensor for damage detections. Many researchers detect various kinds of damage of CFRP laminates by using the ERCM. However, the method has unsolved issues such as an effect of temperature change and an effect of cyclically loading in service. This study experimentally deals with the effects on the ERCM. At first it was revealed that the relationship between temperature and electrical resistance change was almost linear. This means that temperature compensation for temperature distribution is possible using the linear relationship. Moreover, it was investigated that the temperature coefficient depends on a fatigue of CFRP. Finally, temperature change was observed in fatigue tests, therefore electrical resistance is formulated as constant temperature. As a result, the electrical resistance decreases with the increase of number of cycles.

A Fiber Optic Doppler Sensor and Its Application in Debonding Detection for Composite Structures

Debonding is one of the most important damage forms in fiber-reinforced composite structures. This work was devoted to the debonding damage detection of lap splice joints in carbon fiber reinforced plastic (CFRP) structures, which is based on guided ultrasonic wave signals captured by using fiber optic Doppler (FOD) sensor with spiral shape. Interferometers based on two types of laser sources, namely the He-Ne laser and the infrared semiconductor laser, are proposed and compared in this study for the purpose of measuring Doppler frequency shift of the FOD sensor. Locations of the FOD sensors are optimized based on mechanical characteristics of lap splice joint. The FOD sensors are subsequently used to detect the guided ultrasonic waves propagating in the CFRP structures. By taking advantage of signal processing approaches, features of the guided wave signals can be revealed. The results demonstrate that debonding in the lap splice joint results in arrival time delay of the first package in the guided wave signals, which can be the characteristic for debonding damage inspection and damage extent estimation.

Effect of Local Off-Axis Fibers Array on Tensile Properties of CFRP Laminae

CFRP (Carbon fiber reinforced plastics) exhibit superior mechanical properties, and are recently applied to large structural materials using the VaRTM (Vacuum assisted Resin Transfer Molding) method. In the vicinity of the resin inlet of this method, and at the joint of CFRP structures, it is pointed out that the disorder in fiber orientation often occurs. In this study, therefore, the effect of local off-axis fibers array on the tensile properties of unidirectional CFRP was basically investigated. An experimental device for making a local off-axis fibers array on a CFRP lamina was newly developed, and various unidirectional CFRP laminae were prepared by changing the fiber orientation angle and the size. The results show that the tensile strength of CFRP laminae with off-axis fibers array decreases with increasing the fiber orientation angle and the length, and depends on the degree of fiber bundle waving. On the other hand, Young's moduli of the CFRP lamnae show an almost constant value, which is relatively close to that of CFRP lamina without off-axis fibers array. Finally, Young's moduli and tensile strength of the CFRP laminae were analyzed from a proposed composite model with an off-axis fibers array, and its validity was discussed.

Characteristics of CFRP Structure of Bending Strength and Rigidity According to Stacking Orientation Angle

Carbon fiber reinforced plastics (CFRP) have high strength in comparison with dimensional stability, invariability of material property, high strength and rigidity, corrosion resistance. Therefore, CFRP is widely used in various fields including space and aviation industries, sports and leisure industries, and general structural members and parts. To achieve structural members of CFRP with high strength and rigidity, theoretical approaches is limited to isotropic material, and empirical studies have been conducted. As with other composite materials, CFRP shows different rigidity and strength due to difference in properties in fiber and matrix material. In this study, CFRP specimen with changing stack orientation angle was manufactured as general structural application, and bending strength and rigidity of the corresponding composite was measured. In addition, square aluminium tube reinforcement is applied to prevent the shortcoming of CFRP. Vacuum compressed CFRP via autoclave with square aluminium tube reinforcement material (Hybrid) was evaluated by changing of orientation angle.

J0405-1-2 CFRP積層板損傷モニタリング用電気抵抗変化法の適用における銅めっき一体電極(知的材料・構造システム(1) 複合材料関連)

For Carbon Fiber Reinforced Plastic (CFRP), electrical conductivity of carbon fiber can be used as a sensor for damage detection. The change of electrical resistance of the CFRP structure has been studied as a self-sensing system for monitoring damages. Todoroki et al. reported that the electrical resistance change method successfully monitored various damages like matrix cracking, fiber breakage and delamination in CFRP laminates. To apply the electrical resistance change method to practical structures, reliability of the electrodes is a key issue. This paper contains the proposal of "Lead-Wire Integrated Electrodes" and the investigation of the reliability of the proposed electrodes under cyclic load. The electrodes were bonded to CFRP surface by using copper plating after polishing of the specimen surface with sandpaper to remove the surface resin. Lead-Wire was put on the electrode and bonded to the electrode by using copper plating. This proposed electrode is no initial defect. The specimen for this experiment was made of unidirectional CFRP prepreg. Volume fraction of specimens used for the fatigue test was almost 65%. Fatigue tests were carried out using a servo-hydraulic testing system. As a result, the electrodes had a fatigue life at the same level as the conventional strain gages under cyclic load.