CFRP Structures Research Articles

Coupling continuous damage and debris fragmentation for energy absorption prediction by cfrp structures during crushing

Energy absorption during crushing is evaluated using a thermodynamic based continuum damage model inspired from the Matzenmiller–Lubliner–Taylors model. It was found that for crash-worthiness applications, it is necessary to couple the progressive ruin of the material to a representation of the matter openings and debris generation. Element kill technique (erosion) and/or cohesive elements are efficient but not predictive. A technique switching finite elements into discrete particles at rupture is used to create debris and accumulated mater during the crushing of the structure. Switching criteria are evaluated using the contribution of the different ruin modes in the damage evolution, energy absorption, and reaction force generation.

Non contact inspection of the fatigue damage state of carbon fiber reinforced polymer by optical surface roughness measurements

This work presents the evaluation of a new non-contact technique to assess the fatigue damage state of CFRP structures by measuring surface roughness parameters. Surface roughness and stiffness degradation have been measured in CFRP coupons cycled with constant amplitude loads, and a Pearson׳s correlation of 0.79 was obtained between both variables. Results suggest that changes on the surface roughness measured in strategic zones of components made of the evaluated CFRP, could be indicative of the level of damage due to fatigue loads. This methodology could be useful for other FRP due to similarities in the fatigue damage process.

Predicting process-induced distortions in composite manufacturing – A pheno-numerical simulation strategy

Process-induced distortions (PID) of high-performance CFRP structures lead to problems with dimensional control if the distortions are not accounted for by compensating the process tool accordingly. There is a need to predict PID via simulation in industry rather than relying on less reliable information such as past experience or measurements on prototype structures. PID are caused by many phenomena, which can make detailed simulation complex and time consuming. This paper presents three simplified techniques for prediction of PID with sufficient accuracy but that are much faster and cheaper to run than traditional complex process models. The three techniques use solid or shell finite elements and use inputs based on measured spring-in of simple L-angles made of the same material and process conditions as the larger scale structures to be simulated. Application of the three techniques to example structures show that the predictions agree well with each other and that they give good predictions compared to experimentally measured distortions of a CFRP box. Given their speed and simplicity they have the potential to be useful for predicting PID for simpler composite structures, particularly early on in the design cycle where quick approximate calculations are of great value to evaluate different design concepts.

Development of a laser shock adhesion test for the assessment of weak adhesive bonded CFRP structures

Adhesive bonding has a great potential for future lightweight high-loaded structures in the aeronautic industry. A prerequisite for such an application is that the bond quality of the adhesive joint can be assessed in a non-destructive way. However, the use of classical Non-Destructive Techniques (NDT) does not allow the evaluation of the adhesion strength of an adhesive bond yet. This paper presents an investigation made on weak composite bonds in order to develop a laser shock wave adhesion test. First, the procedure to produce controlled weak bonds is described. CFRP bonded samples are prepared in a specific way and characterized by ultrasonic techniques to assess the absence of any detectable defect. Then, for some of the samples, their bond strength is evaluated by mechanical destructive tests and other samples are loaded by various intensity lasers shocks. The obtained results help to understand the behavior of the composite bonds under laser shock loading, thanks to two post-mortem techniques. The correlation between the laser parameters and the induced damage is demonstrated. The potential of the laser shock technique to discriminate different bond qualities is shown, and the need for the test optimization is discussed.

Monitoring of water absorption in CFRP laminates using embedded fiber Bragg grating sensors

Practical monitoring technologies are needed for evaluating water absorption/desorption in actual CFRP structures. In this study, the monitoring of water absorption in laminated CFRPs was demonstrated by a simple evaluation using fiber Bragg grating (FBG) sensors. The swelling strain and coefficient of moisture expansion (CME) of unidirectional (UD) laminates were estimated from the axial strain of FBGs. The measured swelling strains of cross-ply laminates agreed with that obtained from transient swelling analysis using the results of UD laminates. The swelling analysis showed that the change in the axial strain of FBG represents the swelling strain of the laminated CFRP. Moreover, another evaluation method employing changes in the FBG spectrum was proposed, and noted its possibility for laminated CFRP with almost 0 in-plane swelling strain. The present results suggested that the FBG measurement technique is useful for monitoring of water absorption in CFRP.

Failure of CFRP airframe sandwich panels under crash-relevant loading conditions

New aircraft fuselage concepts have to prove equivalent crashworthiness standards compared to conventional metallic fuselages for certification. Brittle failure mechanisms of CFRP structures make the verification of equivalent crashworthiness for novel CFRP fuselage concepts challenging since conventional metal fuselages absorb a significant part of the kinetic energy by plasticization. In this context, the damage initiation and failure of twin-walled fuselage panels were investigated under crash relevant bending–compression loads. Since the sandwich failure is initiated by core failure, a trigger concept for CFRP composite sandwich panels was developed based on local modifications in the fold pattern of the core for controlled failure initiation. By locally adjusting the collapse strength of the core in normal direction, the failure position and failure load can be adapted according to the defined kinematic hinge requirements. The core trigger concept was validated in experiments with triggered and untriggered sandwich panels under identical loading conditions.

Investigation of laser pulse fatigue effect on unpainted and painted CFRP structures

Laser ultrasonic based nondestructive evaluation (NDE) techniques are being increasingly used in aerospace industries. Generally, the service lifetime for an aircraft could be more than 25years. Thus, the composite structures of the aircraft could be susceptible to laser pulse fatigue damage caused by the laser pulse energy in the long-term repetitive maintenance inspection. In this paper, the effect of repeat laser pulse scanning on the mechanical characteristics of unpainted and painted CFRP specimens (USN175BX Carbon UD prepreg, Bisphenol A epoxy resin) is investigated to verify the reliability regarding the use of laser ultrasonic scanning based NDE techniques on the inspection of the CFRP structure. A high-speed laser ultrasonic scanning system is setup to perform repeat scanning of 1300 times on both CFRP specimens at the five laser pulse energy levels using the 532-nm and 1064-nm Q-switched continuous wave lasers. Elastic modulus assessment based on the ultrasonic Lamb wave pitch–catch method is used and the surface condition of the scanned area is investigated by a microscope. As a result, the laser pulse fluences that is shown in this paper are suitable for the long-term repetitive maintenance inspection in unpainted and painted CFRP structure even if it demonstrates an embrittlement phenomenon similar to the modulus measurement resolution in the unpainted CFRP specimen. In addition, the laser pulse fluence for maximum signal-to-noise ratio without any damage is investigated in both unpainted and painted CFRP specimens. As a result, both 102.45mJ/cm2 in unpainted CFRP specimen and 51mJ/cm2 in painted CFRP specimen can be the laser pulse energy for the maximum signal-to-noise ratio without any damage.

Study of trimming damages of CFRP structures in function of the machining processes and their impact on the mechanical behavior

The main focus of this paper is to investigate the defects generated by different machining processes (namely burr tool machining, abrasive water jet machining ‘AWJM’ and abrasive diamond cutter ‘ADS’) and their impact on the mechanical behavior of CFRP in quasi-static (compression and inter-laminar shear) and tensile–tensile fatigue tests. The cutting conditions are selected so that different levels of degradation can be obtained. The machined surface is characterized using roughness measuring devices with and without contact and SEM observations. The results show that the defects generated during the trimming process with a cutting tool are fibers pull-out and resin degradation. These defects are mainly located in the layers with the fibers oriented at −45° and 90°. However, when using abrasive water jet and abrasive diamond processes, the defects generated have the form of streaks and are not dependent on the fiber orientation. Furthermore, the results of quasi-static tests performed on specimens machined by cutting tools show that AWJ specimens offer a better resistance in compression but the ADS samples offer higher inter-laminar-shear strengths. Moreover, the results of fatigue tests show that specimens machined with a burr tool offer higher endurance limit. Finally, it is concluded that the type and the mode of the mechanical loading (quasi-static fatigue) affect the mechanical response of CFRP and favor a given machining process.

Расчетно-экспериментальный анализ двух типов структур из углепластика для крупногабаритных ракетно-космических конструкций

Расчетно-экспериментальный анализ двух типов структур из углепластика для крупногабаритных ракетно-космических конструкций

Design, development and testing of multi-functional non-linear ultrasonic instrumentation for the detection of defects and damage in CFRP materials and structures

Abstract A multi-functional non-linear ultrasonic testing approach is presented for in situ and ex-situ detection of a variety of defects (e.g. micro-cracking, delamination and disbonding) induced by various damage mechanisms (stress, impact, heat) in CFRP materials and structures. Such multi-functionality is provided via programmable and re-configurable instrumentation that incorporates a wide range of non-linear ultrasonic testing regimes, including harmonic and overtone generation, inter-modulation product generation, resonant frequency shift and pulse-inversion techniques. The capabilities of this multi-functional approach to defect detection are demonstrated by examining CFRP samples subjected to various forms of damage, specifically stress, impact and heat induced damage. We show that the multi-functional non-linear approach is well-suited to the detection of such forms of damage and that the pulse-inversion technique, largely ‘ignored’ in the CFRP literature, potentially provides a powerful, but as yet un-tapped, simple and effective route to the defect and damage detection.

Current distribution characteristics of CFRP panels

AbstractCFRP (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

CHIEF EDITORS' NOTE

IJSSD has entered into its thirteenth year. As at 31 December 2012, 12 volumes comprising 52 issues and containing 482 technical and discussion papers were published. Topics covered in these papers range from structural stability and dynamics of structural elements to complex mega structures to nanostructures. With increasing pressure from researchers to publish their ndings in IJSSD promptly, we are pleased to announce that the number of issues will be increased to 8 with e®ect from this year 2013. In line with our goal to publish high quality papers in IJSSD and for the journal to enjoy a high impact factor, we have decided this year to introduce the IJSSD Best Cited Paper Award 2013. This award is presented to authors whose paper has received the most citations over the last ve years (i.e. from 2008 to 2012) from publications captured in the Web of Science. Based on this criterion, we are pleased to present this award to R. Degenhardt, A. Kling, H. Klein, W. Hillger, H.C. Goetting, R. Zimmermann, K. Rohwer and A. Gleiter. Their paper is entitled \Experiments on buckling and postbuckling of thin-walled CFRP structures using advanced measurement systems, and it appeared in Volume 7, Issue 2, pp. 337 358, June 2007. This award, in the form of a certi cate, will recognize the signi cant contribution to the journal by these authors. We hope to receive the continuing support of the IJSSD editorial board members and readers in contributing high quality papers, providing critical review reports on the manuscripts sent to them for review, and in promoting the journal to their institution libraries and colleagues around the world.

An agent-based concept for planning and control of the production of carbon fibre reinforced plastics aircraft structures with mobile production units

This article is concerned with the control of the production of aircraft primary structures made of carbon fibre reinforced plastics. To increase the parallelization of the production, a scenario that implies new design paradigms is discussed. In order to realize this parallelization, mobile production entities work concurrently at the same primary structure of the fuselage. The present article proposes a software architecture for the control of such a production system. It is proposed that components are organized hierarchically. In enhancement to previous proposals, two perspectives of hierarchy are used here: grouping by functionality and grouping by timing context. The core of the architecture is a market-based multi-agent system, where the agents may operate in multiple timing contexts. Such a design yields advantages in terms of fast integration of new functionalities and scale-up of the production system. In brief, this article introduces a top-level control architecture for parallelized production of large reliable CFRP structures in a scalable production system.

In situ thermal diffusivity determination of anisotropic composite structures: Transverse diffusivity measurement

An in situ transient method has been developed for determining transverse thermal diffusivity (αz) of anisotropic composite shells under close to ambient conditions. The problems posed by anisotropy, large thicknesses, linear losses due to convection and radiation and unavoidable lateral conduction loss due to partial heating of large structures have been addressed. The basic approach proposed here is to select appropriate heater size, so that the characteristic time for in-plane conduction is always large compared to the record length (i) preferably of the entire experiment or (ii) at least of the rising part of the time–temperature curve recorded on the rear surface, following an appropriate square pulse excitation. This has ensured adherence to one dimensional heat propagation condition in spite of (i) anisotropy, (ii) partial heating of surface and (iii) long time of transit through thicker shells. Linear losses have been corrected by standard methods available in the literature. Finally, this in situ technique has been validated with simulation and experiments on cylindrical cfrp structures of 5–15mm thickness.

Damage Monitoring of CFRP Plate Using Self-Sensing TDR Method

Laminated CFRP structures are applied to many aerospace structures. Although CFRP laminates have high specific strength and specific stiffness, an impact load easily creates damage such as delamination cracking, matrix cracking, and fiber breakage. Damage to laminated CFRP is usually difficult to detect by visual inspection. As a result, damage monitoring systems are required for large laminated CFRP structures. Many researchers have already proposed self-sensing monitoring systems which measure electrical resistance changes in the laminated CFRP. This method, however, requires a lot of electrodes. In the present study, a new time domain reflectometry (TDR) method using an electrical pulse signal and reinforcement carbon fiber sensors is adopted as a monitoring method. The method is applied to a 2-m long CFRP strip plate specimen to detect an ideal damage mechanical notch using a reflected pulse signal. In addition, the effect of the orthotropic conductance of the CFRP plate is experimentally investigated using multiple electrodes, and is shown to be negligible.

Damage Monitoring of CFRP Plate Using Self-Sensing TDR Method

CFRP laminated structures are applied to many aerospace structures. Although the CFRP laminates have high specific strength and specific stiffness, an impact load easily creates damage of the CFRP laminates such as delamination cracking, matrix cracking and fiber breakages. The damages of the laminated CFRP are usually difficult to be detected by visual inspections. This requires a damage monitoring system for large laminated CFRP structures. Many researchers have already proposed self-sensing monitoring system using electrical resistance changes of the laminated CFRP. The method, however, requires a lot of electrodes. In the present study, Time Domain Reflectometry method is adopted using the reinforcement carbon fibers as sensors. The new self-sensing TDR method uses an electrical pulse signal and observes the reflected signal. The method is applied to a long CFRP strip plate specimen of 2 m length and an ideal damage of mechanical notch is detected from the reflected pulse signal. Effect of the orthotropic conductance of CFRP plate is experimentally investigated here using multiple electrodes. As a result, the effect of the orthotropic conductance is shown to be negligible.

Rapid Repair Concept for CFRP Structures Monitored by Structural Health Monitoring Systems

Rapid Repair Concept for CFRP Structures Monitored by Structural Health Monitoring Systems

On the effect of chain stay impact on the structural safety of CFRP structures in mountain biking

Carbon composites (CFRP) are commonly used in mountain bike frames allowing very high stiffness and strength at low weights. But CFRP is also sensitive to transversal impact loads, which damage the material from within and can affect the structural safety. A mountain bike frame suffers from different types of impacts, like crashes and stone impacts as well as contacts of the chain on the chain stay. In rough terrain or by passing obstacles, the chain starts oscillating vertically, on the slack span as well as on the tight span of the drive train. Oscillations with high amplitudes, caused for example by jumps, will lead to impacts of the chain on the chain stay. It is known that recurring impacts damage the surface of the chain stay, but there is only little knowledge if the impact energy leads to considerable damage inside the material, affecting the structures integrity. To analyze the occurring energy levels and to investigate the effect of chain stay impact on the structural safety, a validated multi body model of a Shimano XT drive train was developed in Simpack. The model is activated by vertical excitation based on acceleration measurements in field. This allows the assessment of the energy levels of the impact events as a function of excitation (e.g. track consistence) and drive train parameters (masses, moments of inertia, geometrical dimensions).

A modelling approach for virtual development of wave based SHM systems

Within the last decade many SHM approaches utilizing elastic waves have been developed. In case of thin-walled structures especially guided waves - excited and sensed by surface-bonded or integrated piezoelectric elements - have shown a high potential. Despite of considerable efforts, some fundamental questions are still to be tackled for industrial application. One way of improving this situation and gaining a deeper insight into wave propagation and wave scattering phenomena is a detailed modelling of the systems. Very promising for this purpose seems to be the spectral element method.This contribution presents a modelling approach for flat shell CFRP structures based on this methodology. Special attention is paid to the incorporation of delamination type damage. The delaminated area is modelled using separated upper and lower elements with conforming meshgrid. After introducing a contact formulation that preserves the high computational efficiency of the approach, the influence of contact on the local displacement field within the delaminated area is analysed. The simulation is validated by comparison to experimental data.

PS07 TDR法を用いたCFRP大型構造の損傷モニタリング(フェロー賞対象ポスターセッション)

CFRP laminated structures are applied to many aerospace structures. Although the CFRP laminates have high specific strength and specific stiffness, delamination cracks are easily created and the delamination cracks are usually difficult to be detected by visual inspections. Recently, high toughness CFRP laminates are adopted to primary structures. The tough CFRP yields small fiber breakages when delamination crack is made in many cases. This requires a detection system of fiber breakages at low cost for large laminated CFRP structures. In the present study, TDR method is adopted. TDR method uses an electrical pulse wave and observes the reflected wave. The method is applied to a 2m CFRP strip specimen and notches are detected from the reflected pulse wave. Besides, the propagation of the pulse wave is investigated by the electromagnetic simulation using FDTD method.