Carbon Fiber Reinforced Polymer Structures Research Articles

Optical sensing of the fatigue damage state of CFRP under realistic aeronautical load sequences.

We present an optical sensing methodology to estimate the fatigue damage state of structures made of carbon fiber reinforced polymer (CFRP), by measuring variations on the surface roughness. Variable amplitude loads (VAL), which represent realistic loads during aeronautical missions of fighter aircraft (FALSTAFF) have been applied to coupons until failure. Stiffness degradation and surface roughness variations have been measured during the life of the coupons obtaining a Pearson correlation of 0.75 between both variables. The data were compared with a previous study for Constant Amplitude Load (CAL) obtaining similar results. Conclusions suggest that the surface roughness measured in strategic zones is a useful technique for structural health monitoring of CFRP structures, and that it is independent of the type of load applied. Surface roughness can be measured in the field by optical techniques such as speckle, confocal perfilometers and interferometry, among others.

Micro-mechanics of failure for fatigue strength prediction of bolted joint structures of carbon fiber reinforced polymer composite

In this paper, the application of theory of micro-mechanics of failure (MMF) is extended to analyze the fatigue progressive failure and predict the strength for the bolted joint structures of carbon fiber reinforced polymer (CFRP) composites. The MMF approach includes the methods of material strength characterization and structure strength prediction for the CFRP structures. The structure strength prediction is realized by developing the user defined material (UMAT) subroutine with Fortran scripts on the ABAQUS platform. The failure mechanism evolution from initial failure to final failure of the bolted joint structures of the UTS50/E51 laminate under the static loading and the fatigue loading are investigated, respectively, in details by using the MMF approach. The experiments under both static and fatigue loading are performed for the bolted joint structures. The predicted strength accuracy of the MMF approach as well as the classical Tsai–Wu and Hashin theories is compared with the experimental results, which shows the MMF approach has the best accuracy. The predicted macro-scale failure appearances of the bolted joint structures under the static tensile loading and the fatigue tensile loading behave as shearing modes. The validation of predicted macro-scale failure appearances is studied by comparing with the tested results.

New analytical method for electric current and multiple delamination cracks for thin CFRP cross-ply laminates using equivalent electric conductance

Carbon fiber reinforced polymer (CFRP) composites have low inter-lamina strength. One of the monitoring technologies is a self-sensing method that uses the electrical resistance change of a CFRP structure for detecting damage. The electric current distribution is vital information for the self-sensing method when optimizing the arrangement of probes to measure electric potential changes. We have developed a new orthotropic electric potential function analysis approach using affine transformation for unidirectional CFRP. In this study, the orthotropic electric potential function analysis method is improved to calculate the electric current of a thin cross-ply CFRP. Two types of stacking sequences for the beam-type cross-ply laminates were calculated to confirm the effectiveness of the improved method. The electrical voltage changes caused by multiple delamination cracks of a cross-ply laminate are new outcomes of this study. The analytical results were compared with computed results using the finite difference method. Consequently, the new equivalent electric conductance method proved to be effective for calculations of the electric current density of a cross-ply CFRP laminate. Furthermore, the new method for calculating the electric potential difference changes caused by multiple delamination cracks using orthotropic distributed doublet analysis, with the equivalent electric conductance, has also proved to be effective.

Assessment of local strain field in adhesive layer of an unsymmetrically repaired CFRP panel using digital image correlation

The present study focuses on experimental investigation of through the thickness displacement and strain field in thin adhesive layer in single sided (unsymmetrical) patch repaired CFRP (carbon fiber reinforced polymer) panel under tensile load. Digital image correlation (DIC) technique is employed to acquire the displacement and strain (longitudinal, peel and shear) field. Experimental determination of shear transfer length based on shear strain field obtained from DIC is introduced to estimate the optimum overlap length which is an essential parameter in patch design for the repair of CFRP structures. Further, DIC experiment with magnified optics is performed to get an insight into complex and localized strain field over thin adhesive layer especially at critical zones leading to damage initiation. The failure mechanism, load displacement behavior, damage initiation and propagation are closely monitored using DIC. The influence of patch edge tapering on strain distribution in adhesive layer is also investigated. The DIC successfully captures the global and localized strain field at critical zones over thin adhesive layer and further helps in monitoring the damage based on strain anomalies. Strains are found to have maximum magnitude at the patch overlap edge and the shear strain level in adhesive layer is higher than the peel strain. Normal tapering increases the peel strain and has negligible influence on shear strain level in adhesive layer. The recommended overlap length is found to be consistent with the recommendation in the literature. Whole field strain pattern and the overlap length obtained from experiment are further compared with the finite element analysis results and they appear to be in good coherence.

Modelling of small CFRP aerostructure parts for X-ray imaging simulation

Purpose – The purpose of this paper is to develop a realistic computational model of carbon fibre reinforced polymer (CFRP) structures dedicated for in-silico investigations of the use of X-ray-based imaging techniques as non-destructive testing (NDT) of CFRP parts. Design/methodology/approach – CFRPs contain layers of carbon-fibres bundles within resin. Bundles’ orientation in the different layers is arranged with respect to each other at a well-defined primary direction. In the model, the bundle was simulated as a circular cylinder. The resulted model is a stack of layers of unidirectional bundles having orientation of 0°/90°/45°/−45°. Two CFRP structures were modelled: a flat CFRP part and a real shaped CFRP clip. A porous layer and non-carbon fibres were inserted within each model, respectively. X-ray projection images were generated with a dedicated simulation programme. Three setups were investigated: radiography, tomosynthesis and cone-beam CT (CBCT). Findings – Results showed that porosity and non-carbon fibres were visible with all X-ray-based techniques. Tomosynthesis and CBCT, however, provide higher quality image of defects. Practical implications – The CFRP computational model is a valuable tool in design, testing and optimization phase of X-ray-based imaging techniques for use in NDT of composite materials. Simulated images are generated within a short time; thus results from virtual optimization and testing are obtained very fast and at low cost. Originality/value – An innovative computational model of CFRP structures, dedicated for X-ray imaging simulations, has been developed. The model is characterized by simplicity in its creation and realistic visual appearance of the produced X-ray images.

An investigation of contact resistance between carbon fiber/epoxy composite laminate and printed silver electrode for damage monitoring

An addressable conducting network (ACN) enables the structural condition to be monitored by the electrical resistance between electrodes on surface of CFRP (carbon fiber reinforced polymer) structure. To improve the reliability of ACN for damage detection, the contact resistance between the electrodes and CFRP laminates needs to be minimized. In this paper, the silver nanoparticles electrodes were fabricated via printed electronics techniques on CFRP composite. The contact resistance between the silver electrodes and CFRP was measured with respect to various fabrication conditions such as the sintering temperature of silver nanoink and the surface roughness of CFRP laminates. The interfaces between silver electrode and carbon fibers were observed using scanning electron microscope (SEM). From the study, it was found that the lowest contact resistance of 0.3664Ω could be achieved when the sintering temperature of the silver nanoink and surface roughness were 120°C and 230nm, respectively.

Hybrid carbon fiber composite lattice truss structures

Carbon fiber reinforced polymer (CFRP) composite sandwich panels with hybrid foam filled CFRP pyramidal lattice cores have been assembled from linear carbon fiber braids and Divinycell H250 polymer foam trapezoids. These have been stitched to 3D woven carbon fiber face sheets and infused with an epoxy resin using a vacuum assisted resin transfer molding process. Sandwich panels with carbon fiber composite truss volumes of 1.5–17.5% of the core volume have been fabricated, and the through-thickness compressive strength and modulus measured, and compared with micromechanical models that establish the relationships between the mechanical properties of the core, its topology and the mechanical properties of the truss and foam. The through thickness modulus and strength of the hybrid cores is found to increase with increasing truss core volume fraction. However, the lattice strength saturates at high CFRP truss volume fraction as the proportion of the truss material contained in the nodes increases. The use of linear carbon fiber braids is shown to facilitate the simpler fabrication of hybrid CFRP structures compared to previously described approaches. Their specific strength, moduli and energy absorption is found to be comparable to those made by alternative approaches.

Performance studies on compressively loaded debonded composite splice joint

Carbon fiber-reinforced polymer structures are being increasingly used in aerospace applications. In this work, analytical, numerical, and experimental investigations are carried out to understand the structural response of debonded carbon fiber-reinforced polymer splice joint subjected to compression load. It is found that debonds are of major importance for the behavior of carbon fiber-reinforced polymer splice-joint structures. A series of carbon fiber-reinforced polymer splice-joint specimens are subjected to compressive loading by introducing an artificial debond at one of the interfaces in the splice joint and compared with specimens without debond. Although the compressive behavior is similar to that of nominal splice joint, the stiffness is considerably decreased in the presence of debond, and also the load-carrying capacity is reduced by 17% approximately. The experimental results are compared with the analytical predictions and numerical simulations using (a) the Rayleigh–Ritz method and (b) the finite element method, respectively, and are in good correlation.

An approach for reducing cost of carbon fibre-reinforced polymer structures using primary materials

The growing use of advanced composites in the aerospace industry has led to the development of new methods and equipment for manufacturing these new materials and structures. In this relatively new field of design, behaviour of production costs relative to different design variables is a challenging task. Carbon “pre-impregnated” (prepregs) composite fibres are costlier and are widely used as main load bearing members in the aircraft structures. As a result of this, 40% of a structural part cost is about its prepreg material. As prepreg material is expensive it is necessary to optimise its usage by avoiding wastages and implementing nesting program in automated prepreg cutting machines. The primary goal of this study is to find out an alternate solution to optimise material usage and to use primary dry fibres in between prepreg layers during lay-up and to evaluate the mechanical and ultrasonic properties. Different lay-up schemes are used to mix dry fibres in between prepreg layers to prepare test specimens as per DIN 29971 standards. This research reveals a method whereby companies can reduce their product cost by 18-20% . This concept has considerable scope for application in industry as the companies can make parts at minimum cost.

Bond defect detection using PTT IRT in concrete structures strengthened with different CFRP systems

In the context of increased use of carbon fibre reinforced polymer (CFRP) to strengthen concrete structures, the bonding conditions between CFRP and existing structures are crucial. Recently, infrared thermography (IR) non-destructive testing (NDT) has emerged as a technique for bond detection. This paper reports in detail the detection of different unbond, debonding and delamination areas within the bond zone between single- and multi-layer CFRP and concrete structures. Pulse thermography (PTT) was chosen for the IR tests. Pulse intervals of 1s, 3s, and 5s were used as thermal loading to all specimens and halogen lamps were positioned at different distances from the investigated surfaces. Each IR test involved the analysis of 600 IR images. Thermal responses were recorded to detect and study defect characterizations. The paper recommends the minimum thermal input to generate well-recognized bond defect detection, and the results show that the maximum thermal signal is proportional to the number of CFRP layers.

Characterizing damage in CFRP structures using flash thermography in reflection and transmission configurations

Carbon fiber reinforced polymer (CFRP) specimens with artificial delaminations and with impact damage have been characterized using active thermography with flash excitation. Systematic investigations have been performed in four different experimental configurations of flash lamps and infrared (IR) camera in transmission as well as in reflection alignment. It is shown here that the diffusivities determined for the sound and for the damaged areas give a good measure for damage characterization. Although reflection measurements also give information about defect depth, reflection measurements from only one side are not sufficient for assessing the whole cross section of the specimens. Thus, depending on sample thickness the lateral size of damage could only be determined from reflections measurements from both sides or from transmission measurements. In this paper, measurement accuracy and limits of flash thermography for the investigation of CFRP specimens are presented in detail together with quantitative data concerning the defects.

OS0702 CFRP構造の損傷検知へのTFDRの適用

Carbon fiber reinforced polymer (CFRP) composites have higher specific strength and specific stiffness than conventional metallic materials. The CFRP structures, however, are easily damaged by impact loading. It is quite difficult to detect internal damages caused by impacts by visual inspection. In the present paper, self-sensing time-frequency-domain reflectometry (TFDR) is proposed as a damage detection method for large CFRP structures. In the TFDR a set of high-frequency burst signals is applied to a structure and damage location is calculated from difference in time of arrival between an input signal and reflected signal from the structure end. By using several signals with different frequencies, other information such as dimensions of damage can be obtained. This is one of the great advantages to using the frequency domain reflectometry. The TFDR uses both methods; one estimates damage location and the other identifies its dimensions. The purpose of this study is to verify the applicability of the TFDR to CFRP structures. In the TFDR, after experiment for damage detection using the time domain reflectometry (TDR) with electromagnetic waves flowing in a long UD CFRP (1980 mm×120 mm) in the transverse direction, the frequency characteristics of the wave in the CFRP structure are investigated by the finite difference time domain (FDTD) analysis. As a result, the experiment indicated that the TDR could detect a notch in the CFRP using waves flowing in the transverse direction. The electromagnetic analyses showed that the TFDR was able to detect damage and identify its location and dimensions of CFRP structures.

Potential emissions savings of lightweight composite aircraft components evaluated through life cycle assessment

A cradle-to-grave life cycle assessment (LCA) of structural aircraft materials has been utilised to assess and com- pare the total emissions produced during manufacturing, use and disposal of aerospace materials and their selected compo- nents. First, a comparison of aluminium, GLARE and carbon fibre reinforced polymer (CFRP) plates was performed to investigate the potential of lightweight composites in reducing aviation emissions. Subsequently, a case study is presented on a tubular component for which more accurate manufacturing data were directly available. A structural steel tube was replaced with a composite tubular component. The analysis has shown that once the composite material is used as a com- ponent in the aircraft, there is a cumulative saving of aircraft fuel and emissions, in particular from CFRP structures. The environmental analysis included the long-term use predictions for CFRPs, involving detailed raw materials production, use and operation, and disposal scenarios.

Structural health monitoring strategy for detection of interlaminar delamination incomposite plates

In this paper, a structural health monitoring strategy for detecting interlaminardelamination in a carbon fiber reinforced polymer structure using Lamb waves is proposed.The delamination is simulated by inserting a Teflon tape between two transverse plies andthe Lamb wave generation and measurement is enabled by using piezoceramic elements.The Lamb wave theoretical propagation and through thickness strain distribution arestudied, in order to determine the optimal configuration of the final system interms of mode and frequency selection, and piezoceramic sizing and spacing,for detection of cross-sectional delamination. Pitch and catch measurements areperformed by comparing wave propagations for different frequencies and alongdamaged and undamaged paths of the structure, and the analysis of results isperformed using the reassigned short time Fourier transform. It appears that in thelow frequency range (below 300 kHz), the A0 mode is sensitive to the damage,while in the high frequency range, S1 and A1 modes are both very sensitive tothe damage while the propagation of the S0 mode is not affected very much.

Damage detection of CFRP laminates using electrical resistance measurement and neural network

As carbon fibers are electrical conductors, the measurement of the electrical resistance appears to be a valuable technique for the in situ detection of various types of damage in carbon fiber reinforced polymers (CFRP) laminates. In such cases, carbon fibers are both the reinforcement and the sensor to detect damage in CFRP laminates. The damage-detecting method of CFRP laminates by electrical resistance measurement that are investigated in this study is made possible by attaching electrodes on the surface of the CFRP structures without special manufacturing. In this paper, we investigate the electrical resistance change as a damage parameter of fatigue damage such as the degradation of residual strength and stiffness. The measured stiffness and electrical resistance change during fatigue tests showed a very similar trend of change. This is because cumulative fatigue damage is represented by the degradation of residual stiffness; these damages also cause change in electrical resistance. Thus, we can use this change in electrical resistance as a damage parameter. We also predict the future damage of composite laminates in fatigue loading from electrical resistance damage model by following a stiffness degradation model. Electrical resistance gradually increased as the stiffness reduced, and showed a very abrupt change when final fatigue failure was imminent. The predicted value showed good agreement with the experimental data except in the final stage, where stiffness and electrical resistance changed abruptly.