Carbon Fiber Reinforced Polymer Samples Research Articles

Delamination detection in CFRP panels using EMI method with temperature compensation

In this paper result of detection and localization of artificially initiated delaminations in carbon fiber reinforced polymer (CFRP) samples were presented. Damage detection was based on electromechanical impedance method (EMI). This method utilizes electromechanical coupling of piezoelectric transducer with host structure. Due to this coupling mechanical resonances of structure can be seen in electrical impedance characteristic of a piezoelectric transducer. In the research real part of electrical impedance (resistance) was measured. Delamination in CFRP sample caused frequency shift of certain resonance frequencies visible in resistance characteristic. In this paper also changing temperature effects on EMI method were investigated. In order to remove this temperature induced effects compensation algorithm based on signal cross-correlation was utilized.

Polymer-matrix composites carbon fibre characterisation and damage inspection using selectively heating thermography (SeHT) through electromagnetic induction

Polymer-matrix composites (PMCs) such as glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP) are being increasingly used in fields of aerospace, renewable energy, civil and architecture. Damages such as broken fibre, impact and delamination are inevitable during either fabrication or lifetime of PMC structures or components. This work proposed selectively heating thermography (SeHT) through electromagnetic induction and infrared temperature measurement for CFRP characterisation and damage inspection. Broken carbon fibres can be easily identified through SeHT whilst inside delamination is difficult to be inspected due to negative influence of temperature gradient, lateral blur effect, and periodic structures. In order to solve this problem, cross correlation (CC) matched filtering algorithm was proposed to enhance the inspection results. Six CFRP laminate samples with impact damages from 2 to 12J were tested to verify the proposed methods. The experimental studies show that SeHT is capable of characterising fibres and inspecting surface broken area, whilst CC phasegram is capable of inspecting inner delamination due to elimination of non-uniform heating effect and lateral blurring effect. The dependence of damage for CFRP sample on impact energy was also investigated. Through SeHT and CC matched filtering, characterisation and inspection for CFRP can be achieved.

The use of electromechanical impedance conductance signatures for detection of weak adhesive bonds of carbon fibre–reinforced polymer

The joints between structural elements should ensure safe usage of the structure. One of the joining method is based on adhesive bonding. However, adhesive bonding has not replaced riveting yet. Rivets are still present even in newest composite aircraft AIRBUS 350. The reliability of the adhesive bonding limits the use of adhesive bonding for primary aircraft structures and there is a search for new non-destructive testing tools allowing to (1) assessment of the surfaces before bonding and (2) assessment of the adhesive bond. The performance of adhesive bonds depends on the physico-chemical properties of the bonded surfaces. The contamination leading to weak bonds may have various origin and be caused by contamination (moisture, release agent, hydraulic fluid and fuel) or poor curing of adhesive. In this work, the research is focused on the development of the method for assessment of the adhesive bonds. Bonded carbon fibre–reinforced polymer samples were considered. Electromechanical impedance technique was proposed. The technique is based on electrical impedance measurements of a piezoelectric transducer attached to the investigated structure. The piezoelectric effect causes the electrical response of a piezoelectric transducer to be related to mechanical response of the structure. The indexes for comparison of the conductance spectra were proposed. Three different cases of possible weak bonds were selected for the investigation. The same cases were investigated by destructive methods by other authors. Such approach allows for direct comparison of the obtained results. It was shown that the proposed method allows for clear separation of weak bond cases from the cases for other samples and free sensors. In terms of weak bond assessment, the frequency change with weak bond level (contamination and level of poor curing) was observed. The obtained results are promising and encourage to future research.

Pulse Compression Approach to Nonstationary Infrared Thermal Wave Imaging for Nondestructive Testing of Carbon Fiber Reinforced Polymers

Infrared thermography (IRT) is one of the promising remote and whole field inspection techniques for nondestructive characterization of various solids. This technique relies on the mapping of surface temperature response to detect the presence of surface and subsurface anomalies within the material. Due to its fast and quantitative testing capabilities, the IRT has gained significant importance in the testing of fiber reinforced polymers (FRP). A carbon FRP sample with flat bottom holes is considered for inspection using nonstationary digitized frequency modulated thermal wave imaging technique. Furthermore, depth scanning performance using frequency domain-based phase approach has been compared with recently proposed time domain phase approach.

Cold Plasma Pretreatment of Carbon Fibre Composite Substrates to Improve Adhesive Bonding Performance

The purpose of the paper is to investigate the effects of low pressure plasma treatment on wettability of carbon fibre reinforced polymer samples and on shear properties of adhesive bonded joints based on these substrates. In particular, two plasma process parameters, exposure time and power input, were optimized, performing contact angle evaluation on lap-shear tests. The plasma treatment was also compared with a conventional mechanical abrasion and untreated and only degreased specimens. The experimental results show that choosing the optimal parameters is possible to improve the wettability of composite substrates and reduce the contact angle.

Real-time damage mechanisms assessment in CFRP samples via acoustic emission Lamb wave modal analysis

This paper proposes a very promising acquisition-analysis procedure to evaluate real-time damage in carbon fiber-reinforced polymer (CFRP) composite plates by means of the acoustic emission (AE) method. It shows how, by using appropriate acquisition frequency filters and very narrow time windows, it is possible to avoid reflection at boundaries and successfully split the A0 and S0 Lamb modes of the AE signals. After that, an appropriate algorithm —based on the comparison of strength of both modes in time and frequency domains— allows one to associate each AE event to a particular damage mechanism (delamination, fiber breaking and matrix micro-cracking). Experimental results from three point bending tests carried out on 22-layer CFRP samples, with delamination artificially induced by a Teflon film, clearly demonstrate the real-time evaluation of the induced delamination and the beginning and growth of new ones.

Hidden defect identification in carbon fibre reinforced polymer plates using magnetic induction tomography

Carbon fibre reinforced polymer (CFRP) materials pose new challenges to the non-destructive evaluation (NDE) techniques. This study addresses the issue of large defect identification in CFRP plates using electromagnetic measurements. A dual plane magnetic induction tomography (MIT) technique is proposed as a method for damage localization in composite parts, where two arrays of planar sensors are utilized to measure the changes in induced voltages due to the changes in electrical conductivity properties. This geometry meets the requirements of damage inspection in plate structures and thus makes the imaging process feasible. The electrical voltage measurements are used as input to inversely map the spatial resolution of the samples in the region of interest. The stability and detectability of the dual plane system is examined using small metallic cubes. Both individual and multiple instances of damage embedded in CFRP samples are created as a representation of the possible manufacturing defects. Experimental study shows that the presence of damage can be identified in both cases using the dual plane MIT system. With advanced sensing design, rapid data collection unit and improvement in resolution, MIT could become a rapid NDE technique for the integrity inspection of composite structures.

The Effect of CFRP Surface Treatment on the Splat Morphology and Coating Adhesion Strength

Metallization of Carbon Fiber-Reinforced Polymer (CFRP) composites aggrandized their application to aircraft, automobile, and wind power industries. Recently, the metallization of CFRP surface using thermal spray technique, especially the cold spray, a solid state deposition technique, is a topic of research. However, a direct cold spray deposition on the CFRP substrate often imposes severe erosion on the surface owing to the high-impact energy of the sprayed particles. This urges the requirement of an interlayer on the CFRP surface. In the present study, the effect of surface treatment on the interlayer adhesion strength is evaluated. The CFRP samples were initially treated mechanically, chemically, and thermally and then an interlayer was developed by atmospheric plasma spray system. The quality of the coating is highly dependent on the splat taxonomy; therefore the present work also devoted to study the splat formation behavior using the splat-collection experiments, where the molten Cu particles impinged on the treated CFRP substrates. These results were correlated with the coating adhesion strength. The coating adhesion strength was measured by pull-out test. The results showed that the surface treatment, particularly the chemical treatment, was fairly successful in improving the adhesion strength.

Laboratory study of soil-CFRP interaction using pull-out test

Soil is a material which is weak in tension; however, different materials such as geotextiles are used to address this inadequacy. In recent years more than one million square metres of geotextiles were used for reinforcing soil. Nevertheless, there are several significant problems associated with geotextiles, such as creep, low modulus of elasticity and susceptibility to aggressive environments. Carbon fibre reinforced polymer (CFRP) was introduced over two decades ago to the field of structural engineering and it can also be used in geotechnical engineering. CFRP has all the benefits associated with geotextiles and it boasts a higher strength, higher modulus, no creep and reliability in aggressive environments. In this investigation, the interface properties of CFRP-sand and fine sand were investigated using the pull-out test. The pull-out test device was designed and assembled using a triaxial loading device and a direct shear device. In the pull-out test, the normal force applied by the triaxial loading and pull-out force is applied by a direct shear device. CFRP samples were prepared in the lab. Precast and cast-in-place samples were tested. The pull-out force and corresponding displacements of each of the materials were recorded and compared.

Laminate fibre structure characterisation of carbon fibre-reinforced polymers by X-ray scatter dark field imaging with a grating interferometer

X-ray scatter dark field imaging (SDFI) using a Talbot-Lau grating interferometer has recently attracted growing interest for the non-destructive testing and evaluation of light materials. In this work, we developed the method further for the characterisation of the laminate structure of carbon fibre reinforced polymers. In particular, we used the anisotropic properties of the small angle scattering signal to image the fibre bundles running in different directions independently with excellent contrast. The results obtained for four different woven carbon fibre reinforced polymer samples were compared to high resolution X-ray computed tomography results. We found that SDFI allows for the visualisation of the weave pattern structure and analysis of the size of the carbon fibre bundle in three dimensions, even if the individual fibres cannot be separated or the absorption contrast between the carbon fibres and the epoxy matrix is very low.

Experimental characterization of thermo-oxidation-induced shrinkage and damage in polymer–matrix composites

This paper focuses on the experimental characterization of thermo-oxidation in carbon-fiber-reinforced polymers (CFRPs) exposed to “high” temperatures (up to 150°C) and “high” oxygen pressures (up to 5 bars), at the microscopic scale. Unidirectional IM7/977-2 composite specimens were aged at 150°C under atmospheric air and under oxygen pressure (1.7 bars and 5 bars): periodic tests were carried out to characterize degradation phenomena after different aging times. The thermo-oxidation-induced resin shrinkage and fiber/matrix debonding were measured on the CFRP sample surface by confocal interferometric microscopy (CIM) and scanning electron microscopy (SEM). The results show that thermo-oxidation-induced degradation strongly depends on aging time, distance between fibers and partial oxygen pressure.

Strengthening of prestressed concrete girders with composites: Installation, design and inspection

The application of fibre reinforced polymer (FRP) or steel reinforced polymer (SRP) materials to the tension side of a reinforced/prestressed concrete member has been accepted as a strengthening technique to increase the load carrying capacity and in some cases can enhance member serviceability. Proper installation and regular inspection of a composite (FRP or SRP) strengthening system is important since quality of the bond is essential to internally transfer forces. This paper describes an experimental programme conducted to study the behaviour of six prestressed concrete bridge girders, which were tested under static and fatigue loading conditions. The test results were combined with the results of 16 other girders tested by the authors to develop structural design guidelines and guidelines on the installation and inspection of composite strengthening systems. The behaviour was also examined using value engineering to evaluate the cost-effectiveness by investigating the overall system performance. Research findings indicate that SRP materials are more structurally efficient than carbon FRP (CFRP) materials. The results of an inspection demonstration programme, including the pull-off testing of over 150 CFRP samples, has shown that the most effective inspection techniques are visual inspection, pull-off testing, and acoustic sounding.

CFRP の吸水による機械的性質の変化

An effect of water absorption on Charpy impact value has been studied for a carbon fiber reinforced polymer (CFRP). Water submersion improves the Charpy impact value, although it softens the CFRP. The Charpy impact value of water submersed CFRP for 2×105 s at 373 K is about 8% higher than that before the treatment. The water submersion increases the mass because of water absorption into the CFRP sample, although hardening agent dissolves into the distilled water.

Fiber print-through mitigation technique for composite mirror replication

The quickest method for generating a lightweight composite optic is to replicate an optical-quality glass tool onto a carbon-fiber-reinforced polymer (CFRP). However, fiber print-through creates an unacceptable sinusoidal surface roughness on replicated CFRP mirrors; chemical and thermal shrinkage during cure are commonly hypothesized to be the dominant causes. In order to mitigate fiber print-through, two methods of generating a polishable resin layer were investigated. The first method employs the application of a resin film to the CFRP surface. The second method, which is a more unconventional approach, generates a cocured resin layer using magnetic fibers. The latter approach is being developed to eliminate the application of additional resin layers to the CFRP surface, since additional layers present structural disadvantages.It was found that the magnetic fiber technique is comparable to the conventional approach in mitigating fiber print-through. Due to the presence of a 0.25-mm-thick buffer above the reinforcing phase, a final polishing step was used to attain optical quality features on all of the replicated specimens. CFRP and magnetic fiber samples were polished to within 50-Å rms roughness (1-µm to 1-mm bandwidth).

HTS SQUIDs for the nondestructive evaluation of composite structures

While LTS and HTS SQUIDs have successfully been applied in the detection of flaws in aircraft grade aluminium structures for well over a decade, interest has recently spread to a type of new material, namely composites. One example, carbon fibre reinforced polymer (CFRP), is increasingly being favoured by the aircraft industry because of its strength to weight ratio and the fact that it is corrosion-resistant. Material and defect characterization using SQUIDs is still at an early stage, but due to expected rapid expansion in the use of such materials, there is ample scope for the application of HTS SQUIDs. Here we have applied HTS SQUID single-layer gradiometers to investigate artificially created defects in CFRP samples.

SQUID-based nondestructive evaluation of carbon fiber reinforced polymer

Recent work with HTS SQUIDs in nondestructive evaluation has concentrated on the detection of flaws in aircraft-grade aluminum, with particular emphasis on surface-breaking tears beside rivets. More complex materials are now also being used in aircraft manufacture, with carbon fiber reinforced polymer (CFRP) being one of the most common. Existing technologies such as ultrasound are particularly well suited to the detection of impact-damaged sites and until now there have only been a few reports of eddy current examination of CFRP samples. Here we present results on samples with regions of heat damage, impact damage and with nonmagnetic inserts using eddy current detection techniques. We compare the signal to noise ratio and spatial resolution for a variety of sensors including HTS SQUIDs and gradiometers and conventional induction coils, and discuss variations in detection efficiency with field component measured.

High-resolution optical study of the impact of carbon-fibre reinforced polymers with different lay-ups

The impact deformation of carbon-fibre reinforced polymer samples with different lay-ups was studied using high-speed optical techniques. The in-plane deformation of the sample was measured using time-resolved fine-grid analysis. The residual velocities of the projectiles were determined using streak photography and hence the energy dissipated during the penetration was obtained. Both microscopic damage and macroscopic damage to the plates were studied. The deformation, energy dissipation and damage all had a strong dependence on the composite lay-up. Unidirectional composites exhibited anisotropic behaviour and were significantly weaker than the other lay-ups. Though this is intuitively reasonable, this paper gives quantitative measures of this effect. Composite samples examined using the fine-grid technique exhibited cracking in the rear ply suprisingly early in the impact with wide-reaching delamination of this ply. Comparative measurements are presented for polymethylmethacrylate which deformed more isotropically until fragmentation.