Carbon Fiber Reinforced Polymer Specimens Research Articles

Independent component thermography for non-destructive testing of defects in polymer composites

Thermographic data processing and analysis is critical for effective infrared thermography non-destructive testing of defects in composite materials. In this research work, the concept of blind source separation is introduced into this field for facilitating subsurface defect detection in polymer composites. In detail, a thermal image generated by infrared thermography is viewed as a linear mixture of several independent and non-Gaussian sources, which can be decomposed by adopting the proposed independent component thermography (ICT) technique. After the decomposition, the non-Gaussian sources are extracted and plotted as a number of images with the same size as the original thermograms. Eventually, the defect signals can be separated from the non-uniform backgrounds which are usually caused by uneven heating. As a result, the location and shape information of the subsurface defective regions is highlighted. The feasibility of the ICT method is illustrated with its application to a carbon fiber reinforced polymer specimen.

Identification of Localized Void Defects in Composite by Recurrence Quantification Analysis of Ultrasonic Backscattered Signal

Voids usually have a detrimental effect on the performance of carbon fiber reinforced polymer (CFRP). This paper presents a nonlinear dynamic method, known as recurrence quantification analysis (RQA), for the identification of localized void defects. First, five standard CFRP specimens with known porosity ranging from 0.2 to 5.94% are tested using ultrasonic pulse-echo technique, and the obtained backscattered signals are subsequently analyzed by RQA. The result shows that two RQA characteristic parameters, the recurrence rate and the Shannon entropy, both clearly increase with increasing porosity. Next, the RQA is performed on another CFRP specimen with unknown porosity. Two regions in this specimen containing localized void defects are detected based on the above finding. This result is finally verified by a destructive experiment. The research demonstrates that RQA is an effective new method for the identification of localized void defects in CFRP.

Three‐dimensional imaging of subsurface delamination in carbon fiber reinforced plastic using photoacoustic wave method

AbstractUltrasonic wave generated by interaction of the material with pulsed lasers is known as photoacoustic (PA) wave. In this study, the PA imaging is applied to the nondestructive testing for carbon fiber reinforced plastic (CFRP). When subsurface flaws are in CFRP, the detection of the flaws is sometimes challenging because the trailing signal from the surface interferes with the scattered signal from the flaws. Here, we developed a PA microscopy (PAM) to reconstruct the location and shape of the subsurface flaws. The PAM employs a confocal arrangement of optical and acoustic lens to maximize the intensity of generated PA waves. To enhance the lateral resolution of the image, we introduced the synthetic aperture focusing technique (SAFT). In the SAFT, it is necessary to consider direction‐dependent group velocities due to the strong anisotropy in CFRP. The performance of the PA imaging combined with the SAFT was checked with artificial delaminations in a cross‐ply CFRP specimen. It was shown that the PA wave had a wide frequency band and our method offered a high spatial resolution image.

Application of Ultrasonic Coda Wave Interferometry for Micro-cracks Monitoring in Woven Fabric Composites

The consequences of a four-point bending test, up to 12 mm, are examined by emitting 1 MHz ultrasonic guided waves in woven carbon fiber reinforced polymer specimens, using coda wave interferometry (CWI), revealing a potential use for nondestructive evaluation. It is known that CWI is more sensitive to realistic damage than the conventional method based on the first arriving time of flight in geophysical, or in civil engineering applications such as concrete structures. However, in composite materials CWI is not well established because of the involved structural complexity. In this paper, CWI is investigated for monitoring the occurrence of realistic defects such as micro-cracks in a woven carbon fiber composite plate. The micro-cracks are generated by a four-point bending test. The damage state is stepwise enhanced by gradually increasing the load level, until failure initiation. The damage is monitored, after each loading, using ultrasound. It is demonstrated that CWI is a powerful tool to detect damage, even low levels, in the sample. Two damage indicators based on CWI, i.e. signals correlation coefficient and relative velocity change, are investigated and appear to be complimentary. Under significant loading levels, the normalized cross-correlation coefficient between the waveforms recorded in the damaged and in the healthy sample (reference at 0 mm), decreases sharply; this first indicator is therefore useful for severe damage detection. It is also demonstrated, by means of a second indicator, that the relative velocity change between a baseline signal taken at zero loading, and the signals taken at various loadings, is linear as a function of the loading, until a critical level is reached; therefore this second indicator, is useful for low damage level detection. The obtained evolution of the relative velocity measurement is supported by relative comparison to the evolution of the bending modulus in function of displacement. The relative velocity change exhibits the same evolution as the bending modulus with loading. It could be used to indicate when the material stiffness has decreased significantly. The research is done in the framework of composite manufacturing quality control and appears to be a promising inspection technique.

Effect of Temperature on Resistivity of CFRP Materials with Added Carbon Powder or Nano-silica

This paper presents an experimental investigation on the effect of temperature on resistivity of Carbon Fiber Reinforced Polymer (CFRP) materials. A series of tests were conducted on three types of CFRP materials, namely pure CFRP material, CFRP with carbon powder (4% in weight) and CFRP with nano-silica (4% in weight). Test results showed that adding carbon powder into the epoxy resin decreases the initial electric resistance R0 and initial volume resistivity ρ0 while adding nano-silica increases R0 and ρ0 compared to pure CFRP material. Preheating cycle test results showed that the volume resistivity of all three types of specimens linearly increases with increasing temperature. CFRP with added nano-silica exhibits higher temperature sensitivity than CFRP with added carbon powder compared to the lowest temperature sensitivity for pure CFRP material. In addition, temperature cycle test results showed that CFRP specimens have approximately stable values of volume resistivity. Both CFRP specimens with added carbon powder or nano-silica exhibit a recognizable trend of first decrease and then increase in volume resistivity with increasing temperature both during heating and cooling cycles. CFRP with added carbon powder mainly shows Negative Temperature Coefficient (NTC) effect in the temperature range of −40 to 40°C and Positive Temperature Coefficient (PTC) effect from 40 to 80°C. CFRP with added nano-silica mainly exhibits PTC effect in the temperature domain of −15 to 80°C and NTC effect from −40 to −15°C. A mathematical-physical model with respect to the thermal effect was presented based on the Eshelby-Mori-Tanaka (EMT) approach and mesomechanics method. The results obtained with the model agree well with the test results considering the temperature domain of PTC effect, which indicates that the proposed model is effective in characterizing the variation of fractional change in resistance (ΔR/R0) at varying temperature.

Tensile properties of CFRP manufactured by resin transfer molding considering stacking sequences at various strain rates

This paper is concerned with the improvement of a tensile test method and the material properties of carbon fiber reinforced polymer manufactured by resin transfer molding considering stacking sequences at various strain rates for auto-body. Auto-body structure experiences the strain rates up to several hundreds per second during car crash. In order to apply the carbon fiber reinforced polymer panel into auto-body structures, it is critical to acquire the material properties of carbon fiber reinforced polymer at various strain rates considering stacking sequences. The tensile test method is improved for acceptable test results. Test specimens are modified for high-speed tensile tests of carbon fiber reinforced polymer in order to achieve designated strain rates and eliminate the effect from unfavorable conditions of inhomogeneity of deformation. Various types of grip tab materials are employed for acceptable failure modes. Tensile tests have been carried out with non-crimp fabric made by 50K high strength carbon fiber (NCF) and woven fabric made by a 2/2 twill pattern of 3K high strength carbon fiber (TWILL) with different stacking sequences of 0° and 90° unidirectional cases as well as [0°/90°] and [45°/−45°] symmetric cases. Digital image correlation method and force equilibrium grid method are adopted for strain and stress measurement of a carbon fiber reinforced polymer specimen during a tensile test. The material properties acquired indicate that the carbon fiber has little rate dependency, while the epoxy matrix has remarkable rate hardening at strain rates from 0.001 s−1 to 100 s−1.

Digital Image Correlation Comparison of Damaged and Undamaged Aeronautical CFRPs During Compression Tests.

The diffusion of composite materials in aeronautical and aerospace applications is attributable to the high specific mechanical properties they offer. In particular, the recent use of Carbon Fiber Reinforced Polymer (CFRP) materials is highly increased. The main disadvantage in using this kind of material is related to the possibility of including damages or defects not visible on the surface that compromise their behavior and make their use extremely unsafe if not properly supervised. The most conventional nondestructive techniques allow the detection of damages when they already compromise the life of these materials. The use of the same techniques makes it harder to monitor in-situ of the progress of damages, especially if they occur inside the materials. The implementation of the innovative strain analysis method, like those based on full-field measurements, could provide additional information about the damage mechanisms by supplying the complete strain distribution of the surface of the sample. The present paper examines the mechanical behavior of two different CFRP specimens, with and without damage, subjected to compressive load in an anti-buckling fixture by using the Digital Image Correlation (DIC). The purpose is to measure the out-of-plane displacements, characteristics of the compression tests, in all the points of the ROI (Region of Interest), using a full-field and noncontact technique. The innovative aspect of this work is therefore to solve this problem through an experimental approach with DIC 3D technique.

Review on carbon fiber reinforced polymer (CFRP) and their mechanical performance

Abstract Materials play a huge role in the shaping and development of human civilization. The need for materials started from the early stone-age where man needed fire to keep him warm and also to cook his food. Many materials such as copper, iron have been used in the past for few of the applications. But with the advancement of technology and new innovations brings about the need for lighter, more compact and many such other properties. So in order to fulfill these conditions a material such as composite material was developed. Carbon fiber Reinforced Polymer (CFRP) is a composite material which is very unique. CFRP has been widely used in aerospace industries. Slowly but gradually due to reduction in cost has lead it to be introduced in the automobile sector. The strength of the CFRP depends on the type of application, right combination of fiber to resin, length, type, orientation of fibers, use of anchors and form (sheet, plate). Temperature is an important factor which is taken into consideration. Curing of the adhesive at elevated temperatures and also if temperatures of the structures to which CFRP are bonded are kept below the glass transition temperature (Tg) it leads to longevity of the structure. CFRP is not affected by moisture by using non-reactive adhesive and in certain cases by treating the specimen with silane. The machinability of the CFRP specimens can be accomplished by using cold air, cryogenic environments and also by traditional methods with slight modifications. Hence it can be found useful in construction industry.

Bending fracture rule for 3D-printed curved continuous-fiber composite

Three-dimensional (3D) printing is an attractive technology to produce complex structures without the need for expensive tools and molds. Additives are usually incorporated with the plastic materials used in 3D printing to increase their strength and rigidity. In particular, carbon fiber-reinforced plastic (CFRP) shows promise as a material for use in 3D printing. However, the strength of CFRP after printing is still unclear, although it is known that its strength is affected by the plastic melting during printing. In this study, we analyzed the fracture behavior of CFRP specimens before and after bending to different curvature radii. From the experimental results, a fracture criterion that described the behavior of the materials by considering tensile and compressive loads was developed. The fracture mechanism was the same for CFRP specimens with different curvature radii. These results increase our understanding of the mechanical properties of CFRP materials used in 3D printing.

Exploration of energy absorption and viscoelastic behavior of CFRPs subjected to low velocity impact

The attention of the present work is focused on the energy absorption and transformation of carbon fiber reinforced polymer (CFRP) composites under low velocity impact. Moreover, the viscoelastic behavior of composites was explored by observing the changes of dent depths via a micrometer and 3D microscope. In addition, the residual strengths of CFRP specimens were investigated by the compression after impact (CAI) test. The results show that more than 88% of impact energy was absorbed by the CFRPs through reversible deformation, irreversible damage and viscoelastic behavior. The impact damage threshold value and the maximum allowable deformation of the CFRP laminates were 9 J and about half of its thickness respectively. The energy is mainly absorbed by deformation when the impact energy was not larger than the threshold value. The energy dissipated by viscoelastic behavior increased with the increase in the impact energy and even exceeded 1.4 J in the case of 21 J, which was much higher than the kinetic energy when the impact energy exceeded the threshold value. The residual strength of the CFRP laminates was related to the energy absorbed by irreversible damage.

A Study on Drilling High-Strength CFRP Laminates: Frictional Heat and Cutting Temperature.

High-strength carbon fiber reinforced polymer (CFRP) composites have become popular materials to be utilized in the aerospace and automotive industries, due to their unique and superior mechanical properties. An understanding of cutting temperatures is rather important when dealing with high-strength CFRPs, since machining defects are likely to occur because of high temperatures (especially in the semi-closed drilling process). The friction behavior at the flank tool-workpiece interface when drilling CFRPs plays a vital role in the heat generation, which still remains poorly understood. The aim of this paper is to address the friction-induced heat based on two specially-designed tribometers to simulate different sliding velocities, similar to those occurring along the flank tool-work interface in drilling. The elastic recovery effect during the drilling process was considered during the tribo-drilling experiments. The drilling temperatures were calculated by the analytical model and verified by the in-situ experimental results gained using the embedded thermocouples into the drills. The results indicate that the magnitudes of the interfacial friction coefficients between the cemented carbide tool and the CFRP specimen are within the range between 0.135–0.168 under the examined conditions. Additionally, the friction caused by the plastic deformation and elastic recovery effects plays a dominant role when the sliding velocity increases. The findings in this paper point out the impact of the friction-induced heat and cutting parameters on the overall drilling temperature.

Active thermographic inspection of carbon fiber reinforced plastic laminates using laser scanning heating

In this study, inspection capability of active thermographic nondestructive inspection using the laser scanning method was examined. Carbon fiber reinforced plastic (CFRP) specimens were heated using a CO2 laser scanning device and their inspection capability was investigated. The experimental results indicate that the temperature and phase behavior after laser scanning heating were similar to those after conventional instantaneous flash heating. We also investigated the effect of the relationship between the scanning direction and fiber direction of the tested CFRPs on defect detection. The numerical and experimental results showed that the observed temperature contrast appearing on the defective surface varied depending on the scanning direction, and the temperature contrast increased when the scanning direction was the same as the fiber direction of the laminates. In addition, it was found that defects in a CFRP specimen located 10 m from the heat source could be detected using the laser scanning heating. These results suggest that using the laser scanning heating method is effective to inspect large objects located at far distances.

A Numerical simulation of delamination caused by internal gas pressure for mid-density CFRP

Interlaminar debonding (delamination) is a major problem for carbon fiber-reinforced plastic (CFRP) when it is used in thermal protection systems subjected to rapid heating, such as in a re-entry situation. In this study, the possibility of avoiding delamination by controlling material porosity was investigated experimentally and numerically. CFRP specimens having various initial porosities (mid-density CFRP) were prepared and rapid heating tests (∼6 MW/m2) were performed, and the specimens were examined to determine whether delamination had occurred. To predict the delamination, heat transfer and pyrolysis gas-induced internal gas pressure growth were simulated numerically as a function of heating time. The parameters regarding temperature-dependent chemical reaction rate, orthotropic coefficients for gas permeability, and flatwise tensile strength were determined by individual experiments using the CFRP specimens. The coupled analysis of thermal transfer and pyrolysis gas flow following Darcy’s law was conducted by finite-difference form, employing the experimentally obtained parameters. When the gas pressure exceeds the flatwise tensile strength of CFRP, delamination is assumed to occur. The numerical results show good agreement with the experimental results, and it is concluded that delamination can be eliminated by using mid-density CFRP.

CFRP manufacturing method by using electro-activated deposition and the effect of reinforcement with carbon fiber circumferentially around the hole

For the efficient manufacturing of carbon fiber reinforced plastics (CFRP), the electro-activated deposition resin molding (ERM) method is developed. The carbon fiber fabric is immersed in an electro-activated deposition solution containing polymer with epoxy groups. By energization, an epoxy resin is precipitated on the surface of the carbon fiber, and impregnation occurred in the solution. In this study, applying the ERM method, CFRP specimen with the hole was manufactured. Furthermore, the reinforcement by arranging the fiber circumferentially around the hole was installed. The carbon fiber fabric was sewn using the tailored fiber placement (TFP) machine. As a results, the effect of the reinforcement was confirmed. The tensile strength of CFRP with the hole and the reinforcement was the same as in the case without the hole. From the finite element analysis, it was confirmed that the reinforcement around the hole reduced the stress concentration.

Characterisation of smart CFRP composites with embedded PZT transducers for nonlinear ultrasonic applications

Embedded piezoelectric lead zirconate titanate (PZT) transducers in carbon fibre reinforced plastic (CFRP) composites are typically electrically insulated by interlaying materials such as polyimide Kapton films between the PZT and the laminate ply. However, the presence of polymeric films may cause debonding at the layer interface, thus reducing the structural performance. This paper proposes an alternative insulation technique in which PZTs are covered by a thin patch of woven E-glass fibre fabric for enhanced adhesion with the surrounding epoxy matrix. An analysis of variance on experimental test results showed that the compressive, flexural and interlaminar shear strengths of plain CFRP specimens were equal to the means of the smart CFRP composite (0.41 < p-value < 0.58), but significantly higher than those of Kapton specimens (0.0001< p-value < 0.05). Moreover, a post-test fractographic analysis indicated that damage opening in Kapton specimens was significantly larger (p-value = 0.03) than that in plain specimens. Brooming failure compression was also the same for the smart CFRP composite and plain samples, whereas Kapton specimens failed by through-thickness shear. Finally, nonlinear ultrasonic experiments were conducted on CFRP laminates with artificial in-plane delamination using glass fibre insulated PZTs. Remarkably, the effect of second harmonic generation was found to be nearly two times higher than conventionally surface-bonded PZTs.

Numerical simulation of dynamic failure behavior for cylindrical carbon fiber reinforced polymer

This paper presents a prediction for the dynamic matrix-dominated failure behavior of a carbon fiber reinforced polymer (CFRP) under impact loading, by using hierarchical multi-scale analysis. In this study, the analysis results were compared to experimental results to verify the validity of the multi-scale model. In the experiment, the split Hopkinson pressure bar (SHPB) test was conducted by using two types of cylindrical CFRP specimens. The commercial finite element analysis (FEA) software Abaqus was used for the analysis. We conducted micro-scale and macro-scale analysis, which corresponded to the experiments. In the micro-scale analysis, a two-dimensional periodic unit-cell (PUC) model, which consisted of 20 fibers, resin, and zero-thickness interface is used. The micromechanical model could simulate the matrix failure and debonding of the fiber/matrix interface by the Christensen failure criterion and cohesive zone modeling, respectively. Additionally, the strain rate dependency failure of matrix was considered. The PUC analysis was conducted under a multi-axial stress state, to obtain the failure envelope. Subsequently, a macro-analysis by FE was conducted with parameters obtained from the micromechanics analysis. The simulation was found to be in good agreement with experimental results. Finally, we investigated the relationship between interfacial strength and the failure load of CFRP.

On crashing behaviors of aluminium/CFRP tubes subjected to axial and oblique loading: An experimental study

Abstract Thin-walled energy-absorbing structures rarely experience in pure axial loading in real crash events, but rather a combination of axial and off-axial loads. In this perspective, it is critical to understand the oblique crushing process of thin-walled structures. To address this issue, circular aluminum and carbon fiber reinforced plastics (CFRP) tubes were experimentally investigated for characterizing their crashworthiness subject to quasi-static axial and oblique compression in this study. The tests were conducted at five different loading angles (θ) of 0°, 5°, 10°, 20° and 30° to the tubal axis. Five sets of specific fixtures were fabricated to apply the desired axial and off-axial loads onto the aluminum and CFRP specimens, respectively. The failure modes, load-displacement curves, crushing force and energy absorption of all the specimens were analyzed; and the effects of loading angle were explored. It is found from the experiments that with the increase in loading angle, aluminum tubes were more prone to collapse in an irregular diamond mode (Id) instead of axisymmetric concertina mode (Ac), while the CFRP tubes collapsed in much more complex failure modes which included splaying mode (Sp), tearing mode (Te), socking mode (So), micro-fragmentation mode (Mf) and catastrophic failure (Cf). As for the energy absorption characteristics, the loading angle (θ) ranged from 0° to 10° had little impact on the energy absorption for the both aluminum and CFRP tubes, nevertheless, the energy absorption of the CFRP tubes decreased more significant from θ = 10° to 30°, while that of aluminum tubes declined fairly steadily.

Study of FRP bars under tension using acoustic emission detection technique

Acoustic emission (AE) signal has proved to be a useful tool for monitoring structures reinforced with FRP composites such as Fiber Reinforced Polymer (FRP) reinforced beams, carbon FRP (CFRP) sheets, or steel fiber reinforced concrete. This work focuses on studying the behavior of pultruded FRP bars, which have been manufactured using continuous fibers. Different configurations of CFRP and glass FRP (GFRP) bar specimens subjected to tensile load were monitored using AE technique. Several algorithms were used for signal processing to analyze AE signals in the time domain and in the time–frequency domain. The signal processing techniques extracted the amplitude, cumulative events, duration, energy, rise time, number of counts, cumulative counts, and frequency peaks of the acoustic signals. The frequency maxima were determined for different amplitude signals using short-time Fourier transform (STFT). Cumulative counts of AE signals showed significant changes in the slope during the tension test, while the stress–strain relationship of the FRP rods showed virtually no deviation from linearity. CFRP bars recorded higher amplitude signals and lower duration, than GFRP bars. The acoustic emission characteristics presented in this work show strong correlations with ultimate load and may prove useful for damage prediction.

Assessing the Contribution of the CFRP Strip of Bearing the Applied Load Using Near-Surface Mounted Strengthening Technique with Innovative High-Strength Self-Compacting Cementitious Adhesive (IHSSC-CA).

Efficient transfer of load between concrete substrate and fibre reinforced polymer (FRP) by the bonding agent is the key factor in any FRP strengthening system. An innovative high-strength self-compacting non-polymer cementitious adhesive (IHSSC-CA) was recently developed by the authors and has been used in a number of studies. Graphene oxide and cementitious materials are used to synthesise the new adhesive. The successful implementation of IHSSC-CA significantly increases carbon FRP (CFRP) strip utilization and the load-bearing capacity of the near-surface mounted (NSM) CFRP strengthening system. A number of tests were used to inspect the interfacial zone in the bonding area of NSM CFRP strips, including physical examination, pore structure analysis, and three-dimensional laser profilometery analysis. It was deduced from the physical inspection of NSM CFRP specimens made with IHSSC-CA that a smooth surface for load transfer was found in the CFRP strip without stress concentrations in some local regions. A smooth surface of the adhesive layer is very important for preventing localized brittle failure in the concrete. The pore structure analysis also confirmed that IHSSC-CA has better composite action between NSM CFRP strips and concrete substrate than other adhesives, resulting in the NSM CFRP specimens made with IHSSC-CA sustaining a greater load. Finally, the results of three-dimensional laser profilometery revealed a greater degree of roughness and less deformation on the surface of the CFRP strip when IHSSC-CA was used compared to other adhesives.

Mechanical characterization of carbon fiber reinforced plastics specimens for aerospace applications

The use of new Composites Fibers Reinforced Plastics materials for structural applications in aerospace industry is fast increasing in the last years. Ensuring high performances requires a deep knowledge of the mechanical behavior of the composite components subjected to different workloads. Components are subjected to strict environmental conditions that are typical of flight settings, as cold temperature and/or elevated temperature close to engines. In the typical assembly of composites, multiple layers are stacked together with a given sequence. Layers could be organized with different angles, different sequences and different technological process for providing specific mechanical properties that need to be studied. The possibility to introduce new parameters, able to improve mechanical properties of composites is also investigated. It refers to an unconventional fibers orientation combined with the through‐the‐thickness stitching on the in‐plane mechanical properties of composites. Conventional carbon fibers orientation is generally referred to a Cartesian coordinate system, in which fibers are arranging in bundles along different angle orientations respect to the zero lamina. The opportunity to use a polar coordinate system of continues carbon tow is introduced to create specimens as well as complex geometry components easily. The application of this technique aims to introduce some advantages in the material's manufacturing process and if compared with the conventional strategy seems to be very promising since it appears to be very efficient in stopping delamination phenomenon. POLYM. COMPOS., 40:716–722, 2019. © 2018 Society of Plastics Engineers