Electric Current Analysis for Thick Laminated CFRP Composites

Compared with metal materials, Carbon Fiber Reinforced Plastic (CFRP) is more excellent in performance, with high specific strength and high specific modulus. And among various CFRP structural elements, CFRP laminated plates have widespread applications. However, some indiscernible and hidden primary damage always appears on the CFRP laminated plate and will continuously evolve and expand after being loaded. This work starts with the analysis of the relationship between the changes of the strain distribution and damage expansion of CFRP laminated plates and proposes the damage recognition method of CFRP laminated plates based on strain information. Then, the CFRP laminate damage monitoring system based on Fiber Bragg Grating (FBG) sensor is established, and the CFRP laminate damage identification experiment based on FBG sensor is made to verify the accuracy of the CFRP laminate damage identification method. The results show that the maximum error between the load when the damage appears on the FBG sensor and that when the damage of CFRP laminated plates expands based on the simulated analysis does not exceed 16%. The accuracy of the damage recognition method of CFRP laminated plates is verified and the damage recognition of CFRP laminated plates and their expansion process is achieved.

In conventional manufacturing processes of composites, Carbon Fibre Reinforced Plastic (CFRP) laminates have been made by stacking unidirectional or woven prepreg sheets. Recently, as a manufacturing process of CFRP, 3D printing of CFRP composites has been developed. The 3D printing process of CFRP composites enables us to fabricate CFRP laminates with arbitrary curvilinear fibre plies. This indicates that the optimization of the in-plane curved carbon fibre placement in a planar ply is strongly required to realize superior 3D printed composites. In the present paper, in-plane curved carbon fibre alignment of a ply with an open hole is optimized in terms of maximization of the fracture strength. For the optimization process, a genetic algorithm is adopted. To describe curved carbon fibre alignments in a planar ply, stream lines of perfect flow is employed. By using the stream lines of the perfect flow, number of optimization parameters is significantly reduced. After the optimization, the fracture strength of CFRP laminate is compared with the results of unidirectional CFRP ply. The curved fibre placement in a planar ply shows superior fracture improvement.

In this study, timber column–sill joints strengthened with carbon fiber-reinforced plastic (CFRP) plates using bonds and screws were subjected to loading tests. Twelve joint specifications were considered depending on different combinations of the CFRP plate surface finish and thickness and the type of bond, and the corresponding effects on the load–displacement behavior of the joints were investigated. Three failure modes were observed in addition to the peeling of the CFRP plate. The specimens that failed owing to screw tear out and screw head pull-through in the CFRP plate or splitting of the sill showed load–displacement curves of a similar shape. Those that failed owing to buckling of the CFRP plate showed a rapid and substantial load decrease due to failure. This failure mode was only observed in 0.50 mm thick CFRP plate specimens. When a peel-ply CFRP plate was installed in the column–sill joint specimens, similar secant stiffnesses were observed in the silicon and epoxy resin-bonded specimens, both with high shear strength. Although the specimens with bonds with a high shear strength showed lower deformation performance, the specimens' maximum load increased with the bond's shear strength. The maximum load improved for a 0.75 mm thick peel-ply silicon- or epoxy-bonded CFRP plate. The load's maximum value after peeling the CFRP plate was 0.79–1.23 times as large as the maximum load of the no-bond specimens and did not change significantly.

Influence of matrix resin on tensile strength properties and splitting fracture toughness in unidirectional carbon fiber-reinforced plastic (CFRP) laminates has been investigated. Tensile tests and splitting fracture toughness tests were carried out on two kinds of CFRPs (carbon/thermoplastic systems ; 3KCF/Nylon 6 and 3KCF/modified Nylon). The modified Nylon was developed to improve the adhesion on the basis of Nylon 6. The performance of the present CFRPs was discussed in comparison with that of carbon/epoxy system (AS4/3501-6). It is noted that the present CFRPs exhibit the high strength and high ductility in 90°and 45°directions to the fibers. This result predicts that the present CFRPs have the potential for high resistance to the splitting in unidirectional laminate and to the transverse cracking in multidirectional laminate. On all the unidirectional CFRPs, the splitting fracture toughness under pure mode II is higher than that under pure mode I and the splitting is controlled by the tensile stress intensity in the wide range from pure mode I to mixed-mode loading. The fracture toughness for splitting in CFRP is drastically increased by changing matrix resin from epoxy to Nylon 6, and furthermore is improved by modified Nylon.

The study aims to explore the preparation of aviation mechanical carbon fiber reinforced plastics (CFRP) and the properties of CFRP composites. Taking the aero box body as an example, the mechanical properties of CFRP are studied. The preparation of CFRP is analyzed by searching the data. CFRP plates are explored according to the stress analysis of composite materials. The finite element analysis software ANSYS Workbench and UG software are adopted to build the 3D model of the aero box body. After adding materials in ANSYS Workbench and simplifying the UG model, the finite element analysis of the model is carried out by computer. The 3D model of the aero box is constructed, the finite element analysis of the aero box is carried out, and the mechanical properties of CFRP are explored. In this study, the possibility of the practical application of CFRP in the aviation box body lightweight is clarified, which gives a direction for the subsequent actual molding and guides the application of CFRP in aviation field.

Carbon fiber reinforced plastic (CFRP) is widely used in fields such as aircraft and construction due to its advantages of light weight, high hardness, wear and corrosion resistance. To quantitatively detect internal defects in CFRP panels, a THz reflective defect detection system was built. A defect analysis algorithm based on multi-layer gradient threshold is proposed. Based on the manufacturing process of CFRP, a THz wave echo function model for multi-layer CFRP structures was derived. A CFRP plate containing debonding defects and crack defects was trial produced, to obtain its THz time-domain spectrum. The experiment completed the reconstruction of THz two-dimensional images of CFRP plates. The test results show that three different depths of test data can be detected, and the calculated diameters of the debonding defect areas are 9.12, 9.86, and 9.93 mm, respectively, with a relative error mean of 3.63%. Crack defects can also be effectively identified, but the quality of their test images will decrease with increasing pre-embedded depth. The linearity between defect depth and testing frequency is 0.98, which verifies the possibility of quantitative calculation. The test results of power spectral density and defect depth show that it has good uniformity.

This paper presents an analytical study on the flexural behavior of reinforced concrete beams strengthened with externally bonded carbon fiber-reinforced plastic (CFRP) laminates. Different failure modes of these strengthened reinforced beams have been reported and these failure modes are generally referred to as brittle failure or ductile failure involving the compression crushing of the concrete, debonding or rupture of the composite laminate and yielding of the steel reinforcement. For the analysis, the stresses and strains of all the components of the beam are related by the material properties, including the stress-strain curves for the concrete, steel and CFRP laminate. The strain distribution is assumed to be compatible within the distorted shape of the cross-section of the concrete beam. The resultant forces on the cross section are balanced with the applied loads for static equilibrium. The analytical solution is derived from the equilibrium equations and the compatibility of the strains, and it is applicable to both singly and doubly reinforced concrete beams strengthened with multi-layers of CFRP laminates. In this study, a simple and direct analytical procedure has been developed to evaluate the flexural capacity of concrete beams strengthened with CFRP and to predict their failure modes. An example is presented to illustrate the computational procedures. For design purposes, the upper and lower limits of CFRP cross-section areas are established to ensure that reinforced concrete beam strengthened with advanced composite materials will fail in a ductile manner. A comparison between the analytical results and the data obtained from the literature has been made and the agreement is very good.

In the past few decades, there has been much attention paid towards carbon fiber reinforced plastics (CFRPs) as a structural material for various applications, such as aerospace, transport, infrastructure, wind energy and so on, because of superior mechanical, electrical, thermal, and chemical characteristics compared to steel or metal. However, they are practically used as a complicated architecture including laminates of different ply orientations and woven fabrics, due to the improvement in an anisotropic mechanical strength. Thus, when the CFRP laminates was used in real products with a complicated architecture, it is difficult to investigate and predict the stress/strain distributions and rupture models. The development of in-situ evaluation techniques capable of providing a stress/strain distribution has been strongly demanded. Recently, we have reported that the mechanoluminescence (ML) techniques using composite films consisting of an ML material and organic compounds can be useful technique for visualizing the dynamic equivalent strain distributions in complicated architectures. The ML is the luminescence phenomenon induced by mechanical actions, such as compression, tension, friction, or torsion. Especially, the ML intensity is proportional to the strain energy in the elastic region. In the present study, therefore, we examined the feasibility of a non-destructive testing (NDT) for carbon fiber reinforced plastic (CFRP) laminates by using a mechanoluminescence (ML) technique. Specifically, we investigated to visualize the relative strain distribution of the CFRP laminates with bidirectional fiber bundles (twill woven) by using the fabricated ML sensor consisting of a mixture of SrAl2O4:Eu (SAOE) ML material, which shows the highest ML intensity among various materials reported, and epoxy resin attached to the surface of CFRP laminates. For comparison, the CFRP with unidirectional fiber bundles was also investigated. An ML sensor was fabricated by using a composite of SAOE powder and epoxy resin. The SAOE powder was prepared by using a solid-state reaction method. SrCO3, α-Al2O3, Eu2O3, and small amount of Ho2O3 powders were mixed in an agate mortar with ethanol solution. The obtained mixture powder was calcined at 800ºC for 1 h in air, and then sintered at 1350ºC for 4 h in reducing atmosphere (H2+Ar). The sensing characteristics of the ML sensors painted on the surface of the CFRP laminates were evaluated under a tensile testing in a dark room at room temperature. The commercial strain gage was also applied on the opposite side of CFRP laminates with a commercial adhesive. Tensile tests were performed with a material testing machine. The ML images of the ML sensor and longitudinal strains during the tensile tests were recorded by using a CCD camera and a uniaxial strain gage connected a data logger, respectively. Both devices were connected to a personal computer to simultaneously obtain the data. For quantitative and reproducible measurement, the ML sensor was once irradiated by blue light-emitting diode (LED) for 1 min and kept under dark condition for 5 min. From recording the ML images in the tensile tests, the ML sensor attached to the CFRP laminates with unidirectional fiber bundles gave uniform ML intensity distribution over the whole of ML sensor, and the ML intensity increased with increasing the tensile load, indicating that the whole of CFRP laminate deforms uniformly by the applying tensile loads. On the other hand, the CFRP laminates with bidirectional fiber bundles exhibited a twill weave ML intensity distribution similar to the original twill weave pattern. Specifically, the ML intensities on the areas of a longitudinal fiber bundle (tensile direction) were found to be higher than those on transverse fiber bundle areas. This indicates that a longitudinal fiber bundle deform much larger than the transverse ones. Therefore, it is concluded that ML sensor could directly visualize the load share between fiber bundles with different directions in CFRP laminate with high spatial resolution. Based on the obtained results in this study, we concluded that the ML sensor could visualize a relative strain distribution of CFRP laminates with both unidirectional and bidirectional fiber bundles. Additionally, the ML sensor could detect a tiny strain difference at the interface between longitudinal fiber bundles even in the elastic deformation region. Thus, it is considered that the ML sensor could be a possible candidate for a dynamic strain distribution technique of the CFRP products.

Impact resistance and damage mechanism of polyurea coated CFRP laminates under quasi-static indentation and low velocity impact

The aim of this study is to investigate the capability of the zero-lag crosscorrelation (ZLCC) imaging condition of an A0 Lamb wave mode in imaging a barely visible impact damage (BVID) in a carbon fiber reinforced plastic (CFRP) using a fully non-contact Lamb wave based non-destructive inspection (NDI) system. A 16- ply [45/0/-45/90]2S CFRP laminate was impacted at three different locations with different impact energies using a drop ball at three drop heights causing three BVIDs with different dimensions, ~7 mm, ~12 mm and ~17.5 mm in diameter. The A0 Lamb wave mode is generated inside the laminate using a circular air-coupled transducer (ACT) and detected along the damaged region using a laser Doppler vibrometer (LDV). The measured wavefield is then decomposed into a forward and backward propagating wavefields by applying a frequency-wavenumber filtering postprocessing technique. The decomposed wavefields are then cross-correlated in the frequency domain using ZLCC method producing a detailed cumulative damage image. The experimental images highlight the three damaged areas with higher ZLCC values compared to other parts of the inspected areas. However, it was found that depending on the damage size and if delaminations are happening all around the impacted spot, the delamination boundary on the right side of the impact is difficult to detect if the generated wavefield is traveling from left to right which interacts with the delaminations of the left side and vice versa. In this case, the region of interest around one of the damages (12 mm) was scanned twice with the excitation being to the left and then to the right of it and overlapping the images have given a good approximation of the actual damaged area. The experimental investigation has shown a good correlation between the ZLCC imaging condition and C-scan images, which demonstrate a strong capability of guided wave ZLCC imaging condition technique in approximating the location and size of relatively small BVIDs in thin composite structures. The presence of three separate delaminations in a 2.4 mm thick CFRP laminate impacted by 13.56, 19, and 27 Joule drop ball energies were detected in a fully non-contact ultrasonic technique. doi: 10.12783/SHM2015/319

Carbon fiber composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. The use of carbon fiber reinforced plastics (CFRP) for the repair of steel structure is also advantageous, for example to avoid welding or bolting which can weaken structure and add fire risk. This study is based on adhesive bonded double lap shear (DLS) joints where the inner adherend is steel and the outer adherend is cross plies unidirectional (UD) CFRP. Overlap lengths of 25-125 mm with CFRP thickness of 3 and 6 mm were considered and two different orientation angles [0o,90o] and [90o,0o] laminates have been considered. The overall objectives of the paper are to investigate the delamination of the CFRP laminates within the DLS joints under quasi-static loading and both experimental and numerical techniques were used. The numerical study was based on 2-D model using strength-limit method, taking into consideration the UD properties of the plies and the resin layers separating these. As a result, it was concluded that the data obtained from 2-D finite element analysis were coherent with experimental results and additional to that fiber orientation angles of the laminate markedly affected the failure load of joints, failure mode and stress distributions appeared in adhesive and composite

Steel structure reinforced engineering makes extensive use of carbon fiber reinforced plastic (CFRP) plates. Although it happens frequently, the interfacial debonding of CFRP plate-reinforced steel beams has been the main cause of failure in flexural strengthened steel beams, which results in the brittle failure of steel structures. This paper proposes a novel percussion method to locate and quantify the debonding area based on the Welch power spectrum density (PSD) estimate, mel frequency cepstral coefficient (MFCC), and convolutional neural network (CNN). According to our knowledge, this is the first attempt to use the percussion method to identify and measure the debonding defects of steel beams strengthened by CFRP plates. Additionally, this experiment takes into account how percussion instruments affect localization and quantification. To begin with, the debonding location is qualitatively detected using the Welch PSD estimate method. The presented percussion method can successfully and accurately detect the interfacial debonding of CFRP plate-reinforced steel beams, according to repeated results. Subsequently, a novel CNN model is constructed to classify the various debonding areas based on the MFCC feature. In the end, the experimental findings show that the localization and quantification of the hammer types vary only slightly. Overall, the percussion-based approach can offer a fresh line of inquiry into the debonding problem on steel beams reinforced with CFRP plates.

An impact load on a thick carbon fiber reinforced plastic (CFRP) plate usually causes a dent. The dent results in a decrease of electrical resistance. Our previous paper demonstrated the effect of the dent in the measurement of electrical resistance change of a thick CFRP plate subjected to impact loading. The present paper focuses on the electrical resistance decrease caused by the dent. Numerous FEM analyses were performed, and the relationship between electrical resistance changes and delamination location was constructed, by measuring the response surfaces. Several experiments were conducted and the measured electrical resistance changes were used to predict damage location from the response surfaces. The present study shows that the dent decreases the electrical resistance change ratio even in the adjacent segments, and the electrical resistance changes caused by the dent enable us to monitor the damage location. FEM analyses for a projected area model are sufficient to construct the relationships without resorting to many experiments.

For a high accuracy antenna in next radio astronomy satellite, a candidate material is carbon fiber reinforced plastics (CFRP), because negative longitudinal coefficient of thermal expansion (CTE) for unidirectional CFRP enables a laminate with 0 CTE through appropriate laminate design. This enables high structural accuracy under large temperature fluctuation like space. On the other hand, when the laminate is subjected to thermal cycles, cyclic thermal stress occurs and causes microscopic damages. In this study, we characterized damage progress in CFRP laminates and resultant variation in mechanical properties under cyclic thermal loading. Three types of matrices, such as polycyanate ester, polyimide and epoxy resin were used to prepare CFRP laminates. Specimens were subjected to thermal cycles from −197°C to 120°C. The test was periodically stopped for surface observation and flexural loading. Transverse cracks in 90° plies accumulated with thermal cycles, whereas flexural modulus remained constant. We also numerically evaluated temperature gradient and resultant thermal stress distribution during cooling by finite element analysis. The result indicates higher transverse stress appeared in the surface of the specimen and saturated to constant value which corresponded with the value calculated based on classical lamination theory.

This paper discusses tool-wear processes in the milling of carbon fiber-reinforced plastic (CFRP) laminates. Plane down-milling tests with unidirectional and cross-directional CFRP laminates were performed using two types of cutting tools made of tungsten carbide and polycrystalline diamond. Measurements of the changes in the cutting forces and tool-wear widths over the cutting distance revealed that the fiber orientation direction in the CFRP laminates relative to the tool-traveling direction is an important parameter to determine the tool-wear processes. Additionally, based on obtained experimental results, a wear parameter to characterize cutting tool wear is introduced. This parameter can accurately explain the relationship between the worn tool-edge profiles and the processed-surface quality.