Simulation on kink-band formation during axial compression of a unidirectional carbon fiber-reinforced plastic constructed by X-ray computed tomography images

X-ray computed tomography was used to obtain cross-sectional images of a unidirectional carbon fiber-reinforced plastic, where fiber locations in each cross-sectional image were identified. The three-dimensional model with fiber waviness was developed by connecting the fiber locations along the fiber direction. Numerical simulation for the initiation and formation of a kink-band during axial compression was performed using the three-dimensional finite element model. The load was increased almost linearly until it reached the compressive strength, after which both load and displacement were decreased, showing snap-back behavior. The matrix yielded locally with the increased axial compression, and fibers started to fall due to insufficient support by the yielded matrix. A kink-band was formed with an increase in the yielded area, and thus, the initiation of a kink-band was defined as the local yielding of the matrix. It was also shown that the kink-band was formed at the longitudinal location at which the average of initial local fiber misalignment angles in the cross-section was relatively large.     

In situ observation of kink-band formation in a unidirectional carbon fiber reinforced plastic by X-ray computed tomography imaging

A compression test of a unidirectional carbon fiber reinforced plastic (CFRP) rectangular coupon was performed in an X-ray computed tomography (CT) system. Internal compressive failure of the CFRP was observed under loading. Two-dimensional fiber microbuckling developed non-uniformly in the specimen, and enlarged locally at one side. Fiber failure then initiated at the side edge and propagated through the whole cross section. A two-dimensional kink-band developed as a result of two-dimensional fiber microbuckling. The out-of-plane and in-plane band widths and band angles were almost the same. The coupon specimen twisted slightly owing to the two-dimensional kink-band, which resulted, macroscopically, in a transverse and through-thickness shear failure mode. The scenario of kink-band failure in a unidirectional CFRP coupon was revealed by X-ray CT imaging.     

X-ray computed tomography used to measure fiber orientation in CFRP laminates

This paper explains a new method to measure the fiber orientation in carbon fiber reinforced plastics (CFRP) laminates from X-ray CT images. In the method, the fiber orientation is analyzed by the application of digital image correlation (DIC) method to the acquired tomographic images. Using DIC, the brightness pattern, which results from the radiodensity difference between fiber and resin, is compared between two different planes in the thickness direction. Then, the three-dimensional displacement of the brightness pattern, which indicates the fiber orientation, can be measured. This study applied the proposed method to a quasi-isotropic CFRP laminate. After X-ray CT imaging, the sample was sectioned and polished. The fiber orientation was then measured experimentally using microscopy. The fiber orientation calculated using the proposed method agrees very well with the experimentally measured one. After demonstrating the validity of the proposed method, we applied it to a plain woven CFRP laminate. Results revealed that an invalid fiber orientation might be calculated for fibers parallel to the plane of the CT image, or for the fiber orientation of the pattern around the outer edge of CT images.     

基于涡流成像的碳纤维增强树脂基复合材料细观结构可视化

对碳纤维增强树脂基复合材料（CFRP）的细观结构成像方法进行了研究,利用涡流成像技术实现了CFRP层合板中纤维方向及纤维缺失、褶皱和空隙过大等缺陷的可视化。首先通过有限元仿真和电路理论分析了CFRP板中涡流的生成机制和分布特性,阐述了基于涡流法的CFRP细观结构成像机制。然后介绍了用于扫描成像的高频涡流检测（HF-ECT）实验系统并确定了涡流探头的形式及其参数。最后利用涡流成像技术分别对单层板、正交层合板和四方向斜交层合板进行了检测,绘制了涡流检测（ECT）信号的三维伪彩图并得到了清晰的纤维纹路分布。通过引入滤波去噪技术和二维快速傅里叶变换（2D-FFT）对图像进行进一步处理,提高了图像分辨率并完成了不同方向上纤维纹路的分离,从而实现对层合板每单向层中缺陷的精确定位。     

An Experimental Study of the Influence of in-Plane Fiber Waviness on Unidirectional Laminates Tensile Properties

As one of the most common process induced defects of automated fiber placement, in-plane fiber waviness and its influences on mechanical properties of fiber reinforced composite lack experimental studies. In this paper, a new approach to prepare the test specimen with in-plane fiber waviness is proposed in consideration of the mismatch between the current test standard and actual fiber trajectory. Based on the generation mechanism of in-plane fiber waviness during automated fiber placement, the magnitude of in-plane fiber waviness is characterized by axial compressive strain of prepreg tow. The elastic constants and tensile strength of unidirectional laminates with in-plane fiber waviness are calculated by off-axis and maximum stress theory. Experimental results show that the tensile properties infade dramatically with increasing magnitude of the waviness, in good agreement with theoretical analyses. When prepreg tow compressive strain reaches 1.2%, the longitudinal tensile modulus and strength of unidirectional laminate decreased by 25.5% and 57.7%, respectively.     

Measurement of fibre waviness in industrial composite components

Fibre orientation is measured from polished sections of the unidirectional plies of two industrial CFRP components made by resin transfer moulding (RTM) or prepreg/vacuum consolidation. The image analysis technique described by Creighton et al. [Composites: Part A 2001; 32: 221–229] is used to determine the fibre orientation over typically 5 × 5 mm sections. The standard deviation in fibre orientation angle is in the range 0.6–1.8°, being smallest for in-plane waviness of the prepreg component. The length and width of the waviness region along and transverse to the fibres is characterised using autocorrelation. The length is in the range 1.1–4.4 mm, being significantly greater in the prepreg than in the RTM component. The width is in the range 0.37–1.30 mm and is broadly similar across the sample types. It is demonstrated that the image analysis method can also be applied to X-ray images, giving good agreement with results from the polished prepreg samples.     

Detection of in-plane fiber waviness in cross-ply CFRP laminates using layer selectable eddy current method

This paper presents an eddy current testing method to detect in-plane fiber waviness in cross-ply carbon fiber reinforced plastic (CFRP) laminates. We propose a method which has high sensitivity to presence of in-plane waviness and can select layers to be inspected. The probe was used to detect artificially induced in-plane waviness in cross-ply CFRP laminates. It was observed that obtained signal has extreme value at the edges and vertex of the waviness, which implies the possibility of precise identification of waviness location. Detectability of subsurface waviness was investigated using 20 layer cross-ply laminate with in-plane waviness at different depths. Experimental results showed that in-plane waviness 18 layers away from the surface could be detected. The minimum misalignment angle of the detected waviness was 7.4°. The effectiveness of the probe and physical background of the obtained signals were verified by finite element method analyses.     

Torsion fatigue behavior of unidirectional carbon/epoxy and glass/epoxy composites

This paper presents results of the feasibility of carbon/epoxy composites (CFRP) as a future helicopter flexbeam material. Torsional behaviors of unidirectional CFRP and glass/epoxy composites (GFRP) with the same resin matrix were investigated. The initial torsional rigidity of CFRP was almost identical to that of GFRP. The torsional rigidities calculated using finite element analyses (FEA) agreed with the experimental results: the torsional rigidities are governed mainly by the material’s shear stiffness. Torsion fatigue tests were also conducted by controlling the angle of twist of the sinusoidal wave under a constant tensile axial load. No catastrophic failure occurred with either GFRP or CFRP, although decreased amplitudes of torque and torsional rigidities were observed according to the number of cycles. Results of X-ray CT inspections and numerical calculation by FEA revealed that degradation of a torsional rigidity is caused mainly by splitting crack propagation along the fiber direction. The torsion fatigue life of CFRP was superior to that of GFRP. Consequently, results confirmed that CFRP exhibits excellent properties as a torsional element of a helicopter flexbeam in terms of torsional rigidity and tension–torsion fatigue behaviors.     

Effects of material and geometric nonlinearities on the tensile and compressive behavior of composite materials with fiber waviness

The effects of fiber waviness on the nonlinear behavior of unidirectional composites under tensile and compressive loadings have been investigated theoretically and experimentally. Unidirectional composites examined were composed of continuous fibers with sinusoidal waviness in a matrix. As a consequence of material and geometric factors, both the tensile and compressive behavior of these composites was generally nonlinear under finite deformation. Analytical models were proposed for predicting the nonlinear tensile and compressive behavior as a function of fiber waviness for three types of fiber waviness pattern: uniform, graded and localized fiber waviness. The material and geometric nonlinearities due to fiber waviness were incorporated into the models based on complementary energy density and an incremental method. Specimens with various degrees of fiber waviness were fabricated. Tensile and compressive tests were conducted on the specimens to obtain the elastic properties and behaviors of the composite materials with fiber waviness. The experimental results were in good agreement with the predictions.     

Time-dependent out-of-plane deformation of UD-CFRP in humid environment

Time-dependent out-of-plane deformation of UD-CFRP (unidirectional CFRP laminate) caused by subjection to a humid environment was examined and analyzed. The UD-CFRP plate showed unpredictable geometrical variation with time in a humid environment, like asymmetric materials. The unpredictability was caused by non-uniform fiber distribution in the thickness direction of the specimen. A three-dimensional diffusion-stress coupling analysis considering the non-uniform fiber distribution was conducted based on finite element analysis. The analytical results showed very good agreement with the experimental results. Furthermore, the relationship between the non-uniform fiber distribution and the out-of-plane deformation with time was obtained quantitatively.     

FEM-aided identification of gauge factors of unidirectional CFRP through multi-point potential measurements

Carbon fiber reinforced plastic (CFRP) has electrical conductivity in both the parallel and transverse directions of the fiber. Because an electrical network may be changed with the applied strain, the electrical conductivity of the CFRP will also be changed for the piezoresistivity. Strain monitoring of CFRP can therefore be conducted, not by using an additional sensor, but by measuring the change in electrical resistance. There have been many studies on the gauge factors of unidirectional CFRPs, although significant mutual differences have been determined in the results reported. It is thought that the differences may be caused by the strong electrical anisotropy and inhomogeneity of the unidirectional CFRP. In this study, a new concept was introduced to precisely measure the gauge factors of a unidirectional CFRP. A finite element analysis was utilized to take into consideration a non-uniform electrical potential field in a unidirectional CFRP. The gauge factors were obtained as a result of minimizing the error sum of the squares of the electrical potentials between the experimental and analytical results. The gauge factor in the fiber direction was affected by this factor in the thickness direction depending on the specimen configuration. The results of the finite element analysis showed the possibility of a unidirectional CFRP showing both positive and negative gauge factors in the fiber direction.     

A comprehensive explanation of compression strength differences between various CFRP materials: Micro-meso model,predictions, parameter studies

The present work proposes a model to predict the compression strength of CFRP as a function of various material characteristics: fiber properties and volume content, non-linear matrix properties, interface properties, residual strains, fiber misalignment, ply waviness, and interlaminar properties. For this, a three-step FE-based approach is developed considering (i) fiber microbuckling, (ii) wavy ply mesobuckling, and (iii) the mutual influence of these two. This approach is then applied to investigate the relative contribution of the mentioned material characteristics on compression strength deficits of NCF (Non-crimp Fabric) CFRP with respect to prepreg, in order to identify ways to improve NCF performance.     

Effect of Yarn Distortion on the Mechanical Properties of Fiber-Bar Composites Reinforced by Three-Dimensional Weaving

A meso-structure model of fiber-bar composites reinforced by three-dimensional weaving (FBCR3DW) is proposed. Optical microscopy images of the preform structure revealed that the fibers along the circumference of the yarn cross-weave were twisted randomly due to alternating yarn winding on either side of the fiber bars during the manufacturing process. Sections of the cross-woven yarn were divided into five regions based on the twist characteristics. Stochastic function theory was used to describe the twist characteristics and to calculate the compliance tensor for each twisted yarn region. The twist characteristics and compliance tensor of each region were then introduced into a finite element model to calculate the elastic properties of the twisted yarn and FBCR3DW; unidirectional tensile stress–strain curves were generated based on the Tsai–Wu failure criterion. Several FBCR3DW specimens with randomly twisted yarns inside the weave structure were used in experimental tests. Our numerical results were in good agreement with the experimental values. Yarn distortion had a significant effect on the elastic properties and axial tensile strength of the yarn; specifically, the influence of yarn distortion on the transverse elastic modulus and transverse shear modulus of FBCR3DW was severe, whereas only a slight effect occurred with regard to the other elastic constants and unidirectional tensile properties. Thus, the proposed method provides an effective reference for modeling fiber composites with a weave structure.     

Feasibility on fiber orientation detection of unidirectional CFRP composite laminates using one-sided pitch–catch ultrasonic technique

It is important to assess fiber orientation, material properties and part defect because strength and stiffness of composites depend on fiber orientation of CFRP (carbon fiber reinforced plastics). A one-sided pitch–catch setup was used in the detection and evaluation of ultrasonic wave behavior and fiber orientation in the unidirectional CFRP composite laminates. Two Rayleigh wave transducers were joined head-to-head and used in the pitch–catch mode on the surface of the composites. The pitch–catch signal was found to be more sensitive than normal incidence backwall echo of longitudinal wave to subtle flaw conditions in the composite. Especially, one-sided ultrasonic measurement was made with using a Rayleigh wave transducers and the Rayleigh ultrasonic waves were extensively characterized in the CFRP composite laminates. Also, a conventional scanner was used in an immersion tank for extracting fiber orientation information in the unidirectional laminate. Therefore, it is thought that the proposed method is useful to evaluate integrity of CFRP laminates.     

Cross sectional shape effects on the performance of post-heated reinforced concrete columns wrapped with FRP composites

An experimental study was conducted to examine how the cross sectional shape affects the strength and ductility of post-heated reinforced concrete columns wrapped with unidirectional fibre reinforced polymer (FRP). Seventeen columns were tested under axial compression. The main variables investigated were the cross sectional shape of the columns, the presence of heat damage and the type of FRP used for repair. The columns were placed into three groups defined by columns without being subjected to heat, post-heated columns and post-heated and repaired columns. The test results showed that the load carrying capacity of post-heated FRP wrapped columns was significantly affected by the column’s original cross sectional shape. For circular sections the strength of post-heated columns was restored up to, or greater than, its original pre-heated strength. However, the strength of post-heated GFRP or CFRP wrapped square columns was recovered to some extent but not to the level of its original pre-heated strength. It was also found that the increase in the ductility of circular columns was more pronounced compared to square columns after wrapping with FRP. For all damaged columns the use of FRP did not restore the column’s stiffness which was lost due to damage caused by heating.     

Numerical Investigation of Delamination in Drilling of Carbon Fiber Reinforced Polymer Composites

Drilling of carbon fiber reinforced polymer (CFRP) is a challenging task in modern manufacturing sector and machining induced delamination is one of the major problems affecting assembly precision. In this work, a new three-dimensional (3D) finite element model is developed to study the chip formation and entrance delamination in drilling of CFRP composites on the microscopic level. Fiber phase, matrix phase and equivalent homogeneous phase in the multi-phase model have different constitutive behaviors, respectively. A comparative drilling test, in which the cement carbide drill and unidirectional CFRP laminate are employed, is conducted to validate the proposedmodel in terms of the delamination and the similar changing trend is obtained. Microscopic mechanism of entrance delamination together with the chip formation process at four special fiber cutting angles (0°, 45°, 90° and 135°) is investigated. Moreover, the peeling force is also predicted. The results show that the delamination occurrence and the chip formation are both strongly dependent on the fiber cutting angle. The length of entrance delamination rises with increasing fiber cutting angles. Negligible delamination at 0° is attributed to the compression by the minor flank face. For 45° and 90°, the delamination resulted from the mode III fracture. At 135°, serious delamination which is driven by the mode I and III fractures is more inclined to occur and the peeling force reaches its maximum. Such numerical models can help understand the mechanism of hole entrance delamination further and provide guidance for the damage-free drilling of CFRP.     

Predicting the flexural behavior of ultra-high-performance fiber-reinforced concrete

To predict the flexural behavior of ultra-high-performance fiber-reinforced concrete (UHPFRC) beams including straight steel fibers with various lengths, micromechanics-based sectional analysis was performed. A linear compressive modeling was adopted on the basis of experiments. The tensile behavior was modeled by considering both pre- and post-cracking tensile behaviors. Pre-cracking behavior was modeled by the rule of mixture. Post-cracking behavior was modeled by a bilinear matrix softening curve and fiber bridging curves, considering three different probability density functions (PDFs) for fiber orientation, i.e., the actual PDF from image analysis and PDFs assuming either random two-dimensional (2-D) or three-dimensional (3-D) fiber orientation. Analytical predictions using the fiber bridging curves with the actual PDF or the PDF assuming 2-D random fiber orientation showed fairly good agreement with the experimental results, whereas analysis using the PDF assuming 3-D random fiber orientation greatly underestimated the experimental results.     

碳纤维复合材料的微波反射特性研究

研究了各种铺层（０°、０°／９０°、±３０°、±４５°、±６０°）方式碳纤维复合材料（ＣＦＲＰ）的皮反射特性。结果表明,单向纤维铺层ＣＦＲＰ的反射率与纤维方向及层板厚度有关;交叉铺层ＣＦＲＰ的反射率很大,但比金属略小,喷涂了防雷击层的单向纤维铺层ＣＦＲＰ,电性能基本上各向员性;喷涂了防雷击层的０°、±４５°、０°／９０°ＣＦＲＰ的反射率比喷涂前有所降低。     

Zerstöurungsfreie Prüfung dünner CFK-Laminate mit innenliegenden Fehlern

Nondestructive Testing of thin CFRP-laminates with Internal Flaws Carbon fiber reinforced plastics (CFRP) are more and more used for primary aircraft structures. Therefore, nondestructive testing is of increasing importance for quality control and maintenance. Test specimens with different flaws show the accuracy by which even very small flaws in CFRP laminates can be detected when using ultrasonic (US) and X-ray-techniques. The US-measurements (C-scans) were effected by a highly resolving US-flaw detector. Details of the system and of testing methods are described. The C-scans are compared with X-ray evaluations. Internal flaws which cannot be found in a normal X-ray image due to X-ray absorption were filled with a contrast agent (TBE); afterwards they appeared very clearly on the X-ray film. A comparison of nondestructive tests and between nondestructive and destructive tests showed a good agreement.     

Experiment and analysis of unidirectional CFRP with a hole and crack as sandwich-form inhomogeneous composite

In modern mechanical industry, many designs are incorporating lightweight materials. Known as the future’s lightweight material; Carbon Fiber Reinforced Plastic (CFRP) contains a fiber structure and therefore is vulnerable to cracks and holes that may be generated during machining. However, in a mechanical structure, it requires bonding the multiple parts using bolts, rivets, etc. In other words, it inevitably requires generating holes in the mechanical structure. Because of such problems, CFRP has been attached with a normally used structural steel, to lighten the weight. It is quite difficult to understand the mechanical features of inhomogeneous composites. In order to increase the usage of inhomogeneous composites that contain multiple light weight mechanical parts, there is a necessity of mechanical basic data. This study used CFRP and SM45C that has the stacking angle [0/60/–60/0]. It has a form of a sandwich in which 5 mm thick SM45C acts as the core, with 2 mm thick unidirectional CFRP attached on both sides of it. Using inhomogeneous composites of such sandwich form, Compact Tension (CT) test and analysis took place. In the test, considering the actual material being used in mechanical industry, a hole was made on the edge of the crack and compared the results according to the location of the hole. SIMADZU corporation’s Universal Testing Machine was used in the test. Setting the test as the basis, we constructed a model for the analysis. In the analysis, like we did in the experiment, we kept in mind the fiber structure of unidirectional CFRP and the conditions of the analysis were identical to the test. As the test result, the magnitude was at its optimal when there was no hole. But if a hole had to be made, it is at its best when placed at the spot where the center of the hole is 20 mm from the edge of the crack. A model with no hole had the reaction force of 14 kN and the forced displacement was 4 mm. A model with a hole placed 20 mm from the edge of the crack had the reaction force of 11 kN with the forced displacement of 10 mm. Test results and analysis differ a lot, but the data after the section when the reaction force rapidly arises are reliable. In other words, in the process of destruction due to the crack propagation, the analysis is reliable.