Fiber Reinforced Plastic Composites Research Articles

Nanoscopic origin of cracks in carbon fibre-reinforced plastic composites

Voids and cracks can fatally degrade structural materials such as metals and ceramics but are tolerated in carbon fibre-reinforced plastic (CFRP) composites if monitored to prevent their growth to a critical size. Thus, the use of CFRPs as aeronautical structural materials requires an understanding of microscopic crack formation. However, this crack-formation mechanism remains unclear because experimental difficulties have hindered studies of relevant phenomena that occur before crack formation. Herein, we report high-resolution (~50 nm) and non-destructive three-dimensional observations of crack initiation and propagation under applied stress. This evaluation reveals that voids and cracks do not simply result from local stresses but instead occur largely through two competing nanoscale mechanisms, namely, fibre/plastic interface debonding and in-plastic crack initiation. Therefore, nanoscopic insights into these heterogeneities are essential for controlling crack initiation and determining reasonable safety margins for CFRP composite use.

Review of natural fiber-reinforced engineering plastic composites, their applications in the transportation sector and processing techniques

Interest in natural fiber-reinforced polymer (NFRP) composites is growing rapidly in the transportation sector, especially as a replacement material for metals and synthetic fiber composites. The heightened interest is directly related to a need to produce lightweight and fuel efficient vehicles. Further, stringent legislation and greater environmental awareness is forcing transportation industries to select materials with a smaller carbon footprint. In such a context, NFRP composite materials are a good choice due to their low cost, low environmental impact, and relatively equivalent properties to metals and other composites. Most prior studies have examined commodity plastics such as polypropylene, polyethylene, and epoxy as the primary polymer matrix in NFRP composites and little work has addressed engineering plastics. Engineering plastics, which includes polycarbonate, polyamides, and polystyrene, are high performance thermoplastics with superior properties but relatively higher cost than commodity plastics. It has been claimed that even after recycling, engineering plastics properties are superior to those of commodity plastics, and thus, utilization of recycled engineering plastic in NFRP composites can help reduce waste and lower composite material costs. The aim of this review article is to explore the current status of engineering plastics reinforced with natural fibers such as flax, hemp, jute, and sisal and to examine their use in automotive, aerospace, and maritime applications. Properties and processing techniques of engineering plastics reinforced with natural fibers are also studied.

Evaluation of the Strength of Bolted Joints of Composite Plates

We determine the bearing strength of many-row bolted joints of layered carbon fiber-reinforced plastic composites according to the level of strains in the vicinity of a single (isolated) hole subjected to contact loading through the bolt. According to results of the tests obtained by the method of digital correlation of images, we construct a fracture diagram on the stress–strain coordinates and establish admissible bearing stresses corresponding to an increase in the residual strains of the diameter of the hole by 2%.

PEEK composites with polyimide sizing SCF as reinforcement: Preparation, characterization, and mechanical properties

In this work, the surface modification of short carbon fibers (SCFs) using polyimide (PI) as a sizing agent was conducted and fully characterized, and SCF-reinforced polyether ether ketone (PEEK) composites were obtained by extrusion and injection molding. The surface characteristics of the PI-coated SCFs were evaluated using scanning electron microscopy and X-ray photoelectron spectroscopy. The results indicated that a uniform PI sizing layer was formed on the surfaces of the SCFs. Thermogravimetric analysis results demonstrated that PI-coated SCFs had better thermal stability than commercial SCFs. The tensile strength and flexural strength of the PI-coated SCF/PEEK composites showed improvements of 11.8% and 16.6% compared with the commercial cases, which were attributed to the PI sizing treatment effectively improving the interfacial adhesion between the SCF and the PEEK matrix. Dynamic mechanical analysis and the morphologies of tensile fracture surfaces suggested better interfacial adhesion between the fibers and the PEEK matrix, which were in good agreement with the mechanical properties. Due to the convenient processing of PI sizing as well as the effectively improved mechanical properties of the composites, the PI-sizing methodology has great potential application in the field of fiber-reinforced high-temperature engineering plastics composites.

Rotary ultrasonic machining of carbon fiber–reinforced plastic composites: effects of ultrasonic frequency

Rotary ultrasonic machining (RUM) is effective and efficient in cutting carbon fiber–reinforced plastic (CFRP) composites. With ultrasonic vibration assistance, both machining efficiency and machining effectiveness can be improved. Ultrasonic vibration has two major variables (including ultrasonic amplitude and ultrasonic frequency), which play important roles in RUM processes. Other intermediate variables, such as ultrasonic power and indentation depth, are directly related to these two variables. Effects of ultrasonic vibration amplitude have been extensively investigated in RUM hole making and surface machining processes. However, the effects of ultrasonic frequency in RUM have not been reported. The effects of ultrasonic frequency under different combinations of tool rotation speed, feed rate, and depth of cut on output variables, including cutting forces, surface roughness, and machined surface characteristics, are investigated, for the first time, in this study. The critical frequency is analyzed for the RUM process. Three different levels of ultrasonic frequencies are generated and applied to the RUM process. The kinematic motions of abrasive grains in RUM with different frequencies are analyzed and discussed. The results show that RUM with higher frequency of ultrasonic vibration is a more effective machining process, leading to cutting force reduction and the machined surface quality improvement.

Numerical treatment of fiber reinforced plastic crushing mechanism in virtual performance solution: Possibilities and limitations

This paper describes both experimental and numerical investigations on the crushing behavior of fiber-reinforced plastic composites (triaxial-braid) for almost rectangular crush specimens. Fiber-reinforced plastic in general is an enormously versatile material group which allows engineers to adapt it to varying tasks, but causes large challenges regarding a trustable numerical forecast in structural simulations. The virtual performance solution (Virtual PAM-CRASH) solver can manage the crushing behavior of fiber-reinforced plastic by a phenomenological implementation, the innovation PLY100. It's possibilities as well as limitations are described briefly in the present paper. These limitations are the incentive for a new approach based on shell elements, which are arranged in a stacked shell order. The delamination process is modelled by using cohesive interface contacts. General ply cards, which are derived from standard material characterization tests, are improved via the inverse method, called MCMC method with an integrated Metropolis algorithm. Besides the numerical simulations, a large experimental drop-weight-tower study was conducted, of which a small extract is presented here. In conclusion, the new numerical approach is able to predict the crushing mechanism and also the mechanical properties during crushing accurately. Small deviations between numerical and experimental results are presented and will be addressed in a future publication.

Composite Materials Made of Basalt Fibres and Biobased Matrix Material for Technical Applications

Basalt fibres are inorganic man-made fibres which, in fibre-reinforced plastic composites (FRP), represent a cost-effective alternative to carbon fibres. Within petrochemical thermoplastic matrices, basalt fibres are established in technical applications. The use of biobased polyamides as matrix material is an innovative approach for the production of FRP with a high proportion of renewable raw materials. In addition to material selection, the compound manufacturing process also defines the properties of the FRP. At the Institute of Lightweight Structures, extensive investigations are carried out into material development and optimization of the associated process technology. As a result, basalt fibre-reinforced, biobased polyamides are produced whose mechanical relevant properties are above those of petrochemical based glass fibre-reinforced polyamides.

NDE Detection Techniques and Characterization of Aluminum Wires Embedded in Honeycomb Sandwich Composite Panels Using Terahertz Waves.

For many years, scientists have been aware of the importance of terahertz waves (T-rays), which have now emerged as an NDE (nondestructive evaluation) technique for certain ranges of the electronic spectrum. The present study deals with T-ray scanning techniques of honeycomb sandwich composite panels with a carbon-fiber-reinforced plastic (CFRP) skin as well as the refractive index (n), and the electrical conductivity (α) of glass fiber-reinforced plastic (GFRP) composites. For this experiment, the degree of penetration to FRP composites is investigated for the THz transmitted power based on the angle in the electric field (E-field) direction vs. the direction of the unidirectional carbon fibers. Also, when CFRP skin honeycomb sandwich panels are manufactured for use in aerospace applications, aluminum wires are twisted together into the one-sided surface of the honeycomb sandwich panels to protect against thunderstorms. The aluminum wires are partly visible because they are embedded in the CFRP skin on the honeycomb sandwich panels. After finishing work with a paintjob, the wires become invisible. Thus, detecting the aluminum wires is a key issue for product monitoring. Based on a simple resistor model, an optimal scanning method is proposed to determine the preferred scan orientation on the baseline of the E-field in the direction of fibers to evaluate the level of transmission of T-rays according to the frequency bandwidth. Thus, the combination of angles required to detect the aluminum wires embedded with carbon fibers on the surface of the composite panels can be determined.

Considering the stress concentration of fiber surfaces in the prediction of the tensile strength of unidirectional carbon fiber-reinforced plastic composites

The tensile strengths of unidirectional carbon fiber-reinforced plastic (CFRP) were predicted by using a spring element model that considers the surface stress concentration on fibers caused by a fracture site in an adjacent fiber. The surface stress concentration on the fibers was experimentally evaluated by implementing multi-fiber fragmentation tests in conjunction with a spring element model simulation. Four types of epoxy materials were utilized to explore the effects of matrix polymer properties on the surface stress concentration of the fibers. The size scaling results, coupled with the results of the spring element model simulation, designed to take into account the surface stress concentration, were reasonably consistent with the experimental data on the tensile strengths of the unidirectional CFRP composites, regardless of the differences in the matrix mechanical properties. Possible mechanisms by which additional stress concentration is generated on an intact fiber surface were analyzed numerically using the finite element method.

Collapse safety margin-oriented seismic retrofit strategy for corroded reinforced concrete frames using fibre reinforced plastics

Considering residual service life, this paper presents a collapse safety margin-oriented seismic retrofit strategy for corroded reinforced concrete (RC) frames using fibre reinforced plastics (FRP). With the assumed uniform corrosion model, corrosion-induced initial damage combined with subsequent earthquake-induced damage is identified by the multi-mode-based global damage model developed previously. The collapse-level earthquake intensity determined by incremental dynamic analyses (IDA) with the damage model and the maximum considered earthquake (MCE)-level intensity considering residual service life are combined to generate the time-variant collapse safety margin assessment of corroded RC structures. Based on this assessment, the collapse safety margin-oriented FRP seismic retrofit strategy is proposed and demonstrated on a 4-storey frame. The damage model originally developed for earthquake scenarios has also exhibited its rationality for characterising corrosion-induced initial damage and its influence on coupled damage development with subsequent earthquake excitation. Seismic retrofitting with FRP composites should consider the effects of the corrosion development stage, target collapse safety margin and residual service life. FRP retrofits can cause decreases in the MCE-level collapse probability and increases in the collapse resistance of corroded RC structures. Retrofitting carried out at different times achieves different efficiencies and different time-variant collapse safety margins within the residual service life.

A simplified modal-based method to quantify delamination in carbon fibre-reinforced plastic beam

ABSTRACTThis study contributes to our knowledge about the detection and recognition of carbon fibre-reinforced plastic (CFRP) composites by using a modal analysis technique. Delamination was artificially induced during the manufacturing process of CFRP beams so that the dimension and location of the artificial delamination were known. The first three longitudinal bending modes were considered for detecting, locating, and evaluating the delamination. Contrary to what is commonly practised, the sensor placed on the CFRP structure had a large mass in order to contribute actively to restricting the delamination and hence modifying the structure’s natural frequencies, thus making it much easier to monitor its health condition. The technique was implemented on a beam with a delamination gap of 9% of the beam’s length, positioned within the span of the beam. The results demonstrate that the proposed technique can be effective not only to locate the delamination but also to measure its extent accurately.

Review of PTB Measurements on Emissivity, Reflectivity and Transmissivity of Semitransparent Fiber-Reinforced Plastic Composites

Due to several excellent material properties, fiber-reinforced plastic (FRP) composites are expediently used in many applications, e.g., in the field of renewable energy and in oil, gas, and transportation applications. They show excellent mechanical stability, low weight and fatigue, and high corrosion resistance. However, their full potential for exploitation, as well as the lifetime of FRP-based structures, is limited due to certain defects and damage mechanisms. One of the most important methods used to ensure the quality of FRPs is non-destructive testing and active thermography. A prerequisite for quantitative active thermography is accurate knowledge of the optical properties of the investigated material (i.e., its spectral emissivity, reflectivity, and transmissivity). The objective of PTB as a partner organization within the European EMRP project titled “Validated Inspection Techniques for Composites in Energy Applications” was to improve the state of the art of this technique. One of the goals in doing so was to significantly reduce the uncertainty of emissivity measurements of FRP materials in the visible and infrared wavelength ranges. Achieving a target value of lower than 0.01 of the emissivity of partially transmitting materials is very challenging, especially at temperatures close to room temperature. Different experimental setups at PTB were employed for these measurements: one setup for spectral emissivity measurements in air and the other for diffuse reflectivity and transmissivity measurements. In this paper, we give a review of PTB measurements on emissivity, reflectivity and transmissivity of semitransparent FRP composites. Part of this work has already been published in Adibekyan et al. (emissivity, reflectivity and transmissivity of semitransparent fiber-reinforced plastic composites. https://www.ndt.net/article/dgzfp-irt-2017/papers/17.pdf, 2018). Here, we present the complete set of data for seven technical relevant materials and compare the results. The directional spectral emissivity was determined at a nominal sample temperature of 40 °C, at angles of observation from 10° to 70° with respect to the surface normal and in a wavelength range from 5 µm to 25 µm. In addition, these spectrally and directionally resolved measurements allow to calculate the total directional emissivity and the hemispherical emissivity. For the determination of the directional–hemispherical spectral transmissivity and directional–hemispherical spectral reflectivity a gold-coated integrating sphere and a vacuum FTIR spectrometer were used. The directional–hemispherical spectral transmissivity was measured under an angle of incidence of 0°; the directional–hemispherical spectral reflectivity was measured under an angle of incidence of 10°. These investigations were performed in the spectral range from 1.0 µm to 16.7 µm and compared in the overlapping wavelength range; the emissivity measurements were performed using the setup for spectral emissivity.

Development of a Decoupled Multiscale Analysis Method for Woven Composites

In this study, development of a decoupled multiscale analysis method for woven composites is conducted. To this end, an elastic-viscoplastic macroscopic constitutive model which is able to express strong anisotropy of composites is introduced, and the material parameters in the constitutive model are determined based on the results of triple-scale homogenization analysis. Moreover, the constitutive model is implemented in the finite element analysis code LS-DYNA. The developed method is applied to 3-point bending analysis of plain-woven carbon fiber-reinforced plastic (CFRP) composites with various types of laminates configurations. It is shown that the present method can analyze their different behavior depending on the laminate configuration with greatly reduced computational costs.

Development and Validation of Multiscale Thermo-Elasto-Viscoplastic Analysis Method for Plain-Woven Composites

In this study, the analysis method for thermomechanical properties of plain-woven composites is developed, and applied to thermoelastoviscoplastic analysis of plain-woven glass fiber-reinforced plastic (GFRP) composites. For this, a time-dependent constitutive equation depending on temperature for matrix materials is incorporated into the micro/meso/macro-scale thermo-elastic homogenization method for plain-woven composites developed by our research group. This method enables us to analyze thermoelastoviscoplastic properties in not only fiber bundles but also fibers and matrix materials in fiber bundles, as well as macroscopic thermal properties. This method is then applied to the thermal expansion analysis of a plain-woven GFRP composite subjected to a macroscopic temperature change from 25°C to 80°C before it is cooled to 25°C. Comparing the analysis results with experimental data, we validate the present method. It is also shown that the present method can evaluate themal residual stress and strain in the composite.

Evaluating random vibration assisted vacuum processing of carbon/epoxy composites in terms of interlaminar shear strength and porosity

In the manufacturing process of carbon fiber-reinforced plastics composites, if the cure pressure that can form the hydrostatic pressure of resin is transferred inhomogeneously to the interior of composite structures, the critical imperfections such as voids and delamination which damage the mechanical properties and restrict subsequent use will be formed. Therefore, the present work reports the experimental research and analysis on the effect of random vibration applied to the curing system of carbon fiber-reinforced plastic laminates, mainly aiming at the reduction of the void content and improvement of mechanical performance under the vacuum pressure. The range of acceleration of vibrations was covered from 5 g to 15 g, for different periods of random vibration. Conducting a facile thermogravimetric analysis-based methodology, the influence of random vibration on the composite density, fiber volume fraction and void content of carbon fiber reinforced plastic laminates was analyzed. And the influence of voids on the interlaminar shear strength of materials was compared in terms of the initiation and propagation of interlaminar failure. In addition, the scanning electron microscopy was employed to observe the fracture surfaces of the composite specimens, which confirmed the different behaviors of fiber–matrix interface of various curing processes. The major benefits of the current research are that, the application of random vibration during the curing process can better wet through fibers by resin and increase the fiber volume fraction while reducing the bubbles and volatile gas of composite laminates at the same time.

Robust topology optimization for multiple fiber-reinforced plastic (FRP) composites under loading uncertainties

This study proposes a non-deterministic robust topology optimization of ply orientation for multiple fiber-reinforced plastic (FRP) materials, such as carbon fiber–reinforced plastic (CFRP) and glass fiber–reinforced plastic (GFRP) composites, under loading uncertainties with both random magnitude and random direction. The robust topology optimization is considered here to minimize the fluctuation of structural performance induced by load uncertainty, in which a joint cost function is formulated to address both the mean and standard deviation of compliance. The sensitivities of the cost function are derived with respect to the design variables in a non-deterministic context. The discrete material optimization (DMO) technique is extended here to accommodate robust topology optimization for FRP composites. To improve the computational efficiency, the DMO approach is revised to reduce the number of design variables by decoupling the selection of FRP materials and fiber orientations. In this study, four material design examples are presented to demonstrate the effectiveness of the proposed methods. The robust topology optimization results exhibit that the composite structures with the proper ply orientations are of more stable performance when the load fluctuates.

Effect of hygrothermal aging on the interlaminar shear strength of a carbon fibre composite

Carbon Fibre Reinforced Plastic (CFRP) composites are used many applications in aerospace structures due to their high specific strength and stiffness. Composite aircraft structures are usually exposed to a range of environmental conditions such as hygrothermal, UV radiation, chemical environments, biological condition etc., in their service life and may degrade. Moisture absorption at different hygrothermal conditions and its effects on properties of CFRP composites is a useful parameter to designers and application engineers. In this study, UD carbon fiber/epoxy composite laminates were fabricated (HexPly® M21E/34%/UD/194/IMA) from prepreg using standard autoclave process. Inter Laminar Shear Strength (ILSS) test specimens were cut and prepared from theses laminates. Hot-wet specimens were aged at two different conditions; 75 0C/ 85% RH, 55 0C/ 100% RH until attaining moisture absorption saturation. Three-point bend tests were conducted as per ASTM test standard specifications using a 25 KN servo-hydraulic test machine to determine ILSS properties for aged and unaged material. Respective hot-wet conditions were maintained even during the mechanical tests. Tests results show that moisture absorption rate increases gradually and attains saturation at about 1 wt. % under both conditions. Inter laminar shear strength reduced significantly by about 33% due to presence of moisture. Microscopic observation of failed samples also investigated to study the fracture behavior and reported in this paper.

Novel patch shape design to repair π/4 quasi-isotropic E-glass/epoxy laminate under uniaxial tensile loading condition

The entitled technical subject in its comprehensive and broadly viewed study is presented by stating that the prevailing glass fiber–reinforced plastic composites play a vital role in replacing the metallic structures in automotive, aerospace, and marine applications and their more usage necessitates the better understanding of such structure’s damage and effective repair practices of the same. In the course of repairing the composite structures, optimum usage of patch material helps to maintain the original strength, eliminate “over design,” and reduce contribution to the substantial performance change of original structure. Optimum usage of repair material brings down the significant percentage of mass and repair cost. In order to apply the concept of optimum patch shapes over actual structures, it is important to design such effective shapes which are equivalent to the conventionally proven repair designs in strength perspective. This technical paper describes one of such ways to design the optimum patch shape using finite element analysis (FEA) tool and in comparison with one of the conventional repair practices, like circular patching, used circular, rectangular, square, elliptical and trapezoidal patch shapes, each of which has its own merits and demerits when compared to the other. But, none of them was arrived at in the perspective to use the minimum material to repair the damage. The object of this work is carried out in order to arrive at the optimum patch shape (minimum material in its in-plane direction) in repairing the damaged structure and it has been demonstrated using π/4 quasi-isotropic E-glass/epoxy specimen with hole using FEA. Moreover, the effectiveness of optimum patch shape design in “strength” and “load transfer efficiency” perspectives is experimentally proven and highlighted by the scope of repair material saving (36%) in repair practice by the usage of this optimized approach, as well.

Friction and wear characteristics of fibre-reinforced plastic composites

The dominance of fibre-reinforced plastics in industries has enhanced the need for research to develop new composites and assess their properties. Among the various property analyses, tribological characteristics of the composites help to understand the friction and wear behaviour. This article presents a comprehensive survey on the tribological behaviour of polymeric composites, which includes the influence of various working parameters such as the velocity of sliding, sliding distance, pressure applied and temperature on the friction and wear. The article also addresses about the importance of the composite constituents such as fibre composition, fibre geometry and fibre orientation on the tribological behaviour. The major reasons causing the tribological failure like the debonding between the fibre and the matrix, failure of the matrix and the fibre are also extensively addressed. This review would be an insight to the industries and tribology researchers towards achieving optimum design for the components made of polymers.

Evaluation of a hybrid cryogenic deburring method to remove uncut fibers on carbon fiber-reinforced plastic composites

This paper presents an evaluation of hybrid cryogenic processes used to eliminate burrs on the exit side of drilled holes in carbon fiber-reinforced plastic composites. These hybrid cryogenic processing methods do not produce environmental hazards and they involve adding a longitudinal ultrasonic motion to the drill cutter and water into a cryogenic environment during the deburring process. There are four methods used for this hybrid cryogenic deburring process: the first method involves ultrasonic vibrations and a cryogenic environment; the second method involves water and a cryogenic environment; the third method involves water, ultrasonic vibrations and a cryogenic environment; and the fourth method involves water, ultrasonic vibrations, a cryogenic environment, and a backup ice layer. The methods were compared in terms of the percentage of removed burrs; the microscopy images of removed burrs are presented in this paper. According to experimental investigations, the fourth method exhibits up to 100% efficiency in removing burrs, followed by the third, the second, and the first method.