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The mechanical properties of polymer composites (PC), in particular, carbon fiber reinforced plastics with operating temperatures up to 150°C, are studied and compared. A number of compression tests of the composite samples reinforced by carbon fiber with a solvent-based and adhesive matrix are carried out according to ASTM D 695 using a thermal test chamber. The strength characteristics of PC samples reinforced by the carbon fiber and adhesive matrix T-107 are found to be higher than that of modern carbon composites with solvent- and melt-based matrices. [ABSTRACT FROM AUTHOR]

The durability of hybrid carbon fiber reinforced plastics (CFRP)/aluminum alloy (Al) structures is receiving increasing attentions in engineering applications. In this paper, the mechanical performance of CFRP/Al electromagnetically riveted lap joints after exposure in neutral salt spray environment for various ageing time was studied. The shear and fatigue tests were conducted. The mechanical property degradation laws and failure mode evolution were obtained. The results showed that the shear and fatigue properties of the CFRP/Al electromagnetically riveted lap joints linearly decreased with the increase of ageing time. In addition, the fatigue test results showed that three weeks was the critical ageing time for failure mode changing. Fatigue crack initiated from the riveted hole (below 3 weeks) and the edge of the overlapping region (over 3 weeks) of Al sheet, respectively. Meanwhile, the fracture analysis showed that the cracks of specimens after long-time ageing (over 3 weeks) were initiated from the several corrosion pits simultaneously. [ABSTRACT FROM AUTHOR]

Laser ultrasonic testing (LUT) can realize contactless and instantaneous non-destructive testing, but its signal-to-noise ratio must be improved in order to measure carbon fiber reinforced plastics (CFRPs). We have developed a mid-infrared (mid-IR) laser source optimal for generating ultrasonic waves in CFRPs by using a wavelength conversion device based on an optical parametric oscillator. This paper reports a comparison of the ultrasonic generation behavior between the mid-IR laser and the Nd:YAG laser. The mid-IR laser generated a significantly larger ultrasonic amplitude in CFRP laminates than a conventional Nd:YAG laser. In addition, our study revealed that the surface epoxy matrix of CFRPs plays an important role in laser ultrasonic generation. [ABSTRACT FROM AUTHOR]

In this paper, an ultrafast femtosecond laser was used to tailor the surface of the PEEK/CFRP (carbon fiber reinforced plastics with polyether ether ketone as resin matrix), using a nanosecond laser as a contrast. By analyzing the effect of femtosecond and nanosecond laser tailoring on the surface morphology, surface chemical composition and wettability of PEEK/CFRP, the internal reasons for the improved of bonding performance of heterojunction between PEEK/CFRP and Al–Li alloy by laser pretreatment were explored. As a result, the shear strength increased considerably from 4.9 MPa to 16.1 MPa and 29.5 MPa after nanosecond laser and femtosecond laser pretreatment, respectively. Finally, the fracture surface and failure modes of heterojunction bonded joints were compared, and the different mechanisms of femtosecond and nanosecond laser tailoring PEEK/CFRP surface were explained. Femtosecond laser tailoring is a competitive process for improving the bonding properties of the heterojunction between PEEK/CFRP and Al–Li alloys because of its productivity, controllability and lack of pollution. [ABSTRACT FROM AUTHOR]

Multifunctional composites combine structural and other physicochemical properties, with major applications in aeronautical, space, telecommunication, automotive, and medical areas. This research evaluates electromagnetic properties of multifunctional composites based on glass fiber woven fabric pre-impregnated with epoxy resin laminated together carbon fiber non-woven veil metalized with Ni. In this way, searching for possible application as radar absorbing structures or electromagnetic interference shielding structures. The scattering parameters, in the frequency range of 8.2 to 12.4 GHz, show that the epoxy resin/glass fiber prepreg allows the transmission of the electromagnetic waves through its microstructure, independently of the glass fiber reinforcement orientation (98% transmission, S24 = −0.09 dB). However, the carbon fiber/Ni veil shows highly reflector behavior (91% reflection, S22 = −0.43 dB). Energy dispersive spectroscopy of the veil, before and after nitric acid attacks, confirmed the Ni coating removal from the carbon fiber surface. Still, the scattering parameters show reflector behavior (77% reflection, S22 = −1.13 dB), attributed to the electrical conductivity of carbon fibers. Multifunctional composites based on glass fiber/epoxy/carbon fiber/Ni veil laminates were processed by hot compression molding. The scattering parameters show that the laminates do not behave as good radar absorbing structures. Nevertheless, the laminates present promising results for application as light weight and low thickness structural composites with electromagnetic interference shielding effectiveness (91.4% reflection for 0.36 mm thickness and 100% for ~ 1.1 mm) for buildings, aircraft, and space components.

Electromagnetic riveting (EMR) has received increasing attention as a new kind of riveting technique in engineering industry. In this paper, EMR process was used to connect carbon fiber reinforced plastics (CFRP) and aluminum alloy (Al) hybrid joints. The mechanical behaviors (including static and fatigue properties) of the electromagnetic riveted lap joints were comprehensively investigated. The microstructure observation and the mechanical property tests were conducted to evaluate the joints performance. The mechanical test results showed that the failure modes of shear specimens were bending of the Al sheet and damage of the CFRP sheet, which was caused by rivet squeezing effect under static loading. However, the failure mode of fatigue specimens under each stress level was all ruptured at the Al sheet and the fracture analysis showed that cracks firstly initiated around the hole of Al sheet. This was caused by the fretting wear between Al sheet and rivet under cyclic loading. Two-parameter Weibull distribution was employed to analyze statistically fatigue cycles results. The S-N curves were drawn for different reliability levels (10%, 36.8%, 50% and 90%) for engineering applications. In addition, Hysteresis loop analysis implied that the specimens had no obvious flaw after EMR. [ABSTRACT FROM AUTHOR]

Electromagnetic riveting (EMR) process is suitable for composite joining due to small damage. In this paper, the EMR experiments were conducted to obtain the fatigue samples. Specimens with normalized driven head dimensions were tested to obtain the fatigue behaviors. The experimental results showed that the driven head dimensions had a remarkable effect on fatigue property. The fatigue life increased first and then decreased with the increasing of D/D 0 ( D and D 0 are the diameters of rivet driven head and original shaft, respectively). The electromagnetic riveted specimens with D/D 0 = 1.5 had the best fatigue properties. The fatigue failure analysis showed that the specimens had three typical fatigue failure modes: failure in the rivet, failure in the Al sheet and failure in the CFRP and Al sheets as influenced by driven head dimensions and stress levels. [ABSTRACT FROM AUTHOR]

This study focuses on the one-step co-curing process of carbon fiber reinforced plastics (CFRP) joined with a steel plate to form a hybrid structure. In this process CFRP laminate and bond to the metal are realized simultaneously by resin infusion, such that the same resin serves for both infusion and adhesion. For comparison, the commonly applied two-step process of adhesive bonding is studied. In this case, the CFRP laminate is fabricated in a first stage through resin infusion of Non Crimp Fabric (NCF) and joined to the steel plate in a further step through adhesive bonding. For this purpose, the commercially available epoxy-based Betamate 1620 is applied. CFRP laminates were fabricated using two different resin systems, namely the epoxy (EP)-based RTM6 and a newly developed fast curing polyurethane (PU) resin. Results show comparable mechanical performance of the PU and EP based CFRP laminates. The strength of the bond of the co-cured samples was in the same order as the samples adhesively bonded with the PU resin and the structural adhesive. The assembly adhesive with higher ductility showed a weaker performance compared to the other tests. It could be shown that the surface roughness had the highest impact on the joint performance under the investigated conditions. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced plastics (CFRP) have good physical properties, such as high specific strength and high specific modulus. However, cutting delamination, tearing and burr, etc. often occur in the machining process of CFRP, which results in the uncontrollability of machining surface quality and serious tool wear. In this paper, milling of CFRP with a staggered cutter was carried out, the cutting-edge radius was investigated in order to characterize the tool wear; the effect of the cutting-edge wear radius on the milling force and surface quality was found, and the formation mechanisms of the 3D surface topography and surface defects were analyzed under the wear state. Experimental results showed that the wear of the staggered PCD cutter was mainly concentrated in the cutting-edge area. With the increase in milling length, the radius of cutting edge gradually became largeer under the action of abrasive wear mechanism, and the flank wear was not obvious. With the intension of tool wear, milling force gradually increased and the machining surface quality of the CFRP deteriorated distinctly, i.e., defects such as bare fiber fracture, groove and hole appeared, and burrs were gradually generated on the workpiece surface. Finally, through a comparative analysis of cutting performance, it was found that the staggered PCD cutter possessed better performance for wear resistance and burr suppression than the straight-teeth cutter. This finding can provide theoretical and technical support for improving the machining quality of carbon fiber composite materials. [ABSTRACT FROM AUTHOR]

The modern Industrial Furnace Technology is characterized by the design of new furnaces for high strength metals and carbon fibers for the light weight construction concepts in automotive industry. The furnaces for the annealing of modern high strength metals (steel, Al), are characterized by innovative cooling sections, with high cooling rates and a concomitant homogeneity, which are controllable for different steel or aluminum grades, variable widths and thicknesses for high productivity strips plants for products used in automotive industry. Resulting from this intensive research and development concerning the improvement of the local heat transfer knowledge for extremely high heat fluxes with gas, mist and/or water cooling is necessary. This research is driven by a competitive situation between steel and aluminum and since a few years also carbon fiber reinforced plastic (CFRP). On the other hand steels with a cheap alloying concept are used successfully in combination with the press hardening technology. The furnaces for the different applications in light weight design are very different between the materials (steel, Al, CFRP) due to different temperature ranges, principles of heat transfer and thermal operations. The environmental aspect of industrial furnaces requires the reduction of the specific CO 2 -Emissions which can be realized with advanced technologies improving the energy efficiency of fuel heated furnaces (high temperature air preheating with recuperative and regenerative burner concepts). For the overall CO 2 -emission reduction of 90 % up to 2050 related to 1990 actually no proved concepts are available. [ABSTRACT FROM AUTHOR]

This research is focused on the use of active infrared thermography as a non-destructive testing technique for damage detection in carbon fiber reinforced plastics (CFRPs). The aim of this study is to examine the efficiency of various mathematical methods in thermographic data processing, with respect to the thermal excitation method and the type of artificial defect in the CFRP specimens. We applied two techniques of active infrared thermography to CFRP samples with artificial cracks and internal delaminations at known locations. An infrared camera recorded the temperature field and generated a sequence of thermal images. To reveal the defects of the CFRP laminate, the thermograms were processed (a) as 2D images, and (b) as if each pixel was a 1D signal over time. We present representative experimental results, which illustrate that the depiction of the norm of the 1st spatial derivative of temperature and the 2D wavelet transforms proved to be most efficient for crack detection, whereas the 1D Fourier and 1D wavelet transforms did not yield clear results. In contrast, delamination damages could be identified through 1D techniques because the 1D Fourier transform as well as the 1D wavelet transform were very accurate. [ABSTRACT FROM AUTHOR]

The production of carbon fiber reinforced (CFR) anionic polyamide 6 (APA6) plates by thermoplastic resin transfer molding (T-RTM) processing is highly sensitive to the presence of polymer additives such as flame retardants (FRs). However, APA6 is flammable, and therefore, it is mandatory to protect the polymer matrix, especially if the produced materials are intended for aerospace applications. Typically used FR for PA6 cannot be incorporated in the T-RTM processes. With hexaphenoxycyclotriphosphazene, an incorporable and effective FR additive for CFR APA6 was discovered. Cone calorimetry measurements indicate a char formation-based mechanism, and the flame retardancy was further confirmed with limiting oxygen index tests, classification by UL94 (V-0), and FAR25 tests. The influence of this FR on the thermal (thermogravimetric analysis and differential scanning calorimetry) and mechanical behavior (flexural, compressive and ultimate tensile strength, and Charpy impact tests) on CFR APA6 produced via T-RTM processing is evaluated and indicates no interaction with the APA6; it works as a passive filler. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This study proposes a skin-added X-lattice composite structure consisting of corrugated ribs and a very thin skin, as an alternative form of anisogrid composites, for the efficient light weight structural concept under compression loading. The high mechanical capability of skin-added X-lattice structures is numerically analyzed, followed by introduction of the related manufacturing method. This manufacturing process is composed of preparation of corrugation units and co-curing of the units with a thin skin. The results of compression tests of fabricated X-lattice panels agree with numerical predictions in terms of buckling loads and buckling modes, while skin-added X-lattice panels exhibited somewhat lower buckling load than the prediction. Feasibility of the proposed skin-added X-lattice is demonstrated in this work by fabrication and mechanical characterization. [ABSTRACT FROM AUTHOR]

The surfaces of carbon fibers were modified by hydrogen plasma treatment to investigate the consequent effects on reinforcement of polyetherimide (PEI) composites. The structural surface properties were characterized by Raman spectroscopy, XPS, FT-IR and SEM. The effectiveness of hydrogen and oxygen plasma treatments in improving the surface roughness, structure and mechanical properties of the composites was demonstrated. The results indicated that hydrogen and oxygen plasma treatment modified the carbon bonding structure and the surface roughness differently. Both an increase in the density of functional groups and changes in the carbon bonding contributed to the enhancement of the PEI matrix. SEM imaging confirmed decreased fiber pull-out for PEI reinforced with plasma-treated carbon fibers because of the enhanced adhesion between the carbon fibers and the PEI. Thus, hydrogen plasma treatment of the carbon fibers led to an enhancement of tensile properties at both room temperature and high temperature (150 °C). This study demonstrates that hydrogen plasma treatment is a promising technique for improving the mechanical properties of carbon-fiber-reinforced polymer composites. [ABSTRACT FROM AUTHOR]

It is important to know the electric current distribution in a laminated carbon fiber reinforced plastics (CFRP) plates for safety analysis of lightning strikes and for damage monitoring of plates based on changes in electrical resistance. An orthotropic electric potential function has been previously proposed, which was effective for modeling of a CFRP cross-ply laminate used in beam structures. However, that method is limited to cross-ply laminates that contain no angle plies, while CFRP plate structures typically feature angle plies. In the present study, a new analysis method is proposed for laminated CFRP plates that contain angled plies. In the new method, two or three angle plies are coupled and assumed to have orthotropic conductivity of double or triple thickness. A new method to calculate the equivalent conductivity is also proposed. The new method is applied to two types of laminates containing angle ply structures. The results obtained are compared with those from a finite difference method and from experimental measurements. The new method is shown to be effective for analysis of the electric current of laminates that contain angled plies. [ABSTRACT FROM AUTHOR]

The interference-fit joint of composite laminates is widely used in assembly of thin-walled components in aviation product, but the interference percentage has a significant effect on squeezed damage which may reduce structural reliability. An investigation is conducted into the in-plane stress distribution and initial damage mechanism of symmetrical carbon fiber reinforced plastics laminates during the interference-fit bolt installation process. Considering the elastic deformation of the bolt, a general stress distribution model around the interference-fit joint is established with complex potential method. The initial damage mechanism of carbon fiber reinforced plastics laminates is characterized and critical interference percentages without damage are obtained with the mixed damage criteria. The effects of ply orientation and interference percentage on damage mechanism of each individual layer are discussed. Then, extensive finite element models with USDFLD subroutine of interference fit process are used to analyze and simulate the stress distribution and squeezed damage which are validated by strain measurement and micrographs by experiments subsequently. It is observed that theoretical solutions fall within the finite element results. The matrix tensile damage occurs first, and the critical interference percentages decrease from 1.10% to 0.85% with bolt diameters varying from 4 to 10 mm. [ABSTRACT FROM AUTHOR]

Multifunctional composites combine structural and other physicochemical properties, with major applications in aeronautical, space, telecommunication, automotive, and medical areas. This research evaluates electromagnetic properties of multifunctional composites based on glass fiber woven fabric pre-impregnated with epoxy resin laminated together carbon fiber non-woven veil metalized with Ni. In this way, searching for possible application as radar absorbing structures or electromagnetic interference shielding structures. The scattering parameters, in the frequency range of 8.2 to 12.4 GHz, show that the epoxy resin/glass fiber prepreg allows the transmission of the electromagnetic waves through its microstructure, independently of the glass fiber reinforcement orientation (98% transmission, S24 = -0.09 dB). However, the carbon fiber/Ni veil shows highly reflector behavior (91% reflection, S22 = -0.43 dB). Energy dispersive spectroscopy of the veil, before and after nitric acid attacks, confirmed the Ni coating removal from the carbon fiber surface. Still, the scattering parameters show reflector behavior (77% reflection, S22 = -1.13 dB), attributed to the electrical conductivity of carbon fibers. Multifunctional composites based on glass fiber/epoxy/carbon fiber/Ni veil laminates were processed by hot compression molding. The scattering parameters show that the laminates do not behave as good radar absorbing structures. Nevertheless, the laminates present promising results for application as light weight and low thickness structural composites with electromagnetic interference shielding effectiveness (91.4% reflection for 0.36 mm thickness and 100% for ~ 1.1 mm) for buildings, aircraft, and space components. [ABSTRACT FROM AUTHOR]

In the present study, rotary ultrasonic machining (RUM) was adopted to perform drilling of carbon fiber reinforced plastics (CFRP) in a cryogenic environment. An L27 orthogonal array was selected to conduct experiments by varying the spindle speed (denoted as N), feed rate (denoted as f), and ultrasonic power (denoted as P). The thrust force (denoted as Fz), exit burr area, and surface roughness (denoted as Ra) were measured to evaluate the machining performance. The influence of process parameters and the regression model were derived for each output quality response. Additionally, multi-objective optimization was performed using desirability analysis, and the predicted levels were used for confirmation. The results indicated that the feed rate (f) contributed more to the thrust force (Fz) by 45.85% and a maximum thrust force was recorded at 0.1 mm/rev. A decrease in spindle speed (N) was associated with an increase in feed rate (f) and ultrasonic power (P), and it resulted in minimum exit burr area. The influence of ultrasonic power (P) was highly significant in reducing burrs with a contribution of 52.45%. Conversely, the surface roughness (Ra) of the drill holes decreased at 3000 rpm, and this was attributed to the brittle fracture of the fibers at a lower temperature. Both N (30.88%) and f (30.83%) had an equal influence on producing a better surface finish in the drill holes. Furthermore, the predicted optimal settings were used to validate the results and were found to be within 95% confidence and prediction interval. Finally, the microscopic images of tool wear, burr formation, and drill hole surface morphology were analyzed and examined. [ABSTRACT FROM AUTHOR]

In practical application, carbon fiber reinforced plastics (CFRP) structures are easy to appear all sorts of invisible damages. So the damages should be timely located and detected for the safety of CFPR structures. In this paper, an acoustic emission (AE) localization system based on fiber Bragg grating (FBG) sensing network and support vector regression (SVR) is proposed for damage localization. AE signals, which are caused by damage, are acquired by high speed FBG interrogation. According to the Shannon wavelet transform, time differences between AE signals are extracted for localization algorithm based on SVR. According to the SVR model, the coordinate of AE source can be accurately predicted without wave velocity. The FBG system and localization algorithm are verified on a 500 mm×500 mm×2 mm CFRP plate. The experimental results show that the average error of localization system is 2.8 mm and the training time is 0.07 s. [ABSTRACT FROM AUTHOR]

Machined surface quality determines the reliability, wear resistance and service life of carbon fiber reinforced plastic (CFRP) workpieces. In this work, the formation mechanism of the surface topography and the machining defects of CFRPs are proposed, and the influence of milling parameters and fiber cutting angles on the surface quality of CFRPs is obtained, which can provide a reference for extended tool life and good surface quality. Trimming and slot milling tests of unidirectional CFRP laminates are performed. The surface roughness of the machined surface is measured, and the influence of milling parameters on the surface roughness is analyzed. A regression model for the surface roughness of CFRP milling is established. A significance test of the regression model is conducted. The machined surface topography of milling CFRP unidirectional laminates with different fiber orientations is analyzed, and the effect of fiber cutting angle on the surface topography of the machined surface is presented by using a digital super depth-of-field microscope and scanning electron microscope (SEM). To study the influence of fiber cutting angle on machining defects, the machined topography under different fiber orientations is analyzed. The slot milling defects and their formation mechanism under different fiber cutting angles are investigated. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced plastics (CFRP) are extensively used in aerospace, marine, and automotive fields. A variety of imaging methods for plate structures are proposed to find the damage as early as possible and ensure the safety of the structure. However, due to the anisotropy of CFRP, current methods still have some limitations for CFRP damage imaging, such as inaccurate localization, large spots and obvious artifacts. To improve the damage imaging performance of CFRP structures, a time-of-flight based weighted imaging method is proposed in this paper. First, a time-of-flight based weighted function is designed, which can be calculated according to the time-of-flight of potential scatterers and scattering signals. Then, the Hausdorff distance between the reference signal and the damage signal is implemented as damage influencing factor to characterize the impact of the damage on the sensing path. Finally, all time-of-flight based weighted functions are multiplied with damage influencing factors and fused for damage imaging and localization. The experimental results show that this method has less artifacts and lower positioning error, and can accurately locate crack damage with different states compared with other imaging methods. [ABSTRACT FROM AUTHOR]

In this study, compression properties of composite honeycomb sandwich panel and tube structures with S-shaped reinforced core was studied. The S-shaped reinforcements was machined from conventional honeycomb core. Quasi-static compression tests showed that the peak force and energy absorption of the proposed sandwich panels were enhanced by up to 85.28% and 31.41% respectively, in comparison to the carbon fiber/aluminum honeycomb sandwich panels. Based on the experimentally validated finite element model, the effects of number of S-shaped reinforcement structures, wall thickness and rotation direction were investigated for mechanical performance of the carbon fiber/aluminum honeycomb sandwich tubes under compression. Numerical results demonstrated that the S-shaped reinforcement structure could change the load transfer path effectively. The mechanical behavior and properties of the honeycomb core with S-shaped reinforcement were significantly influenced by changing the number and distribution of S-shaped reinforcements with different rotating directions. The homodromous structure caused shear damage, while the heterodromous structure showed more stability. • A honeycomb core with S-shaped reinforcements, which could effectively enhanced peak load and energy absorption, was proposed. [ABSTRACT FROM AUTHOR]

A method for connecting a driving shaft to a three-dimensional composite disk via a polymer ring was examined to achieve stable rotation at high rotation speeds. Polyoxymethylene (POM) was adopted as the polymer ring material. Compression test was conducted to determine the limit of linear viscoelasticity, and creep resistance of POM was examined to evaluate its long-term durability. Structural design using finite element analysis including long-term creep resistance has shown that this method was predicted to be effective up to a tip speed of 1210 m/s. A three-dimensional composite disk was reinforced to three directions in accordance with the cylindrical coordinates. A trial composite disk with outer and inner diameters of 304 and 41 mm, respectively, was manufactured and tested up to a tip speed of 908 m/s. Vibration amplitudes were measured using gap and laser sensors. The vibration phenomenon and methods of suppressing vibration were discussed for higher rotation speeds.

The use of carbon fiber reinforced plastic (CFRP) has contributed in producing light-weighted and strong aircraft structures. However, the low impact resistance of CFRP makes it easier for internal damages to occur. By using thin-ply prepeg with thickness of less than 0.05 mm, laminates with smaller differences in fiber orientation angle and with the same thickness as the conventional laminates can be formed. This study investigates and compares the mechanical properties and damage behaviours between quasi-isotropic laminates with fiber orientation angle mismatch of 45 degrees (45QI) and laminates with small fiber orientation angle mismatch of 15 degrees (15QI). Both laminates are loaded in tension in 0, 7.5, 15 and 22.5 degrees. Low velocity impact tests are also conducted. From tensile testing, 15QI laminates shows more isotropic properties in strength than 45QI. Damages were observed by using microscopic and X-ray images. Crack propagation in width direction can be prevented in 15QI laminates. From low velocity impact testing, we understand that impact responses are not depending so much on the fiber orientation angle mismatch. In terms of internal damage, 15QI laminates has smaller delaminated area near the impact point compared to 45QI laminates.

Potentials of integrating new technologies, particularly carbon fiber reinforced plastics, are not fully considered conventional reengineering processes. This paper firstly addresses the benefit by integration of functions, reduction of weight and fuel consumption, substitution of rare materials and customer individuality. Consequently, these aspects have been evaluated in three case studies, comprising representative cultural goods (eg. pipe organs), components of sport-aircraft (eg. ultralight helicopters) and urban-transport (eg. multifunctional stroller-bicycle trailer). Therefore, the following methods have been applied: functional analysis, reverse engineering and measuring technologies. [ABSTRACT FROM AUTHOR]

Traditional autoclave molding process is energy-consuming and time-consuming. To deal with this issue, a novel method for curing the high performance carbon fiber reinforced plastics (CFRP) by high-pressure microwave is presented. The high-pressure microwave curing equipment has been developed, which enables the microwave energy be imported into the autoclave under high pressure, and the temperature of the component could be real-time monitored and controlled. A series of tests have been carried out to study the curing effects of this equipment. The results show that, comparing with traditional autoclave curing, the high-pressure microwave curing save energy and time significantly. Furthermore, the properties of the CFRP irradiated by microwave under the high-pressure outweigh those cured by autoclave. Specifically, there are no obvious defects in the CFRP components cured by high-pressure microwave; the CFRP exhibits more ductile behavior which cured by high-pressure microwave. The mechanical properties of the specimens are enhanced. The failure of the CFRP mainly occur in the inner of the resin matrix, which indicate that the interfacial strength is stronger. In conclusion, it is promising to improve the interfacial properties and mechanical property of CFRP in an efficient and economical way by high-pressure microwave curing. [ABSTRACT FROM AUTHOR]

The article describes research results of damping properties of carbon fiber reinforced plastics (CFRP). The effect of stress/strain levels on the damping value is studied. Research is conducted on flat samples (plates) with different lay-up schemes from 1-layered to 12-layered. The paper contains information about the modal and harmonic tests on the samples and their numerical modeling. [ABSTRACT FROM AUTHOR]

The influence of porosity on the ultrasonic wave propagation in unidirectional carbon-fiber-reinforced composite laminates is investigated based on the two-dimensional finite element analysis and measurements. Random distributions of pores with different contents and size are considered in the analysis, together with the effects of viscoelastic plies and interlaminar resin-rich regions. The transient reflection waveforms are calculated from the frequency-domain finite-element solutions by the inverse Fourier transform. As the measures for porosity characterization, the ultrasonic wave velocity, attenuation coefficient, and interlaminar interface echo characteristics are examined for 24-ply unidirectional composite laminates. As a result, the wave velocity decreases with the porosity content in a manner insensitive to the pore size. On the other hand, the attenuation coefficient increases both with the porosity content and with the pore size. The time–frequency analysis of the reflection waveforms shows that the temporal decay rate of interlaminar interface echoes at the stop-band frequency is a good indicator of the porosity content. The measured porosity-content dependence of the wave velocity is better reproduced by the numerical simulations when the interlayer interfacial stiffnesses are adjusted according to the porosity content, indicating that not only the porosity features but also the interlaminar interfacial properties vary with curing conditions. [ABSTRACT FROM AUTHOR]

The homogenization of material parameters layer by layer is a mostly sufficient method for the simulation of Lamb wave propagation in carbon fiber reinforced plastics. But since current experimental investigations of the wave behavior in undamaged CFRP structures reveal an effect called “quasi-continuous mode conversion”, which can not be reproduced with this layer-wise homogenization procedure, an improved material model for the finite element analysis of the Lamb wave propagation is presented in this work. The used 2D plane strain state model considers the random distribution of fibers in the matrix material on the microscale by homogenized subdomains and thereby properly capture the “quasi-continuous mode conversion” effect. Numerical investigations of the influence of the material model on the developing wave modes in a unidirectional layer will complete the work. [ABSTRACT FROM AUTHOR]

Fiber reinforced polymers (FRPs) show advantageous physical-mechanical, thermal, and dielectric characteristics, making them promising candidates for weight reduction in structural applications. However, machinability is often difficult because of the specificity of their structure. This paper highlights the latest advances in CFRP drilling. Key papers are analyzed with respect to workpiece materials, geometrical tool features, and input variables (such as variation in process parameters). The influence of tool geometry and process parameters on workpiece delamination and hole quality/integrity represents the primary focus of this review. In addition, some new data are presented and discussed. [ABSTRACT FROM AUTHOR]

In this paper the effect of the embroidery based textile manufacturing technology Tailored Fiber Placement (TFP) on the stiffness and stress behavior of continuous fiber reinforced plastics is analyzed. Therefore the geometrical impact of the used stitching yarn on the fiber waviness is experimentally determined in a single unidirectional carbon fiber reinforced plastic layer made by TFP. Based on experimental data a parametrical three-dimensional representative unit cell (RUC) is developed to numerically determine the local fiber volume content as well as the stress and strain distribution under uniaxial loading. The numerical results are found in good agreement with the results obtained from tensile tests. A significant variation of local strain is found in fiber direction within the RUC while due to the effect of TFP the variation of global strain is observed to be insignificant. [ABSTRACT FROM AUTHOR]

The surface machining of carbon fiber reinforced plastics materials is a challenging process, given the heterogeneity and anisotropic nature of composites, which, combined with the abrasiveness of the fibers, can produce some surface damage and extensive tool wear. The cutting temperature is one of the most important factors associated with the tool wear rate and machinability of these materials, which are also affected by the mechanical and thermal properties of the workpiece material and the cutting conditions. In this work, the cutting temperature, cutting forces, and composite surface roughness were measured under different cutting conditions for the end milling of unidirectional carbon fiber reinforced plastics. Cutting speeds ranging from 200 to 350 m/min; a feed rate of 0.063 mm/rev; fiber orientations of 0, 45, 90, and 135°; and a 0.5 mm depth of cut were considered. The results show that the cutting speed and fiber orientation have a significant influence on the cutting temperature and cutting forces. The maximum and minimum cutting forces and temperatures were achieved for fiber orientations of 90 and 0°, respectively. [ABSTRACT FROM AUTHOR]

In this study, a new cutting model for the machining of unidirectional CFRP laminates was developed to examine the mechanism for changes in the cutting forces with tool wear. This model is based on Zhang's model and divides the cutting zone into two characteristic deformation regions; chipping and pressing. Furthermore, CFRP milling tests were performed using two kinds of cutting tools: tungsten carbide (WC–Co) and polycrystalline diamond (PCD). Based on the experimental results with the developed cutting model, the net cutting forces to make a cut chip and press CFRP laminates under the flank surface of the tool were quantitatively evaluated. Consequently, it was found that the former does not depend on the progress of the tool wear, but the later increases with tool wear. Additionally, based on the results in this study, a new technique for reducing cutting forces during CFRP machining, which is based on the use of two layer tool that has a wear resistance distribution at around the tool edge was introduced. [ABSTRACT FROM AUTHOR]

The fracture toughness of carbon fiber reinforced plastic laminates was investigated in term of modifying the epoxy matrix with the pristine and functionalized stacked-cup carbon nanofibers. The results showed that the highest enhancements in Mode I (13.6%) and Mode II (45.3%) fracture toughness were achieved by adding 0.5 wt% and 1.0 wt% functionalized nanofibers, respectively. The toughening mechanism was analyzed based on the morphology evaluations of the fracture surfaces. The evidence of bridging, pull-out, peeling-off and unravelling of nanofibers were clearly observed, which contributed to dissipating more energy for crack propagation, resulting in the improvement of fracture toughness. [ABSTRACT FROM AUTHOR]

Multi-damage identification of carbon fiber reinforced plastics (CFRP) structure is very important to ensure the structural safety. In this paper, the multi-damage identification system of CFRP using fiber Bragg grating (FBG) sensors and multi-classification relevance vector machine (RVM) method was proposed. First, the multi-damage identification method was researched by an active actuation method. In addition, the structural dynamic response signals were detected by the FBG sensors. Then, the multi-damage characteristics extraction was completed by Fourier transform, reliefF, and principal component analysis algorithms. With the multi-damage characteristics as the input and their corresponding multi-damage state as the output, the multi-classification RVM model was trained to identify the multi-damage state. Finally, the multi-damage identification system was established and verified on a CFRP plate with dimensions of 500 mm $\times \,\, 500$ mm $\times \,\, 2$ mm. The results showed that the proposed multi-damage identification method can accurately identify the CFRP structural multi-damage state. This paper provided a reliable method for the CFRP structural multi-damage identification. [ABSTRACT FROM PUBLISHER]

The increasing demand for products made of carbon fiber reinforced plastics (CFRP) raises the risk of damages occurring during their use in a large variety and number. Depending on the type of damage, the product cannot be used any longer. CFRPs are characterized by high manufacturing costs and geometric individuality of the products, which limits the availability of replacement. From an economic and ecological point of view, several repair methods especially in the aerospace industry have been developed in the last years. The decision, which of these repair techniques has to be applied, depends on the extent of damage and different requirements of the product. Parts made of CFRP can often be repaired by manual removing of the damaged area followed by a re-lamination. This paper shows an actual approach to the CFRP-repair. Thereby influencing factors like the material removal, surface pretreatment and material composition will be discussed. By comparing the major factors, which lead to an increase of mechanical properties, suitable repair techniques could be defined. The optimum was found by experimental methods. [ABSTRACT FROM AUTHOR]

Carbon fiber reinforced plastics (CFRPs) structures are very susceptible to invisible damage induced by the low velocity impact. The impact area localization can be useful information for detecting this damage. In this paper, the low velocity impact area localization system using fiber Bragg grating (FBG) sensors and area localization algorithm based on extreme learning machine (ELM) were investigated. The FBG sensors were used to detect the impact signal. Fourier transform, ReliefF algorithm, and principal component analysis technology were used to extract the impact signal characteristic. The ELM technology was used to realize the impact area localization. Finally, the impact area localization system was established and verified on a CFRP plate with 240-mm $\times 240$ -mm experiment area. The experimental results showed that the proposed system made accurate identification 35 times for 36 times experiments. The area localization accuracy was 96.9%. In addition, the precision of the area localization system was 40-mm $\times 40$ -mm area. This paper provided a reliable method for CFRP low velocity impact area localization. [ABSTRACT FROM PUBLISHER]