Unidirectional Carbon Fiber Reinforced Polymer Laminate Research Articles

CFRP strengthening of web perforated CFS lipped channel short columns: An experimental study

Web perforations provided in the columns or studs of light-gauge steel buildings to meet the functional requirements, decrease the axial capacity and lead to localized failure. Hence, the aim of this study is to experimentally investigate the influence of carbon fiber reinforced polymer (CFRP) strengthening on the stability, axial strength, and stiffness of the CFS web-perforated lipped channel short columns (of 600 mm length) subjected to monotonic axial compression. A total of 20 specimens, provided with circular perforation at mid-height and at the centre of the web, were tested by adopting different CFRP skin-strengthening configurations, considering Uni-Directional CFRP (CF_UD) and Bi-Directional CFRP (CF_BD) laminas or laminates. From the tests conducted, it was observed that CF_UD and CF_UD / UD were found to result in an increase in axial capacity of 11.12% and 22.23% for the cases of single- and double-layered CFRP, respectively. Similarly, the axial stiffness increased by 9.23% and 26.07%, respectively. However, the only considered triple-layered laminate, CF_UD / UD / BD has outperformed among the entire adopted strengthening configurations, with an increase in axial strength and stiffness of 28.90% and 38.33%, respectively, in comparison to the bare steel specimens. Notably, a local-distortional interactive buckling mode of failure was noticed in the cases of both bare steel and CFRP-strengthened specimens.

Cross-scale data-based damage identification of CFRP laminates using acoustic emission and deep learning

Damage characterization of laminated composites has undergone extensive research, leading to the development of several damage models that incorporate acoustic emission (AE). However, feature extraction from AE data will cause the loss of certain information in the construction of damage models, which manifestly falls short of feature analysis in the frequency domain. In this study, a novel cross-scale data-based damage identification methodology for carbon fiber-reinforced polymer (CFRP) laminates is proposed by combining the AE technique with deep learning approach. Elementary experiments involved with single damage mode are designed to avoid blind inference, and the cross-scale correlation of AE features between component materials and CFRP laminates is established by wavelet packet transform (WPT). The time–frequency spectrums of AE signals of CFRP laminates by continuous wavelet transform (CWT), which adequately preserve the frequency domain information, are adopted as inputs to the convolutional neural network (CNN) model. The proposed methodology achieves a high accuracy of 96.3% in detecting and classifying damage modes (i.e., matrix cracking, matrix/fiber debonding, and fiber breakage) in unidirectional CFRP laminates.

Hierarchical mode I interlaminar toughening of unidirectional CFRP laminates by the synergistic effects of CNT powders and veils

While carbon fiber reinforced polymer (CFRP) composites are widely used in structural components because of their excellent mechanical properties, delamination between reinforcing plies remains a well-known Achilles heel, weakening the composite structure and limiting their more widespread applications. This work tried to improve the interlaminar properties of unidirectional CFRP laminates by the synergistic effects of CNT powders and veils. The short CNT powders (average length <1 μm) were adopted for toughening the matrix, which effectively avoids the “self-filtration effect” commonly found in the VARTM process and allows the CNTs to penetrate between the tiny gap of the carbon fibers and be uniformly dispersed in the composites. In addition, an ultrathin CNT fiber veil (∼100 nm in thickness) was used as the interleaf material, offering the opportunity to reinforce the interlaminar bonding with minimal weight penalty and effectively avoid reductions in in-plane properties. The results showed that 0.5 wt% is the optimum loading level of CNT powders for matrix toughening. The Mode I interlaminar fracture toughness (propagation value, GIC,prop) of the unidirectional CFRP composites increased as much as 32 %, 62 % and 123 % when integrating CNT veil, CNT powder and CNT veil + powder, respectively. The figure of merit for interlaminar reinforcement, consisting of the change in interlaminar properties normalized by interleaf thickness and ply thickness, comes out as high as 2460, which far outweighs the state of the art.

Establishment of instantaneous milling force prediction model for multi-directional CFRP laminate

Carbon fiber reinforced polymer (CFRP) is widely used in the aerospace field due to its light weight and high strength. The CFRP milling process is prone to damage such as burrs and tears. The cutting force is closely related to the damage of CFRP and tool wear. In this paper, a back propagation (BP) neural network model of cutting force and edge force coefficients was established. The model considers the effects of instantaneous uncut chip thickness, fiber cutting angle, spindle speed, and axial depth of cut. The unidirectional CFRP laminate instantaneous milling model considering the cutting edge force was further established. The instantaneous milling force prediction model was extended to multi-directional CFRP laminates. And the relationship between the damage mechanism of CFRP and the instantaneous milling force was analyzed. Experiments have proved that the instantaneous milling force prediction model built in this paper has high accuracy.

Effect of tool wear on drilling unidirectional CFRP laminates in different fiber cutting angles

Drilling performance of carbon fiber reinforced polymer (CFRP) aggravates significantly with serious tool wear caused by highly abrasive fibers. Because of the anisotropy of CFRP, the angle between fiber orientation and cutting direction, namely, fiber cutting angle, also plays a crucial part in the drilling performance. To enhance drilling performance, this paper initially conducts a comprehensive experimental study on the effects of tool wear on thrust force, exit temperature, and hole wall defect in different fiber cutting angles in the drilling of unidirectional CFRP. The results demonstrate that the profile of worn cutting edge is an approximate elliptical arc and the half axis lengths of the elliptical arc are utilized to evaluate cutting edge wear. Tool wear on the cutting edge, flank face, and rake face all deteriorates with the number of drilled holes. Tool wear has a more significant influence on thrust force in the fiber cutting angle region of 0~90° than in the fiber cutting angle region of 90~180°. With increasing tool wear, the exit temperature distribution can still be approximate to an ellipse. The variation of exit temperature caused by tool wear in the major axis direction is still no less than that in other directions. In addition, on hole wall, tool wear causes severe crack defects in the fiber cutting angle region of 0~90° and cavity defects in the fiber cutting angle region of 90~180°.

Conductivity inversion of unidirectional CFRP laminate based on ECT using neural network

For the electrical anisotropy of carbon fiber reinforced polymer (CFRP), conductivity of unidirectional CFRP laminate in three directions was inverted in this paper. The three-dimensional eddy current electromagnetic model of unidirectional composites was constructed by ANSYS software, and the influence of the electrical conductivity of the material on the detection signal of the probe in the longitudinal, transverse and thickness directions was studied. In order to improve the amplitude of the probe output signal induced by the change of conductivity, the optimal detection angle of the eddy current probe was determined. On this basis, the relationship between the conductivity and the detection signal was studied to estimate the initial values of the conductivity based on the experimental data obtained by the eddy current testing (ECT). According to the forward model, the theoretical probe voltage under the estimated conductivity were calculated. The database consisting of conductivity and corresponding theoretical results was built for the neural network to construct the mapping that can estimate conductivity by experimental results. Using neural network for iteration, the conductivity was inverted quickly and precisely.

Machinability study of unidirectional CFRP laminates by slot milling

Slot milling of unidirectional carbon fiber reinforced polymer (CFRP) laminates in four directions relative to fiber orientation is conducted to effectively study the machinability of CFRP composites with respect to process parameters and fiber orientation angle (relative to cutting speed) simultaneously and fiber orientation angle can vary from 0 to 180∘ continuously. First, slot milling experiments with a full factorial design are conducted to include all fiber orientation angles from 0 to 180∘. Then, the cutting forces with respect to fiber orientation angle and chip thickness are decoupled and results are analyzed. The results show that both fiber orientation angle and chip thickness have significant effect on cutting forces and the specific cutting forces with respect to chip thickness are governed by power law for fiber orientation angle lower than 135∘.

Thermal characteristics of unidirectional carbon fiber reinforced polymer laminates during orthogonal cutting

Carbon fiber reinforced polymer has been used as a major material for primary load-bearing structural components in aviation industry. But its poor heat resistance is an important factor affecting the machining performance, because high cutting temperature above glass transition temperature of resin matrix (normally 300°C or below) may lead to the degradation of the resin matrix. In this study, orthogonal machining experiments were conducted to investigate the effects of cutting parameters, cutting tool geometric parameters, and material parameters on cutting temperature, and the prediction model of cutting temperature about fiber orientation angle ( θ) was built. Cutting temperature was measured by semiartificial thermocouple method. The experimental results revealed that the influence of cutting parameters on cutting temperature was not affected by fiber orientation angle of carbon fiber reinforced polymer. Cutting tool geometric parameters have little effect on cutting temperature. Unlike metal materials, cutting temperature was greatly influenced by θ. Cutting temperature for θ &lt; 90° was significantly higher than that for θ &gt; 90°. The maximum temperature occurred at θ = 90°. The influence of fiber orientation angle was shown in two aspects: changing the springback of unidirectional-carbon fiber reinforced polymer laminates in cutting process, changing material removal mechanism, which affected cutting temperature eventually.

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.

Experimental study on effects of fiber cutting angle in milling of high-strength unidirectional carbon fiber–reinforced polymer laminates

It is still a challenge to machine high-strength carbon fiber–reinforced polymer with high quality due to its poor machinability. Fiber direction is the critical factor. This article aims to investigate the effects of fiber cutting angle in milling of high-strength unidirectional carbon fiber–reinforced polymer laminates with regard to milling forces, machined surface morphology and surface roughness. The edge trimming and slot milling tests were conducted. The largest radial and tangential forces were observed on 135° fiber cutting angle followed by 90° while the smallest milling forces were observed on 45° fiber cutting angle. Totally, four basic material fracture mechanisms, that is, fiber–matrix debonding, bending-induced fiber fracture, shear-induced fiber fracture and compression-induced fiber fracture, were observed by the analysis of fracture morphology for a single fiber. The four basic material fracture mechanisms dominate the material fracture behavior during the cutting of carbon fiber–reinforced polymer. Besides, it is indicated that surface roughness of the machined surfaces is highly related to the type of the surface defects. Surface cavities caused by fiber–matrix debonding and bending-induced fiber fractures on 45° fiber cutting angle were observed to be the main factors leading to the decline of surface finish in milling of carbon fiber–reinforced polymer laminates.

Study on Damage Evaluation and Machinability of UD-CFRP for the Orthogonal Cutting Operation Using Scanning Acoustic Microscopy and the Finite Element Method.

Owing to high specific strength and designability, unidirectional carbon fiber reinforced polymer (UD-CFRP) has been utilized in numerous fields to replace conventional metal materials. Post machining processes are always required for UD-CFRP to achieve dimensional tolerance and assembly specifications. Due to inhomogeneity and anisotropy, UD-CFRP differs greatly from metal materials in machining and failure mechanism. To improve the efficiency and avoid machining-induced damage, this paper undertook to study the correlations between cutting parameters, fiber orientation angle, cutting forces, and cutting-induced damage for UD-CFRP laminate. Scanning acoustic microscopy (SAM) was employed and one-/two-dimensional damage factors were then created to quantitatively characterize the damage of the laminate workpieces. According to the 3D Hashin’s criteria a numerical model was further proposed in terms of the finite element method (FEM). A good agreement between simulation and experimental results was validated for the prediction and structural optimization of the UD-CFRP.

Occurrence and formation mechanism of surface cavity defects during orthogonal milling of CFRP laminates

Carbon fiber reinforced polymer (CFRP) has been commonly used in the aviation industry and related machining demands are increasing. During machining CFRP, severe machining defects, such as delamination, burrs and subsurface damages etc., usually occur due to the prominent anisotropy and heterogeneity of CFRP. Surface cavity defects are a major manifestation of subsurface damages leading to the decline of the machined surface quality. This paper aims to investigate the occurrence and formation mechanism of the surface cavity defects in a continuum of fiber cutting angles (θ) ranging from 0°–180° by circular orthogonal milling of unidirectional CFRP laminate disks. The experimental results showed that the surface cavities only occur on 50–60% of the against fiber cutting region (0° < θ < 90°). It was found that the surface cavities were mainly caused by the occurrence and propagation of fiber-matrix debonding followed by bending-induced fiber fractures and shear-induced fiber fractures. Results also showed that the surface cavities could be avoided by employing “inclination milling” strategy during edge trimming of typical multidirectional CFRP laminates.

Study on the cutting mechanics characteristics of high-strength UD-CFRP laminates based on orthogonal cutting method

High-strength unidirectional carbon fiber reinforced polymer (UD-CFRP) has gradually become one major material for primary load-bearing structural components of aircrafts, and related machining demands are also ever increasing. Owing to its prominent anisotropy and heterogeneity, UD-CFRP laminate has rather poor machinability. This paper conducted orthogonal cutting tests on T700/800 high-strength UD-CFRP laminates, and investigated machining mechanism by studying cutting mechanics characteristics in cutting process, in order to provide the basis for improving their machinability. Experimental results showed that the cutting force and specific cutting energy of T700 and T800 UD-CFRP laminates were all significantly directional; and at the conditions of same fiber orientation angle, greater cutting force and specific cutting energy were found for cutting of T800 UD-CFRP than that for cutting of T700. As the cutting speed increased, main cutting force and radial thrust force both decreased and specific energy map shrank rapidly, indicating reduced cutting energy consumption, and improved machinability of CFRPs. With the increase in cutting depth, main cutting force and radial thrust force both exhibited increasing trends, but specific cutting energy map narrowed, indicating improved machinability.

CFRP-Strengthening and Long-Term Performance of Fatigue Critical Welds of a Steel Box Girder

Empa’s research efforts in the 1990s provided evidence that a considerable increase of the fatigue strength of welded aluminum beams can be achieved by externally bonding pultruded carbon fiber reinforced polymer (CFRP) laminates using rubber-toughened epoxies over the fatigue-weak welding zone on their tensile flange. The reinforcing effect obtained is determined by the stiffness of the unidirectional CFRP laminate which has twice the elastic modulus of aluminum. One can therefore easily follow that an unstressed CFRP laminate reinforcement of welded beams made of steel will not lead to a substantial increase in fatigue strength of the steel structure. This consideration led to the idea of prestressing an external reinforcement of the welded zone. The present investigation describes experimental studies to identify the adhesive system suitable for achieving high creep and fatigue strength of the prestressed CFRP patch. Experimental results (Wöhler-fields) of shear-lap-specimens and welded steel beams reinforced with prestressed CFRP laminates are presented. The paper concludes by presenting a field application, the reinforcement of a steel pendulum by adhesively bonded prestressed CFRP laminates to the tensile flanges of the welded box girder. Inspections carried out periodically on this structure revealed neither prestress losses nor crack initiation after nine years of service.

SQP법을 사용한 복합재 조류력 발전용 블레이드의 스파 캡에 대한 두께 최적화

In this study, the thickness optimization for uni-directional (UD) glass fiber reinforced polymer (GFRP) laminates of the spar cap of composite tidal blades was performed under the tip deflection constrains. The spar cap was composed of GFRP composites and carbon fiber reinforced polymer (CFRP) composites. The stress distributions in the blade as well as its material costs for the optimized results were additionally investigated. The optimized thickness was obtained by interacting a sequential quadratic programming (SQP) algorithm and an ABAQUS software to calculate an objective function. It was confirmed that the thickness of UD GFRP increased with a decrease of the restrained tip deflection when a thickness of UD CFRP laminates was constrained to 9 mm. The weight of the optimized spar-cap increased up to 96.2% while the maximum longitudinal tensile stress decreased up to 24.6%. The thickness of UD GFRP laminates increased with a decrease of the thickness of UD CFRP laminates when the tip deflection was constrained to 126.83 mm. The weight increased up to 40.1%, but the material cost decreased up to 16.97%. Finally, the relationships among the weight, internal tensile stress, and material costs were presented based on the optimized thicknesses of the spar cap.

Mechanistic force modeling for milling of unidirectional carbon fiber reinforced polymer laminates

Carbon fiber reinforced polymer (CFRP) usage in the aerospace industry has been steadily increasing due to its superior material properties such as high strength, low weight, high resistance to corrosion, and a low thermal expansion coefficient. In addition, CFRP parts are produced near-net-shape, a process that eliminates rough machining operations. However, machining operations such as drilling, side milling, and slotting are still necessary to give the CFRP parts their final shape. A majority of the studies on machining of CFRP laminates are on drilling. The number of studies on milling of CFRPs is quite limited. In this study, a mechanistic cutting force model for milling CFRPs is proposed based on experimentally collected cutting force data during slot milling of unidirectional CFRP laminates using two different polycrystalline diamond cutters. Cutting force coefficients in radial and tangential directions are calculated as a function of fiber cutting angle. The relationship is represented with simple sine functions. The mechanistic model is shown to be capable of predicting cutting forces during milling of multidirectional CFRP laminates. The experimental milling force measurements and predicted milling forces agree well with each other. Surface milling experiments were also conducted to investigate the relationship between milling forces and surface quality. Some suggestions on surface milling of CFRP laminates are given based on these observations.

Milling Force Modelling of Multidirectional Carbon Fiber Reinforced Polymer Laminates

Carbon fiber reinforced polymer (CFRP) usage in the aerospace industry has been steadily increasing due to its superior material properties such as high strength, low weight, high resistance to corrosion, and a low thermal expansion coefficient. In addition, CFRP parts are produced near-net-shape, a process that eliminates rough machining operations. However, machining operations such as drilling, side milling, and slotting are still necessary to give the CFRP parts their final shape. A majority of the studies on machining of CFRP laminates are on drilling. The number of studies on milling of CFRPs is quite limited. In this study, a mechanistic cutting force model for milling CFRPs is proposed based on experimentally collected cutting force data during slot milling of unidirectional CFRP laminates using a polycrystalline diamond cutter. Cutting force coefficients in radial and tangential directions are calculated as a function of fiber cutting angle. The mechanistic model is shown to be capable of predicting cutting forces during milling of multidirectional CFRP laminates and capable of investigating stability of machining.

Dual-Polarized Near-Field Microwave Reflectometer for Noninvasive Inspection of Carbon Fiber Reinforced Polymer-Strengthened Structures

Carbon fiber reinforced polymer (CFRP) composites are increasingly being used to strengthen various components of civil, aerospace, and automotive infrastructures. Subsequent to the application of CFRP, evaluation of the structural integrity of the strengthened members becomes a critical issue. Microwave noninvasive inspection techniques have been successfully used for this purpose. To this end, a novel near-field microwave inspection system that employs a dual-polarized reflectometer for detecting defects, such as disbonds between CFRP laminates and strengthened structures, was developed. It is shown that this system is capable of automatic removal of the influence of undesired standoff distance (or surface roughness) variations. It can simultaneously generate three images of a defect: two at orthogonal polarizations and one after the influence of standoff distance variations is removed using the information provided by the two images. This paper discusses the unique features of a unidirectional CFRP laminate that enabled the design of this unique system, as well as the design of the dual-polarized reflectometer at the X-band (8.2-12.4 GHz). The detail design of the reflectometer, as well as the results of several measurements conducted on an actual bridge, whose members were strengthened with CFRP laminates, are also presented, illustrating the practical utility of this system.