Multidirectional CFRP Research Articles

Experimental Investigation of Bouncing-Back of Multidirectional CFRP Laminates during Ultrasonic Assisted Edge Trimming

Edge trimming process is needed for finishing CFRP components to the required accuracy and surface quality. The bouncing–back effect of CFRP components is very challenging owing to spring back of trimmed edge after cutting tool pass. Ultrasonic assisted machining (UAM) is an efficient method used to enhance the quality of CFRP parts due to the reduced contact time between the tool and workpiece. This paper experimentally investigates the effect of edge trimming variables on the cutting forces and the magnitude of the bouncing back. Diamond abrasive end mills were utilized during ultrasonic assisted edge trimming of CFRP. The processes variables include spindle speed, feed rate, radial depth of cut, fiber orientations, and up/down strategy. Statistical analysis was conducted to determine the most significant factor on performance characteristics. Regression equation was also developed to predict the value of bouncing back. The results showed that depth of cut and feed rate have a significant effect on bouncing back among the process variables.

Optimization of cutting conditions in slotting of multidirectional CFRP laminate

For metallic or composite materials, the judicious choice of cutting conditions depends on several factors that may be of such objectives (time, cost of production, material removal rate, etc.) or constraints (cutting force, temperature in the machining area, consumed power, etc.). The quality of the results depends on the optimization method and the efficiency of the algorithm involved. In this paper, graphical and particle swarm optimization (PSO) methods are proposed. They aim to determine the optimal cutting conditions (cutting speed and feed per tooth) in slotting of multidirectional carbon fiber reinforced plastic laminate (CFRP), referenced G803/914, with three knurled tools having different geometries. The experiences that led to the measures of roughness, temperature, cutting efforts, and consumed power are made in the same working conditions with cutting speed ranging from 80 to 200 m/min and feed per tooth from 0.008 to 0.060 mm/rev/tooth. The results illustrate that for the graphical method, the optimum cutting speed depends on the performance “maximum total removal rate” and is the same for all the studied knurled tools while optimum feed per tooth depends on the “roughness” performance: its value depends on the tool geometry. For the PSO technique, optimum cutting speed and feed per tooth values are variable and depend on the tool geometry.

Temperature measurement and machining damage in slotting of multidirectional CFRP laminate

ABSTRACTCompared to metallic materials, carbon fiber-reinforced plastics (CFRPs) have lower thermal conductivity and minor thermal expansion coefficient. Despite this, their machining can generate accuracy errors if the cutting temperature is not controlled. In this paper, an experimental study of slotting of multidirectional CFRP laminate (G803/914) with three micrograin carbide burr tools with different geometries is considered in order to investigate tool-workpiece contact point temperature, chip temperature, machined surface damage, subsurface defects and tool degradation. The experiment is made on a computer numerical control (CNC) machine with cutting speed ranging from 80 to 200 m/min and feed per tooth from 0.008 to 0.060 mm/rev/tooth. The data were analyzed in order to establish empirical models showing the dependence of cutting temperature on tool geometry and cutting conditions. Based on the results, it is concluded that cutting speed is the factor influencing cutting temperature the most, the heat generated during slotting is removed mainly by chips and the chip temperature is greater than the tool-workpiece contact temperature of about 18.5°C on average for the three burr tools.

Visualization and size estimation of fiber waviness in multidirectional CFRP laminates using eddy current imaging

This paper presents an eddy current method to visualize fiber waviness in multidirectional carbon fiber reinforced plastics. Since eddy currents induced by a driver coil flow along carbon fibers, waviness can be visualized if the eddy current path is visualized. We proposed a new complex plane analysis method to visualize an eddy current path from magnetic field measurements. The validity of the method was verified by finite element method analyses. Experiments were performed for multidirectional carbon fiber reinforced plastic specimens. In-plane waviness with misalignment angle from 6.9° to 24.9° were artificially induced in the specimens. An eddy current path was visualized from magnetic field data and it corresponded to the shape of induced waviness. The sizes of the wavy eddy current path were compared with waviness sizes measured by X-ray computed tomography and from optical images. Experimental results indicate that the surface waviness size can be estimated accurately, whereas the subsurface waviness size is underestimated.

Influence of Fiber Orientation on Machined Surface Quality in Milling of Unidirectional CFRP Laminates

Machined surface quality is the deciding factor when evaluating the machinability of CFRP. This present work concerns the influence of fiber orientation on the machined surface quality of the machined surface in terms of surface morphology and surface roughness during milling of unidirectional T800/X850 CFRP laminates. Four group milling tests are conducted under the fiber orientation angle of 0°, 45°, 90° and 135°, respectively. For the fiber orientation angle of 0°, the machined defects are mainly fiber pull-out and fiber brittle fracture owing to interfacial debonding between the fibers and matrix resin. For the fiber orientation angle of 45°, the machined defects are mainly resin cavities and the surface morphology is rough and presents wavy fractures. For the fiber orientation angle of 90°, smooth or neat surface is observed except for the surface as the cutting tool cutting in the workpiece on which severe cracks are observed. For the fiber orientation angle of 135°, the surface is smooth with less fibers pull-out. Evaluation profile and surface roughness of the machined surfaces were measured as well. Dramatically fluctuate of the evaluation profile is observed for the fiber orientation angle of 45° with a high surface roughness Ra. Verification tests were also conducted on the multidirectional CFRP (cross-ply) laminates. It is indicated that the presence of the fiber orientation angle of 45° is the main factor leading to the decline of the machined surface quality.

Hole Surface Topography and Tool Wear in CFRP Drilling

This paper deals with the hole surface quality when CFRP drilling. Mechanical instrumented operations have enabled to follow precisely the damage on the tool margins and the main cutting edges. The amount of uncut fibers according to the drill wear was evaluated with a roundness/cylindricity device versus the fiber's orientation and the tool damage. Thus, according to the hole's diameter tolerance, a wear criterion was defined to limit the number of holes achievable. This method has been carried out on unidirectional (UD) CFRP and then transposed on multidirectional (MD) CFRP. A model developed under Matlab was applied to predict the global topography of the hole surface.

A mechanistic approach to investigate drilling of UD-CFRP laminates with PCD drills

Carbon fiber reinforced plastics (CFRPs) possess desirable material properties that satisfy the aerospace industry's high strength to weight ratio objective. Therefore, CFRPs are commonly used in structural parts, either alone or together with aluminum and titanium alloys. Drilling of CFRPs has been studied extensively in the literature in recent years, with special emphasis on process parameters and delamination. This study identifies mechanical properties of uni-directional CFRPs through drilling tests. Drilling of uni-directional CFRP plates with and without pilot holes has been performed, and cutting and edge force coefficients are identified. A polycrystalline diamond (PCD) drill was used in tests since this type of drill is commonly used in practice. Finally, validation tests on multi directional CFRP laminates have been performed and good results have been obtained.

Surface Morphology in Milling Multidirectional Carbon Fiber Reinforced Polymer Laminates

Carbon fiber reinforced plastics (CFRP) use in many industries applications has seen a dramatic increase over the last decade. Milling is the most practical machining operation for removing excess material. The work presented details the effect of different cutting parameters on the surface roughness and integrity of machined multidirectional CFRP laminates. The results indicate that the surface morphology mainly relates to the fiber orientation. Increasing cutting speed leads to severe softening, degradation and burning of the matrix material that binds fibers together. The feed speed has little effect on the surface morphology. And the roughness value Ra increases with the feed rate, and decreases with the cutting speed.

Machinability and surface quality during high speed trimming of multi directional CFRP

In this paper, high speed trimming of a multidirectional CFRP using unused and used burr tools is considered in order to investigate the influence of the machining parameters (e.g., feed speed, cutting speed and cutting distance) on the cutting forces, machining temperature, and the machined surface quality. To estimate the effect of the tools’ wear and cutting parameters on the cutting forces and surface roughness, a statistical method (ANOVA) has been used. When considering an unused tool, the recorded temperatures were below the glass transition temperature of the composite material (Tg). In the case of used tools, these temperatures were mostly higher than the Tg. Furthermore, SEM observations of the machined surface showed damaged areas. These areas were wider when the cutting distance increased. Statistical analyses have shown that the machining parameters have a significant influence on the variation of the machined surface quality and the cutting forces.

Characterization of crack propagation in mode I delamination of multidirectional CFRP laminates

During the experimental characterization of the mode I interlaminar fracture toughness of multidirectional composite laminates, the crack tends to migrate from the propagation plane (crack jumping) invalidating the tests. In an earlier numerical study [9], we reported that this problem could be eliminated by choosing the appropriate bending stiffness of the beam arms.In the current paper, six stacking sequences were defined numerically and checked experimentally to validate the methodology presented in the numerical study. The results showed that crack jumping could be avoided by increasing the stiffness of the crack arms. Micrographies of the tested specimens showed that the delamination was not a perfect interlaminar fracture, as is usually considered. Instead, the propagation of delamination occurred with fiber tearing. The toughness values showed a dependency upon both the amount of fiber bridging and the interface angles.

Bond Behavior between Concrete and Multi-Directional CFRP Laminates Using the MF-EBR Strengthening Technique

The strengthening technique based on the use of multi-directional laminates of CFRP simultaneously glued and fixed with anchors (MF-EBR) has been recently purposed. An experimental program composed of pullout tests was carried out in order to study the bond behavior between concrete and MF-EBR system. The diameter and number of anchors, the pre-stress level applied to the anchors, and the strengthening procedures were the main analyzed parameters. The present work describes the carried-out tests and presents and analyzes the most significant obtained results.

Bond Behavior between Concrete and Multi-Directional CFRP Laminates Using the MF-EBR Strengthening Technique

The strengthening technique based on the use of multi-directional laminates of CFRP simultaneously glued and fixed with anchors (MF-EBR) has been recently purposed. An experimental program composed of pullout tests was carried out in order to study the bond behavior between concrete and MF-EBR system. The diameter and number of anchors, the pre-stress level applied to the anchors, and the strengthening procedures were the main analyzed parameters. The present work describes the carried-out tests and presents and analyzes the most significant obtained results.

CFRP Drilling Model: Fiber Orientation Influence on Mechanical Load and Delamination

This paper presents a combined experimental and theoretical investigation and proposes a mechanical load modeling of a multidirectional layer CFRP drilling to avoid fibers delamination by a better choice of their orientations. A first step consists in working with unidirectional CFRP to correlate cutting mode (opening, shear and bending) with force level depending on the angular tool position. The discretisation of the cutting edge is employed to know the exact contribution of each elementary part of the edge, resulting in a complete drill operation modeling (tip penetration, full engagement and exit). A second step consists in using this unidirectional approach to model a multidirectional layer CFRP drilling. In this case, layers are cut simultaneously which induce brutal mechanical load variation. A summation method is used to estimate the force level depending on fiber arrangement. Optimal orientation combinations are underlined by this approach, in order to improve the CFRP design, by considering the manufacturing concerns.

Bond between Concrete and Multi-Directional CFRP Laminates

Recently, laminates of multi-directional carbon fiber reinforced polymers (MDL-CFRP) have been developed for Civil Engineering applications. A MDL-CFRP laminate has fibers in distinct directions that can be arranged in order to optimize stiffness and/or strength requisites. These laminates can be conceived in order to be fixed to structural elements with anchors, resulting high effective strengthening systems. To evaluate the strengthening potentialities of this type of laminates, pullout tests were carried out. The influence of the number of anchors, their geometric location and the applied pre-stress are analyzed. The present work describes the carried-out tests and presents and analyzes the most significant obtained results.

Investigation on Drilling-Induced Delamination of CFRP with Infiltration Method

Carbon fiber reinforced plastics (CFRP) are typical difficult-to-cut materials because of their anisotropic mechanical properties and poor heat conductivity. Drilling is the most common process in the manufacturing of CFRP products in which delamination is the most potentially damaging defect. In the present work the delamination damage around drilled hole of multi-directional CFRP is analyzed by means of infiltration inspection method with gold chloride solution, and the effects of drill rotational rate and feed as well as drill geometry on delamination are investigated, the damage degrees in laminas near the hole entrance and the hole exit are compared. The experiment proves the infiltration method is applicable for the investigation. The results show that the nearer the distance to the hole entrance or exit, the larger the remains area of infiltration agent is; the remains mark near the hole exit is larger and more irregular than that near the hole entrance, and it has the largest length in the direction along fiber length; a chisel-free drill, a high drill rotation rate combining with a small feed brings on the decrease of delamination damage.

809 一方向CFRPの室温における主軸等寿命線図の同定(OS9-(3)オーガナイズドセッション《複合材料の変形と破壊およびマルチスケール計算技術》)

The constant fatigue life (CFL) diagrams for a unidirectional CFRP laminate in its in-plane principal directions are identified. First, constant amplitude fatigue tests were performed at various stress ratios in the longitudinal, transverse and a shear dominated off-axis directions, respectively. On the basis of the fatigue test data, the constant fatigue life diagrams for the unidirectional CFRP laminate subjected to longitudinal, transverse and in-plane shear loading are identified. While the shape of the CFL diagram for longitudinal loading is similar to that observed so far for multidirectional CFRP laminates, a significant difference is observed in the shapes of the CFL diagrams for transverse and in-plane shear loading. Finally, a modified CFL diagram is developed to improve the accuracy of prediction of the transverse and in-plane shear CFL diagrams for the unidirectional composite.

108 一方向CFRPの主軸等寿命線図および繊維とマトリックスの疲労関数の同定(OS1-2 微視構造を有する材料の変形と破壊 : 材料特性の評価)

The constant fatigue life (CFL) diagrams for a unidirectional (UD) CFRP laminate in its in-plane principal directions are identified. First, constant amplitude fatigue tests were performed at various stress ratios in the longitudinal, transverse and shear-dominated off-axis directions, respectively. On the basis of the fatigue test data, the constant fatigue life diagrams for the UD CFRP laminate subjected to longitudinal, transverse and in-plane shear loading are identified. While the shape of the CFL diagram for longitudinal loading is similar to that observed so far for multidirectional CFRP laminates, a significant difference is observed in the shapes of the CFL diagrams for transverse and in-plane shear loading. Finally, a modified CFL diagram is developed to improve the accuracy of prediction of the transverse and in-plane shear CFL diagrams for the unidirectional composite.

S0406-3-3 MMF/ATMによるCFRP構造物の長期強度の予測(プラスチック基複合材料の加工と評価(3))

This paper deals with the constituent-based characterization of long-term strength of CFRP structure components with the combined method of micromechanics failure (MMF) and the accelerated testing methodology (ATM). The time-temperature dependent master curves of MMF/ATM critical parameters were constructed by tensile and compressive tests for the longitudinal and transverse directions of unidirectional CFRP under various temperatures based on the time-temperature superposition principle which holds for the viscoelastic creep compliance of matrix resin. These master curves can be used for predicting the strength of composite structures with multi-directional CFRP laminates at any time and temperatures. The applicability of MMF/ATM combined method to predict the long-term strength of CFRP structures was experimentally confirmed by the open hole compressive strength of the quasi-isotropic CFRP laminates.

Nonlinear constant fatigue life diagrams for carbon/epoxy laminates at room temperature

Exploration of a full shape of constant fatigue life (CFL) diagram and development of an efficient CFL diagram-based fatigue life prediction method are attempted for multidirectional CFRP laminates. On three kinds of CFRP laminates of [45/90/−45/0]2s, [0/60/−60]2s and [0/90]3s lay-ups, tension–tension, tension–compression and compression–compression fatigue tests are performed at room temperature for two different stress ratios each. Experimental results clearly show that a stress ratio has a significant influence on the fatigue behavior of those CFRP laminates, and the CFL diagrams delineated using alternating stress and mean stress become asymmetric about the alternating stress axis. The alternating stress component of fatigue load for a given constant value of fatigue life turns maximum in the case of fatigue loading at a critical stress ratio that is nearly equal to the ratio of compressive strength to the tensile one. The shape of CFL diagrams progressively changes from a straight line to a nonlinear curve as a given constant value of fatigue life increases. A new and efficient method for accurately predicting an asymmetric nonlinear CFL diagram is then developed which is based on the static strengths in tension and compression and the reference S–N relationship fitted to the fatigue data for the critical stress ratio. The theoretical CFL diagram constructed following the proposed procedure agrees well with the experimental CFL diagram, regardless of the type of CFRP laminate. It is also demonstrated that the S–N relationships predicted using the proposed CFL diagram-based fatigue life prediction method adequately coincide with the experimental results for fatigue loading with a variety of different stress ratios in the range of fatigue life up to 106 cycles.

Effects of Temperature on Mode II Fracture Toughness of Multidirectional CFRP Laminates

End notched flexure (ENF) tests were performed to investigate the effects of temperature and fiber orientation on Mode II interlaminar fracture behavior, GIIC (GII at the crack initiation), of carbon fiber-reinforced epoxy composites, T800H/#3631. The values of GIIC for three kinds of laminates, [012//012], [22.5/-22.5/08/-22.5/22.5//-22.5/22.5/08/22.5/-22.5] and [45/-45/08/-45/45//-45/45/08/45/-45], with a pre-cracked interface, that is // in each laminate, were obtained at three temperatures, i.e. -100°C, 25°C and 150°C. It is shown that GIIC is obviously affected by the temperature and fiber orientation. The scanning electron microscope (SEM) observation was also carried out to investigate the fracture surface. SEM analysis suggested that the decreased Mode II interlaminar fracture toughness for all kinds of specimens at high temperature could be attributed to temperature-induced matrix property change or fiber-matrix interfacial weakening.