Carbon Fiber Reinforced Plastic Laminates Research Articles

Multipoint cure monitoring of CFRP laminates using a flexible matrix sensor

Since a carbon fiber reinforced plastic (CFRP) structure is complicated in the adoption of integral molding, local molding faults such as under curing and dry spots are liable to occur. To solve this problem, the distribution of the degree of curing for the entire composite structure must be measured. In the present study, we propose a patch-type flexible matrix sensor based on permittivity measurements. Multiple electrodes and wirings are readily fabricated simultaneously using a photolithographic process. Moreover, the sensor has only m + n wirings for m × n sensors, and is thereby suitable for multipoint cure monitoring. We also constructed a method for estimating the degree of curing considering the effect of frequency dependence of the permittivity of resin and viscosity variation due to temperature change. Experiments of multipoint cure monitoring are carried out using a CFRP plate and an actual aircraft structure. As a result, we confirmed the effectiveness of this method by comparing with results using a conventional differential scanning calorimeter.

Transverse crack growth behavior considering free-edge effect in quasi-isotropic CFRP laminates under high-cycle fatigue loading

The high-cycle fatigue characteristics focused on the behavior of the transverse crack growth up to 10 8 cycles were investigated using quasi-isotropic carbon fiber reinforced plastic (CFRP) laminates whose stacking sequence was [−45/0/45/90] s . To assess the fatigue behavior in the high-cycle region, fatigue tests were conducted at a frequency of 100 Hz in addition to 5 Hz. In this study, to evaluate quantitative characteristics of the transverse crack growth in the high-cycle region, the energy release rate considering the free-edge effect was calculated. Transverse crack growth behavior was evaluated based on a modified Paris law approach. The results revealed that transverse crack growth was delayed under the test conditions of the applied stress level of σ max/ σ b = 0.2.

HVI tests on CFRP laminates at low temperature

The paper reports a result of low temperature hypervelocity impact (HVI) tests of aluminum sphere against a 16-ply quasi-isotropic Carbon Fiber Reinforced Plastic (CFRP) laminate plate at speed ranging from 1.4 to 5.4 km/s in air at 10 Pa. The result was compared with room temperature impacts. At low speed impact on CFRP plates, fracture patterns of specimens varied depending on their temperatures, whereas at high-speed impact, any significant differences in the fracture patterns around penetration holes and independent of the temperatures.

Seismic Rehabilitation of Reinforced Concrete Frame Interior Beam-Column Joints with FRP Composites

An experimental research program is described regarding the use of externally applied carbon fiber-reinforced plastic (CFRP) jackets for seismic rehabilitation of reinforced concrete interior beam-column joints, which were designed for gravity loads. The joints had steel reinforcement details that are known to be inadequate by current seismic codes in terms of joint shear capacity due to the absence of transverse steel hoops and bond capacity of beam bottom steel reinforcing bars at the joint. Lap splicing of beam bottom steel reinforcement at the joint using externally applied longitudinal CFRP composite laminates is investigated. Improvement of joint shear capacity using diagonal CFRP composite laminates is another strengthening scheme employed. Concrete crack widths for the as-built specimens and the extent of CFRP delamination for the rehabilitated specimens at various drift ratios are reported. The test results indicate that CFRP jackets are an effective rehabilitation measure for improving the seismic performance of existing beam-column joints with inadequate seismic details in terms of increased joint shear strength and inelastic rotation capacity. In addition, CFRP laminates are effective rehabilitation measures for overcoming problems associated with beam bottom steel bars that have inadequate embedment into the beam-column joints.

The extrinsic influence of carbon fibre reinforced plastic laminates to strengthen steel structures

The intrinsic advantages of strengthening the steel-based structures by the use of fibre reinforced plastic (FRP) material have not yet been fully exploited. In this paper, a succinct overview of recent studies made to enhance the strength of steel beams using FRP laminates is presented. The results presented and discussed in this paper were obtained by closely studying the behaviour of steel beams strengthened with carbon FRP material. An attempt is made to succinctly summarise the findings for two different types of strengthening of the steel beams using carbon FRP laminates. The first type of beams focuses on enhancing the strength of steel in flexure while the second focuses on increasing the shear strength of the beams. Three beams were designed so as to cause them to fail in flexure. Of the beams studied, two were strengthened using carbon FRP strips attached to the tension flange. One of the beams was tested to facilitate comparison of their behaviour to the two beams which are strengthened in flexure. Three other beams were designed such that they failed predominantly in shear. Of these three, two were strengthened with carbon FRP strips attached to the webs while the third beam was used as a control beam for the purpose of drawing comparisons. Preliminary results revealed a noticeable increase in the strength for both the flexure strengthened beams and the beams strengthened in shear. The observed increase in shear strength of the beams was 26% while the increase in strength for the beams tested in flexure was 15%. This study convincingly shows that it is possible to strengthen steel beams using carbon FRP laminates in both flexure and in shear.

Actuator Capabilities of Piezoelectric Devices Embedded in Composite Laminate

In this work the actuator performance of a ceramic piezoelectric device is studied. Its ability of deforming a Carbon Fiber Reinforced Plastic (CFRP) laminate is measured in the absence/presence of mechanical constraints. Deformation and bending of the CFRP laminate is obtained when a voltage is applied to the piezoelectric device. This deformation was assessed using ESPI technique and a very good agreement with the expected values for the unconstrained case was observed. When completely embedded a reduction of 86% in the piezoelectric response is expected. Finite Element Analysis confirmed the obtained experimental data and a very good linearity in the response was observed.

Investigations on the thermal effect caused by laser cutting with respect to static strength of CFRP

To increase production volume and efficiency in the area of CFRP (carbon fiber-reinforced plastics) component production, fast, flexible and cost-efficient technologies are needed. One process that is necessary during CFRP component production is trimming and cutting. Although laser cutting in principle meets these requirements, it is often not used for component trimming and contour cutting, due to insufficient knowledge about the influences of thermal machining on the material behavior. It is a common argument that lasers, as a thermally acting tool, may damage the CFRP, thus reducing its strength properties. This, however, has never been proven or disproven. Therefore, this paper presents investigations on the influence of laser cutting on the static strength of a CFRP laminate. The material is cut using three different high-power laser sources: a pulsed Nd:YAG laser, a disk laser and a CO 2 laser. Appropriate cutting parameters have been found, and the results in cut quality and heat-affected zone are discussed. With these parameter sets, specimens for tensile strength and bending tests have been prepared. These specimens have been tested under static tensile and bending conditions, and the results have been compared to conventional milling as well as abrasive water-jet cut samples. Though a clear dependency of the static strength values on the heat-affected zone was detected, all strengths were found to be far above the material values given by the producer of the laminate.

Influence of Drill Geometry in Drilling Carbon Fiber Reinforced Plastics

Twist drill is widely used in hole-making process in industries, due their low production cost and ease of regrinding after wear. However, drilling of fiber reinforced plastics with twist drill often results in defects and damages, such as delamination, debonding, spalling and fiber pullout. The chisel edge of twist drill is the mainly influence for the thrust force and the hole quality in drilling carbon fiber reinforced plastic (CFRP) laminates. Pre-drilled pilot hole or reduce chisel edge can eliminate the threat for twist drill in drilling induced-delamination. Drilling-induced thrust force was selected as quality character factors to optimize the drilling parameters (drill type, feed rate and spindle speed) to get the smaller the better machining characteristics by Taguchi method. The results show that the feed rate and drill type are the most significant factor affecting the induced-thrust in drilling CFRP laminates.

Temperature Dependence of Accumulation of Fiber Breakages under Tensile Loading for Unidirectional CFRP Laminates

The temperature dependence of the tensile damage process as well as the tensile strength in unidirectional carbon fiber-reinforced plastic (CFRP) laminates is investigated experimentally and numerically. The constant strain rate (CSR) tensile tests for CFRP laminates are carried out at room temperature (RT) and 120°C. The damage process under tensile loading is recorded by using a digital microscope to comprehend the accumulation of fiber breakages observed with increasing strain. The fiber breakages observed under tensile loading increase strongly in the vicinity of ultimate failure load, and this behavior decreases with increasing temperature. The clusters of fiber breakages observed at RT cannot be observed at 120°C. These experimental results lead to the fact that the number of fiber breakages and the existence of cluster of fiber breakages are influenced by the temperature dependent stress-strain relation of matrix resin. Then, a new simulation based on finite element model considering the stress-strain relation of matrix resin is proposed. The damage process and strength predicted numerically by the proposed simulation agree well with experimental results. Therefore, the temperature dependent mechanism for the damage process, including the generation of clusters as well as the strength, is quantitatively shown.

Finite Element Study on Delamination Identification in Quasi-Isotropic CFRP Laminate by Residual Stress Release Using the Electric Potential Change Method

The electric potential change method (EPCM) was applied to identify delamination in quasi-isotropic carbon fiber reinforced plastic (CFRP) laminate. The authors have introduced EPCM previously to detect delamination in a cross-ply CFRP laminate although it was difficult to apply to quasi-isotropic CFRP laminate because of its complicated electric anisotropy. In this study, a new concept was introduced to resolve this problem. Residual stress that developed during the curing process in CFRP laminate fabrication was utilized. This residual stress was released locally by the creation of a delamination which resulted in a local strain variation on the laminate surface. Since the electric conductivity of CFRP may change with applied strain because of its piezoresistivity a CFRP cloth was used as a strain sensor. The release of residual stress was detected as an electric potential change of the CFRP cloth by applying electric current to the laminate. The delamination location and size were estimated from electric potential changes. A finite element study was performed to investigate the applicability of the method. Two steps of finite element analysis were performed to calculate the electric potential change due to delaminations and matrix cracks, structural analyses to calculate strain variation due to release of the residual stress by the delamination and then electric field analyses to calculate electric potential change due to strain variation. The delamination location and size were estimated from electric potential changes using a response surface methodology. A numerical simulation was successful in estimating delaminations in quasi-isotropic CFRP laminate.

1319 ひずみ検知型電位差法による擬似等方CFRP積層板はく離同定のための数値解析(J14-4 知的材料・構造システム(4) 計測・モニタリングIII,ジョイントセッション,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Since carbon fiber reinforced plastic (CFRP) laminate has superior specific strength and stiffness as compared with metals, it has been increasingly applying to aerospace structures. It is, however, sensitive to an impact which causes a delamination in the laminate. The delamination strongly reduces the mechanical property of the laminate with difficulty of visual inspection. Simple structural health monitoring method is required to maintain the structural reliability of CFRP structures. Many researchers have been reported strain-sensing system of CFRP in which piezoresistivity of CFRP laminate is utilized. It is, however, not reported the delamination sensing. The delamination releases the residual stress in the laminate which was caused in the fabrication process. The delamination may be detected by measuring electric potential changes due to the release of residual stress. In this paper, FEM analyses are performed to investigate the applicability of the method for delamination detection in the quasi-isotropic CFRP laminates.

1516 渦電流探傷法によるCFRP積層板の層間はく離検出(S10-3 非破壊評価とモニタリングIII,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Carbon fiber reinforced plastic (CFRP) laminates are very sensitive to an impact which causes delaminations in the laminate. Since delamination reduces the mechanical property of the laminate, nondestructive inspection is required to maintain the structural reliability of the CFRP structure. Eddy current test (ECT) has been studied to be applied to CFRP laminate. Strong electric anisotropy, however, makes application of the method difficult. Since delamination is in-plane damage in the laminate, conventional ECT has a problem for making current-loop which should be impeded by the damage to be detected. In the present paper, rectangular twin probe for electrically anisotropic CFRP laminate is introduced to resolve the problems. The probe is made of two rectangular coils put side-by-side to induce strong uniform electric current. Delamination was made in CFRP cross-ply laminates by means of the indentation test; the applicability of the ECT with rectangular twin probe is examined experimentally.

Detection of Matrix Crack Density of CFRP using an Electrical Potential Change Method with Multiple Probes

Carbon Fiber Reinforced Plastic (CFRP) laminates are adopted for fuel tank structures of next generation space rockets or automobiles. Matrix cracks may cause fuel leak or trigger fatigue damage. A monitoring system of the matrix crack density is required. The authors have developed an electrical resistance change method for the monitoring of delamination cracks in CFRP laminates. Reinforcement fibers are used as a self-sensing system. In the present study, the electric potential method is adopted for matrix crack density monitoring. Finite element analysis (FEA) was performed to investigate the possibility of monitoring matrix crack density using multiple electrodes mounted on a single surface of a specimen. The FEA reveals the matrix crack density increases electrical resistance for a target segment between electrodes. Experimental confirmation was also performed using cross-ply laminates. Eight electrodes were mounted on a single surface of a specimen using silver paste after polishing of the specimen surface with sandpaper. The two outermost electrodes applied electrical current, and the inner electrodes measured electric voltage changes. The slope of electrical resistance during reloading is revealed to be an appropriate index for the detection of matrix crack density.

The geometrical effect between stages in step drilling of composite materials

The delamination-induced of step drill with step angle, stage ratio, feed rate and spindle speed in drilling Carbon Fibre Reinforced Plastic (CFRP) laminate was experimentally investigated in this study. Delamination can be minimised by changing the drill geometry of step drill. A 0.6 stage ratio of step drill with a larger step angle of 120? produced a smaller thrust force than the one with an 80? step angle during exit of the step drill. However, the effect of stage ratio on the delamination is more critical when the step angle is small.

Initiation and Propagation of Impact-Induced Damage in CFRP Laminates

This paper investigated the initiation and propagation characteristics of impact-induced damage in carbon-fiber-reinforced-plastic (CFRP) laminates with different stacking sequences and thicknesses under low-velocity impact. Impact force histories were measured with a drop-weight impact tester. A strain gauge was attached on the back face of CFRP laminates to measure exactly when a matrix crack on its back face was initiated. It was found from fractographic observation that impact-induced damage in CFRP laminates was initiated at the matrix crack on the back face of CFRP laminates due to bending deformation during impact. Finite element analysis was conducted using the impact forces derived from the experimental results of the impact test. Its results clarified that the tensile stress normal to the fiber on the back face of the specimen was the criterion to initiate impact damage in CFRP laminates.

Application of Stress Analysis by Digital Image Correlation and Intelligent Hybrid Method to Unidirectional CFRP Laminate

Fiber reinforced composites are heterogeneous and anisotropic. The applicability of the stress analysis methods on such heterogeneous and anisotropic materials is not well known. In the present study, an attempt is made to apply the digital image correlation method and the intelligent hybrid method to a carbon fiber reinforced plastic (CFRP) laminate. A material used is carbon/epoxy system. Laminate configuration is unidirectional. Tensile load is applied in off-axis (45 degrees) direction as well as longitudinal (0 degree) and transverse (90 degrees) directions on a CFRP laminate. Displacement, strain and stress fields due to the tensile loading in the CFRP unidirectional laminate are analyzed. The constitutive equation considering material anisotropy is built into the intelligent hybrid method used at the time of analyses. The validity of the algorithm is checked through comparison between results of the present method, experimental results from strain gauge method, and the analytical results from finite element method (FEM).

Parametric study on thrust force of core drill

Abstract Drilling-induced delamination is among the major concerns of applying the fiber-reinforced composite materials in various industries. Core drill possesses the advantage for reducing the delamination by distributing the drilling thrust force toward the drill periphery. The thrust force of core drill varying with the conditions in drilling carbon fiber reinforced plastic (CFRP) is experimentally investigated in this study. The experimental results show that reduced thickness of core drill, large grit size of diamond, low feed rate and medium spindle speed are effective in reducing the thrust force. The grit size of diamond is the most significant factor among the four control factors, while the drill thickness shows limited influence. The correlation between thrust force and cutting parameters is obtained by multi-variable linear regression and compared with the experimental results. A feasible method for the evaluation of thrust force in drilling CFRP laminate with errors within 16% is derived.

Neural network modeling of strength enhancement for CFRP confined concrete cylinders

This study presents the application of neural networks (NN) for the modeling of strength enhancement of CFRP (carbon fiber-reinforced plastic) confined concrete cylinders. The proposed NN model is based on experimental results collected from literature. It represents the ultimate strength of concrete cylinders after CFRP confinement which is also given in explicit form in terms of diameter, unconfined concrete strength, tensile strength CFRP laminate and total thickness of CFRP layer used. The accuracy of the proposed NN model is quite satisfactory as compared to experimental results. Moreover the results of proposed NN model are compared with 10 different theoretical models proposed by researchers so far and are found to be by far more accurate.

A Method for Identifying Asymmetric Dissimilar Constant Fatigue Life Diagrams for CFRP Laminates

The full shape of constant fatigue life (CFL) diagram has been identified for three kinds of carbon fiber-reinforced plastic (CFRP) laminates of [45/90/-45/0]2s, [0/60/-60]2s and [0/90]3s lay-ups, respectively. First, the effects of mean stress on fatigue behavior are observed for the quasi-isotropic [45/90/-45/0]2s CFRP laminate. It is clearly observed that the CFL diagram turns out asymmetric and the peak of the CFL diagram is slightly offset to the right of the alternating stress axis. The CFL plots indicate that the alternating stress component of fatigue load becomes maximal at a critical stress ratio closely equal to the compressive strength to tensile one (i.e. a C/T strength ratio). It is also demonstrated that CFL curves for different constant values of life are linear in the range of a short life, but they turn quadratic in the range of a longer life. Similar features are observed for the other kinds of CFRP laminates. Then, to construct the asymmetric dissimilar shape of CFL diagrams, a new methodology is developed on the basis of the static strengths in tension and compression and the reference S-N relationship for the critical stress ratio. An efficient procedure for determining the reference S-N relationship for the critical stress ratio by using fatigue data for another stress ratio is also developed.

Delamination monitoring of CFRP laminate using the two-stage electric potential change method with equivalent electric conductivity

Carbon fiber reinforced plastics laminate (CFRP) has a high specific strength and stiffness compared to conventional metals, although it is very sensitive to impact. A low impact leads to a delamination and results in the deterioration of the structural reliability. The present study employs a two-stage electric potential change method (EPCM) to identify delamination. In the methods, delamination is estimated using response surfaces, which require numerous experiments. An equivalent electric conductivity method is introduced to reduce this number. Delaminations are successfully estimated using response surfaces based on the results of the finite element method with equivalent electric conductivity.