Flexural retrofit of a bridge subjected to overweight trucks using CFRP laminates

The reinforced concrete spans of a bridge subjected to extreme vehicular loads are investigated and retrofitted with carbon fiber reinforced polymer (CFRP) laminates. A finite element model of the bridge superstructure was created to determine the forces resulting from extreme loads. A moment–curvature analysis was subsequently carried out to investigate the flexural characteristics of the reinforced concrete sections prior to and after strengthening with CFRP laminates. The analytical modeling concluded that significant strength can be gained at the ultimate limit state, while relatively small increase in strength is observed at service load levels. The increase in flexural resistance at ultimate does provide an adequate margin of safety against further overloading. The analytical investigation and the retrofitting work are presented herein.     

Strengthening of thin-webbed castellated beam using CFRP

Abstract

The utilisation of fibre reinforced polymers (FRP) in the rehabilitation of steel structures increased in recent years. This article presents the advantages of using carbon fibre reinforced polymers (CFRP) in strengthening of the thin-webbed castellated beam (TCB). The performances of CFRP strengthened TCBs were analysed using finite element (FE) tool ABAQUS^®. Nonlinear FE analysis was carried out to find the optimum geometrical. Nine different designations of TCBs were arrived based on weld lengths, perforation sizes and strengthening technique (three each). Though all three techniques increased the strength considerably, the third one had much greater efficiency and was more suitable.     

Improvement of interlaminar mechanical properties of CFRP laminates using VGCF

In order to improve the interlaminar mechanical properties of CFRP laminates, hybrid CFRP/VGCF laminates have been fabricated by using a newly-developed method, i.e., powder method, where the powder of vapor grown carbon fiber (VGCF) is added at the mid-plane of [0°/0°]₁₄ CFRP laminates. Experimental results of double cantilever beam (DCB) tests indicate the improvement on the interlaminar mechanical properties of Mode-I fracture behavior with much higher critical load P_C and fracture toughness G_IC with VGCF interlayer. Crack propagation and fracture surface have also been observed to interpret this improvement mechanism. Moreover, based on experimental G_IC, numerical simulations using finite element method (FEM) with cohesive elements have been carried out to analyze the delamination propagation. The interlaminar tensile strength of hybrid CFRP/VGCF laminates, which is obtained by matching the numerical load–COD (crack opening displacement) curves to experimental ones, is higher than that of base CFRP laminates.     

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.     

Fatigue simulation for titanium/CFRP hybrid laminates using cohesive elements

This paper presents a new numerical approach for predicting fatigue crack growth in fiber-metal laminate (FML). Cohesive elements are used to express the complicated damage consisting of transverse cracking, splitting, and interlaminar delamination. The damage growth in the cohesive elements due to cyclic loading is represented by the conventional damage-mechanics model. The simulation was applied to notched Ti/CFRP hybrid laminates of two stacking configurations. In both cases, the crack growth rate in the titanium layer and the delamination shape agreed well with experiments reported in the literature. Complementary analysis for crack extension in the metal sheet is performed out of consideration of the damage in internal FRP layers. The numerical results demonstrated that the underlying damage modes in the FRP layer must be taken into account to predict the fatigue crack growth at the metal layer in FMLs.     

Tensile behaviour of patch-repaired CFRP laminates

The present study examines the tensile behaviour of composite structures repaired by bonding external patches. Various patches of different stacking sequences placed on both sides of the parent plate were considered. Damage development and the failure process of the repaired plates were analyzed and a parent plate fracture model has been proposed. Optimised patch repairs were calculated by finite element modelling. It was found that high stress concentration along the longitudinal edges of circular patches and/or at the transverse edges of the hole leads to early damage initiation in the parent plate. However, the position of damage initiation and the process of damage progression depend particularly on the properties of repair patches. In order to optimise patch repairs, finite element modelling was used and it was founded possible to attain over 90% of the strength of an unnotched specimen. The optimised patch design can be characterised by an optimal strength ratio R^*, which should be minimized when selecting repair parameters.     

Normal and oblique penetration of woven CFRP laminates by a high velocity steel sphere

In this research, two thicknesses of a woven CFRP laminate have been subjected to impact by a steel sphere in a velocity regime ranging from 170 to 374 m/s. Impact and penetration of targets at normal and oblique incidence were studied using high speed video. For the normal incidence targets at the higher velocities of impact, a conical mass of laminate was ejected ahead of the projectile. Furthermore, despite the energy transferred to the plate increasing with impact energy, the degree of delamination in the thicker targets decreased indicating a change in projectile penetration mechanism. Eventually, the degree of delamination in the thicker targets appeared to approach an asymptotic level whereas for the thinner targets the degree of delamination appeared constant regardless of impact energy. For oblique targets, more of the kinetic energy was transferred from the projectile when compared to the same thickness of target that had been subjected to a normal incidence impact. However, this was merely due to a geometrical effect. Further, thicker panels appeared to behave more efficiently by absorbing more kinetic energy per effective linear thickness at the lower impact energies where petalling is a dominant factor in the penetration. This advantage appeared to disappear as the impact energy was increased.     

Mechanical properties of CFRP laminates manufactured from unidirectional prepregs using CSCNT-dispersed epoxy

This study presents the experimental results of the mechanical properties of three-phase CFRP laminates consisting of traditional carbon fibers and epoxy matrix modified using cup-stacked carbon nanotubes (CSCNTs) in comparison to those of CFRP laminates without CSCNTs. The prepreg system of carbon fibers impregnated with CSCNT-dispersed epoxy is developed, and successful fabrication of three-phase CFRP laminates is achieved using an autoclave. Basic mechanical properties of unidirectional laminates (stiffness, strength, fracture toughness, etc.) are summarized. Next, quasi-isotropic laminates are subjected to tension, compression, flexural, and compression after impact (CAI) tests. Improvement of stiffness and strength and no adverse effects on mechanical properties due to CSCNT dispersion are experimentally verified.     

DCB test simulation of stitched CFRP laminates using interlaminar tension test results

A simulation model for the delamination extension of stitched CFRP laminates and 3-D orthogonal interlocked fabric composites (3-D OIFC) has been developed using a 2-D finite element method incorporating interlaminar tension test results to simulate the experimental results of their DCB tests. The mechanical properties of through-the-thickness fiber were determined from the results of interlaminar tension tests in which the specimen included only one through-the-thickness yarn. The fracture phenomena around the through-the-thickness thread, such as debonding from the in-plane layer, slack absorption, fiber bridging, and the pull-out of broken threads from the in-plane layers, are also introduced into the FEM model. The present FEM simulation results were compared to DCB test results for certain stitched laminates and a 3-D OIFC, and the simulation results showed good agreement with the experimental results of DCB tests, including the load–displacement curve and Mode I strain energy release rate (GI). While it was difficult to estimate GI accurately when the DCB test specimen included different types of z-fiber fracture modes, the present model of FEM analysis can simulate the experimental results of DCB tests of stitched laminates and 3-D OIFC. It is suggested that the GI of CFRP with arbitrary z-fiber densities can be predicted by using this FEM analysis model together with interlaminar tension test results.     

Damage detection of CFRP laminates using electrical resistance measurement and neural network

As carbon fibers are electrical conductors, the measurement of the electrical resistance appears to be a valuable technique for the in situ detection of various types of damage in carbon fiber reinforced polymers (CFRP) laminates. In such cases, carbon fibers are both the reinforcement and the sensor to detect damage in CFRP laminates. The damage-detecting method of CFRP laminates by electrical resistance measurement that are investigated in this study is made possible by attaching electrodes on the surface of the CFRP structures without special manufacturing.

In this paper, we investigate the electrical resistance change as a damage parameter of fatigue damage such as the degradation of residual strength and stiffness. The measured stiffness and electrical resistance change during fatigue tests showed a very similar trend of change. This is because cumulative fatigue damage is represented by the degradation of residual stiffness; these damages also cause change in electrical resistance. Thus, we can use this change in electrical resistance as a damage parameter. We also predict the future damage of composite laminates in fatigue loading from electrical resistance damage model by following a stiffness degradation model. Electrical resistance gradually increased as the stiffness reduced, and showed a very abrupt change when final fatigue failure was imminent. The predicted value showed good agreement with the experimental data except in the final stage, where stiffness and electrical resistance changed abruptly.     

Detection of impact damage in CFRP laminates

The initiation and propagation of damage in carbon fibre composites subjected to impact loading has been investigated. High velocity impact tests were conducted on a variety of stacking configurations using a nitrogen operated gas gun. The damage processes were characterised using X-radiography, ultrasonic C-scanning, optical microscopy and the deply technique. The merits and weaknesses of applying such damage detection techniques to the monitoring of impact damage in composites are discussed. The consequence of the various fracture mechanisms on residual tensile strength is also considered.     

Wireless detection of internal delamination cracks in CFRP laminates using oscillating frequency changes

It is difficult to detect delamination of rotating composite components like helicopter and wind turbine blades while in-service with a wired system. In the present study, a wireless system using a tiny oscillation circuit for detecting delamination of carbon/epoxy composites is proposed. In this system, a tiny oscillation circuit is attached to the composite component. When delamination of the component occurs, electrical resistance changes, which causes a change in the oscillating frequency of the circuit. Since this system uses the composite structure itself as a sensor and the oscillating circuit is very small, it is applicable to rotating components. The electrical resistance change and oscillating frequency change due to delamination is experimentally measured using carbon/epoxy specimens. The effects of temperature changes are also measured. The wireless method is found to successfully detect embedded delamination, and to estimate the size of the delamination. The effect of temperature change is minimized by means of a temperature compensation circuit.     

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.     

Prediction for progression of transverse cracking in CFRP cross-ply laminates using Monte Carlo method

This study predicted transverse cracking progression in laminates including 90° plies. The refined stress field (RSF) model, which takes into account thermal residual strain for plies including transverse cracks is formulated, and the energy release rate associated with transverse cracking is calculated using this model. For comparison, the energy release rate based on the continuum damage mechanics (CDM) model is formulated. Next, transverse cracking progression in CFRP cross-ply laminates including 90° plies is predicted based on both stress and energy criteria using the Monte Carlo method. The results indicated that the RSF model and the CDM model proposed in this study can predict the experiment results for the relationship between transverse crack density and ply strain in 90° ply. The models presented in this paper can be applied to an arbitrary laminate including 90° plies.     

Thermal and mechanical fatigue analysis of CFRP laminates

[0°/90°]_s and [±45°]_s CFRP laminated plates were analysed using a finite element formulation for their fatigue behaviour. A fatigue criterion which is based on the laminate interlaminar stresses and the basic lamina fatigue parameters was used. Thermal effects were included in the formulation. In particular, initial thermal stresses resulting from the curing of the laminate were also included in the analysis. The results showed that both laminates had predicted S-N behaviour similar to that from experiments of past investigators. Also, the fatigue behaviour for the [±45°]_s laminate between room temperature and the curing temperature were found to be the same. However, in the case of the [0°/90°]_s laminate the fatigue strength at high temperatures was found to be lower than that at low temperatures.     

Microscopic fatigue damage progress in CFRP cross-ply laminates

Damage progress in toughened-type carbon fibre-reinforced plastic (CFRP) cross-ply laminates under tensile fatigue loading was measured using the replica technique. The laminate configuration was [0/90_m/0], where m = 4, 8 and 12. The damage parameters, transverse crack density and delamination ratio, were determined. A power-law model was proposed, relating the cyclic strain range and the number of cycles at transverse crack initiation. Based on experimental data, a simple shear-lag analysis combined with the modified Paris law was conducted to model the transverse crack multiplication. An extension of the shearlag analysis for laminates containing delaminations initiating from the tips of the transverse cracks was used to conduct a modified Paris law analysis for delamination growth.     

Impact behaviour of stiffened CFRP sections

Dropweight impact tests have been performed on thin CFRP panels stiffened with blade or T-stiffeners and comparisons made with similar plain panels. The change in structural response of the panels is governed by the amount of damage sustained during impact. The increase in panel stiffness is associated with the suppression of backface cracking but larger areas of delamination.     

On the fatigue life prediction of CFRP laminates using the Electrical Resistance Change method

The electro-mechanical response (Electrical Resistance Change method) as a damage index of quasi-isotropic Carbon Fiber Reinforced (CFRPs) laminates under fatigue loading was investigated. The effect of dispersed Multi-Wall Carbon Nanotubes (MWCNT) into the epoxy matrix was additionally evaluated and compared with neat epoxy CFRPs. The longitudinal resistance change of the specimens was monitored throughout the fatigue experiment. Three different stress levels were tested. The frequency and the ratio (R) of the minimum applied load (stress) to the maximum applied load (stress) were kept constant for the different stress levels. The temperature of the specimen was also monitored throughout the process in order to deduce its effect on the electrical resistance of the specimen. The electrical behavior of the quasi-isotropic CFRP deviated from the commonly observed electrical response of unidirectional or cross-ply CFRPs due to the presence of the 45° layers. During initial stages of loading the resistance drops and afterwards it follows a positive slope up to final fracture. This repeatable pattern was observed for both the neat and the CNT-doped specimens, with the latter having smoother electrical recordings. The effect of temperature was calculated to be limited for the specific material and test/measurement configuration. The electro-mechanical response was correlated to stiffness degradation and acoustic emission findings enabling the identification of the specific regions during the fatigue life referring to specific mechanisms of damage accumulation. More specifically the experimental results revealed that the occurrence of the initial drop of the electrical resistance is linked with the occurrence of the Characteristic Damage State (CDS), associated with a specific percentage of stiffness reduction. This finding was used in order to predict the remaining life independently from the applied stress level with a high degree of confidence, assuming a constant stress level throughout the whole lifetime. The remaining life prediction for the CNT-doped specimens had higher coefficient of confidence (R²).     

Detection of edge delamination in CFRP laminates under cyclic loading using small-diameter FBG sensors

Small-diameter FBG sensors were applied for the detection of edge delamination in carbon fiber reinforced plastic (CFRP) quasi-isotropic laminates. Reflection spectra from the embedded fiber Bragg grating (FBG) sensor were measured at various lengths of delamination initiated from the edge of the specimen under cyclic loading. The form of the spectrum changed sensitively as the edge delamination grew. For confirmation of the measured results, the strain distribution in the FBG sensor was calculated by FEM analysis and the spectrum was simulated from the strain distribution theoretically. The change in the form of the measured spectrum was consistent with that of the calculated spectrum. From these results, the spectrum was found to depend on the size and location of the edge delamination. Moreover, the intensity ratio of the two peaks in the spectrum was proposed as an effective indicator for the quantitative evaluation of the edge delamination size.     

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.