• Journal of the Korean Society of Manufacturing Process Engineers
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• v.19 no.2
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• pp.75-80
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• 2020

In this study, we designed car frames for collision analysis using carbon fiber reinforced polymer (CFRP) as the lighter composite material. The impact conditions were 100 percent frontal impact, 40 percent frontal impact, and 90 degrees side impact. The impact analysis measured the maximum stress at velocities of 20km/h and 40km/h for each condition and evaluated the vulnerable points in the car frame. Additional supports have been designed both to improve the weak points in existing vehicle frames, and to be taken into account when new parts are assembled. Our impact analysis compared the results of maximum stress on the car frame with and without the support.

• Korean Journal of Materials Research
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• v.2 no.4
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• pp.298-305
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• 1992

The carbonization behaviors of CFRP fabricated with 2D-woven fabric and matrix phenolic resin have been studied. The changes in dimension were observed in the temperature range of 365-37$0^{\circ}C$ in the thickness direction, 118-12$0^{\circ}C$ in the normal direction each other by TMA analysis. Observation with the optical microscope shows that the formed cracks and pores during the fabrication of CFRP were propagated with the increase of pyrolysis temperaure. New cracks and pores were formed in the pyrolysis temperature range of 400-50$0^{\circ}C$ In line with the formation and propagation of cracks, porosity was increased and density was decreased rapidly in the pyrolysis temperature range of from 40$0^{\circ}C$ to 70$0^{\circ}C$. Therefore heating rate in the carbonization process need to be controlled carefully by intervals.

• Journal of the Korea Concrete Institute
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• v.18 no.2 s.92
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• pp.293-298
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• 2006

The reinforced concrete(RC) structures strengthened with fiber reinforced plastic(FRP) has been accepted by the construction engineering community for rehabilitation. FRP composites can present many advantages like a corrosion resistance, strength-weight ratio, relatively short application time, and cost effectiveness. The beams under design load, however, are cracked and result in degrading the strength. It is difficult to recognize cracks and deflections on the surface of the concrete members retrofitted with FRP through the life cycle. For these reasons, if they result in the effects, which were below the expected strength, we must monitor the state of concrete structures all the time in order to take an appropriate measure. Fiber Bragg Grating(FBG) sensor excel as monitoring of investigating the stress state of the retrofitted beams with FRP. The main objective of this study is to measure strain by experiment and analyze the behavior of RC beams retrofitted with FRP using FBG sensor. The kinds of FRP which were used in research are carbon, glass and improved hybrid FRP(IFRP) that has capacity than any other FRP. Other variables are the length of FRP, the number of sheet.

• Composites Research
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• v.25 no.4
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• pp.98-104
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• 2012

In recent years, degenerative spinal instability has been effectively treated with a cage. However, little attention is focused on the stiffness of the cage. Recent advances in the medical implant industry have resulted in the use of medical carbon fiber reinforced polymer (CFRP) cages. The biomechanical advantages of using different cage material in terms of stability and stresses in bone graft are not fully understood. A previously validated three-dimensional, nonlinear finite element model of an intact L2-L5 segment was modified to simulate posterior interbody fusion cages made of CFRP and titanium at the L4-L5 disc with pedicle screw, to investigate the effect of cage stiffness on the biomechanics of the fused segment in the lumbar region. From the results, it could be found that the use of a CFRP cage would not only reduce stress shielding, but it might also have led to increased bony fusion.

• Advances in Computational Design
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• v.2 no.2
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• pp.147-168
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• 2017

Over the past couple decades, externally bonded fiber reinforced polymer (FRP) composites have emerged as a repair and strengthening material for many concrete infrastructure applications. This paper presents an analytical investigation of the use of carbon FRP (CFRP) for a specific problem that occurs in concrete bridge girders wherein the girder ends are damaged by excessive exposure to deicing salts and numerous freezing/thawing cycles. A 3D finite element (FE) model of a full scale prestressed concrete (PC) I-girder is used to investigate the effect of damage to the cover concrete and stirrups in the end region of the girder. Parametric studies are performed using externally bonded CFRP shear laminates to determine the most effective repair schemes for the damaged end region under a short shear span-to-depth ratio. Experimental results on shear pull off tests of CFRP laminates that have undergone accelerated aging are used to calibrate a bond stress-slip model for the interface between the FRP and concrete substrate and approximate the reduced bond stress-slip properties associated with exposure to the environment that causes this type of end region damage. The results of these analyses indicate that this particular application of this material can be effective in recovering the original strength of PC bridge girders with damaged end regions, even after environmental aging.

• Smart Structures and Systems
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• v.26 no.2
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• pp.157-174
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• 2020

The guided wave technique is commonly used in structural health monitoring as the guided waves can propagate far in the structures without much energy loss. The guided waves are conventionally generated by the surface-mounted piezoelectric wafer active sensor (PWAS). However, there is still lack of understanding of the wave propagation in layered structures, especially in structures made of anisotropic materials such as carbon fiber reinforced polymer (CFRP) composites. In this paper, the Rayleigh-Lamb wave strain tuning curves in a PWAS-mounted unidirectional CFRP plate are analytically derived using the normal mode expansion (NME) method. The excitation frequency spectrum is then multiplied by the tuning curves to calculate the frequency response spectrum. The corresponding time domain responses are obtained through the inverse Fourier transform. The theoretical calculations are validated through finite element analysis and an experimental study. The PWAS responses under the free, debonded and bonded CFRP conditions are investigated and compared. The results demonstrate that the amplitude and travelling time of wave packet can be used to evaluate the CFRP bonding conditions. The method can work on a baseline-free manner.

• Computers and Concrete
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• v.21 no.6
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• pp.607-622
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• 2018

A non-linear finite element model (FEM) was constructed using a three-dimensional software (ATENA-3D) to investigate the effect of strengthening on the behavior of prestressed hollow-core (PHC) slabs with or without openings. The slabs were strengthened using near surface mounted (NSM)-carbon fiber reinforced polymer (CFRP) strips. The constructed model was validated against experimental results that were previously reported by the authors. The validated FEM was then used to conduct an extensive parametric study to examine the influence of prestressing reinforcement ratio, compressive strength of concrete and strengthening reinforcement ratio on the behavior of such slabs. The FEM results showed good agreement with the experimental results where it captured the cracking, yielding, and ultimate loads as well as the mid-span deflection with a reasonable accuracy. Also, an overall enhancement in the structural performance of these slabs was achieved with an increase in prestressing reinforcement ratio, compressive strength of concrete, external reinforcement ratio. The presence of openings with different dimensions along the flexural or shear spans reduced significantly the capacity of the PHC slabs. However, strengthening these slabs with 2 and 4 (64 and $128mm^2$ that represent reinforcement ratios of 0.046 and 0.092%) CFRP strips was successful in restoring the original strength of the slab and enhancing post-cracking stiffness and load carrying capacity.

• International Journal of Concrete Structures and Materials
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• v.11 no.1
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• pp.115-133
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• 2017

To investigate the effectiveness of using fiber reinforced polymer (FRP) sheets in confining heat-damaged columns, 15 circular RC column specimens were tested under axial compression. The effects of heating duration, stiffness and thickness of the FRP wrapping sheets were examined. Two specimen groups, six each, were subjected to elevated temperatures of $500^{\circ}C$ for 2 and 3 h, respectively. Eight of the heat-damaged specimens were wrapped with unidirectional carbon and glass FRP sheets. Test results confirmed that elevated temperatures adversely affect the axial load resistance and stiffness of the columns while increasing their ductility and toughness. Full wrapping with FRP sheets increased the axial load capacity and toughness of the damaged columns. A single layer of the carbon sheets managed to restore the original axial resistance of the columns heated for 2 h yet, two layers were needed to restore the axial resistance of columns heated for 3 h. Glass FRP sheets were found to be less effective; using two layers of glass sheets managed to restore the axial load carrying capacity of columns heated for 2 h only. Confining the heat-damaged columns with FRP circumferential wraps failed in recovering the original axial stiffness of the columns. Test results confirmed that FRP-confining models adopted by international design guidelines should address the increased confinement efficiency in heat-damaged circular RC columns.

• Smart Structures and Systems
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• v.28 no.6
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• pp.737-744
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• 2021

In this study, structural health monitoring (SHM) methods of carbon fiber reinforced plastics (CFRPs) were investigated using electrical resistance. The developed sensing technique monitored electrical resistance in accordance with the impact damage of a CFRP. The changes in electrical resistances with multiple electrode sets enabled SHM without extra sensors so that this technique can be called self-sensing. Moreover, this study proposed electrodes only at peripheral side of a structure to minimize the number of electrodes compared to those in an array which has square number of sensors as the sensing area increases. For the intensive investigation, electromechanical sensitivity in terms of electrode distance was analyzed and optimized under drop weight impact testing. Then, SHM methods with electrodes in an array and electrodes in peripheral edges were comparatively investigated. The developed methods successfully localized impact damages into 2D coordinates. Furthermore, damage severity can be shown with a damage map by calculating electrical resistance change ratio. Therefore, structural health self-sensing system using electrical resistance was successfully developed with the minimum number of electrodes.

• Journal of the Korea institute for structural maintenance and inspection
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• v.24 no.6
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• pp.25-33
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• 2020

To investigate the relationship between strain distribution and tensile properties of brittle material, five types of tensile coupon of carbon fiber reinforced polymer (CFRP) modified the tab portion in order to have a strain distribution including S0, SD1, SD2, SV1, SV2 were tested. The ultimate stress and strain of SD2 and SV2 which was intended to have larger strain distribution were smaller than those of SD1 and SV1, that was more clearly shown in the test results of the symmetric coupons (SV series) than the asymmetric coupons (SD series). In addition, the ultimate stress and strain of most coupons with strain distribution in this study were decreased when compared to the control group with uniform strain. These results were analyzed in various ways through 1) the average of the strain values directly measured by the strain gages, 2) the converted strain calculated by dividing the total deformation by the effective length, and 3) the ultimate effective strain derived from both the elastic modulus and the ultimate load. The values measured by strain gage indicates response of the local region precisely, but it does not represent the response from whole section. However, the converted strain and effective strain can supplement disadvantage of gage because they represent the average response of whole section. In particular, the effective strain can provide rupture strain conservatively, which can be utilized in practice, when the value obtained by strain gage was not effective due to gage damage or abnormal gage readings near ultimate load. This value provides a value that can be used even when partial rupture has occurred and is reasonably useful for specimens with strain distribution.