• Proceedings of the Korean Society of Precision Engineering Conference
• /
• 2003.06a
• /
• pp.1386-1388
• /
• 2003

In this study. the fatigue behavior of cracked aluminum repaired by unidirectional graphite/epoxy composites was experimentally investigated. The aluminum used was 7075-T6 and the patch used was four plied unidirectional ([0]$_4$) composites. The composite patch was adhesively bonded to the cracked aluminum using secondary bonding procedure. Two different specimens of cracked aluminum and cracked aluminum repaired with patch were used in the fatigue tests. Load ratio and the frequency applied in the fatigue tests were 0 and 10 Hz, respectively. The results showed that the fatigue behavior of cracked aluminum was improved by repairing the cracked area with composite patch. Specifically, the specimen repaired by composite patch showed 30% more improved fatigue behavior than regular specimen.

• The Transactions of the Korean Institute of Electrical Engineers C
• /
• v.50 no.4
• /
• pp.153-160
• /
• 2001

In order to analyze the degradation process of epoxy/glass fiber for outdoor condition, FRP laminate was exposed to high temperature. Then, the degradation process was evaluated by comparing contact angle, surface potential, surface resistivity, and XPS. The experimental results showed that the amount of weight loss, contact angle, surface potential and surface resistivity increased up to 200 $^{\circ}C$ as a function of temperature. These phenomena show the existence of hydrophobic surface. With the change to the hydrophobic surface and the electrical potential and resistivity on FRP surface increased. In XPS to analyze surface chemical structures, the increased hydrophobicity in thermal increase of unsaturated double bond in carbon chains. Aslo, thermal treatment caused the discoloration on the point of treated surface. These phenomena were attributed to the generations of ether group.

• Journal of Adhesion and Interface
• /
• v.10 no.4
• /
• pp.162-168
• /
• 2009

As a preliminary study of optimum composite properties under cryogenic temperature, the comparison of interfacial properties of carbon or glass fibers reinforced epoxy composites was evaluated at ambient and intermediate low temperature, i.e., 25 and $-10^{\circ}C$ by using micromechanical techniques. Under tensile and compressive loading conditions, their mechanical modulus at low temperature was higher than that atambient temperature. Interfacial shear strength (IFSS) at ambient and low temperatures was compared to each other, depending on epoxy matrix toughness and apparent modulus at the interface. The IFSS was much higher at low temperature than that at room temperature because of the increased epoxy matrix modulus. Statistical distributions of tensile strengths of glass and carbon fibers were evaluated for different temperature ranges, which is dependent upon fiber's inherent flaws and rigidity.

• Proceedings of the Korean Society For Composite Materials Conference
• /
• 2002.10a
• /
• pp.88-91
• /
• 2002

The two dimensional size effect of specimen gauge section (length x width) was investigated on the compressive behavior of a T300/924 [45/-45/0/90]3s, carbon fiber-epoxy laminate. A modified ICSTM compression test fixture was used together with an anti-buckling device to test 3mm thick specimens with a 30$\times$30, 50$\times$50, 70$\times$70, and 90mm$\times$90mm gauge length by width section. In all cases failure was sudden and occurred mainly within the gauge length. Post failure examination suggests that $0^{\circ}$ fiber microbuckling is the critical damage mechanism that causes final failure. This is the matrix dominated failure mode and its triggering depends very much on initial fiber waviness. It is suggested that manufacturing process and quality may play a significant role in determining the compressive strength. When the anti-buckling device was used on specimens, it was showed that the compressive strength with the device was slightly greater than that without the device due to surface friction between the specimen and the device by pretoque in bolts of the device. In the analysis result on influence of the anti-buckling device using the finite element method, it was found that the compressive strength with the anti-buckling device by loaded bolts was about 7% higher than actual compressive strength. Additionally, compressive tests on specimen with an open hole were performed. The local stress concentration arising from the hole dominates the strength of the laminate rather than the stresses in the bulk of the material. It is observed that the remote failure stress decreases with increasing hole size and specimen width but is generally well above the value one might predict from the elastic stress concentration factor. This suggests that the material is not ideally brittle and some stress relief occurs around the hole. X-ray radiography reveals that damage in the form of fiber microbuckling and delamination initiates at the edge of the hole at approximately 80% of the failure load and extends stably under increasing load before becoming unstable at a critical length of 2-3mm (depends on specimen geometry). This damage growth and failure are analysed by a linear cohesive zone model. Using the independently measured laminate parameters of unnotched compressive strength and in-plane fracture toughness the model predicts successfully the notched strength as a function of hole size and width.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.11 no.5
• /
• pp.89-98
• /
• 2007

This study investigates the deformation behavior, related to debonding failure, of adhesion and repairing-strengthening materials of RC construction. A strain-stress curve shows that when the stress of specimens reached the highest and then fails, the strain value of cement mortar is $2.0{\times}10^{-3}$, while concrete was indicated at around $1.3{\times}10^{-3}$, epoxy resins are $0.8{\times}10^{-3}$, polymer mortar is $2.5{\times}10^{-3}$, steel plate is $2.5{\times}10^{-3}$, and carbon bar was $9.1{\times}10^{-3}$, respectively. For a thermal expansion coefficient with temperature variation, those basis materials, cement mortar and concrete, exhibited around $10{\mu}{\varepsilon}/{^{\circ}C}$, but adhesive materials, such as epoxy resins and polymer mortar, were $41{\sim}54{\mu}{\varepsilon}/{^{\circ}C}$ and $-0.5{\sim}0.7{\mu}{\varepsilon}/{^{\circ}C}$, respectively. In the case of steel plate is similar to basic materials but carbon fiber is indicates at $-1.7{\mu}{\varepsilon}/{^{\circ}C}$, which is the lowest value. Especially, between basic and adhesive materials, the thermal expansion coefficient was highly different. Although the coefficient depends on the type of epoxy resins, it is clear that the epoxy resins are susceptible to be debonded in nature, when the difference of environmental temperature varies more than $20{\sim}35{^{\circ}C}$.

• Journal of the Korean Society for Precision Engineering
• /
• v.34 no.2
• /
• pp.139-143
• /
• 2017

Finite element analysis was performed for a split-type CFRP bicycle frame, which was designed to apply a compression molding process with carbon fiber prepreg for a conventional bicycle. An epoxy adhesive material for joining the frames was selected by the extent of stress at joint interfaces. The split-type bicycle frame was then formed and its weak parts examined by the boundary conditions according to reliability tests. The results verified the reliability of the bicycle frame after modification of these weak parts. The finished product was manufactured by using this developed split-type bicycle frame.

• Proceedings of the Korea Concrete Institute Conference
• /
• 2001.11a
• /
• pp.491-496
• /
• 2001

In recent years, the research and development about the new material proceed rapidly and actively in the building industry. As building structures become bigger, higher and more specialized, so does the demand for material with higher strength. In the future, we will need to research repair and rehabilitation to make high strength concrete mixed steel fibrous building safe. The carbon fiber reinforced plastic bonding method is widely used in reinforcing the existing concrete structure among the various methods. The repair of initiate loaded reinforced high-strength concrete beams mixed steel fibrous with epoxy bonded Carbon Fiber Sheets(CFS) was investigated experimentally. The CFS thickness and length were varied to assess the peel failure at the curtailment of CFS, The behaviour of the repaired beams was represented by load-longitudinal steel strain relation and failure modes were discussed. The test results indicate that CFS is very effective for strengthening the demand beams and controlling deflections of reinforced high strength concrete beams mixed steel fibrous happen diagonal crack, the increase in the number of CFS layers over two layers didn't effect the increase in the strength of beams.

• Corrosion Science and Technology
• /
• v.19 no.2
• /
• pp.75-81
• /
• 2020

To reduce structural weight, light metals, including aluminum and magnesium alloys, have been widely used in various industries such as aircraft, transportation and automobiles. Recently, composite materials such as Carbon Fiber Reinforced Plastics (CFRP) and Graphite Epoxy Composite Material (GECM) have also been applied. However, aluminum and its alloys suffer corrosion from various factors, which include aggressive ions, pH, solution temperature and galvanic contact by potential difference. Moreover, carbon fiber in CFRP and GECM is a very efficient cathode, and very noble in the galvanic series. Galvanic contact between carbon fiber composites and metals in electrolytes such as rain or seawater, is highly undesirable. Notwithstanding the potentially dangerous effects of chloride and temperature, there is little research on galvanic corrosion according to chloride concentration and temperature. This work focused on the effects of chloride concentration and solution temperature on AA7075T6. The increased galvanic corrosion between CRFP and AA7075T6 was evaluated by electrochemical experiments, and these effects were elucidated.

• Journal of the Korean Society for Aeronautical & Space Sciences
• /
• v.47 no.1
• /
• pp.35-40
• /
• 2019

The objective of this paper is to identify the fatigue properties of carbon-fiber composite which is widely applied for the development of aircraft structures and obtain data for full-scale fatigue test. The durability and damage tolerance evaluation of composite structures is achieved by fatigue tests and parameters such as fatigue life factor and load enhancement factor. The specimens are made with carbon-fiber/epoxy UD tape and fabric prepreg. Fatigue tests are performed with several stress ratios and lay-up patterns. The Weibull shape parameters are analyzed by Sendeckyj model and individual fatigue lives with Weibull distribution. And the fatigue life factor and load enhancement factor considering reliability are evaluated.

• Journal of the Korea institute for structural maintenance and inspection
• /
• v.5 no.2
• /
• pp.93-102
• /
• 2001

The objective of this experimental study is to evaluate the strengthening effects of concrete compression confined with Epoxy-boned compound fiber sheets. An analytical model is proposed to construct a stress-strain relationship for confined concrete. Test results are summarized as followed. While non-FRP lateral confinement specimens appeared sudden failure after shell concrete was torn off, specimens confined laterally with FRP were showed that their failure. Specially, Glass fiber lateral confined specimens occurred obviously increase ductility ability. Hence, concrete specimen with lateral confinement by Hi-carborn and Aramide. Glass fiber simultaneously can be increased in not only strength but also a lot of ductility ability.