• Journal of the Korean Society for Nondestructive Testing
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• v.21 no.5
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• pp.532-541
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• 2001

Developments of nondestructive evaluation techniques for reduction of strength or toughness by aging of material have been carried out, and the method using electrical resistivity is one of them. In this study, to examine the application of electrical resistivity to the evaluation of degradation of metallic alloy, ten different non-magnetic materials were selected as test materials. Electrical resistivities measured by DC two-point probe method and those measured by non-contact type eddy current method were compared with each other. In addition, to examine the application possibility of four-point probe technology in field, the electrical resistivities for 1Cr-lMo-0.25V steel measured by DC two-point probe method and four-point probe method were compared with each other Differences between two measured values for the 1Cr-1Mo-0.25V steel were 0.6%. Therefore, the four-point probe method can be applied to the estimation of the degradation of metallic alloy. ect.

• Structural Engineering and Mechanics
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• v.60 no.6
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• pp.1063-1077
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• 2016

Components manufactured from composite materials are frequently subjected to superimposed mechanical and thermal loadings during their operating service. Both types of loadings may cause fracture and failure of composite structures. When composite cross-ply laminates of type [$0_m/90_n]_s$ are subjected to uni-axial tensile loading, different types of damage are set-up and developed such as matrix cracking: transverse and longitudinal cracks, delamination between disoriented layers and broken fibers. The development of these modes of damage can be detrimental for the stiffness of the laminates. From the experimental point of view, transverse cracking is known as the first mode of damage. In this regard, the objective of the present paper is to investigate the effect of transverse cracking in cross-ply laminate under thermo-mechanical degradation. A Finite Element (FE) simulation of damage evolution in composite crossply laminates of type [$0_m/90_n]_s$ subjected to uni-axial tensile loading is carried out. The effect of transverse cracking on the cross-ply laminate strength under thermo-mechanical degradation is investigated numerically. The results obtained by prediction of the numerical model developed in this investigation demonstrate the influence of the transverse cracking on the bearing capacity and resistance to damage as well as its effects on the variation of the mechanical properties such as Young's modulus, Poisson's ratio and coefficient of thermal expansion. The results obtained are in good agreement with those predicted by the Shear-lag analytical model as well as with the obtained experimental results available in the literature.

• Corrosion Science and Technology
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• v.21 no.2
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• pp.130-137
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• 2022

In this work, the degradation of high-pressure superheater tubes exposed to the flame of a duct burner in a heat recovery steam generator of a district heating system was evaluated. To assess the deterioration of the used superheater tube, the microstructure, microhardness, and tensile properties were investigated by comparison to an unused tube. The study found that a fin bound at the outer surface of the used tube became fragile only in the location facing the flame. This indicates that the tube was directly exposed to the flame from the duct burner or underwent abnormal overheating. While the unused tube showed a uniform value in hardness and equiaxial grain structure, the used tube revealed a decrease in hardness up to 105 HV and an increase in grain size with a plate-like morphology in the location facing the flame. The coarsening of the grain structure by the flame weakened the mechanical properties of yield strength, tensile strength, and elongation.

• Nuclear Engineering and Technology
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• v.56 no.5
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• pp.1854-1862
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• 2024

This study aims to investigate the strain-dependent-deformation property of Gyeongju bentonite buffer material. A series of unconfined compressive tests were performed with cylindrical specimens prepared at varying dry densities (𝜌_d = 1.58 g/cm³ to 1.74 g/cm³) using cold isostatic pressing technique. It is found that as 𝜌_d increase, the unconfined compressive strength (q_u), failure strain, and elastic modulus (E) of Gyeongju compacted bentonite (GCB) increases. Normalized elastic modulus (E_sec/E_max) degradation curves of GCB specimens are fitted using Ramberg-Osgood model and the elastic threshold strain (𝜀_e,th) is determined through the fitted curves. The strain-dependency of E and Poisson's ratio (v) of GCB were observed. E and v were measured constant below 𝜀_e,th of 0.14 %. Then, E decreases while v increases after exceeding the strain threshold. The E_sec/E_max degradation curves of GCB in this study suggests wider linear range and higher linearity than those of sedimentary clay in previous study. On top of that, the influence of 𝜌_d is observed on E_sec/E_max degradation curves of GCB, showing a slight increase in 𝜀_e,th with increase in 𝜌_d. Furthermore, an empirical model of q_u with 𝜌_d and a correlation model between q_u and E are proposed for Gyeongju bentonite buffer materials.

• Journal of Conservation Science
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• v.32 no.4
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• pp.501-510
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• 2016

In this study, we investigated the degradation of adhesives when exposed to ultraviolet light irradiation using samples of lacquer (L), treated lacquer (TL), lacquer mixed with glue (LG), and urushiol mixed with glue (UG). Four types of film specimens were collected under the ultraviolet exposure time, and gloss test, tensile shear strength test, scanning electron microscope analysis, and infrared spectroscopic analysis were conducted for the specimens. LG and UG showed lowering rate of gloss is somewhat later than L. Also, it was observed that with increasing exposure time to ultraviolet irradiation, the surface of L began to show spherical pits and cracks when the polysaccharide layers started to be exposed, whereas the surfaces of LG and UG remained smooth. The Infrared spectra of L and TL showed that the intensity of the overall peak decreased with increasing ultraviolet irradiation time. There was no change in the peak intensity of LG, but for UG, the peaks at $3013cm^{-1}$, $1593cm^{-1}$ and so on disappeared and the overall intensity declined. The tensile shear strength of LG and UG was maintained or increased as compared to the initial test, whereas the tensile shear strength of L decreased sharply after 600 h. LG and UG exhibited fewer changes as a result of high temperature and humidity conditions, and they retained their strength under UV exposure. These results indicate that LG and UG are more durable than L when subjected to environmental change.

• Journal of the Korea Concrete Institute
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• v.20 no.3
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• pp.375-381
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• 2008

Various problems such as durability degradation may happen when extra water is added to concrete. Because of these reasons, the change of water content is managed by using rapid evaluation method of unit water content such as electric capacity method, heat drying method making use of micro wave, unit capacity mass method among various methods. Especially, in Japan, guidance for the change of water content ($\pm$ 10, 15, 20 kg/$m^3$ etc.) were regulated and used. However, it is the real situation that the guidance which were regulated in South Korea evaluate suitability only considering production and measurement error under the circumstances which are not considering the degree of durability degradation. Therefore, this study tries to investigate the influence of addition of extra water in the concrete on the durability degradation of concrete when it was added by artificial manipulation or by management error. From the test results, a guideline of the contents of extra water for the quality control is suggested with the consideration of the degree of durability degradation and the probable error resulted from the addition of extra water. The contents of extra water for tests are set as 0, 15, 25, 35 kg/$m^3$. To examine the durability degradation of concrete, freezing and thawing, carbonation, chloride penetration and compressive strength are tested.

• KOREAN JOURNAL OF PACKAGING SCIENCE & TECHNOLOGY
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• v.28 no.3
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• pp.165-170
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• 2022

If petroleum- derived plastic like a bio-based plastic was degradation, awareness like a global warming and environmental disasters will be decreased. Plastic film was produced by adding ferric ions according to concentration by using a pro-oxidant in polyolefin resin. Changes in tensile strength, elongation, and molecular weight were evaluated according to the UV irradiation time. Increasing the amount of ferric ions resulted in more significant declines of physical properties, and also resulted in greater changes in molecular weight. After 100 hours of UV irradiation, tensile strength declined significantly in the film containing pro-oxidant as compared to the control. A similar effect was also observed in terms of elongation. The film containing pro-oxidant showed a 73.8% decrease in molecular weight after 100 hours of UV irradiation. The appropriate use of pro-oxidant can not only degrade plastic film but also control the time of degradation at the petroleum-derived plastic films. Further studies are necessary to investigate the conditions of plastic film degradation.

• Proceedings of the Earthquake Engineering Society of Korea Conference
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• 2000.10a
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• pp.364-371
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• 2000

The pseudo dynamic test has been carried out so as to investigate the seismic performance of RC bridge piers strengthened with and without glass fiber sheets. The Lessons from severe demage of many infrastructures in Kobe(1995) and Northridge(1996) earthquakes have emphasized the need to develop the retrofit measures to enhance flexural strength, ductility and shear strength of RC bridge piers nonseismically designed before 1992. Therefore, the objective of this experimental research is to investigate the seismic behavior of circular reinforced concrete bridge piers by the pseudo dynamic test. and then to enhance the ductility of concrete piers strengthening with glass fiber sheets in the plastic hinge region. 7 circular RC bridge piers were made in a 1/3.4 scale. Important test parameters are confinement steel ratio, retrofitting. load pattern, etc. The seismic behavior of circular concrete piers under artificial ground motions has been evaluated through strength and stiffness degradation, energy dissipation. It can be concluded that existing bridge piers wrapped with glass fibers in the plastic hinge regions could have enough seismic performance.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1991.10a
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• pp.29-32
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• 1991

It is well known that the fatigue damage process in composite materials is very complicated due to complex failure mechanisms that comprise debounding, matrix cracking, delamination and fiber splitting of laminates. Therefore, the residual strength, instead of a single dominant crack length, is chosen to describe the criticality of the damage accumulated in the sublaminate. In this study, two models for residual strength degradation established by Yang-Liu and Tanimoto-Ishikawa that are capable of predicting the statistical distribution of both fatigue life and residual strength have been investigated and compared. Statistical methodologies for fatigue life prediction of composite materials have frequently been adopted. However, these are usually based on a simplified probabilistic approach considering only the variation of fatigue test data. The main object of this work is to propose a fatigue reliability analysis model which accounts for the effect of all sources of variation such as fabrication and workmanship, error in the fatigue model, load itself, etc. The proposed model is examined using the previous experimental data of GFRP and it is shown that it can be practically applied for fatigue problems in composite materials.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.05a
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• pp.17-21
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• 2002

A finite element method based on the two-dimensional progressive failure analysis is presented for characterizing the strength and failure of the unidirectional-fabric hybrid laminated composite joints under pin loading. The 8-node laminated shell element is incorporated in the updated Lagrangian formulation. Various failure criteria including the maximum stress, Tsai-Wu, Yamada-Sun, and combinations of them are used in conjunction with the complete unloading stiffness degradation method. For the verification, joint tests are conducted for the specimens with various geometries. Although there are some differences depending on the geometry, the finite element model using the Yamada-Sun or the combined Yamada-Sun and Tsai-Wu criterion predicts the failure strength best.