• Korean Journal of Materials Research
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• v.25 no.10
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• pp.571-576
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• 2015

The effects of coating thickness on the delamination and fracture behavior of thermal barrier coating (TBC) systems were investigated with cyclic flame thermal fatigue (FTF) and thermal shock (TS) tests. The top and bond coats of the TBCs were prepared by electron beam-physical vapor deposition and low pressure plasma spray methods, respectively, with a thickness ratio of 2:1 in the top and bond coats. The thicknesses of the top coat were 200 and $500{\mu}m$, and those of the bond coat were 100 and $250{\mu}m$. FTF tests were performed until 1140 cycles at a surface temperature of $1100^{\circ}C$ for a dwell time of 5 min. TS tests were also done until more than 50 % delamination or 1140 cycles with a dwell time of 60 min. After the FTF for 1140 cycles, the interface microstructures of each TBC exhibited a sound condition without cracking or delamination. In the TS, the TBCs of 200 and $500{\mu}m$ were fully delaminated (> 50 %) within 171 and 440 cycles, respectively. These results enabled us to control the thickness of TBC systems and to propose an efficient coating in protecting the substrate in cyclic thermal exposure environments.

• Journal of Powder Materials
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• v.5 no.3
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• pp.184-191
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• 1998

The fabrication process and properties of SiC particulate preforms with high volume fraction above 50% were investigated. The SiC particulate preforms were fabricated by vacuum-assisted extraction method after wet mixing of SiC particulates of 48 ${\mu}m$ in diameter, $SiO_2$ as inorganic binder, cationic starch as organic binder and polyacrylamide as dispersant in distilled water. The SiC particulate preforms were consolidated by vacuum-assisted extraction, and were followed by drying and calcination. The drying processes were consisted with natural drying at $25^{\circ}C$ for 36 hrs and forced drying at 10$0^{\circ}C$ for 12 hrs in order to prevent the micro-cracking of SiC particulates preform. The compressive strengths of SiC particulate preforms were dependent on the inorganic binder content, calcination temperature and calcination time. The compressive strength of SiC preform increased from 0.47 MPa to 1.79 MPa with increasing the inorganic binder content from 1% to 4% due to the increase of $SiO_2$ flocculant between the interfaces of SiC particulates. The compressive strength of SiC preform increased from 0.90 MPa to 3.21 MPa with increasing the calcination temperatures from 800 to 120$0^{\circ}C$ under identical calcination time of 4hrs. The compressive strength of SiC preform increased from 0.92 to 1.95 MPa with increasing the calcination time from 2 hrs to f hrs at calcination temperature of 110$0^{\circ}C$. The increase of compressive strength of SiC preform with increasing the calcination temperature and time is due to the formation of crystobalite $SiO_2$ phase at the interfaces of SiC particulates.

• Transactions of Materials Processing
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• v.26 no.2
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• pp.121-126
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• 2017

The hot stamping process is an innovative forming method that could prevent the cracking of high strength steel sheets. The formability test of boron steel sheet using forming limit diagrams at elevated temperature is very complicated and time consuming job. In this paper, an alternative test method to evaluate the formability of boron steel in hot stamping has proposed. It measured the FLD0 instead of whole strain combinations of FLD with the tensile test machine and specially designed test rig. Test results shows that the proposed test method can simulate the plain strain condition fracture and can make the FLD of boron steel sheet at elevated temperature with less effort.

• Structural Engineering and Mechanics
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• v.84 no.2
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• pp.253-268
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• 2022

Using non-destructive Acoustic Emission (AE) and optical Digital Image Correlation (DIC) methods, the moment-curvature behavior of steel and GFRP bars reinforced concrete beams under flexure was explored in this study. In the tension zone, laboratory studies were carried out on steel-RC and GFRP-RC beams with varying percentages of longitudinal reinforcement ratios of 0.33 %, 0.52%, and 1.11%. The distinct mechanism of cracking initiation and fracture progression of failure in steel-RC and GFRP-RC beams were effectively correlated and picked up using AE waveform characteristics of the number of AE hits and their amplitudes, AE energy as well as average frequency and duration. AE XY event plots and longitudinal strain profiles using DIC gives an online and real-time visual display of progressive AE activity and strains respectively to efficaciously depict the crack evolution and their advancement in steel-RC and GFRP-RC beams. They display a close matching with the micro and macro-cracks visually observed in the actual beams at various stages of loading.

• Computers and Concrete
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• v.18 no.5
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• pp.1041-1051
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• 2016

In this study, the construction of a reinforced concrete bridge pier was analyzed from durability point of view. The goal of the study is to analyze the crack iniation condition due to construction and present some recommendations for construction conditions of the reinforced concrete bridge pier. The bridge is located at the western port area of Shenzhen, where the climate is high temperature and humidity. To control the cracking of concrete, a construction simulation was carried out for a heat transfer problem as well as a thermal stress problem. A shrinkage model for heat produced due to cement hydration and a Burger constitutive model to simulate the creep effect are used. The modelling based on Femmasse(C) is verified by comparing with the testing results of a real underground abutment. For the bridge pier, the temperature and stress distribution, as well as their evolution with time are shown. To simulate the construction condition, four initial concrete temperatures ($5^{\circ}C$, $10^{\circ}C$, $15^{\circ}C$, $20^{\circ}C$) and three demoulding time tips (48h, 72h, 96h) are investigated. From the results, it is concluded that a high initial concrete temperature could result in a high extreme internal temperature, which causes the early peak temperature and the larger principle stresses. The demoulding time seems to be less important for the chosen study cases. Currently used 72 hours in the construction practice may be a reasonable choice.

• Computers and Concrete
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• v.19 no.5
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• pp.579-588
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• 2017

Creep and shrinkage are the main types of volume change with time in concrete. These changes cause deflection, cracking and stresses that affect durability, serviceability, long-term reliability and structural integrity of civil engineering infrastructure. Although laboratory test may be undertaken to determine the deformation properties of concrete, these are time-consuming, often expensive and generally not a practical option. Therefore, relatively simple empirically design code models are relied to predict the creep strain. This paper reviews the accuracy of creep and shrinkage predictions of reinforced concrete (RC) shear walls structures strengthened with carbon fibre reinforced polymer (CFRP) plates, which is characterized by a widthwise varying fibre volume fraction. This review is yielded by three commonly used international "code type" models. The assessed are the: CEB-FIP MC 90 model, ACI 209 model and Bazant & Baweja (B3) model. The time-dependent behavior was investigated to analyze their seismic behavior. In the numerical formulation, the adherents and the adhesives are all modelled as shear wall elements, using the mixed finite element method. Several tests were used to demonstrate the accuracy and effectiveness of the proposed method. Numerical results from the present analysis are presented to illustrate the significance of the time-dependency of the lateral displacements and eigenfrequencies modes.

• Structural Engineering and Mechanics
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• v.19 no.5
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• pp.581-602
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• 2005

In this paper the accumulation of local damage during the cyclic loading in reinforced high-strength concrete columns is experimentally investigated. Two identical column specimens with annular cross-section and spiral reinforcement were designed and two tests, up to failure, under the action of a constant vertical concentrated force and a time-dependent concentrated horizontal force, were carried out at the LNEC shaking tables facility. Sine type signals, controlled in amplitude, frequency and time duration were used for these experiments. The concept of local damage based on local stiffness degradation is considered in detail and illustrated by experimental results. The specimens were designed and reinforced in such a way that the accumulation of damage was predicted by dominating deformations (cracking and crushing of the concrete) while the increasing of the loading values was a dominating factor of damage. It was observed that the local damage of HSC columns has exposed their anisotropic local behaviour. The damage accumulation was slightly different from the expected in accordance with the continuum damage concept, and a partial random character was observed.

• Corrosion Science and Technology
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• v.22 no.6
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• pp.457-465
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• 2023

Irradiation-assisted stress corrosion cracking (IASCC) is one of the main degradation mechanisms of austenitic stainless steels, which are used as reactor internal materials. Slow strain rate testing (SSRT) has been widely applied to evaluate the IASCC initiation characteristics of proton-irradiated tensile specimens. Tensile specimens require low surface roughness for micro-crack observation, and electropolishing is the most important specimen pre-treatment process used for this. In this study, optimal electropolishing conditions were examined through analyzing results of polarization experiments and surface roughness measurements after electropolishing. Corrosion cell and electropolishing equipment were fabricated for polarization tests and electropolishing experiments using SSRT specimens. The experimental parameters were electropolishing time, current density, electrolyte temperature, and stirring speed. The optimal electropolishing conditions for SSRT tensile specimens made of type 316 stainless steel were evaluated as a polishing time of 180 seconds, a current density of 0.15 A/cm², an electrolyte temperature of 60 ℃, and a stirring speed of 200 RPM.

• Corrosion Science and Technology
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• v.10 no.2
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• pp.52-59
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• 2011

Corrosion of metallic materials occurs by the reaction with corrosive environment such as atmosphere, marine, soil, urban, high temperature etc. In general, reduction of thickness and cracking and degradation are resulted from corrosion. Corrosion in all industrial facilities and infrastructure causes large economic losses as well as a large number of accidents. Economic loss by corrosion has been reported to be nearly 1-6% of GNP or GDP. In order to reduce corrosion damage of industrial facilities, corrosion map as well as a systematic investigation of the loss of corrosion in each industrial sector is needed. The Corrosion Science Society of Korea in collaboration with 15 universities and institutes has started to survey on the cost of corrosion and corrosion map of Korea since 2005. This work presents the results of the survey on cost of corrosion by Uhlig, Hoar, and input-output methods, and the evaluation of atmospheric corrosion rate of carbon steel, weathering steel, galvanized steel, copper, and aluminum in Korea. The total corrosion cost was estimated in terms of the percentage of the GDP of industry sectors and the total GDP of Korea. According to the result of Input/output method, corrosion cost of Korea was calculated as 2.9% to GDP (2005). Time of wetness was shown to be categories 3 to 4 in all exposure areas. A definite seasonal difference was observed in Korea. In summer and fall, time of wetness was higher than in other seasons. Because of short exposure period (12 months), significant corrosion trends depending upon materials and exposure corrosion environments were not revealed even though increased mass loss and decreased corrosion rate by exposure time.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.4A
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• pp.353-359
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• 2010

Cementitious matrix based composites are vulnerable to the drying shrinkage crack during the curing process. In this study, the drying shrinkage induced fracture behavior of the fiber reinforced concrete is simulated and the effects of the fiber reinforcement conditions on the fracture characteristics are analysed. The numerical model is composed of conduit elements and rigid-body-spring elements on the identical irregular lattice topology, where the drying shrinkage is presented by the coupling of nonmechanical-mechanical behaviors handled by those respective element types. Semi-discrete fiber elements are applied within the rigid-body-spring network to model the fiber reinforcement. The shrinkage parameters are calibrated through the KS F 2424 free drying shrinkage test simulation and comparison of the time-shrinkage strain curves. Next, the KS F 2595 restrained drying shrinkage test is simulated for various fiber volume fractions and the numerical model is verified by comparison of the crack initiating time with the previous experimental results. In addition, the drying shrinkage cracking phenomenon is analysed with change in the length and the surface shape of the fibers, the measurement of the maximum crack width in the numerical experiment indicates the judgement of the crack controlling effect.