• Journal of the Korean Society for Heat Treatment
• /
• v.36 no.2
• /
• pp.61-68
• /
• 2023

Ti and Ti alloys are used in the automobile and aerospace industries due to their high specific strength and excellent corrosion resistance. However their application is limited due to poor formability at room temperature and high unit cost. In order to overcome these issues, dissimilarly jointed materials, such as cladding materials, are widely investigated to utilize them in each industrial field because of an enhanced plasticity and relatively low cost. Among various dissimilar bonding processes, the rolled cladding process is widely used in Ti alloys, but has a disadvantage of low bonding strength. Although this problem can be solved through post-heat treatment, the mechanical properties at the bonded interface are deteriorated due to residual stress generated during post-heat treatment. Therefore, in this study, the microstructure change and residual stress trends at the interfaces of Ti/Al cladding materials were studied with increasing post-heat treatment temperature. As a result, compared to the as-rolled specimens, no difference in microstructure was observed in the specimens after postheat treatment at 300, 400, and 500℃. However, a new intermetallic compound layer was formed between Ti and Al when post-heat treatment was performed at a temperature of 600℃ or higher. Then, it was also confirmed that compressive residual stress with a large deviation was formed in Ti due to the difference in thermal expansion coefficient and modulus of elasticity between Ti Grade II and Al 1050.

• Earthquakes and Structures
• /
• v.23 no.5
• /
• pp.473-491
• /
• 2022

The research investigates experimentally the effect of confinement on structural behavior at the ends of beam-column in reinforced concrete (RC) frames. In the experimental study, five specimens consisting of 1/3-scaled RC frames having single-bay, representing the traditional deficiencies of existing buildings constructed without receiving proper engineering service is investigated. The RC frame specimens were produced to represent most of the existing buildings in Turkey that have damage potential. To decrease the probable damage to the existing buildings exposed to earthquakes, the carbon Textile Reinforced Mortar (TRM) strengthening technique (fully wrapping) was used on the ends of the RC frame elements to increase the energy dissipation and deformation capacity. The specimens were tested under reversed cyclic lateral loading with constant axial loads. They were constructed satisfying the weak column-strong beam condition and consisting of low-strength concrete, such as compressive strength of 15 MPa. The test results were compared and evaluated considering stiffness, strength, energy dissipation capacity, structural damping, ductility, and damage propagation in detail. Comprehensive investigations of these experimental results reveal that the strengthening of a brittle frame with fully-TRM wrapping with non-anchored was effective in increasing the stiffness, ductility, and energy dissipation capacities of RC bare frames. It was also observed that the frame-only-retrofitting with an infill wall is not enough to increase the ductility capacity. In this case, both the frame and infill wall must be retrofitted with TRM composite to increase the stiffness, lateral load carrying, ductility and energy dissipation capacities of RC frames. The presented strengthening method can be an alternative strengthening technique to enhance the seismic performance of existing or moderately damaged RC buildings.

• Geomechanics and Engineering
• /
• v.33 no.1
• /
• pp.29-40
• /
• 2023

Xanthan gum and starch compound biopolymer (XS), an environmentally friendly soil-binding material produced from natural resources, has been suggested as a slope protection material to enhance soil strength and erosion resistance. Insufficient wet strength and the consequent durability concerns remain, despite XS biopolymer-soil treatment showing high strength and erosion resistance in the dried state, even with a small dosage of soil mass. These concerns need to be solved to improve the field applicability and post-stability of this treatment. This study explored the utilization of an alkaline-based cation crosslinking method using calcium hydroxide and sodium hydroxide to induce non-thermal gelation, resulting in the enhancement of the wet strength and durability of biopolymer-treated soil. Laboratory experiments were conducted to assess the unconfined compressive strength and cyclic wetting-drying durability performance of the treated soil using a selected recipe based on a preliminary gel formation test. The results demonstrated that the uniformity of the gel structure and gelling time varied depending on the ratio of crosslinkers to biopolymer; consequently, the strength of the soil was affected. Subsequently, site soil treated with the recipe, which showed the best performance in indoor assessment, was implemented on the field slope at the bridge abutment via compaction and pressurized spraying methods to assess feasibility in field implementation. Moreover, the variation in surface soil hardness was monitored periodically for one year. Both slopes implemented by the two construction methods showed sufficient stability against detachment and scouring, with a higher soil hardness index than the natural slope for a year.

• Journal of the Korea Institute of Military Science and Technology
• /
• v.26 no.1
• /
• pp.1-9
• /
• 2023

The skin structure of sensor pod mounted on the exterior of aircraft can be exposed to the acoustic dynamic load and static load such as aerodynamic pressure and inertial load during flight. Fatigue life prediction of structural model under acoustic load should be performed and also differential stiffness of model modified by static load should be considered. The acoustic noise test spectrum of MIL-STD-810G was applied to the structural model and the stress response power spectral density (PSD) was calculated. The frequency response analysis was performed with or without prestress induced by inplane static load, and the response spectrum was compared. Time series data was generated using the calculated PSD, and the time and frequency domain fatigue life were predicted and compared. The variation of stress response spectrum due to static load and predicted fatigue life according to the different structural model considering mean stress were examined and decreasing fatigue life was observed in the model with prestress of compressive static load.

• Journal of the Korean Recycled Construction Resources Institute
• /
• v.11 no.1
• /
• pp.48-54
• /
• 2023

In this research, The basic physical properties and EMP shielding performance by thickness were evaluated for optimum composition of EMP shield concrete that can be applied on-site by mixing carbon-based materials with high conductivity into concrete that uses electric furnace oxidized slag (EOS). As a result of the evaluation, it was confirmed that the slump decreased as the amount of mixed carbon fib er (CF) increased, and increased when milled carb on (MCF) was mixed. As for the compressive strength, it was confirmed that EOS enhanced the strength compared to NA, and it was confirmed that the strength decreased when CF and MCF were mixed. As the thickness of the EMP shielding measurement increases, the shielding rate increases, and it was confirmed that the type of conductive material and the thickness of the test specimen have a greater influence on the shielding rate than the Amount of conductive material added. As a result of a comparative evaluation, EOS CF 0.2 is considered suitable for EMP shield concrete formulation.

• Journal of the Korean Geotechnical Society
• /
• v.23 no.3
• /
• pp.41-50
• /
• 2007

The results from normally consolidated isotropic drained and undrained triaxial compression tests (NCIU and NCID) on sand with high silt content were presented in this paper. The experiments were performed on specimens of Nak-dong River sand with 63% silt content under effective confined pressures, 100 kPa to 400 kPa. From test results, Sandy silt became initially compressive but eventually appeared to provide dilatancy response throughout the entire stress-strain curve The behavior of sandy silt was more difficult to characterize than that of clay and sand due to lower plastic characteristic. Especially, the samples exhibited dilatancy development during shear after failure. The shear behavior and shear strength parameters of sandy silt can be determined as stress-strain behaviors are described by the Mohr-Coulomb failure criterion. The shear behaviors were observed increasing dilatancy volume change tendency with strain-softening tendency after failure. In this paper, the behavior of dilatancy depends on not only sand content but also fine content with low-cohesion during shear in the samples of sandy silt.

• Journal of the Korean Geotechnical Society
• /
• v.25 no.10
• /
• pp.5-15
• /
• 2009

This paper presents the results of full-scale uplift load tests performed on 24 passive anchors grouted to various lengths at Okchun and Changnyong site. Rock anchors were installed over a wide range of rock types and qualities with a fixed anchored depth of 1~6 m. The majority of installations used D51 mm high grade steel rebar to induce rock failure prior to rod failure. However, a few installations included the use of D32 mm rebar at relatively deeper anchored depth so as to induce rod failure. In many tests, rock failure was reached and the ultimate loads were recorded along with observations of the shape and extent of the failure surface. In addition to field tests, laboratory pullout tests were conducted to determine bond strength and bond stress-shear slip relation at the tendon/grout interface when a corrosion protection sheath is installed in the cement-based grout. The test results show that the ultimate tendon-grout bond strength is measured from 18~25% of unconfined compressive strength of grout. One of the important results from these tests is that the measured strains along the corrosion protection sheath were so small that practically the reduction of bond strength by the presence of sheath would be negligible.

• Journal of the Korean Geotechnical Society
• /
• v.24 no.4
• /
• pp.67-78
• /
• 2008

In this study, artificial cemented sand made of a few portland cement and Nak-Dong river sand was researched closely to investigate cementing effect quantitatively through unconfined tests and triaxial tests. The peak strength and elastic modulus increased and dilation of cemented sand was restricted by the cementation, but after breakage of the cementation, dilation and negative excess pore water pressure increased. In stress-strain curve, strain-softening behavior appeared in drained condition but strain-hardening behavior was appeared in undrained condition as a result of the increase of effective stress. The test was quantitatively analyzed by multiple regression models, correlating each response variable with input variable. The equations are valid only over the range investigated. Its adjusted coefficient of determination was $0.81{\sim}0.91$, and dry density is important factor for estimating strength of cemented sand.

• Journal of Powder Materials
• /
• v.30 no.2
• /
• pp.146-155
• /
• 2023

The Ti-6Al-4V lattice structure is widely used in the aerospace industry owing to its high specific strength, specific stiffness, and energy absorption. The quality, performance, and surface roughness of the additively manufactured parts are significantly dependent on various process parameters. Therefore, it is important to study process parameter optimization for relative density and surface roughness control. Here, the part density and surface roughness are examined according to the hatching space, laser power, and scan rotation during laser-powder bed fusion (LPBF), and the optimal process parameters for LPBF are investigated. It has high density and low surface roughness in the specific process parameter ranges of hatching space (0.06-0.12 mm), laser power (225-325 W), and scan rotation (15°). In addition, to investigate the compressive behavior of the lattice structure, a finite element analysis is performed based on the homogenization method. Finite element analysis using the homogenization method indicates that the number of elements decreases from 437,710 to 27 and the analysis time decreases from 3,360 to 9 s. In addition, to verify the reliability of this method, stress-strain data from the compression test and analysis are compared.

• Land and Housing Review
• /
• v.14 no.2
• /
• pp.137-145
• /
• 2023

In this study, the characteristics such as flowability, bleeding rate, and strength of the CLSM (Controlled Low Strength Material) according to physical properties such as particle size distribution and particulate content of the pond ash were investigated as part of the practical development of technology for CLSM using pond ash. As a result of analyzing the properties of the collected pond ash, it was found that the density and particle size distribution characteristics were different. And that the bleeding ratio did not satisfy the standard in the case of the specimen with a large amount of fly ash and a lot of addition of mixing water. As a result of the compressive strength test, the strength development of 0.5 MPa or more for four hours was found to be satisfactory for the specimens using hemihydrate gypsum with a unit binder amount of 200 or more, and the remaining gypsum showed poor strength development. Although it was determined that landfill coal ash can be used as a CLSM material, it is necessary to identify and apply the physical and chemical characteristics of coal ash buried in the ash treatment plant of each power generation company.