• Polymer(Korea)
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• v.32 no.5
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• pp.440-445
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• 2008

The failure mechanism and failure morphology of linear low density polyethylene (LLDPE) tubing under hydrostatic pressure were investigated. Microscopic observations using video microscope and scanning electron microscope indicate that the failure mode is a brittle fracture including cracks propagated from inner wall to outer wall. In addition, oxidation induction time and Fourier transform infrared spectroscopy results show the presence of exothermic peak and the increase in carbonyl index on the surface of fractured LLDPE tubing, due to thermal-degradation. An accelerated life test methodology and testing system for LLDPE tubing are developed using the relationship between stresses and life characteristics by means of thermal acceleration. Statistical approaches using the Arrhenius model and Weibull distribution are implemented to estimate the long-term life time of LLDPE tubing under hydrostatic pressure. Consequently, the long-term life time of LLDPE tubing at the operating temperature of $25^{\circ}C$ could be predicted and also be analyzed.

• Transactions of Materials Processing
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• v.31 no.6
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• pp.376-383
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• 2022

The potential damage region of a WC-Co cemented carbide die is investigated for cold forging process of a wheel-nut by numerical simulation with its chemical composition considered. Numerical simulation is utilized to calculate internal stress, especially for the WC-Co die, during the forging process. Finite element model is established, in which the elasto-plastic properties are applied to the work-piece of bulk steel, and elastic properties are considered for the lower die insert of the WC-Co alloy. This stress analysis enables to distinguish the potential damage regions of the WC-Co die. The regions from calculation are comparatively analyzed along with the crack area observed in the die after repetitive manufacturing. Effect of chemical composition of the WC-Co is also evaluated on characteristics of potential damage region of the die with variance of mechanical properties considered. Derived from Mohr-Coulomb fracture model, furthermore, a new stress index is presented and used for die stress analysis. This index inherently considers hydrostatic pressure and is then capable of deducing wide range of its distribution for representing stress state by modification of its parameter implying pressure sensitivity.

• Explosives and Blasting
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• v.37 no.1
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• pp.14-23
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• 2019

It has been an always issue for the blasting or the impact analysis to consider the strength characteristics of the rock materials associate with loading rate dependency. Due to the nature of transient loading, the dynamic rock test requires a careful technique to achieve the stress equilibrium state of the specimen. In this study, to investigate the relationship between the rock dynamic strength and the stress equilibrium state, a series of dynamic uniaxial compression tests for Pocheon granite were performed. As a result, the unbalanced stress state on the specimen can lead to the premature failure on the specimen and the less estimation of dynamic strength characteristic as well as the overestimation of strain rate. Consequently, a careful consideration of rock fracture process to achieve the dynamic force balance on the specimen should be required to make an reasonable evaluation of rock dynamic strength.

• Journal of Powder Materials
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• v.26 no.4
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• pp.290-298
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• 2019

$TiO_2$-particles containing Co grains are fabricated via thermal hydrogenation and selective oxidation of TiCo alloy. For comparison, $TiO_2$-Co composite powders are prepared by two kinds of methods which were the mechanical carbonization and oxidation process, and the conventional mixing process. The microstructural characteristics of the prepared composites are analyzed by X-ray diffraction, field-emission scattering electron microscopy, and transmission electron microscopy. In addition, the composite powders are sintered at $800^{\circ}C$ by spark plasma sintering. The flexural strength and fracture toughness of the sintered samples prepared by thermal hydrogenation and mechanical carbonization are found to be higher than those of the samples prepared by the conventional mixing process. Moreover, the microstructures of sintered samples prepared by thermal hydrogenation and mechanical carbonization processes are found to be similar. The difference in the mechanical properties of sintered samples prepared by thermal hydrogenation and mechanical carbonization processes is attributed to the different sizes of metallic Co particles in the samples.

• Tunnel and Underground Space
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• v.29 no.3
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• pp.157-171
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• 2019

The fracturing by liquid carbon dioxide ($LCO_2$) as a fracking fluid has been an alternative to mitigate the environmental issues often caused by the conventional hydraulic fracking since it facilitates the fluid permeation owing to its low viscosity. This study presents how $LCO_2$ injection influences the breakdown pressure, acoustic emission, and fracture morphology. Three fracturing fluids such as $LCO_2$, water, and oil are injected with different pressurization rate to the synthetic and porous mortar specimens. Also, the shale which has been a major target formation in conventional fracking practices is also tested to examine the failure characteristics. The results show that $LCO_2$ injection induces more tortuous and undulated fractures, and particularly the larger fractures are developed in cases of shale specimen. On the other hand, the relationship between the fracturing fluids and the breakdown pressure shows opposite tendency in the tests of mortar and shale specimens.

• Korean Journal of Acupuncture
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• v.35 no.4
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• pp.174-186
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• 2018

Objectives : The aim of this survey was to investigate the general characteristics of sports injury patients in Korean medical institutions by Korean Medicine Doctors (KMDs) and the treatment patterns of Korean medicine for ankle sprain which is one of the common sports injuries. Methods : An online survey was conducted on KMDs interested in sports injuries to assess their experience and perception of sports injuries including diagnosis, intervention, and treatment plan for ankle sprain. Results : A total of 276 KMDs participated in the survey. They answered that 12% (median, range 0~80%) of patients visiting a Korean medical institution had sports injuries. Sports injuries frequently occur in the ankle, back, shoulder, knee, and elbow and the most common sports injury was sprain/strain. Many participants were aware that sports injuries should be treated differently from other musculoskeletal diseases in general. They reported that confirmation of fracture, swelling, and tenderness was essential to diagnose ankle sprains. Acupuncture was the most commonly used treatment and reported to need 2 to 6 weeks of treatment period, depending on the severity. Conclusions : Based on the findings regarding treatment of sports injury patients by KMDs, this study would be utilized as basic information for pragmatic clinical research design related to sports medicine in the future.

• Clinical and Experimental Pediatrics
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• v.62 no.3
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• pp.90-94
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• 2019

Purpose: Several published policy statements have warned against the risks associated with trampoline use and recommended safety guidelines. However, few studies have focused on trampoline-related injuries in Korea. This study aimed to assess the incidence and characteristics of pediatric trampoline-related injuries presented to Ulsan University Hospital. Methods: We retrospectively reviewed the medical records of children aged <16 years with trampoline-related injuries who visited our Emergency Department between 2008 and 2017. Results: Over the 10-year period, 178 trampoline-related injuries were reported, which represented a significant increase (P=0.016). Most (87.6%) of the injuries occurred during the last 5 study years, and a rapid increase in injuries was observed in children aged <6 years. Lower extremity injuries (62.4%) were the most common, followed by injuries of the upper extremities, head and face, and trunk, including injuries to the neck and spine. Sixty-seven children (37.6%) had fractures, and proximal tibia fractures were the most common. Fractures were significantly more common in younger children (<6 years old) than in older children (P=0.026). Conclusion: In Korea, the mechanism of trampoline injury is similar to that of injuries incurred in indoor trampoline parks but is characterized by smaller spaces and multiple users. Trampoline use and the incidence of trampoline-related injuries in children aged <6 years are increasing rapidly. Prohibiting the use of trampolines for children aged <6 years, restricting simultaneous use by multiple children, and ensuring adult supervision should be strictly emphasized. Public awareness and policy guidelines are needed to reduce the incidence of trampoline-related injuries.

• Archives of Plastic Surgery
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• v.46 no.5
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• pp.414-420
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• 2019

Background Congenital muscular torticollis (CMT) is characterized by persistent head tilt toward the affected side. No consensus exists regarding the cause of this disorder. In this study, we analyzed various clinical factors in patients with CMT who were treated with surgical release. This analysis enabled us to identify potential causative factors of CMT and to establish a basis for surgical interventions. Methods In total, 584 patients who underwent surgical intervention for CMT from October 2007 to December 2016 were included in this study. Their demographic characteristics, birthrelated factors, and clinical features were analyzed. Results Data from 525 patients were analyzed in this study after exclusion of those with insufficient information. Before birth, 31 patients (5.9%) were diagnosed with oligohydramnios, and 87 (16.6%) had a breech presentation. Seven (1.3%) cases of clavicle fracture and two (0.4%) cases of cephalohematoma were noted at birth. Before surgery, 397 patients (75.6%) underwent physiotherapy and 128 patients (24.4%) did not. The duration of physiotherapy ranged from 1 to 50 months (average, 6 months). Conclusions Our study shows that 16.6% of the CMT patients presented in the breech position, which is a much higher rate than that observed in the general population (3%-4%). We hypothesize that being in the breech position as a fetus appears to exert a significant influence on shortening and fibrosis of the sternocleidomastoid muscle.

• Geomechanics and Engineering
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• v.17 no.1
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• pp.47-56
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• 2019

Crossing (X-type) flaws are commonly encountered in rock mass. However, the crack coalescence and failure mechanisms of rock mass with X-type flaws remain unclear. In this study, we investigate the compressive failure process of rock-like specimens containing two X-type flaws aligned in the loading direction. For comparison purposes, compressive failure behavior of specimens containing two aligned single flaws is also studied. By examining the crack coalescence behavior, two characteristics for the aligned X-type flaws under uniaxial compression are revealed. The flaws tend to coalesce by cracks emanating from flaw tips along a potential path that is parallel to the maximum compressive stress direction. The flaws are more likely to coalesce along the coalescence path linked by flaw tips with greater maximum circumferential stress if there are several potential coalescence paths almost parallel to the maximum compressive stress direction. In addition, we find that some of the specimens containing two aligned X-type flaws exhibit higher strengths than that of the specimens containing two single parallel flaws. The two underlying reasons that may influence the strengths of specimens containing two aligned X-type flaws are the values of flaw tips maximum circumferential stresses and maximum shear stresses, as well as the shear crack propagation tendencies of some secondary flaws. The research reported here provides increased understanding of the fundamental nature of rock/rock-like material failure in uniaxial compression.

• Applied Microscopy
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• v.49
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• pp.3.1-3.7
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• 2019

Graphene, which is one of the most promising materials for its state-of-the-art applications, has received extensive attention because of its superior mechanical properties. However, there is little experimental evidence related to the mechanical properties of graphene at the atomic level because of the challenges associated with transferring atomically-thin two-dimensional (2D) materials onto microelectromechanical systems (MEMS) devices. In this study, we show successful dry transfer with a gel material of a stable, clean, and free-standing exfoliated graphene film onto a push-to-pull (PTP) device, which is a MEMS device used for uniaxial tensile testing in in situ transmission electron microscopy (TEM). Through the results of optical microscopy, Raman spectroscopy, and TEM, we demonstrate high quality exfoliated graphene on the PTP device. Finally, the stress-strain results corresponding to propagating cracks in folded graphene were simultaneously obtained during the tensile tests in TEM. The zigzag and armchair edges of graphene confirmed that the fracture occurred in association with the hexagonal lattice structure of graphene while the tensile testing. In the wake of the results, we envision the dedicated preparation and in situ TEM tensile experiments advance the understanding of the relationship between the mechanical properties and structural characteristics of 2D materials.