• Journal of Korean Tunnelling and Underground Space Association
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• v.22 no.5
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• pp.529-541
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• 2020

Although TBM's cutterhead requires design review for fatigue failure due to wear-induced section loss as well as heavy load during excavation, it is difficult to find a case of fatigue analysis for TBM cutterhead at present. In this study, a stress-life design review was conducted on cutter heads with a diameter of 8.2 m using S-N curves as a safety life design concept. Also, we introduced the fatigue design method of construction equipment and the method of assessing fatigue damage and explained the results of the fatigue analysis on the TBM cutter head with a diameter of 8.2 m. The S-N curve has been shown to play a key role in fatigue design and can also be used to assess how much fatigue damage a structure is suffering from at this point in time. In the future, it is necessary to find out when fatigue problems occur during using the equipment and when it is good to conduct safety inspections of the equipment.

• The Journal of Korean Academy of Prosthodontics
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• v.17 no.1
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• pp.23-34
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• 1979

In this study, the adhesive strength of three commercial polycarboxylate cements to ten types of dental casting alloys, such as gold, palladium, silver, indium, copper, nickel, chromium, and human enamel and dentine were measured and compared with that of a conventional zinc phosphate cement. The $8.0mm{\times}3.0mm$ cylindrical alloy specimens were made by casting. The enamel specimens were prepared from the labial surface of human upper incisor, and the dentine specimens were prepared from the occulusal surface of the human molar respectively. Sound extracted human teeth, which had been kept in a fresh condition since, extraction, were mounted in a wax box with a cold-curing acrylic resin to expose the flattened area. The mounted teeth were then placed in a Specimen Cutter (Technicut) and were cut down under a water spray, and then the flat area on the all specimens were ground by hand with 400 and 600 grit wet silicone carbide paper. Two such specimens were then cemented together face-to-face with freshly mixed cement, and moderate finger pressure was applied to squeeze the cement to a thin and uniform film. All cemented specimens were then kept in a thermostatic humidor cabinet regulated at $23{\pm}2^{\circ}C.$ and more than 95 per cent relative humidity and tested after 24 hours and 1 week. Link chain was attached to each alloy specimen to reduce the rigidity of the jig assembly, and then all the specimens were mounted in the grips of the Instron Universal Testing Machine, and a tensile load was delivered to the adhering surface at a cross head speed of 0.20 mm/min. The loads to which the specimens were subjected were recorded on a chart moving at 0.50 mm/min. The adhesive strength was determined by measuring the load when the specimen separated from the cement block and by dividing the load by the area. The test was performed in a room at $23{\pm}2^{\circ}C.$ and $50{\pm}10$ per cent relative humidity. A minimum of five specimens were tested each material and those which deviated more than 15 per cent from the mean were discarded and new specimens prepared. From the experiments, the following results were obtained. 1) It was found that the adhesive strength of the polycarboxylate cement to all alloys tested was considerably greater than that of the zinc phosphate cement. 2) The adhesive strength of the polycarboxylate cements was superior to the non precious alloys, such as the copper, indium, nickel and chromium alloys, but it was inferior to the precious gold, silver and palladium alloys. 3) Surface treatment of the alloy was found to be an important factor in achieving adhesion. It appears that a polycarboxylate cement will adhere better to a smooth surface than to a rough one. This contrasts with zinc phosphate cements, where a rough helps mechanical interlocking. 4) The adhesion of the polycarboxylate cement with enamel was found superior to its adhesion with dentine.

• Journal of the Korean Wood Science and Technology
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• v.50 no.1
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• pp.1-11
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• 2022

To evaluate the bearing strength of red pine cross-laminated timber (CLT) with self-tapping screw (STS), which is widely used as a fastener for connection in CLT building, the bearing test was conducted. Accoring to the STS's diameters (8, 10, 12 mm), the bearing test specimens with half hole were manufactured. Bearing strength was compared and reviewed in consideration of the configuration in STS and the loading direction to the grain of red pine. As a result of the bearing test on the STS's diameter, the yield bearing load increases as the larger diameter of the STS in all directions of the red pine. The bearing strength of the thread part (thread + tip) was higher than the shank part (shank + shank cutter). In compared with the directions to the grain of red pine, the bearing strength of the cross section parallel to the loading direction was the highest, and the tangent section was the lowest bearing strength. The average bearing strength of the loading direction in parallel to the grain was 23.43 MPa, which was about 45% higher than the average 16.16 MPa in perpendicular to the grain. The predicted bearing strength calculated by Eurocode (EN) and Korean Building Code (KBC)'s equation was lower than the experimental value. It is nessesary to propose the new equations of bearing strength reflected the configuration information of STS.

• Proceedings of the Korean Society for Agricultural Machinery Conference
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• 2017.04a
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• pp.65-65
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• 2017

밭작물 중 양파와 마늘은 높은 노동투하시간, 노임의 상승, 복잡한 수거 과정 등으로 최근 재배면적이 감소하는 추세이다. 밭농업 활성화 방안으로 생산비 절감과 작업 속도 향상을 위한 수확기계 개발 연구로 줄기절단기가 개발되었다. 이 연구는 줄기절단기 예취날의 설치각이 절단부하에 미치는 영향을 분립체 해석 기법으로 분석하고자 하였다. 실험에 사용한 시뮬레이션은 EDEM으로 인장탄성계수, 전단탄성 계수, 중첩길이, 입자 반경 및 질량, 상대속도를 이용하여 충돌시 힘을 해석하고 인장 및 전단강도, 한계, 결합 길이를 통해 입자간 결합을 설정할 수 있다. 시제품으로 설계된 줄기절단기의 치수를 활용하여 프로그램 상에서 줄기 절단 시뮬레이션을 진행하여 절단부하 결과를 얻도록 하였고 칼날 설치각을 30도, 45도, 60도로 변경하여 각각의 부하를 분석하여 경향성을 파악하고자 하였다. 실험에서 사용한 프로그램은 EDEM 2.7.1 Academic Research 버전이며 시뮬레이션을 진행한 PC의 사양은 Intel(R) $Core^{TM}$ i7-4790 CPU @ 3.60GHz, Memory 16.0GB이다. 줄기 절단 시뮬레이션에 적용시킨 줄기 모델은 마늘의 조건을 적용시켜 직경 1mm의 입자로 이루어진 지름 12mm, 높이 214mm의 원통형 모델이며 60.32N의 최대절단력을 가지고 있다. 줄기절단기는 2개의 회전날을 가지고 있으나 좌우대칭을 적용하여 절반에 대한 해석으로 하나의 회전날로 절단을 하도록 줄기 모델은 4조로 하여 3열을 140mm 간격으로 위치시켰다. 줄기절단기의 칼날은 반경 350mm로 회전하며 진행속도는 1.65m/s, 회전속도는 1680rpm으로 작업하도록 하였다. 시뮬레이션은 0.5초의 시간에 대해 해석하도록 하였으며 0.003초 간격으로 칼날에 가해지는 힘을 구하여 저장하도록 하였다. 시뮬레이션 해석시간은 약 116시간이었으며 설치각별 시간에 따른 칼날에 가해지는 압축력 값과 그래프를 얻을 수 있었다. 대부분의 시간에서 절단이 이루어지지 않으므로 0의 값을 나타내었으며 절단이 이루어지는 시점에 절단부하가 나타났다. 결과 해석을 위해 그래프의 피크 값들을 이용하였으며 그 중 상위 6개의 값으로 분석하였다. 30도, 45도, 60도의 설치각에 따른 절단부하의 평균 값은 각각 105.4N, 160.5N, 215.9N으로 나타났다. 설치각에 따른 절단부하의 경향성은 유의수준은 3.93%로 각각의 차이가 유의미하게 나타났으며 상관계수는 0.489로 증가하는 경향이 보였다. 그러나 $R^2$는 0.2394로 낮은 값을 보여 데이터 처리 방법의 개선과 적절한 회귀 모델의 적용이 필요하다. 향후 포장시험을 진행하고 복합적으로 분석하여 경향성을 자세히 분석하고자 한다.

• Journal of Korean Tunnelling and Underground Space Association
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• v.21 no.4
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• pp.545-560
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• 2019

During EPB TBM tunnelling, an appropriate application of additives such as foam and polymer is an essential factor to secure the stability of TBM as well as tunnelling performance. From the '90s, there have been many studies on the optimal injection of additives worldwidely contrary to the domestic situation. Therefore, in this paper, the foam, which is widely adopted for soil conditioning, was selected as an additive in order to investigate the effect of foam injection on TBM performance through a series of laboratory excavation tests. The excavation experiments were carried out on artificial sandy soil specimens with consideration of the variance of FIR (Foam Injection Ratio), FER (Foam Expansion Ratio) and $C_f$ (Surfactant Concentration), which indicate the amount and quality of the foam. During the tests, torque values were measured, and the workability of conditioned soil was evaluated by comparing the slump values of muck after each experiment. In addition, a weight loss of the replaceable aluminum cutter bits installed on the blade was measured to estimate the degree of abrasion. Finally, the foam injection ratio for the optimal TBM excavation for the typical soil specimen was determined by comparing the measured torque, slump value and abrasion. Note that the foam injection conditions satisfying the appropriate level of machine load, mechanical wear and workability are essential in the EPB TBM operational design.