• 한국자동차공학회논문집
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• 제10권2호
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• pp.96-100
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• 2002

The purpose of this paper is presented a rational method of predicting dynamic/impact tensile strength of high tensile steel materials widely used fur structural material of automobiles. It is known that the ultimate strength is related with the loading speed and the Lethargy Coefficient from the tensile test. The Dynamic Lethargy Coefficient is proportional to the disorientation of the molecular structure and indicates the magnitude of defects resulting from the probability of breaking the bonds responsible for its strength. The coefficient is obtained from the simple tensile test such as failure time and stresses at fracture. These factors not only affect the static strength but also have a great influence on the dynamic/impact characteristics of the joist and the adjacent structures. This strength is used to analyze the failure life prediction of mechanical system by virtue of its material fracture. The impact tensile test is performed to evaluate the life parameters due to loading speed with the proposed method. Also the evaluation of the dynamic/impact effect on the material tensile strength characteristics is compared with the result of Campbell-Cooper equation to verify the proposed method.

• 터널과지하공간
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• 제13권3호
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• pp.180-186
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• 2003

동적 인장강도를 평가하기 위해 수치해석기법을 이용하여 홉킨슨효과를 가정한 동적 인장시험의 암석 파쇄 과정을 해석하였다. 동적 인장 파쇄 과정과 정적 인장 파쇄 과정에 명확한 차이가 있음을 밝혔으며, 동적 인장강도와 정적 인장강도의 상위는 응력집중현상과 응력재분배에 의해 발생된다는 것을 지적하였다. 그러나 정적파쇄와 동적파쇄 과정에 큰 차이가 있음에도 불구하고 정적 인장강도와 동적 인장강도는 미시적 인장강도의 균질성 계수가 증가함에 따라 미시적 인장강도의 평균치에 근접하였다. 본 연구로부터 암석의 불균질성은 동적 인장강도에 영향을 미치는 요인이며, 동적 인장강도와 정적 인장강도의 상위를 발생시키는 중요한 요인임을 지적하였다.

• Geomechanics and Engineering
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• 제29권4호
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• pp.451-461
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• 2022

The roadways surrounded by rock and coal will lose their stability or even collapse under rock burst. Rock burst mainly involves an evolution of dynamic loading which behaves quite differently from static or quasi-static loading. To compare the dynamic response of coal and rocks with different static strengths, three different rocks and bituminous coal were selected for testing at three different dynamic loadings. It's found that the dynamic compression strength of rocks and bituminous coal is much greater than the static compression strength. The dynamic compression strength and dynamic increase factor of the rocks both increase linearly with the increase of the strain rate, while those of the bituminous coal are irregular due to the characteristics of multi-fracture and heterogeneity. Moreover, the absorbed energy of the rocks and bituminous coal both increase linearly with an increase in the strain rate. And the ratio of absorbed energy to the total energy of bituminous coal is greater than that of rocks. With the increase of dynamic loading, the failure degree of the sample increases, with the increase of the static compressive strength, the damage degree also increases. The static compassion strength of the bituminous coal is lower than that of rocks, so the number of small-scale fragments was the largest after bituminous coal rupture.

• 한국세라믹학회지
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• 제34권10호
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• pp.1053-1059
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• 1997

The dynamic fatigue behavior of alumina ceramics was observed at room temperature using four point bending system. The dynamic fatigue fracture strength and the dynamic fatigue lifetime were observed as a function of crosshead speed and the notch length. The notched specimen showed the smaller deviation in dynamic fatigue fracture strength than the unnotched specimen. The crack growth exponent n and the material constant A of the notched specimen could be represented as functions of the notch length. Fracture strength of the specimen calculated from the notch length, when the notch length was regarded as the crack size, was in good agreement with the measured 4 point bending strength. Fracture surface of the specimen showed the different fracture modes according to the crosshead speed. The four point flexural strength, fracture toughness, Young's modulus and Weibull modulus of the alumina were measured as 360 MPa, 3.91 MPa.m1/2, 159GPa, 17.64, respectively.

• 대한물리치료과학회지
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• 제30권3호
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• pp.59-71
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• 2023

Background: This study was to investigate the effects of coordinative locomotor training(CLT) using elastic bands on pain, dynamic balance, muscle strength, and muscle activity of female college students with patellofemoral pain syndrome(PFPS). Design: Randomized Controlled Trial Methods: Twenty-six female college students with PFPS were assigned randomly to an experimental(n=13) or control(n=13) group. The experimental group performed CLT using an elastic band. The control group performed squat exercises to strengthen muscle strength. The 30-minute inter- vention was applied a total of twelve times, three times a week for four weeks. All subjects measured the pain, the muscle strength, the dynamic balance, and the muscle activity(VL/VM ratio) before and after intervention. A paired t-test was used for the determination of differences before and after treatment, and an independent t-test was used for the determination of differences between treatment groups. Results: As a result of comparison within groups, the experimental group showed significant differences in pain, muscle strength, dynamic balance, and VL/VM ratio after the experiment(p<0.05), and the control group showed significant differences in pain, muscle strength, and dynamic balance after the experiment(p<0.05). In comparison between the two groups, the experimental group showed more significant differences in pain, dynamic balance, and VL/VM ratio than the control group(p<0.05), and the control group showed more significant differences in muscle strength than the experimental group(p<0.05). Conclusion: Based on these results, CLT using elastic bands effectively improved the pain, muscle strength, dynamic balance, and VL/VM ratio of female college students with PFPS.

• 한국운동역학회지
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• 제33권2호
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• pp.45-51
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• 2023

Objective: The aim of this study was to examine the effect of strength training targeting medial quadriceps/hamstrings and non-targeting strength training on dynamic balance. Method: A total of 51 healthy subjects were randomly assigned to control, targeting strength training, or non-targeting training groups. To measure the dynamic balance, the star excursion balance test (SEBT) was performed before and after training. The SEBT parameters were compared using repeated measures ANOVA, and post-hoc paired t test at a significance level of 0.05. Results: Greater anterior (p= .011), anteromedial (p= .001), medial (p< .001), lateral (p< .001), and anterolateral (p= .001) reach distances were found between pre- and post-training in the strength training targeting medial thigh muscles group. Only greater lateral reach distance was found after non-targeting strength training (p= .029). In addition, no differences were found for any SEBT scores in the control group. Conclusion: Strength training targeting medial quadriceps and hamstrings can improve the dynamic balance, thereby it positively affected in lower extremity injury risk, whereas non-targeting strength training rarely changes the dynamic balance.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2003년도 춘계학술대회
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• pp.537-543
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• 2003

This paper presents the improved procedure to assess static and dynamic strength of crawler type excavators. A fully integrated model including front attachment and chassis was prepared for structural analysis. In this paper, two types of loading input methods were investigated and the method imposing digging force directly on bucket tooth was more convenient than imposing cylinder reaction force on cylinder pin even if the two methods showed no discrepancy in analysis results. Static strength analysis was carried out for eight analysis scenarios based on two extreme digging positions, maximum digging reach position and maximum digging force positions. The results from static strength analysis were compared with measured stresses, cylinder pressures and digging forces and showed a good quantitative agreement with measured data. Dynamic strength analysis was carried out for simple reciprocation of boom cylinders. It was recognized that the effect of compressive stiffness of hydraulic oil was very important for dynamic structural behavior. The results from dynamic strength analysis including hydraulic oil stiffness were also compared with measured acceleration data and showed a qualitative agreement with measured data.

• Journal of Welding and Joining
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• 제21권6호
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• pp.55-63
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• 2003

Welded joint generally has heterogeneity of strength, material, and fracture toughness and it is important to understand the characteristics of material strength and fracture of welded joint considering heterogeneous effect. Characteristics of strength and fracture of an undermatched joint under dynamic loading was studied by round-bar tension tests and thermal elastic-plastic analyses in this paper. The strength and fracture of the undermatched joints should be evaluated based on the effects of the strain rate and the temperature including temperature rise during the dynamic loading. The differences of fracture characteristics like such as ductile-to-brittle transition behavior are well precisely explained from the stress-strain distribution obtained by numerical analysis.

• International journal of advanced smart convergence
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• 제11권3호
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• pp.197-205
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• 2022

In order to investigate the effect of an 8-week elastic band exercise program according to age on the isokinetic strength and dynamic equilibrium of the knee, 10 women under 49 years old and 10 women over 50 years old were selected. Dynamic equilibrium was measured through Y-Balance test one week before the exercise program, and power and endurance were confirmed by measuring isokinetic muscle strength of the knee. After measurement, small tool exercise was performed for 8 weeks. After 8 weeks of exercise, isokinetic muscle strength and dynamic equilibrium were measured. As a result, isokinetic muscle strength, which checks muscle strength and muscular endurance, increased after measurement than before measurement regardless of age, and dynamic equilibrium increased after measurement rather than before measurement, and the group under 49 years of age was higher than the group over 50 years old. We think that small tool exercise improves isokinetic muscle strength and is effective in neuromuscular development, improving dynamic stability ability, which is an important factor in preventing falls.

• 터널과지하공간
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• 제22권5호
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• pp.339-345
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• 2012

동적하중 하에서 암석의 변형거동 및 파괴강도는 발파, 지진 또는 암반돌출의 지진동에 의한 지하구조물의 안정해석에 중요하다. 본 연구에서는 스플릿 홉킨슨 압력봉(SHPB) 충격실험시스템을 석회암시료의 일축압축시험과 압열인장시험에 적용하여 고변형률과 동적강도 평가를 수행하였다. 충격봉의 발사속도를 제어하여 파괴강도 이하 동적응력 상태에서의 석회암시료의 동적응력-변형거동을 분석하였다. 시료 내 파괴여부 및 파괴양상을 파악하기 위하여 Micro-focus X-ray 단층촬영을 수행하였다. 동적 압축강도 실험결과, 석회암시료의 동적압축강도는 변형율속도 의존성을 보였으며, 동적 압축파괴강도는 140 MPa 이상으로 평가되었다. 동적 압열 인장 실험결과, 석회암시료의 동적 압열인장강도는 21 MPa 이상으로 정적 압열인장강도보다 3배 이상 높은 것으로 평가되었다.