• Journal of Welding and Joining
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• 제7권3호
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• pp.19-27
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• 1989

Both the SS41 steel and the M.E.F(martensite encapsulated islands of frrite) dual phase steel made of SS41 steel by heat treatment were welded by friction welding, and then manufactured machinemade Vnotch standard Charpy impact specimens and precracked with a fatigue system at BM(base metal), HAZ(heat affected zone) and WZ(weld interface Zone). The impact test of them was performed with an instrumented impact test machine at a number of temperatures in constant loading velocity and the dynamic fracture characteristics were studied on bases of the absorbed energy, dynamic fracture toughness and fractography from the test. The results obtained are as follows; At the room temperature, the absorbed energy is HAZ.geq.WZ.geq.BM in case of the M.E.F. dual phase steel: BM.geq.HAZ.geq.WZ in case of the SS41 steel, HAZ.geq.BM.geq.WZ at the low temperature. The absorbed energy is decreased markedly with the temperature lowering; it is highly dependent on the temperature. The dynamic fracture toughness of the M.E.F. dual phase steel is HAZ.geq.WZ.geq.BM at the room temperature; BM.geq.WZ.geq.HAZ below-60.deg. C. Therefore the reliability of friction welding is uncertain at the low temperature(below-60.deg. C). The dynamic fracture toughness of the SS41 steel; HZA.geq.WZ.geq.BM at overall temperature region. The flaw formed by rotational upsetting pressure was shown y SEM; in this region. The absorbed energy per unit area and dynamic fracture toughness were low relative to other region.

• Advanced Composite Materials
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• 제17권4호
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• pp.373-407
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• 2008

This paper will examine the possibilities of the virtual tests of composite structures by simulating mechanical behaviors by using supercomputing technologies, which have now become easily available and powerful but relatively inexpensive. We will describe mainly the applications of large-scale finite element analysis using the direct numerical simulation (DNS), which describes composite material properties considering individual constituent properties. DNS approach is based on the full microscopic concepts, which can provide detailed information about the local interaction between the constituents and micro-failure mechanisms by separate modeling of each constituent. Various composite materials such as metal matrix composites (MMCs), active fiber composites (AFCs), boron/epoxy cross-ply laminates and 3-D orthogonal woven composites are selected as verification examples of DNS. The effective elastic moduli and impact structural characteristics of the composites are determined using the DNS models. These DNS models can also give the global and local information about deformations and influences of high local in-plane and interlaminar stresses induced by transverse impact loading at a microscopic level inside the materials. Furthermore, the multi-scale models based on DNS concepts considering microscopic and macroscopic structures simultaneously are also developed and a numerical low-velocity impact simulation is performed using these multi-scale DNS models. Through these various applications of DNS models, it can be shown that the DNS approach can provide insights of various structural behaviors of composite structures.

• 대한기계학회논문집A
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• 제26권5호
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• pp.932-938
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• 2002

In structural applications, brittle materials such as soda-lime glasses and ceramics are usually subjected to multiaxial stress state. Brittle materials with cracks or damage by foreign object impacts are apt to fracture abruptly from cracks, because of their properities of very high strength and low fracture toughness. But in most cases, the residual strength of structural members with damage has been tested under uniaxial stress condition such as the 4-point bend test. Depending upon the crack pattern developed, the strength under multiaxial stress state might be different from the one under uniaxial. A comparative study was carried out to investigate the influence of stress state on the residual strength evaluation. In comparable tests, the residual strength under biaxial stress state by the ball-on-ring test was greater than that under the uniaxial one by the 4-point bend test, when a small size indendation crack was introduced. In the case that crack having an angle of 90deg. to the applied stress direction, the ratio of biaxial to uniaxial flexure strength was about 1.12. The residual strength was different from crack angles to loading direction when it was evaluated by the 4-point bend test. The ratio of residual strength of 45deg. crack to 90deg. one was about 1.20. In the case of specimen cracked by a spherical impact, it was shown that an overall decrease in flexure strength with increasing impact velocity, and the critical impact velocity for formation of a radial and/or cone crack was about 30m/s. In those cases that relatively large cracks were developed as compared with the case of indented cracks, the ratio of residual strength under biaxial stress state to one uniaxial became small.

• 한국자동차공학회논문집
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• 제17권4호
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• pp.118-125
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• 2009

Bumper is designed to protect the automotive frame without damage at low velocity. Expanded polypropylene (EPP) foam is used in the bumper as an energy absorbing material. In order to exactly predict the energy absorbing performance of the foam material under impact loading condition, it is important to use high strain rate material properties. In this study, a new apparatus for dynamic compression tests was developed to investigate the high strain rate behavior of EPP foams. Three kinds of EPP foams which have different expansion ratios were tested to investigate the quasi-static and dynamic compression behavior. Quasi-static compressions were performed at low strain rates of 0.001/s, 0.1/s and 1/s. The dynamic compressions were carried out at high strain rates of 50/s and 100/s with the developed apparatus. It was observed that the EPP foam has significant strain rate effect as compared to quasi-static behavior.

• 대한기계학회논문집
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• 제14권6호
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• pp.1417-1425
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• 1990

본 연구에서는 취성재료인 유리판이 충격을 받을 때 생기는 크랙패턴 특히, 콘 크랙의 발생 현상을 이론적으로 규명하여 취성재료의 충격파괴방지에 도움이 되게 하고져 하였으며, 판두께방향의 변형을 고려한 제1보에서의 삼차원 동탄성이론에 의한 응력해석방법을 이용하여 충돌점 및 충돌점근방에서의 변형율분포를 해석하였다. 또 한 고속 및 자유낙하 충격시험을 행하여 얻은 크랙의 패턴과 본 이론해석 결과인 변형 률 분포의 수치계산 결과와 비교함으로써 콘 크랙의 발생현상을 3차원 동탄성이론을 이용한 본 충격응력해석 방법에 의한 규명하였다. 변형률 분포의 해석은 국부변형을 고려한 Hertz의 접촉이론과 Lagrange의 고전판 이론을 이용하여 구한 충격하중계수의 크기에 따라 충격하중의 함수근사식을 바꿔가며 해석하였으며 충돌점으로 부터 0.1cm 간격으로 5cm범위까지를 해석하였다.

• 대한기계학회논문집A
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• 제37권2호
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• pp.187-192
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• 2013

피라미드형 금속 내부구조체를 가진 얇은 금속 샌드위치 판재는 외판들과 내부구조체를 연속적 프로젝션 용접하여 제작한다. 이 용접공정에서 발생하는 용접압력 때문에 내부구조체의 변형에 의한 내부구조체의 결함이 발생한다. 내부구조체의 결함은 저속 충격하중이 부가된 얇은 금속 샌드위치 판재의 응답에 영향을 미친다. 이 논문에서는 피라미드형 금속 내부구조체를 가진 얇은 금속 샌드위치 판재의 지배적인 결함 모드를 도출하고자 한다. 3 차원 유한요소해석을 통하여 여러가지 결함모드에 대한 얇은 금속 샌드위치 판재의 충격 응답 변화를 고찰하였다. 또한 유한요소해석 결과와 낙하 충격시험 결과를 비교/분석 하였다. 이 결과로부터 피라미드형 금속 내부구조체를 가진 얇은 금속 샌드위치 판재의 지배적인 결함모드를 4 개 지주들에 대칭 결함이 적용된 전방형 대칭 결함모드로 선정하였다.

• Geomechanics and Engineering
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• 제37권5호
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• pp.443-452
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• 2024

The use of sand-tire chip mixtures in construction industry is a sustainable and environmentally friendly approach that addresses both waste tire disposal and soil improvement needs. However, the addition of tire chip particles to natural soils decreases maximum shear modulus (G_max), but increases compressibility, which can be potential drawbacks. This study examines the effect of overconsolidation stress history on the maximum shear modulus (G_max) of rigid-soft mixtures with varying size ratios (SR) and tire chip contents (TC) by measuring the wave velocity through a 1-D compression test during loading and unloading. The results demonstrate that the G_max of tested mixtures in the normally consolidated state increased with increasing SR and decreasing TC. However, the tested mixtures with a smaller SR exhibited a greater increase in G_max during unloading because of the active pore-filling behavior of the smaller rubber particles and the consequent increased connectivity between sand particles. The SR-dependent impact of the overconsolidation stress history on G_max was verified using the ratio between the swelling and compression indices. Most importantly, this study reveals that the excessive settlement and lower G_max of rigid-soft mixtures can be overcome by introducing an overconsolidated state in sand-tire chip mixtures with low TC.

• Composites Research
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• 제18권3호
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• pp.38-46
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• 2005

3차원 복합재료의 뛰어난 특성을 확인하기 위하여 저속충격 시험을 하였다. 복합재료의 3차원 구조는 자동적층 공정 (ATP, Automated Tape Placement)과 스티칭 (stitching) 방법으로 제조하였다. 이 방법은 일정한 폭을 가지는 탄소섬유/에폭시 프리프레그 테이프를 균일한 간격을 두고 층 별로 서로 직교 적층한 후 비어 있는 공간 사이를 케블라 섬유로 스티칭하는 성형법이다. 새로운 3차원 복합재료와 기존의 프리프레그 시트(sheet)를 사용한 2차원 복합재료와의 충격특성을 비교하기 위하여 저속충격 시험을 하였으며, C-scan에 의한 충격손상 면적 확인 및 충격 후 압축시험을 하였다. 3D 복합재는 스티칭을 하기 위한 간격으로 인하여 복합재료의 전체 섬유 체적율이 낮아졌기 때문에 충격 전 압축 강도는 2D 복합재에 비해 낮았으나 충격 후 파손면적은 약 $30-40\%$의 감소를 보였으며, 충격 전 압축 강도에 패한 충격후 압축강도 비율은 약 $5-10\%$의 증가를 보였다. 스티칭에 의해 충격 후 압축강도는 전반적으로 향상되었으나, 30J의 충격 에너지부터는 그 효과가 감소하였으며 35J 이상의 충격에서는 스티칭 효과가 없었다.

• 한국운동역학회지
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• 제17권2호
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• pp.1-9
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• 2007

Walking is not only an essential component of the human mobility, but also is a good exercise. Inability to walk freely can reduce an individual's quality of life and independence substantially. Being a relatively low impact activity, walking is particularly good for the elderly and research has shown that regular walking in the elderly reduces the chance of fall-related injuries and mental diseases as well. In spite of the documented benefits of regular walking, it is still difficult to walk without the aid of assistive devices for the frail elderly who have lower extremity problems. Assistive walking devices(AWD), such as crutches, canes, hiking-poles, T-Poles and walkers, are often prescribed to the elderly to make their walking be safe and efficient. Many researchers have demonstrated the effects of AWDs such as reducing lower extremity loading, improved dynamic/gait stability, yet, no study has been done for gait pattern when the elderly gait with AWDs. Therefore, the purpose of this study was to examine whether T-Poles, one of the AWDs, change the elderly gait pattern. Eight community-dwelling female elderly participated in this study. Laboratory kinematics during walking with T-Poles(PW) and with out T-Poles(NPW) was assessed. PW showed significant increase in step width, stride length, gait velocity and decrease in swing time. No significances were found in lower body joint angles but meaningful trend and pattern were found. Maybe the reason was due to the participants. Our participants were healthy enough so that the effect of T-Poles was minimum. PW also showed typical gait phases which are no single support phase during a gait cycle. It indicates that walking with T-Poles may guarantee safe and confident walking to the frail elderly.