• 한국해양공학회지
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• 제10권2호
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• pp.61-68
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• 1996

Weibull distribution function is applied very successfully to the strength of brittle materials such as ceramics and the weakest link model is applied to explain the ovents. This paper deals with the effect of specimen size on the strength of ceramics. The values of tensile strength were calculated by the Monte-Calro simuation. The tensile strength obtained was plotted on Weibull probabillity papers and represented by the 3-parameter Weibull distribution. The strength distribution function was compared with the theoretical weibull distribution. As a result, it was found that the Weibull shape parameter was changed due to the size and there was a possibility of a false indication as if the weakest link model holds good. We should be very careful when we apply the Weibull statistics to estimate the strength of products.

• 마이크로전자및패키징학회지
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• 제12권1호
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• pp.41-46
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• 2005

무전해 Ni(P)과 솔더와의 반응 중 발견되는 취성파괴 현상과 계면 화학반응시의 금속간화합물 spalling과의 연관성을 전단 파괴실험을 통하여 체계적으로 연구하였다. 취성파괴는 무전해 Ni(P)과 Sn-3.5Ag 솔더와의 반응 후에만 나타났고 Sn-3.0Ag-0.5Cu 솔더와의 반응시에는 연성파괴만이 관찰되었다. Sn-3.0Ag-0.5Cu 솔더와의 반응시 $(Ni,Cu)_3Sn_4$와 $(Cu,Ni)_6Sn_5$의 삼성분계 금속간화합물이 생성되었고 spatting은 발생하지 않았다. 반면, Sn-3.5Ag 솔더와의 반응시에는 $Ni_3Sn_4$ 금속간화합물이 spatting된 솔더패드에서 취성파괴가 발생하였다. 파괴표면을 면밀히 분석한 결과 취성파괴는 $Ni_3Sn_4$ 금속간화합물과 Ni(P) 금속층 사이에 형성된 $Ni_3SnP$ 층에서 발생하는 것을 알 수 있었다. $Ni_3SnP$ 금속간화합물 층은 $Ni_3Sn_4$ 금속간화합물이 spatting되는 과정 중에 두껍게 성장하므로 무진해 Ni(P) 사용시 기계적 신뢰성을 보장하기 위해서는 금속간화합물의 spatting을 방지하는 것이 매우 중요하다.

• Advances in concrete construction
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• 제5권3호
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• pp.221-240
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• 2017

The modeling of the mechanical behavior of quasi-brittle materials is still a challenge task, mainly in failure processes when fracture and plasticity phenomena become important actors in dissipative processes which occur in materials like concrete, as instance. Many homogenization-based approaches have been proposed to deal with heterogeneous materials in the last years. In this context, a computational homogenization modeling for concrete is presented in this work using the concept of Representative Volume Element (RVE). The material is considered as a three-phase material consisting of interface zone (ITZ), matrix and inclusions-each constituent modeled by an independent constitutive model. The Representative Volume Element (RVE) consists of inclusions idealized as circular shapes symmetrically and nonsymmetrically placed into the specimen. The interface zone is modeled by means of cohesive contact finite elements. The inclusion is modeled as linear elastic and matrix region is considered as elastoplastic material. A set of examples is presented in order to show the potentialities and limitations of the proposed modeling. The consideration of the fracture processes in the ITZ is fundamental to capture complex macroscopic characteristics of the material using simple constitutive models at mesoscopic level.

• 한국재료학회지
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• 제4권5호
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• pp.590-599
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• 1994

취성재료의 균열첨단에서 전위의 이동에 대한 거동을 이해하기 위하여 단결정의 알루미나에 대한 취성-연성 전이(BDT)에 대한 한 연구가 진행되었다. 여러 온도에서, 예비균열된 시편으로 4점 굽힘시험을 이용하여 임계응력확대계수와 항복강도가 측정되었다. 그 결과로, BDT온도는 변형속도와 시편 방향에 따라 달랐다.:(1120)파단면에 대하여 $4.2 \times 10^{-6}$와 $4.2 \times 10^{-7}s^{-1}$에서 BDT온도는 각각$1034^{\circ}C$, $1150^{\circ}C$이었다. 또한 4점굽힘 시험을 이용하여 연성영역에서 균열첨단으로 부터 방출된 전위의 이동거리과 방향은 에칭 피트법에 의해서 측정되었다. 이중 에칭법을 이용하여 즉정된 사파이어에서 전위의 이동속도는 모델링 연구에 응용되었다.

• 한국원자력학회:학술대회논문집
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• 한국원자력학회 2005년도 추계학술발표회
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• pp.358-359
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• 2005

• 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.

• 대한기계학회논문집A
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• 제37권2호
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• pp.213-218
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• 2013

초음파 가공(Ultrasonic Machining:USM)은 새로운 기계가공기술 분야 중의 하나이다. 초음파 가공 과정은 비열, 비화학, 그리고 비전도의 방법이기 때문에 공작물 재료의 물리적, 화학적 변화가 없다. 이러한 특성으로 인해 초음파 가공기술은 유리, 세라믹 등과 같은 취성재료의 가공에 적합하다. 그러나 단점으로는 초음파 진동을 이용하여 취성재료를 가공하는 경우 크랙이 빈번하게 발생한다. 본 논문에서는 유리와 세라믹의 미세 구멍가공에서 다구찌 방법을 이용하여 크랙발생을 최소화하는 최적의 가공조건을 얻고자 하였다. 이를 통해 공작물의 입구 및 출구에서 발생하는 크랙 현상을 감소시켰다.

• 한국표면공학회지
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• 제39권5호
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• pp.240-244
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• 2006

Tribaloy 800 (T800) powder is coated on the Inconel 718 substrate by the optimal High Velocity Oxy-Fuel (HVOF) thermal spray coating process developed by this laboratory. For the study of the possibility of replacing of the widely used classical chrome plating, friction, wear properties and sliding wear mechanism of coatings are investigated using reciprocating sliding tester both at room and at an elevated temperature of $1000^{\circ}F\;(538^{\circ}C). Both at room temperature and at $538^{\circ}C$, friction coefficients and wear debris of coatings are drastically reduced compared to those of non-coated surface of Inconel 718 substrate. Friction coefficients and wear traces of both coated and non-coated surfaces are drastically reduced at higher temperature of $538^{\circ}C$ compared with those at room temperature. At high temperature, the brittle oxides such as $CoO,\;Co_3O_4,\;MoO_2,\;MoO_3$ are formed rapidly on the sliding surfaces, and the brittle oxide phases are easily attrited by reciprocating slides at high temperature through complicated mixed wear mechanisms. The sliding surfaces are worn by the mixed mechanisms such as oxidative wear, abrasion, slurry erosion. The brittle oxide particles and melts and partial-melts play roles as solid and liquid lubricant reducing friction coefficient and wear. These show that the coating is highly recommendable for the durability improvement coating on the surfaces vulnerable to frictional heat and wear.

• 한국기계가공학회지
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• 제16권5호
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• pp.63-68
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• 2017

This study determines the selection of an appropriate jig material for the blade edge of the medical sapphire knife. The physical properties of the jig material affects the edge shape and chipping phenomenon in machining of the medical sapphire knife. If a grinding wheel is used, brittle workpieces such as sapphire are easily damaged by the propagation of cracks because the grinding force significantly increases. It is important to constantly maintain the grinding force in the grinding process of the brittle materials. The grinding force can be kept constantly by inducing the elastic deformation of the Jig material because the elastic deformation of brittle work-piece is negligibly low. The chipping phenomenon may be reduced by selecting the proper Jig material. Aluminum, copper, stainless steels and carbon steel were used as Jig materials. The experiment was conducted using a cast iron grinding wheel, which was installed on a conventional grinding machine with the ELID grinding system. The thickness and width of the chipping area were measured using an optical microscope and FE-SEM to analyze the shape of the blade edge. According to the experiment result, the chipping phenomenon decreased, and the sharp edge was formed when the jig materials with low elastic modulus were used.

• 한국재료학회지
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• 제11권6호
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• pp.508-511
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• 2001

사파이어 단결정의 취성-연성전이에 대한 실험을 행하였다. 사파이어 단결정의 취성-연성전이온도는 변형율 3.3$\times$ $10^{-5}$/sec에서 $1000\pm$$25^{\circ}C$ 그리고 변형률 3.3$\times$$10^{-5}$/sec에서는 1100$\pm$26$^{\circ}C$이었다. 취성-연성전이모델을 이용하여 Prism Plane slip {1120} <1100> 전위속도의 활성화에너지를 계산하였으며, 그 결과 활성화에너지는 4.6$\pm$2.3eV의 범위를 가졌다. 이 활성화에너지는 에치-퍼트법을 이용하여 전위속도측정으로부터 구한 결과치 3.8eV와 유사하였다.