• 한국지반공학회논문집
• /
• 제24권9호
• /
• pp.41-48
• /
• 2008

본 연구에서는 수축균열 단계를 나타낼 수 있는 도해적인 방법을 제안하였다. 우선 발생된 균열들을 균열폭의 크기 순서대로 나열하여 균열 분포를 구하였다. 다음에 균열폭을 정규화하여 0에서 1사이의 값으로 나타내었다. 마지막으로 Brooks와 Corey(1964), Fredlund와 Xing(1994), van Genuchten(1980)이 제안한바 있는 S형 곡선모델에 실험 결과를 적용시켰다. 분석 결과 van Genuchten의 식이 Brooks와 Corey식보다 정확도가 크게 높은 것으로 나타났으며, Fredlund와 Xing식보다도 높게 나타나 van Genuchten의 식을 적용하였다. 결과적으로 수축균열의 단계는 정규화 된 균열폭 분포가 3개의 직선부로 나누이는 도해적인 방법으로 나타낼 수 있었다. 제안된 방법의 적용성을 보기 위해 시료의 두께에 변화를 주며 시험을 실시하였다. 측정된 데이터를 제안된 모델에 적용하여 본 결과 높은 상관성을 보여 주었다. 따라서 수축 균열은 초기수축단계, 이차수축단계 그리고 잔류수축단계의 3단계로 모사할 수 있었다. 또한 각 단계에서의 균열 폭의 범위를 제시하였다.

• 한국원자력학회:학술대회논문집
• /
• 한국원자력학회 1996년도 춘계학술발표회논문집(3)
• /
• pp.145-150
• /
• 1996

Acoustic Emission (AE) technique was applied to stress corrosion cracking of Inconel 600 to investigate the AE capability of detecting crack growth and to obtain the relation between AE characteristics and crack mechanism. The specimens were heat-treated in two conditions (600$^{\circ}C$ for 30 hrs or 700 $^{\circ}C$ for 1 hr) and undergone CERT at two extension rates ( 2.5${\times}$10$^{-5}$ or 1.25${\times}$10$^{－4}$(mm/s)). It was found that the AE peak amplitude from plastic deformation was generally smaller than about 48dB (0.25mV), while Intergranular stress corrosion cracking (IGSCC) and ductile fracture produced higher values of 49 to 70dB (0.3mV to 3mV). The slopes of cumulative amplitude distribution (b-values) were linearly dependent on IGSCC susceptibility and the higher the susceptibility, the smaller the b-value. The monitoring of combined AE parameters such as event rate, amplitude, count and energy can provide effective means to clearly identify the transition from crack initiation and small crack growth to rapid growth of dominant cracks.

• 한국정밀공학회지
• /
• 제12권2호
• /
• pp.154-161
• /
• 1995

In order to analysis of the stress distribution around flaws and the interaction effects between fatigue cracks, stress around micro hole was analyzed by Finite Element Method(F.E.M.) and micro hole specimens were tested using rotary bending fatigue machine and twisting fatigue machine to identify stress effects for fatigue cracks initiating from micro holes and interaction effects between micro holes. The results are as follows : Interaction effects of .sigma. $_{y}$for the micro hole side is larger than the large micro hole side when the interval between micro holes is near. Stress concentration factor increase as the diameter of micro hole becomes smaller. But, stress field of micro hole is smaller than that of large micro hole at h .leq. r (h:depth of micro hole, r:radius of micro hole) and that of large hole is larger than that of small micro hole at h >r expect the small range from micro hole.e.

• 한국세라믹학회지
• /
• 제22권5호
• /
• pp.76-84
• /
• 1985

Mechanical property enhancing mechanisms of $Al_2O_3-ZrO_2$ two phase ceramic composites were studied for several compositions of different $ZrO_2$/$Al_2O_3$ ratio. Microstructural analysis of $Al_2O_3-ZrO_3$(pure) composites indicated that pre-existing microcrack due to larger $ZrO_2$ particle at grain boundary extended along alumina grain boundaries within process zone. Microcracks also nucleated when very small $ZrO_2$ particles at the grain boundaries transformed to monoclinic phase at near of main crack tip. These types of microcracks could contribute to the toughening achieved by creating additional crack surface area during crack propagation. Microstructural analyses also showed that the average grain size and abnormal grain size of $Al_2O_3$ were decreased with increasing $ZrO_2$ vol% in $Al_2O_3$ matrix. As a result it could be concluded as follows In TEX>$Al_2O_3-ZrO_3$(pure) system 1. Microcrack nucleation (stress-induced microcracking) and extension was effective mechanism for absorpiton of fracture energy 2, More narrow distribution and smaller grain size of $Al_2O_3$ due to $ZrO_2$particles mainly contributed to main-tatin the strength and hardness.

• Journal of Welding and Joining
• /
• 제17권1호
• /
• pp.124-132
• /
• 1999

Thermal fatigue strength of the solder joints is the most critical issue for TSOP(Thin Small Outline Package) because the leads of this package are extremely short and thermal deformation cannot be absorbed by the deflection of the lead. And the TSOP body can be subject to early fatigue failures in thermal cycle environments. This paper was discussed distribution of thermal stresses at near the joint between silicon chip and die pad and investigated their reliability of solder joints of TSOP with 42 alloy clad lead frame on printed circuit board through FEM and 3 different thermal cycling tests. It has been found that the stress concentration around the encapsulated edge structure for internal crack between the silicon chip and Cu alloy die pad. And using 42 alloy clad, The reliability of TSOP body was improved. In case of using 42 alloy clad die pad(t=0.03mm). $$\sigma$_{VMmax}$ is 69Mpa. It is showed that 15% improvement of the strength in the TSOP body in comparison with using Cu alloy die pad $($\sigma$_{VMmax}$=81MPa). In solder joint of TSOP, the maximum equivalent plastic strain and Von Mises stress concentrate on the heel of solder fillet and crack was initiated in it's region and propagated through the interface between lead and solder. Finally, the modified Manson-Coffin equation and relationship of the ratio of $N_{f}$ to nest(η) and cumulative fracture probability(f) with respect to the deviations of the 50% fracture probability life $(N_{f 50%})$ were achieved.

• 한국세라믹학회지
• /
• 제29권3호
• /
• pp.211-215
• /
• 1992

Crack free PbTiO3 ceramics were produced by sol-gel processing using alkoxide, which has not been reported to be successful. The PbTiO3 gels were prepared from Ti alkoxide and lead acetate without any dopants. They were calcined at $600^{\circ}C$ and miled to produce fine PbTiO3 powder. It was pressed into discs and they were sintered at 110$0^{\circ}C$ for a few hours. The sintered ceramics were relativley hard and dense as having about 96% of theoretical density of PbTiO3. Fabrication of pure PbTiO3 ceramics by sol-gel processing is possibly due to their small grain size and uniform distribution of residual stress created during cubic-tetragonal transition over large number of small grains in fine grain PbTiO3 ceramics.

• 비파괴검사학회지
• /
• 제8권1호
• /
• pp.12-21
• /
• 1988

The behaviour of pressure vessel made of SS41 steel was investigated during hydrotest. AE tests were carried out for the vessels in as-manufactured, V-notched and weld-cracked state using microcomputer-based AE instrumentation. The following results were obtained: 1) In the case of source location using cylindrical program, to minimize the error of source location, the difference of max. Delta T values measured from each sensor should be kept as small as possible. 2) When crack grew, AE event rate increased continuously but AE event from the inclusions occurred intermittently, so by analyzing event rate, the source of AE could be derived. 3) From the spot welding part of supporter, many events with low energy occurred independent of Kaiser effect, which could be confirmed by analyzing energy parameter. 4) The b-value from the tensile specimen of was lower than that from normal specimen and the b-value from crack propagation was lower than that from deformation, so by analyzing peak amplitude distribution, the source mechanism could be derived from the b-value.

• 대한조선학회지
• /
• 제27권3호
• /
• pp.100-106
• /
• 1990

본(本) 연구(硏究)에서는 304스테인리스강을 사용한 평활재와 작은 인공피트재의 고온 피로시험(疲勞試驗)과 크리프 시험(試驗)을 $593^{\circ}C$의 대기중에서 실시하고, 표면의 분포균열을 실온에서 레프리카법으로 연속적으로 관찰하고, 표면균열의 발생, 성장, 합체 거동을 관측하여, 균열 밀도(密度) 길이의 분포등의 정량적인 통계적(統計的) 성질(性質)에 대하여 검토하는 동시에, 파괴역학적으로 시험편의 수명을 지배하는 주균열의 성장거동 및 수명예측을 실시한 것이다. 또 실온의 결과와 고온피로(疲勞) 및 크리프 경우의 미소 분포균열의 통계적(統計的) 결과를 비교 검토하였다.

• 대한조선학회논문집
• /
• 제29권4호
• /
• pp.234-243
• /
• 1992

각종 구조물, 기계요소의 파괴(破壞)에 있어서 상당수는 피로파괴(疲勞破壞)가 그 원인인 것이 많고, 그 설계 및 안전성 평가를 위해 신뢰성해석법(信賴性解析法)을 도입함에 있어서 피로수명(疲勞壽命)의 확률분포(確率分布)를 파악하는 것은 구조물에 작용하는 외력의 추정(推定)과 함께 중요하다. 피로수명(疲勞壽命)의 확률분포(確率分布)를 구하는데는 대수정규분포(對數正規分布) 혹은 Weibull분포(分布) 등이 제안되어 있지만, Weibull분포(分布)가 그 뛰어난 응용성과 정확성 때문에 피로수명분포(疲勞壽命分布)에 가장 자주 쓰이고 있다. 따라서, 본 연구에서는 응용범위가 넓고, 데이터의 특성을 가장 잘 표현하는 것으로 알려진 Weibull분포(分布)를 사용하였다. 어떤 응력상태 하에서의 피로균열발생수명(疲勞龜裂發生壽命)의 확률분포(確率分布)를 구하기 위해서는 일반적으로 수십개의 피로시험(疲勞試驗)을 행하여 많은 데이터를 얻어야 한다. 그러나, 그러한 많은 시험은 시험시간, 경비등의 면에서 어려움이 있다. 따라서, 본 연구에서는 피로균열발생분포를 구하기 위하여 적은 시험편으로 보다 많은 데이터를 얻을 수 있는 100개의 응력집중부(應力集中部)를 가진 변단면(變斷面)외팔보형 시험편(試驗片)을 개발하였다. 피로시험(疲勞試驗)은 일정진폭완전량진(一定振幅完全兩振) 굽힘피로하중에서 실시하였고, 피로균열발생수명(疲勞龜裂發生壽命)의 확률분포(確率分布)를 추정(推定)하는데에는 여러가지 방법들이 있으나, 본 연구에서는 미지(未知)의 모수추정(母數推定)에 그 유용성이 확인된 Bayesian 신뢰성해석법(信賴性解析法), Skewness법과 최소자승법을 사용하여 그 정도를 비교하였다.

• 한국철도학회:학술대회논문집
• /
• 한국철도학회 2010년도 춘계학술대회 논문집
• /
• pp.597-603
• /
• 2010

Metallic sintered brake pads are often applied to Mid/High Speed Train due to its high strength and thermal characteristics. Because of its imbalance contact between discs and pads can greatly influence the life span, one sided wear, discs attack/crack and threat the safety of the train during operation. In this research, we analyzed pressure/temperature distribution between brake pads and disks. Analyzed data had been verified and modified to conduct further tests of Flexible brake pads with small/full-scale dynamo test. Flexible brake pads were installed to TTX train to conduct further tests to identify the differences between Rigid brake pads and Flexible brake pads. In result, Flexible brake pads showed outstanding disk thermal stability, one sided wear, noise and life of pad than rigid brake pad.