• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2003년도 춘계학술발표대회 논문집
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• pp.108-111
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• 2003

The Objective of this work was to design Surface Antenna Structure (SAS) and investigate fatigue behavior of SAS that was asymmetric sandwich structure. This term, SAS, indicates that structural surface becomes antenna. Constituent materials were selected considering electrical properties, dielectric constant and tangent loss as well as mechanical properties. For the antenna performance, SSFIP elements inserted into structural layers were designed for satellite communication at a resonant frequency of 12.5 GHz and final demonstration article was $16\;{\times}\;8$ array antenna. From electrical measurements, it was shown that antenna performances were in good agreement with design requirements. In cyclic 4-point bending, flexure behavior was investigated by static and fatigue test. Fatigue life curve of SAS was obtained. The fatigue load was determined experimentally at a 0.75(1.875kN) load level. SAS concept is the first serious attempt at integration for both antenna and composite engineers and promises innovative future communication technology.

• 한국세라믹학회지
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• 제35권6호
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• pp.551-558
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• 1998

Detection of microcrack in {{{{ {Al }_{2 } {O }_{3 } }} ceramics were studided by AE(acoustic emission) technique with 4-point bending test in order to evaluate the fracture process and formation of microcrack. Fully-dense alu-mina ceramics having a different grain size were fabricated by varing the hot-pressing temperature. The grain size of alumina increased with increasing the hot-pressing temperature whereas the bending strength decreasd. The microcracks were observed by SEM and TEM. The generation of AE event increased with increasing the applied load and many AE event was generated at maximum applied load. Alumina with smaller grain size shows the generation of many AE event resulting in an increase of microcrack formation. An intergranular fracture is predominantly observed in fine-grained alumina whereas intragranular fracture occurs predominantly in coarse-grained alumina,. Analysis of micorstructure and AE prove that primary mi-crocracks occur within grain-boundaries of alumina. The larger microcracking were formed by the growth and/or coalesence of primary microcracks. Then the materials become to fracuture by main crack gen-eration at the maximum applied load.

• 대한치과교정학회지
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• 제36권5호
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• pp.349-359
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• 2006

니켈-티타늄 합금은 높은 spring-back성질, 초탄성 효과, 형상기억 효과 등의 장점을 가지고 있으나 성형이 어렵다는 단점을 가지고 있다. 본 연구에서는 니켈-티타늄 와이어의 굴곡을 위한 열처리 시에 나타나는 부하-변위 곡선의 변화 및 상전이 온도 변화와 같은 물성 변화 양상을 조사하고자 하였다. 수종의 니켈-티타늄 와이어를 열처리를 시행하지 않은 군, 전기저항 열처리 장치를 이용하여 와이어 단면적에 따라 제조회사에서 권장하는 전류를 기초로 하여 $00.016"\;{\times}\;00.022"$ (5 A, 7 sec), $0.018"\;{\times}\;0.025"$ (6 A, 7 sec) 그리고 $0.0215"\;{\times}\;0.028"$ (7 A, 8 sec) 와이어를 열처리만 시행한 실험군, 열처리를 시행하여 굴곡을 부여한 실험군 그리고 열처리 시간을 1초 증가시켜 굴곡을 부여한 실험군으로 분류하여 3점 굴곡 실험과 시차주사열량측정을 하여 다음과 같은 결과를 얻었다. $0.016"\;{\times}\;0.022"$, $0.018"\;{\times}\;0.025"$ 그리고 $0.0215"\;{\times}\;0.028"$ 니켈-티타늄 와이어에서 굴곡을 부여하지 않고 열처리만 시행한 실험군이 열처리를 시행하지 않은 대조군에 비해 부하-변위 곡선이 상방 이동되어 초탄성 현상에 의한 평탄역(loading and unloading plateau)의 힘이 더 증가되었다. $0.016"\;{\times}\;0.022"$, $0.018"\;{\times}\;0.025"$ 그리고 $0.0215"\;{\times}\;0.028"$ 와이어에서 열처리만 시행한 실험군이 열처리를 시행하지 않은 군보다 더 낮은 austenite finish ($A_f$) 온도를 보였다. $0.018"\;{\times}\;0.025"$ 및 $0.0215"0.028"$ 와이어에서 열처리를 시행하여 굴곡을 부여한 실험군은 열처리만 시행한 실험군과 열처리를 시행하지 않은 대조군에 비해 부하-변위 곡선이 상방 이동되었으며, 열처리 시간을 1초 증가시켜 굴곡을 부여한 실험군에서 가장 높은 부하-변위 곡선을 나타냈다. $0.018"\;{\times}\;0.025"$ 그리고 $0.0215"\;{\times}\;0.028"$ 와이어에서 $A_f$ 온도는 열처리 시간을 1초 증가시켜 굴곡을 부여한 실험군에서 가장 낮게 관찰되었고 열처리를 시행하여 굴곡을 부여한 실험군, 열처리만 시행한 실험군 그리고 열처리를 시행하지 않은 대조군 순으로 높게 관찰되었다. 이상의 결과를 종합할 때, 임상에서 니켈-티타늄 합금 와이어에 굴곡을 부여하기 위해 열처리하는 경우 초탄성 특성은 유지될 수 있으나, 부하-변위 곡선의 상방 증가가 나타나므로, 와이어에 의한 교정력이 증가될 수 있음에 유의하여야 한다.

• 한국복합재료학회:학술대회논문집
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• 한국복합재료학회 2001년도 춘계학술발표대회 논문집
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• pp.78-82
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• 2001

Impact behaviors of Aluminum Honeycombs Sandwich Panel(AHSP) by drop weight test were investigated. Two types of specimens with 1/2" and 1/4" cell size were tested by two impactors which are weight of $5.25\textrm{kg}_{\textrm{f}}$ and $11.9\textrm{kg}_{\textrm{f}}$. Parametric studies were achieved including the impactor weight and impact sites which consist face, long-edge, short-edge, and point of the specimen. Face one of impact sites was the strongest and short-edge one of impact sites was the weakest. The damaged area of AHSP was enlarged with the increase of impactor weight that is equal to impact energy. After 3 point bending test, fracture modes of AHSP were analyzed with AE counts. Lower facesheet was fractured in the long-edge direction and then separated between facesheet and core. In the short-edge direction after core wrinkled, lower facesheet tear occurred. Impact behavior by FE analysis were increased localized damage in fast velocity because the faster velocity of the impact was, the smaller the stress of core was. Consequently, impactor weight had an effect on widely damaged area, while the impact velocity was caused on the localized damaged area.aged area.

• 한국세라믹학회지
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• 제36권9호
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• pp.997-1005
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• 1999

• 한국도로학회:학술대회논문집
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• 한국도로학회 2005년도 학술발표회 논문집
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• pp.101-108
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• 2005

• 대한기계학회논문집A
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• 제31권4호
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• pp.425-431
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• 2007

[ $Al_2O_3/SiC$ ]composite ceramics were sintered to evaluate the bending strength and elastic wave characteristics. The three-point bending test was carried out under room temperature. The elastic wave was detected by fracture wave detector. The crack healing behavior was investigated from 1373 K to 1723 K. The bending strength of $Al_2O_3/SiC$ composite by nanocomposite is higher than that of $Al_2O_3$ monolithic. Crack-healing behavior depended on an amount of additive powder $Y_2O_3$. In $Al_2O_3/SiC$ composite ceramics with 3 wt. % $Y_2O_3$ for additive powder, the bending strength at 1573 K is about 100% increase than that of the smooth specimens. From the result of wavelet analysis of elastic wave signal, the smooth specimen and heat treated specimen of $Al_2O_3$ monolithic and $Al_2O_3/SiC$ composite ceramics showed characteristics of frequency about 58 kHz. The strength of $Al_2O_3/SiC$ composite ceramics was a little higher than those of $Al_2O_3$ monolithic. The dominant frequencies were high with increasing of $Y_2O_3$ for additive powder. The dominant frequencies had direct connection with the bending strength.

• Restorative Dentistry and Endodontics
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• 제15권2호
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• pp.17-33
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• 1990

The purpose of this study was to evaluate the fracture toughness of dental composite resins and to investigate the filler factor affecting the fracture behaviour on which the degree of fracture toughness depends. Six kinds of commercially available composite resin;, including two of each macrofilled, microfilled, and hybrid type were used for this study, The plane strain fracture toughness ($K_{10}$) was determined by three-point bending test using the single edge notch specimen according to the ASTM-E399. The specimens were fabricated with visible light curing or self curing of each composite resin previously inserted into a metal mold, and three-point bending test was conducted with cross-head speed of 0.1mm/min following a day's storage of the specimens in $37^{\circ}C$ distilled water. The filler volume fractions were determined by the standard ashing test according to the ISO-4049. Acoustic Emission(AE), a nondestructive testing method detecting the elastic wave released from the localized sources In material under a certain stress, was detected during three-point bending test and its analyzed data was compared with, canning electron fractographs of each specimen. The results were as follows : 1. The filler content of composite resin material was found to be highest in the hybrid type followed by the macrofilled type, and the microfilled type. 2. It was found that the value of plane strain fracture toughness of composite resin material was in the range from 0.69 MPa$\sqrt{m}$ to 1 46 MPa$\sqrt{m}$ and highest In the macrofilled type followed by the hybrid type, and the microfilled type. 3. The consequence of Acoustic Emission analysis revealed that the plane strain fracture toughness increased according as the count of Acoustic Emission events increased. 4. The higher the plane strain fracture toughness became, the higher degree of surface roughness and irregularity the fractographs demonstrated.

• Corrosion Science and Technology
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• 제7권4호
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• pp.224-232
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• 2008

During the hot rolling process, steels develop an oxide scale on their surface. This scale can affect the mechanical properties of the rolled steel and its surface aspect. The main problem comes from the mechanical integrity of the oxide scales which could delaminate or crack, leading eventually to later oxide incrustation within the steel. The objective of the present work is to qualify the mechanical integrity of the iron oxide scales during the hot rolling process. The laboratory experiments use a four point bending test to simulate the mechanical solicitation which takes place during the rolling sequence of the steel slabs. The oxide scales grow on a mild steel at $900^{\circ}C$ under wet or dry atmosphere and the oxidized steel is then mecahnically tested at $900^{\circ}C$ or $700^{\circ}C$. The high temperature four point bending tests are completed with microstructural observations and with the record of acoustic emission to follow in-situ the mechanical damages of the oxide scales. The results show the role of water vapor which promotes the scale adherence, and the role of the temperature as the oxide are more damaged at $700^{\circ}C$ than at $900^{\circ}C$.

• 콘크리트학회논문집
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• 제24권5호
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• pp.525-533
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• 2012

이 연구에서는 UHPCC에서 섬유혼입률에 따른 초기균열강도 및 휨인장강도의 변화를 0~5 vol.% 범위에서 조사하였으며, 노치의 여부에 따른 영향을 파악하기 위해 노치가 없는 보에 대한 4점 재하실험 및 노치 낸 보에 대한 3점 재하실험을 같이 실시하였다. 실험 결과로부터 섬유혼입률이 증가함에 따라 휨인장강도는 선형적으로 강도가 향상됨을 확인할 수 있었고, 초기균열강도의 경우에는 1 vol.% 이상에서는 강도향상을 나타내었으나 그 이하의 섬유혼입에서는 강도향상 효과가 거의 없는 것으로 나타났다. 노치 유무에 따른 휨 실험으로부터 구한 UHPCC의 초기균열발생강도 및 휨 인장강도를 비교했을 때, 섬유혼입률에 따라 노치의 영향이 변하는 것으로 나타났다. 섬유혼입률이 증가함에 따라 노치에서의 응력집중의 영향이 감소하여 강도 차이가 점차 줄어들었으며, 높은 섬유혼입률에서는 노치에 의한 응력집중효과는 없어지고 균열면의 상태 및 크기효과의 영향이 지배적으로 작용하여 노치낸 보의 강도가 좀 더 크게 나타났다.