• Steel and Composite Structures
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• 제31권1호
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• pp.43-51
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• 2019

This paper is concerned with the numerical calculation of mixed-mode stress intensity factors (SIFs) of 2-D isotropic functionally graded materials (FGMs) by the natural element method (more exactly, Petrov-Galerkin NEM). The spatial variation of elastic modulus in non-homogeneous FGMs is reflected into the modified interaction integral ${\tilde{M}}^{(1,2)}$. The local NEM grid near the crack tip is refined, and the directly approximated strain and stress fields by PG-NEM are enhanced and smoothened by the patch recovery technique. Two numerical examples with the exponentially varying elastic modulus are taken to illustrate the proposed method. The mixed-mode SIFs are parametrically computed with respect to the exponent index in the elastic modulus and external loading and the crack angle and compared with the other reported results. It has been justified from the numerical results that the present method successfully and accurately calculates the mixed-mode stress intensity factors of 2-D non-homogeneous functionally graded materials.

• 분석과학
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• 제9권2호
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• pp.179-191
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• 1996

초내열 718 합금의 고온 변형 거동을 이해하기 위하여 rotating grade의 718 합금을 이용하여 온도 $927{\sim}1066^{\circ}C$, 변형속도 $5{\times}10^{-4}{\sim}5{\times}10^0sec^{-1}$ 범위에서 진변형량 0.7까지 압축실험을 수행하였다. 최대 유동 응력은 변형 속도가 증가하고 시험 온도가 감소함에 따라 증가하였다. 변형 속도 $5{\times}10^{-1}sec^{-1}$을 제외한 대부분의 설험 조건에서 가공 연화현상이 관찰되었다. 가공 연화는 저온, 고변형 속도에서는 주로 동적 회복 및 변형 쌍정에 의해 일어나는 반면 고온, 저변형 속도 조건에서는 동적 재결정에 의해 발생하였으며 $5{\times}10^{-1}sec^{-1}$의 변형 속도 조건에서는 동적 재결정된 결정 입자들의 재가공 경화에 의해 가동 경화현상이 나타났다. 변형 속도 감도(m)는 변형 속도가 낮은 경우에는 0.3 정도로서 주로 동적 재결정에 의해 변형 거동이 나타남을 반영하였으며 고변형 속도에서는 0.1 정도로서 동적 회복과 변형 쌍정의 발생으로 718 합금의 변형이 이루어짐을 알 수 있었다.

• 열처리공학회지
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• 제30권6호
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• pp.251-257
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• 2017

The effect of annealing temperature on the martensitic transformation behavior, tensile deformation chracteristics and shape recovery etc., has been studied in TiNi based shape memory ribbon fabricated by coldrolling of wire. TiNi based shape memory wire (${\phi}=500{\mu}m$) of which structure is intermetallic compound could be cold-rolled without process annealing up to the reduction rate in thickness of 50%, but a few cracks appear in cold-rolled ribbon in the reduction rate in thickness of 65%. The $B2{\rightarrow}R{\rightarrow}B19^{\prime}$ martensitic transformation or $B2{\rightarrow}B19^{\prime}$ martensitic transformation occurs in annealing conditions dissipating lattice defects introduced by coldrolling. However, in case of higher reduction rate or lower annealing temperature, martensitic transformation in cold-rolled and then annealed ribbons does not occur. The maximum shape recovery rate of cold-rolled ribbons with the reduction rate of 35 and 65% could be achieved at annealing temperatures of 250 and $350^{\circ}C$, respectively. The shape recovery rate seems to be related to the stress level of plateau region on stress-strain curve.

• 한국염색가공학회지
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• 제7권4호
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• pp.25-32
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• 1995

This study was carried out to examine the influences of fatigue and perspiration on the deterioration of chrome leather for shoes. The samples were treated with acidic and alkaline artificial perspirations and applied cyclic tensile stress, and the elastic recovery from fatigue strain, tensile strength and elongation, stiffness etc. by regions and directions were determined. The physical properties generally indicate considerable difference by regions and directions due to the thickness, entangling state, and arrangement direction of fiber buldles within the leather. By the treatment of artificial perspirations, the length, weight, strength, elastic recovery from fatigue of the lether are decreased and the thicknes, elongation, and stiffness are increased; and their extents by the alkaline perspiration are higher than those by the acidic one. The elastic recovery from fatigue of samples are decreased with increasing fatigue cylcle; and indicate regressive relations against the logarithm of the elapsed time, and the means of regression coefficients are 2.73 > 2.55 > 2.22 for samples treated by the acidic perspiration, untreated, and treated by the alkaline perspiration, respectively. The tensile strength is decreased and elogation is increased by fatigue, but the changes are diminished likely due to a rearrangment of fiber bundles with increasing fatigue cycle. And the samples become a little flexible immediately after fatigue; but stiffen again with elastic recovery from fatigue strain, consequently for some cases their stiffness after recovery are rather greater than those before fatigre.

• 한국의류산업학회지
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• 제22권2호
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• pp.250-260
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• 2020

The purpose of the present study was to explore the most effective body region and cooling temperature to alleviate heat strain of workers in hot environments. We developed liquid cooling hood, vest, sleeves and socks and applied the water temperatures of 10, 15, 20, and 25℃ through the liquid cooling garments in a hot and humid environment (33℃ air temperature and 70% RH air humidity). A healthy young male participated in a total of 16 experimental trials (four cooling garments × four cooling temperatures) with the following protocol: 10-min rest, 40-min exercise on a treadmill and 10-min recovery. The results showed that rectal temperature, mean skin temperature, and ratings of perceived exertion during exercise; heart rate and diastolic blood pressure during recovery; and total sweat rate were lower for the vest condition than other garment conditions(p < .05). However, there was no differences in mean skin temperature among the four cooling garments when we compared the values converted by covering area(%BSA). When we classified the results by cooling temperature, there were no consistent differences in thermoregulatory and cardiovascular responses among the four temperatures, but 25℃ water temperature was evaluated as being the most ineffective cooling temperature in terms of subjective responses. In conclusion, the results indicated that wearing cooling vest with < 20℃ cooling temperature can alleviate heat strain of workers in hot and humid environments. If the peripheral body regions are cooled with liquid cooling garments, larger cooling area with lower cooling temperature than 10℃ would be effective to reduce heat strain of workers. Further studies with a vaild number of subjects are required.

• 소성∙가공
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• 제11권2호
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• pp.171-178
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• 2002

The elastic deflections of the cold forging die influence the dimensional accuracy of forged parts. The die dimension is continuously changed during the loading, unloading, and ejecting stage. In this paper, we evaluated the elastic deflections of cold forging die during the loading, unloding and ejecting stage with experimental and FEM analysis. Uni-axial strain gages are used to measure elastic strain of die during each forging stage. Strain gages are attached un the upper surface of die. A commercial F.E.M. code, DEFORM$-2D^{TM}$ is used to predict the elastic strains of die, to be compared those by experiments. Two modelling approaches are used to define the reasonable analysis method. The first of the two modelling approaches is to regard the die as rigid body over forging cycle. And then, the die stress is analyzed by loading the die with pressure from the deformed part. The other is to regard the die as elastic body from forging cycle. The elastic strain of tool is calculated and the tool is elastically deformed at each strep. The calculated results under the elastic die assumption are well agreed wish experimental data using the strain gages.

• 접착 및 계면
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• 제9권3호
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• pp.20-26
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• 2008

형상기억합금(SMA)의 구조는 부가된 온도 혹은 응력에 의해 마텐자이트로부터 오스테나이트로의 변화가 가능하다는 것은 잘 알려져 있다. 형상기억합금섬유의 자체 형상회복력으로 인해 응력과 온도가 적용되는 동안에 응력이나 경화 모니터링 센서 또는 작동기로서 사용되었다. 초탄성 현상은 연속적인 기계적 하중 하에서나 온도변화 중에 응력-변형률 곡선에서 확인되었다. 반복하중 실험을 통해 응력-변형률 곡선에서 나타난 초탄성 현상 구간이 나타나는 응력 이력현상이 발생함을 확인하였다. 이것은 형상기억합금섬유 혹은 에폭시에 함침된 형상기억합금섬유 복합재료가 반복하중으로 계면물성 저하로 인한 형상기억 회복 성능의 저하를 의미한다. 강성도가 큰 에폭시 사용과 형상기억합금섬유의 표면처리 이후 형상기억합금섬유와 에폭시 사이의 계면결합력의 증대에도 불구하고 유사한 불완전한 초탄성을 보여 주었다. 단-형상기억합금섬유/에폭시 복합재료 내부에 남은 잔류 열과 이에 따른 잔류 응력으로 인해 에폭시에 함침된 단-형상기억합금섬유에서는 경화과정에서 불완전한 회복을 나타났다.

• 한국재료학회지
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• 제26권4호
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• pp.167-172
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• 2016

The high temperature deformation behavior of $Ni_3Al$ and $Ni_3(Al,Mo)$ single crystals that were oriented near <112> was investigated at low strain rates in the temperature range above the flow stress peak temperature. Three types of behavior were found under the present experimental conditions. In the relatively high strain rate region, the strain rate dependence of the flow stress is small, and the deformation may be controlled by the dislocation glide mainly on the {001} slip plane in both crystals. At low strain rates, the octahedral glide is still active in $Ni_3Al$ above the peak temperature, but the active slip system in $Ni_3(Al,Mo)$ changes from octahedral glide to cube glide at the peak temperature. These results suggest that the deformation rate controlling mechanism of $Ni_3Al$ is viscous glide of dislocations by the <110>{111} slip, whereas that of $Ni_3(Al,Mo)$ is a recovery process of dislocation climb in the substructures formed by the <110>{001} slip. The results of TEM observation show that the characteristics of dislocation structures are uniform distribution in $Ni_3Al$ and subboundary formation in $Ni_3(Al,Mo)$. Activation energies for deformation in $Ni_3Al$ and $Ni_3(Al,Mo)$ were obtained in the low strain rate region. The values of the activation energy are 360 kJ/mol for $Ni_3Al$ and 300 kJ/mol for $Ni_3(Al,Mo)$.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 1997년도 추계학술대회논문집
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• pp.53-56
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• 1997

The multistage precision forming process for aperture of electronic guns consists mainly of drawing and swaging operations. The multistage forming sequence is analyzed in view of production industry, and based on the industrial investigation. The processes are simulated by the finite element code MARC. The elastic recovery, strain and stress distribution, and the die forces are summarized.

• Bulletin of the Korean Chemical Society
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• 제32권spc8호
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• pp.2988-2996
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• 2011

Shape memory polyurethane (SMPU), grafted with a fluorescent dye (Rhodamine, Mehylene violet, or Fluorescein) through an allophanate linking, was tested for the fluorescence and the shape recovery effect. The main chain of SMPU was composed of 4,4'-methylenebis(phenylisocyanate) (MDI), poly(tetramethyleneglycol) (PTMG), and 1,4-butanediol (BD), and a fluorescent dye was connected through a second MDI linked to the carbamate moiety of the main chain. Three series of SMPU, differing according to their dye content, were prepared to compare their shape recovery and fluorescence properties. In tensile mechanical property, maximum stress increased up to 350% compared to the linear SMPU, and strain remained above 2000%. Shape recovery went to as high as 97%, and remained almost same after repetitive shape recovery test cycles. Finally, the fluorescence emission of SMPU was demonstrated in the luminescence spectrum and fluorescent light emission pictures. In addition, the response of SMPU to external stimuli such as metal ions was investigated.