• 대한기계학회논문집A
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• 제20권7호
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• pp.2148-2158
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• 1996

Uncoupled, quasi-static and linear thermoviscoelastic problems are analyzed in time domain by the finite element approximation which is developed using the principle of virtual work and viscoelasticity matrices instead of shear and bulk relaxation functions as in usual formulations. The material is assumed to be isotropic, homegeneous and thermorheologically simple, which means that the temperature-time equivalence postulate is effective. The stress-strain laws are expressed by relaxation-type hereditary integrals. In spatial and time discritizations, isoparametric quadratic quadrilateral finite elements and linear time variations are adopted. For explicit derivations, the viscoelastic material is assumed to behave standard linear solid in shear and elastically in dilatation. Two-dimensional examples are solved under general temperature distributions T = T(x, t), and compared with other opproximate solutions to show the versatility of the presented analysis.

• Clinical Pain
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• 제18권2호
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• pp.121-125
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• 2019

Arthritis of hip joints deteriorates the quality of life in ankylosing spondylitis (AS) patients. Secondary to the articular inflammatory process, the shortened hip-girdle muscles contribute to the decreased joint mobility which may lead to the functional impairment. As the limitation of range of motion (ROM) usually progress slowly, clinicians regard it as a chronic condition and prescribe long-term therapy. However, by short-term intensive multimodal treatment, a 20-year-old man diagnosed as AS with severely limited hip joint ROM who relied on crutches doubled the joint angle and could walk independently only within 2 weeks. The combination included intra-articular steroid injection, electrical twitch obtaining intramuscular stimulation, extracorporeal shock wave therapy, heat, manual therapy, and stretching exercises. The management focused on the relaxation of hip-girdle muscles as well as the direct control of intra-articular inflammation. Hereby, we emphasize the effectiveness of intensive multimodal treatment in improving the function even within a short period.

• Korea-Australia Rheology Journal
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• 제18권2호
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• pp.67-81
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• 2006

Using a strain-controlled rheometer, the dynamic viscoelastic properties of aqueous xanthan gum solutions with different concentrations were measured over a wide range of strain amplitudes and then the linear viscoelastic behavior in small amplitude oscillatory shear flow fields was investigated over a broad range of angular frequencies. In this article, both the strain amplitude and concentration dependencies of dynamic viscoelastic behavior were reported at full length from the experimental data obtained from strain-sweep tests. In addition, the linear viscoelastic behavior was explained in detail and the effects of angular frequency and concentration on this behavior were discussed using the well-known power-law type equations. Finally, a fractional derivative model originally developed by Ma and Barbosa-Canovas (1996) was employed to make a quantitative description of a linear viscoelastic behavior and then the applicability of this model was examined with a brief comment on its limitations. Main findings obtained from this study can be summarized as follows: (1) At strain amplitude range larger than 10%, the storage modulus shows a nonlinear strain-thinning behavior, indicating a decrease in storage modulus as an increase in strain amplitude. (2) At strain amplitude range larger than 80%, the loss modulus exhibits an exceptional nonlinear strain-overshoot behavior, indicating that the loss modulus is first increased up to a certain strain amplitude(${\gamma}_0{\approx}150%$) beyond which followed by a decrease in loss modulus with an increase in strain amplitude. (3) At sufficiently large strain amplitude range (${\gamma}_0>200%$), a viscous behavior becomes superior to an elastic behavior. (4) An ability to flow without fracture at large strain amplitudes is one of the most important differences between typical strong gel systems and concentrated xanthan gum solutions. (5) The linear viscoelastic behavior of concentrated xanthan gum solutions is dominated by an elastic nature rather than a viscous nature and a gel-like structure is present in these systems. (6) As the polymer concentration is increased, xanthan gum solutions become more elastic and can be characterized by a slower relaxation mechanism. (7) Concentrated xanthan gum solutions do not form a chemically cross-linked stable (strong) gel but exhibit a weak gel-like behavior. (8) A fractional derivative model may be an attractive means for predicting a linear viscoelastic behavior of concentrated xanthan gum solutions but classified as a semi-empirical relationship because there exists no real physical meaning for the model parameters.

• 방사성폐기물학회지
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• 제18권2_spc호
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• pp.237-245
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• 2020

사용후핵연료를 저장하는 볼트체결 저장용기의 격납경계를 형성하는 주된 구성요소는 금속 밀봉재이다. 이러한 금속 밀봉재는 열과 방사선에 의해 그 성질이 저하된다. 또한, 금속 밀봉재가 강한 열에 장기간 노출되면 크리프 현상이 발생한다. 이러한 크리프는 밀봉시스템에 응력 이완을 가져와서, 결국 밀봉재의 건전성을 해치게 된다. 이러한 응력 이완은 금속 밀봉재의 밀봉성능 저하로 이어지며, 저하의 정도에 따라 저장용기의 누설을 야기할 수 있다. 또한, 볼트 체결력의 감소도 밀봉성능 저하에 영향을 미친다. 본 논문에서는 금속 밀봉재의 격납건전성과 볼트체결력 감소를 평가하기 위해 수행한 가속화 시험의 결과에 대하여 기술한다. 전 시험기간 동안 각 시편에서의 누설률, 볼트 변형률, 금속 밀봉재 주변 온도를 계측하여 분석하였고, 금속 밀봉재는 저장기간 50년 동안 격납건전성을 유지함을 입증하였다. 또한, 가속화 시험의 타당성에 대해서 기술하였다.

• 대한토목학회논문집
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• 제26권1A호
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• pp.67-74
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• 2006

장대교량 건설의 증가와 강교량의 합리화 추세에 따라 최근 후판 강재의 사용이 늘어가고 있다. 후판 부재의 현장이음에서는 M30 고장력볼트와 같은 대형볼트의 사용이 불가피하나 현재 국내 시방기준에는 대형볼트의 사용에 대한 규정이 없는 실정이다. 따라서 본 연구에서는 대형볼트인 M30 고장력볼트의 사용성을 검토할 목적으로 볼트의 크기와 부재의 판두께를 변수로 하여 릴렉세이션과 미끄러짐 거동을 시험적으로 분석하였다. 또한, 유한요소해석을 통하여 판두께의 차이에 따른 내부압축응력을 분석하였으며, 이를 볼트축과 볼트구멍 내벽에 부착한 변형률게이지로부터 얻어진 응력분포와 비교하였다. 그 결과 M30 고장력볼트를 사용한 경우에도 릴렉세이션은 10%를 초과하지 않았으며, 미끄러짐 계수도 시방기준을 초과하는 충분한 내력을 확보하는 것으로 나타나 사용상에 문제가 없는 것으로 판단된다.

• Tribology and Lubricants
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• 제37권6호
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• pp.224-231
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• 2021

Recently, the waterproofing performance of high-voltage connectors in automotive vehicles has attracted increased interest. In this study, an optimal cross-sectional shape was derived to obtain uniform contact pressure and strain by considering stress relaxation problems caused by initial tension when mounting a seal. A high strain of 52.1 was distributed in the round region, owing to excessive initial tension. The finite element method (FEM) analysis indicated that the strain corresponding to the optimal initial tensile was 11. We adopted six design factors to optimize the seal cross-section and three factors as the main design factors. An orthogonal arrangement table was prepared using Minitab. FEM analyses of 16 study models were conducted to determine the optimized model. The contact pressure of the optimization model is the most evenly distributed while satisfying the waterproof performance of 0.47 MPa. Compared to the initial model, the difference in strain decreases from 35.5% to 19.6%. Finally, the derived cross-sectional shape can reduce the strain of the round region by 33.8% and the differences in the contact pressure at the upper and lower surfaces by 42% and 76%, respectively.

• 생명과학회지
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• 제13권5호
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• pp.634-641
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• 2003

외부에서 가해진 비소 스트레스가 내피유무에 따른 흰쥐 대동맥의 수축력 증가와 이완반응에 미치는 영향을 알아보고자 본 실험을 실시하였다. 혈압의 측정은 생리기록계를 이용하였고, 내피 유무에 따른 혈관의 수축력은 Organ bath에 조직을 걸고 자동조절 생리기록계를 이용하여 측정하였다. 중추신경계를 파괴시킨 흰쥐의 생체 내 실험에서 비소처리로 vasopressin과 phenylephrine에 의한 혈관 수축력은 대조군에 비해 각각 19.1%와 46.6%로 대동맥의 혈압은 상승되었다. 0, 0.5, 1, 2 및 4 mM As을 처리한 적출 대동맥 실험에서 phenylephrine $(10^{-6}M)$을 가했을 때 5시간까지는 혈관 수축력의 변화가 미미했으나 8시간째 비소 처리군은 대조군에 비해 39% 증가된 값을 보여 수축력이 더욱 유의하게 증가되었다. 내피 제거 시 저농도 비소처리에서 다소 신속한 수축반응을 보였으나 고농도 비소 처리시에는 내피유무에 따른 차이가 유의적이지 않았다. 이완제 sodium nitroprusside와 acetylcoline를 처리했을 때 대조군에 비해 다소 증가된 이완력을 보였고, 시간경과에 따라 내피 비의존적인 nitroprusside와 달리 내피 의존적인 acetylcholine에서 이완력이 대조군에 비해 다소 촉진되었다. 이상의 결과에서 4 mM As처리시 혈관의 수축력은 증가되었으나 내피유무에 따른 차이는 유의적이지 않았고, 내피가 혈관의 이완력을 다소 촉진시킨 것으로 나타났다. 결론적으로 비소처리한 혈관은 내피유무와 무관하게 수축력이 증가되어 장기간 고농도 비소에 노출 시 고혈압을 유발할 우려가 있을 것으로 여겨진다.

• 한국지반신소재학회논문집
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• 제21권4호
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• pp.69-78
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• 2022

지오튜브 공법은 해안지역 해안선 침식방지와 준설토 매립 호안사면의 뒤채움 필터석 대체용으로도 널리 활용되고 있다. 본 연구는 지오텍스타일에 의해 보강되어 제작된 공시체를 현장 상태를 고려하여 등방 구속압력 10kPa, 50kPa, 100kPa 이하로 최소화하여 연직응력 재하 시 변형 거동을 파악하였다. 시험결과 공시체의 연직 변형율 7%까지는 구속압(≤100kPa)에 관계없이 지오텍스타일 조직의 이완에 따른 인장력이 발휘되는 초기 Strain Hardening 영향으로 응력-변위 거동은 동일하였다. 변형율 7%이상부터는 구속압력이 작은 공시체는 재하 시 변형이 커서 보강 지오텍스타일의 인장 저항력을 증가시키므로 Strain Hardening에 의해 파괴 시 축차응력은 상대적으로 증가하였다. 파괴 후는 급격하게 Strain Softening에 의해 취성파괴형태를 나타내면서 저하되었다. 이는 일반적인 삼축압축시험에서 셀 압이 증가되면서 전단응력도 크게 증가되는 현상과 다르다. 지오텍스타일 등방구속시험에서는 지오텍스타일의 인장변위가 일차적으로 영향을 받기 때문에 탄성계수를 급격하게 증가시키는 탄성 거동을 나타내고 있다.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2001년도 추계학술대회 논문집
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• pp.285-288
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• 2001

Hypereutectic Al-25Si-X alloys, expected to be applied to the cylinder-liner-part of the engine-block of an automobile due to the excellent wear resistance, low density and low thermal expansion coefficient has been fabricated through a spray forming process. The obtained microstructure of the hypereutectic Al-25Si-X alloy appeared to consist of Al matrix and equiaxed Si particles of average diameter of $5-7{\mu}m$. To characterize the deformation behavior of this alloy, a series of load relaxation and compression tests have been conducted at temperatures ranging from RT to $500^{\circ}C$. The strain rate sensitivity parameter (m) of this alloy has been found to be very low (0.1) below foot and reached 0.2 at $500^{\circ}C$. During the deformation above 300'c in compression, strain softening has been observed. The diagram of extrusion pressure vs. ram-speed has been constructed, providing the extrusion condition of Al-25Si-X alloys.

• 터널과지하공간
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• 제5권4호
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• pp.363-375
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• 1995

Critical literature review in this study revealed that there can be a significant influence of the residual stress on the engineering properties of rock. The review also showed that few number of research works on the quantification of the influence was attributed to the limitation of the two classical measurement techniques, viz, X-ray diffraction and mechanical relaxation method. In this study, a new way of approach was sought based on the assumption that residual stress up to the failure. A series of hoop tests conducted onthe samples from the limb of Carboniferous Limestone in Clevedon, England, revealed that (i) there is no preferential orientations of microcracks and minerals which have been widely believed as the main source of the strength anisotropy of rock; (ii) the anisotropy of the tensile strength of the limestone results from the influence of the residual stress; (iii) since jointing commenced within the fold, residual stored strain energy has been released preferentially in the direction perpendicular to the major joints(o$^{\circ}$ and 90$^{\circ}$); (ⅳ) during the hoop test making it much easier to create tensile fracture in these directons, viz 45$^{\circ}$ and 135$^{\circ}$)was released during the hoop test making it much easier to create tensile fracture in these directions, viz 45$^{\circ}$and 135$^{\circ}$;(v) the direction in which the stored strain energy may be presumed to be the least, required the greatest work to cause failure.