• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2008년도 춘계 학술발표회 초청강연 및 논문집
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• pp.251-258
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• 2008

The use of reinforced earth walls in permanent structures is getting it's popularity. Despite a number of advantages of reinforced earth walls over conventional concrete retaining walls, there exit concerns over long-term residual deformation when subjected to repeated and/or cyclic loads, during their service period. In this investigation, the effect of preloading in reducing long term residiual deformation of back-to-back reinforced soil wall under sustained and/or repeated loading enviormentment using a series of reduced-scale model tests. It is found that the preloading technique can be an effective means of controlling residual deformations of reinforced soils under varisous loading conditions.

• Journal of Periodontal and Implant Science
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• 제54권3호
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• pp.189-204
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• 2024

Purpose: This pilot study was conducted to develop a novel test setup for the in vitro assessment of the primary stability of dental implants. This was achieved by characterising their long-term behaviour based on the continuous recording of micromotions resulting from dynamic and cyclic loading. Methods: Twenty screw implants, each 11 mm in length and either 3.8 mm (for premolars) or 4.3 mm (for molars) in diameter, were inserted into the posterior region of 5 synthetic mandibular models. Physiological masticatory loads were simulated by superimposing cyclic buccal-lingual movement of the mandible with a vertically applied masticatory force. Using an optical 3-dimensional (3D) measuring system, the micromotions of the dental crowns relative to the alveolar bone resulting from alternating off-centre loads were concurrently determined over 10,000 test cycles. Results: The buccal-lingual deflections of the dental crowns significantly increased from cycle 10 to cycle 10,000 (P<0.05). The deflections increased sharply during the first 500 cycles before approaching a plateau. Premolars exhibited greater maximum deflections than molars. The bone regions located mesially and distally adjacent to the loaded implants demonstrated deflections that occurred synchronously and in the same direction as the applied loads. The overall spatial movement of the implants over time followed an hourglass-shaped loosening pattern with a characteristic pivot point 5.5±1.1 mm from the apical end. Conclusions: In synthetic mandibular models, the cyclic reciprocal loading of dental implants with an average masticatory force produces significant loosening. The evasive movements observed in the alveolar bone suggest that its anatomy and yielding could significantly influence the force distribution and, consequently, the mechanical behaviour of dental implants. The 3D visualisation of the overall implant movement under functional cyclic loading complements known methods and can contribute to the development of implant designs and surgical techniques by providing a more profound understanding of dynamic bone-implant interactions.

• The Journal of Advanced Prosthodontics
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• 제7권4호
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• pp.323-328
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• 2015

PURPOSE. This in-vitro study was designed to evaluate retention forces, microleakage and plastic deformation of a prefabricated 2-implant bar attachment system (SFI-Bar, Cendres+$M{\acute{e}}taux$, Switzerland). MATERIALS AND METHODS. Two SFI implant-adapters were torqued with 35 Ncm into two implant analogues. Before the tube bars were finally sealed, the inner cavity of the tube bar was filled with liquid red dye to evaluate microleakage. As tube bar sealing agents three different materials were used (AGC Cem (AGC, resin based), Cervitec Plus (CP; varnish) and Gapseal (GS; silicone based). Four groups with eight specimens each were tested (GS, GS+AGC, AGC, CP). For cyclic loading, the attachment system was assembled parallel to the female counterparts in a chewing simulator. The mean retention forces of the initial and final ten cycles were statistically evaluated (ANOVA, ${\alpha}{\leq}.05$). RESULTS. All groups showed a significant loss of retention forces. Their means differed between 30-39 N initially and 22-28 N after 50,000 loading cycles. No significant statistical differences could be found between the groups at the beginning (P=.224), at the end (P=.257) or between the loss of retention forces (P=.288). Microleakage occurred initially only in some groups but after 10,000 loading cycles all groups exhibited microleakage. CONCLUSION. Long-term retention forces of the SFI-Bar remained above 20 N which can be considered clinically sufficient. The sealing agents in this study are not suitable to prevent microleakage.

• Computers and Concrete
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• 제30권4호
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• pp.277-287
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• 2022

Due to the scarcity of extortionate experimental data, fatigue failure of the reinforced concrete (RC) element might be achieved economically adopting nonlinear finite element (FE) analysis as an alternative approach. However, conventional implicit dynamic analysis is expensive, quasi-static method overlooks interaction effects and inertia, direct cyclic analysis computes stabilized responses. Apart from this, explicit dynamic analysis may provide a numerical operating system for factual long-term responses. The study explores the fatigue behavior based on a simplified explicit dynamic solution employing nonlinear time domain analysis. Among fourteen RC beams, one beam is selected to validate under static loading, one under fatigue with the experimental study and other twelve to check the detail fatigue behavior. The SWOT (Strength, Weakness, Opportunities, Threats) analysis has been carried out to pinpoint the detail scenario in the adoption of numerical approach as an alternative to the experimental study. Excellent agreement of FE and experimental results is seen. The 3D nonlinear RC beam model at service fatigue limits is truthful to be used as an expedient contrivance to envisage the precise fatigue behavior. The simplified analysis approach for RC beam under fatigue offers savings in computation to predict responses providing acceptable accuracy rather than the complicated laboratory investigation. At higher frequency, the flexural failure occurs a bit earlier gradually compared to the repeated loading case of lower frequency. The deflection increases by 6%-10% at the end of first cycle for beams with increasing frequency of cyclic loading. However, at the end of fatigue loading, greater deflection occur earlier for higher load range because of more rapid stiffness degradation. For higher frequency, a slight boost in concrete compressive strains at an initial stage of loading has been seen indicating somewhat stepper increment. Stiffness degradation in larger loading cycle at same duration escalates the upsurge of the rate of strain in case of higher frequency.

• 폴리머
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• 제36권1호
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• pp.88-92
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• 2012

일정 하중에 지속적으로 노출되는 고분자의 장기간 사용을 위해서는 재료의 수명을 평가할 수 있는 가속화된 시험 방법이 필요하다. 반복 피로하중 시험법은 이러한 방법들 중 하나로 많은 관심을 받고 있다. 본 연구에서는 X-선 회절법을 이용하여 고밀도 폴리에틸렌의 반복 피로하중에 의한 구조적 변화와 크리프 변형을 비교하고자 하였다. 이를 위하여 별도의 인장시험기를 제조, X-선 회절기에 부착하여 장시간 변형 과정을 성공적으로 관찰하였다. 그 결과 크리프와 반복 피로하중 사이의 거시적이고 뚜렷한 차이에도 불구하고 결정화도, 결정크기 및 면간거리와 같은 결정의 미세구조는 두 방법에서 거의 동일하게 관찰되었다. 그러나 항복점 전(BYP), 항복점(YP) 그리고 항복점 후(AYP)로 각각 변형시킨 후 시험한 시료의 경우 AYP와 다른 두 시료간 뚜렷한 구조적 차이를 확인할 수 있었다.

• 한국지반공학회논문집
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• 제24권6호
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• pp.101-116
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• 2008

보강토 옹벽의 영구구조물로서의 적용성이 증가함에 따라 지속 혹은 반복하중 등 다양한 하중조건하에서 장기적인 잔류변형이 설계/시공시 주요 관점이 되고 있다. 본 연구에서는 교량구조물의 피어 및 Back-to-Back(BTB) 형태의 보강토 옹벽을 대상으로 사용하중 작용 이전에 작용시키는 선행하중이 보강토 구조물의 잔류변위를 제어에 미치는 영향을 고찰하였다. 이를 위해 모형 피어와 BTB 옹벽을 축조하여 단순재하, 지속하중, 반복하중 등 다양한 하중조건 및 시공조건에 대한 모형실험을 수행하였다. 그 결과 선행재하 공법은 다양한 하중 및 시공조건에서 잔류변위 제어에 탁월한 것으로 나타났다.

• 대한심미치과학회지
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• 제27권2호
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• pp.75-81
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• 2018

본 연구의 목적은 기존의 다른 포스트-코어 재료와의 유한요소분석(FEA)의 비교를 통한 치과용 포스트-코어 재료로서의 고성능 폴리머 PEKK의 생체역학 및 장기 안전성을 평가하는 데 있다. 상악 중절치를 모델링 하였으며, 구개 표면에서 치아의 장축에 $45^{\circ}$의 각도로 50N의 반복 하중을 가했다. 전통적으로 사용 된 포스트 코어 재료와 비교하기 위해 3가지 재료 (금, 유리 섬유 및 PEKK)를 시뮬레이션하여 결과를 상호 비교 해 보았다. 상아질보다 낮은 탄성 계수를 갖는 PEKK는 종래의 포스트 코어 재료보다 비교적 치근에 유리한 응력 분포를 보여 주었다. 그러나, PEKK 포스트-코어 시스템은 금속 또는 유리 섬유 포스트-코어 시스템보다 장기간의 반복하중에 대해 타락 및 크라운 파손의 가능성이 더 높을 가능성이 있다.

• The Journal of Advanced Prosthodontics
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• 제7권5호
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• pp.375-379
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• 2015

PURPOSE. The purpose of this study was to evaluate the influence of tightening technique and the screw coating on the loosening torque of screws used for Universal Abutment fixation after cyclic loading. MATERIALS AND METHODS. Forty implants (Titamax Ti Cortical, HE, Neodent) (n=10) were submerged in acrylic resin and four tightening techniques for Universal Abutment fixation were evaluated: A - torque with 32 Ncm (control); B - torque with 32 Ncm holding the torque meter for 20 seconds; C - torque with 32 Ncm and retorque after 10 minutes; D - torque (32 Ncm) holding the torque meter for 20 seconds and retorque after 10 minutes as initially. Samples were divided into subgroups according to the screw used: conventional titanium screw or diamond like carbon-coated (DLC) screw. Metallic crowns were fabricated for each abutment. Samples were submitted to cyclic loading at $10^6$ cycles and 130 N of force. Data were analyzed by two-way ANOVA and Tukey's test (5%). RESULTS. The tightening technique did not show significant influence on the loosening torque of screws (P=.509). Conventional titanium screws showed significant higher loosening torque values than DLC (P=.000). CONCLUSION. The use of conventional titanium screw is more important than the tightening techniques employed in this study to provide long-term stability to Universal Abutment screws.

• 한국지반신소재학회논문집
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• 제11권3호
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• pp.1-9
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• 2012

흙노반으로 이루어진 철도 구간의 경우 반복적인 하중재하에 따라 탄성상태로 복귀하는 회복변형과 영구 변형이 동시에 발생한다. 따라서 열차하중이 반복적으로 작용하는 철도하부 지반에서의 변형예측을 위하여 반복하중에 대한 장기변형 예측이 필요하다. 본 논문에서는 최적함수비 범위에서의 최적함수비의 일반토사를 대상으로 진동삼축압축시험과 원형토조시험을 통해 반복하중에 따른 변형 특성을 비교하였으며, 파워함수 모델을 이용하여 각 재료별 축차응력과 반복횟수를 고려한예측모델식을 제안하였다.

• Smart Structures and Systems
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• 제18권6호
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• pp.1203-1215
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• 2016

When strain sensing cables are under long-term stress and cyclic loading, creep may occur in the jacket material and each layer of the cable structure may slide relative to other layers, causing fatigue in the cables. This study proposes a device for testing the fatigue characteristics of three types of cables operating under different conditions to establish a decay model for observing the patterns of strain decay. The fatigue characteristics of cables encased in polyurethane (PU), GFRP-reinforced, and wire rope-reinforced jackets were compared. The findings are outlined as follows. The cable strain decayed exponentially, and the decay process involved quick decay, slow decay, and stabilization stages. Moreover, the strain decay increased with the initial strain and tensile frequency. The shorter the unstrained period was, the more similar the initial strain levels of the strain decay curves were to the stabilized strain levels of the first cyclic elongation. As the unstrained period increased, the initial strain levels of the strain decay curves approached those of the first cyclic elongation. The tested sensing cables differed in the amount and rate of strain decay. The wire rope-reinforced cable exhibited the smallest amount and rate of decay, whereas the GFRP-reinforced cable demonstrated the largest.