• Elastomers and Composites
• /
• 제37권4호
• /
• pp.258-264
• /
• 2002

본 연구에서는 열처리에 의한 실리카의 표면특성과 실리카/폴리우레탄 복합재료의 기계적 계면물성에 대하여 고찰하였다. 열처리에 의한 실리카의 표면특성은 Fourier Transform-IR(FT-IR), Solid-state 29Si NMR spectroscopy, 그리고 접촉각을 통하여 알아보았으며 실리카/폴리우레탄 복합재료의 기계적 계면물성은 인열에너지 (GIIIC)를 측정하여 관찰하였다. 본 실험결과, 열처리 온도가 증가함에 따라 실리카의 Si-OH가 축합하여 Si-O-Si를 형성하고 표면자유에너지의 비극성 요소가 증가하는 것을 확인할 수 있었다. 이러한 결과로부터, 증가된 실리카의 표면자유에너지의 비극성요소는 폴리우레탄 내에 실리카의 분산성을 향상시켜 결과적으로 실리카/폴리우레탄 복합재료의 기계적 계면물성인 인열에너지가 증가된 것으로 사료된다.

• Steel and Composite Structures
• /
• 제46권3호
• /
• pp.385-401
• /
• 2023

Steel is becoming increasingly popular due to its high strength, excellent ductility, great assembly performance, and recyclability. In reality, steel structures serving for a long time in atmospheric, industrial, and marine environments inevitably suffer from corrosion, which significantly decreases the durability and the service life with the exposure time. For the mechanical properties of corroded steel, experimental studies are mainly conducted. The existing numerical analyses only evaluate the mechanical properties based on corroded morphology at the isolated time-in-point, ignoring that this morphology varies continuously with corrosion time. To solve this problem, the relationships between pit depth expectation, standard deviation, and corrosion time are initially constructed based on a large amount of wet-dry cyclic accelerated test data. Successively, based on that, an in-situ pitting evolution method for evaluating the residual tensile strength of corroded steel is proposed. To verify the method, 20 repeated simulations of mass loss rates and mechanical properties are adopted against the test results. Then, numerical analyses are conducted on 135 models of corrosion pits with different aspect ratios and uneven corrosion degree on two corroded surfaces. Results show that the power function with exponents of 1.483 and 1.091 can well describe the increase in pit depth expectation and standard deviation with corrosion time, respectively. The effect of the commonly used pit aspect ratios of 0.10-0.25 on yield strength and ultimate strength is negligible. Besides, pit number ratio α equating to 0.6 is the critical value for the strength degradation. When α is less than 0.6, the pit number increases with α, accelerating the degradation of strength. Otherwise, the strength degradation is weakened. In addition, a power function model is adopted to characterize the degradation of yield strength and ultimate strength with corrosion time, which is revised by initial steel plate thickness.

• Steel and Composite Structures
• /
• 제42권1호
• /
• pp.1-22
• /
• 2022

In cold-formed steel (CFS) structures, such as trusses, transmission towers and portal frames, the use of back-to-back built-up CFS unequal angle sections are becoming increasingly popular. In such an arrangement, intermediate welds or screw fasteners are required at discrete points along the length, preventing the angle sections from buckling independently. Limited research is available in the literature on axial strength of back-to-back built-up CFS unequal angle sections. The issue is addressed herein. This paper presents an experimental investigation reported by the authors on back-to-back built-up CFS unequal angle sections with intermediate stiffeners under axial compression. The load-axial shortening behaviour along with the deformed shapes at failure are reported. A nonlinear finite element (FE) model was then developed, which includes material non-linearity, geometric imperfections and modelling of intermediate fasteners. The FE model was validated against the experimental test results, which showed good agreement, both in terms of failure loads and deformed shapes at failure. The validated finite element model was then used for the purpose of a parametric study comprising 96 models to investigate the effect of longer to shorter leg ratios, stiffener provided in the longer leg, thicknesses and lengths on axial strength of back-to-back built-up CFS unequal angle sections. Four different thicknesses and seven different lengths (stub to slender columns) with three overall widths to the overall depth (B/D) ratios were investigated in the parametric study. Axial strengths obtained from the experimental tests and FE analyses were used to assess the performance of the current design guidelines as per the Direct Strength Method (DSM); obtained comparisons show that the current DSM is conservative by only 7% and 5% on average, while predicting the axial strengths of back-to-back built-up CFS unequal angle sections with and without the stiffener, respectively.

• 대한토목학회논문집
• /
• 제29권3A호
• /
• pp.243-250
• /
• 2009

이 연구의 목적은 PSC 거더교를 통상적인 경간 이상으로 장경간화할 수 있는 기법들을 고찰하고, 각 기법이 경간에 미치는 영향을 평가하는 체계적인 방법론을 제안하는 것이다. 전편 논문에서는 평가에 필요한 그래프를 구성하는 기본적인 수식들을 유도한 바 있다. 여기에서는 이러한 방법론을 PSC 거더교 예제에 적용하여 각 장경간화 기법들이 미치는 영향을 정량적으로 분석하였다. 제안된 도식적인 방법은 각 기법이 장경간화에 기여하는 이유를 일목요연하게 파악할 수 있고, 더 나아가 장경간화를 위한 개선점을 용이하게 도출할 수 있는 장점이 있음을 보였다. 분석 결과 여러가지 기법들 중 특히 거더의 고강도화, 바닥판 합성 전 2차 긴장을 실시하는 다단계 긴장 기법, Decked PSC 거더 기법 등이 장경간화에 크게 기여하는 것으로 나타났다. 장경간 거더의 증가된 자중에 대처할 수 있는 시공성 및 경제성만 뒷받침된다면 PSC 거더교도 기존 박스거더교의 영역인 50~70 m 경간까지 충분히 설계 및 시공될 수 있을 것으로 기대된다.

• 대한토목학회논문집
• /
• 제29권2A호
• /
• pp.109-118
• /
• 2009

이 연구에서는 CFT부재를 거더로 사용하는 신형식 강합성 교량 시스템 개발을 최종목표로 하고, CFT부재의 휨거동 특성을 규명하기 위한 부재실험을 수행하였다. 실험체는 충전재 강도 및 합성조건 변화 등 설계에 영향을 미치는 주요 매개변수를 고려하여 제작하였고, 휨재하 실험의 결과분석을 통해 CFT부재가 갖는 내력성능을 효율적으로 설계에 반영하기 위한 방안을 고찰하였다. 충전강관은 중공강관에 비해 휨강도 및 연성이 크게 증가함을 확인할 수 있었으며, 콘크리트의 취성적 재료 특성이 강관의 구속효과로 보완되는 결과를 확인하였다. 내부 전단연결재를 둠으로써 충전재와 강관 사이의 미끄러짐을 억제하여 완전합성단면으로 거동하였고, ㄱ형 전단연결재의 보강재 효과가 휨내력 증가에 다소 기여하고 있음을 알 수 있었다.

• Steel and Composite Structures
• /
• 제44권4호
• /
• pp.503-517
• /
• 2022

This paper presents the mechanical buckling of bi-directional functionally graded sandwich beams (BFGSW) with various boundary conditions employing a quasi-3D beam theory, including an integral term in the displacement field, which reduces the number of unknowns and governing equations. The beams are composed of three layers. The core is made from two constituents and varies across the thickness; however, the covering layers of the beams are made of bidirectional functionally graded material (BFGSW) and vary smoothly along the beam length and thickness directions. The power gradation model is considered to estimate the variation of material properties. The used formulation reflects the transverse shear effect and uses only three variables without including the correction factor used in the first shear deformation theory (FSDT) proposed by Timoshenko. The principle of virtual forces is used to obtain stability equations. Moreover, the impacts of the control of the power-law index, layer thickness ratio, length-to-depth ratio, and boundary conditions on buckling response are demonstrated. Our contribution in the present work is applying an analytical solution to investigate the stability behavior of bidirectional FG sandwich beams under various boundary conditions.

• Steel and Composite Structures
• /
• 제44권5호
• /
• pp.619-632
• /
• 2022

There are numerous structural details (Longitudinal beam, web plate, U-ribs and I-ribs) in the top and bottom plates of steel box girders, which have significant influences on the longitudinal stress (normal stress) distribution. Clarifying the influence of these structural details on the normal stress distribution is important. In this paper, the ultra-wide steel box girder with large cantilevers of the Jinhai Bridge in China, which is the widest cable-stayed bridge in the world, has been analyzed. A 1:4.5 scale laboratory model of the steel box girder has been manufactured, and the influence of structural details on the normal stress distribution in the top and bottom plates for four different load cases has been analyzed in detail. Furthermore, a three-dimensional finite element model has been established to further investigate the influence regularity of structural details on the normal stress. The experimental and finite element analysis (FEA) results have shown that different structural details of the top and bottom plates have varying effects on the normal stress distribution. Notably, the U-ribs and I-ribs of the top and bottom plates introduce periodicity to the normal stress distribution. The period of the influence of U-ribs on the normal stress distribution is the sum of the single U-rib width and the U-rib spacing, and that of the influence of I-ribs on the normal stress distribution is equal to the spacing of the I-ribs. Furthermore, the same structural details but located at different positions, will have a different effect on the normal stress distribution.

• Restorative Dentistry and Endodontics
• /
• 제47권4호
• /
• pp.36.1-36.17
• /
• 2022

Objectives: This study aimed to evaluate the effect of aging on the marginal quality of glass hybrid (GH) material used to elevate dentin gingival margins, and to analyze the consistency of the results obtained by 3 in vitro methods. Materials and Methods: Ten teeth received compound class II cavities with subgingival margins. The dentin gingival margins were elevated with GH, followed by resin composite. The GH/gingival dentin interfaces were examined through digital microscopy, scanning electron microscopy (SEM) using resin replicas, and according to the World Dental Federation (FDI) criteria. After initial evaluations, all teeth were subjected to 10,000 thermal cycles, followed by repeating the same marginal evaluations and energy dispersive spectroscopy (EDS) analysis for the interfacial zone of 2 specimens. Marginal quality was expressed as the percentage of continuous margin at ×200 for microscopic techniques and as the frequency of each score for FDI ranking. Data were analyzed using the paired sample t-test, Wilcoxon signed-rank test, and Pearson and Spearmen correlation coefficients. Results: None of the testing techniques proved the significance of the aging factor. Moderate and strong significant correlations were found between the testing techniques. The EDS results suggested the presence of an ion-exchange layer along the GH/gingival dentin interface of aged specimens. Conclusions: The marginal quality of the GH/dentin gingival interface defied aging by thermocycling. The replica SEM and FDI ranking results had stronger correlations with each other than either showed with the digital microscopy results.

• Restorative Dentistry and Endodontics
• /
• 제47권4호
• /
• pp.43.1-43.13
• /
• 2022

Objectives: This study compared the surface gloss (SG), gloss retention (GR), and color stability (CS) of 2 universal resin composites after chemical (CA) and mechanical (MA) aging. Materials and Methods: Twenty disc-shaped samples of G-ænial A'Chord (GC-Europe) and Filtek Universal (3M-ESPE) were polished with sequential abrasive papers. For CA, specimens were stored in 1 mL of 75% ethanol for 15 days at 37℃, and readings (SG, GR, and CS) were obtained at baseline and 5, 10, and 15 days. For MA, specimens were subjected to 10,750 simulated brushing cycles. SG and CS were evaluated after every 3,583 cycles. SG was measured with a glossmeter (geometrical configuration: 60°), and values were expressed in gloss units. Color was measured with a spectrophotometer using the CIE-L^*a^*b^* color system. The Student's t-test, 1-way analysis of variance, and Scheffé test were used for statistical analysis (α = 0.05). Results: G-ænial presented significantly higher SG values than Filtek (p = 0.02), with GR reductions of 5.2% (CA) and 5.3% (MA) for G-ænial and 7.6% (CA) and 7.2% (MA) for Filtek. The aging protocol had no statistically significant effect on SG or GR (p = 0.25) from baseline to the final readings. G-ænial-MA presented the lowest color difference (ΔE = 1.8), and G-ænial-CA and Filtek-CA had the largest changes (ΔE = 8.6 and ΔE = 11.8, respectively). Conclusion: G-ænial presented higher SG values and better CS. Both restorative materials demonstrated acceptable GR and CS. Aging protocols impacted these properties negatively.

• 한국재료학회지
• /
• 제33권11호
• /
• pp.447-457
• /
• 2023

Single-atom Pd clusters anchored on t-BaTiO₃ material was synthesized using hydrothermal and ultrasonic methods for the effective piezoelectric catalytic degradation of pollutants using vibration energy. XRD patterns of BaTiO₃ loaded with monoatomic Pd were obtained before and after calcining, and showed typical cubic-phase BTO. TEM and HAADF-STEM images indicated single-atom Pd clusters were successfully introduced into the BaTiO₃. The piezoelectric current density of the prepared Pd-BaTiO₃ binary composite was significantly higher than that of the pristine BaTiO₃. Under mechanical vibration, the nanomaterial exhibited a tetracycline decomposition rate of ~95 % within 7 h, which is much higher than the degradation rate of 56.7 % observed with pure BaTiO₃. Many of the piezo-induced electrons escaped to the Pd-doped BaTiO₃ interface because of Pd's excellent conductivity. Single-atom Pd clusters help promote the separation of the piezo-induced electrons, thereby achieving synergistic catalysis. This work demonstrates the feasibility of combining ultrasonic technology with the piezoelectric effect and provides a promising strategy for the development of ultrasonic and piezoelectric materials.