• 대한금속재료학회지
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• 제47권2호
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• pp.59-70
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• 2009

This study is concerned with the correlation between microstructure and mechanical properties in heat affected zones (HAZs) of API X80 pipeline steels containing complex oxides. Three kinds of specimens were fabricated by varying alloying elements of Ti, Al, and Mg to form complex oxides, and their microstructures, Vickers hardness, Charpy impact properties were investigated. The number of complex oxides increased as the excess amount of Ti, Al, and Mg was included in the steels. The simulated HAZs containing a number of oxides showed a high volume fraction of acicular ferrite region because oxides acted as nucleation sites for acicular ferrite. According to the correlation study between thermal input, volume fraction of acicular ferrite region, and Charpy impact properties, the ductile fracture occurred predominantly when the volume fraction of acicular ferrite region was 65% or higher, and the Charpy absorbed energy was excellent over 200 J. When the volume fraction of acicular ferrite region was 35% or lower, the Charpy absorbed energy was poor below 50 J as the brittle cleavage fracture occurred. These findings suggested that the active nucleation of acicular ferrite in the oxide-containing steel HAZs was associated with the great improvement of Charpy impact properties of the HAZs.

• 대한금속재료학회지
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• 제47권12호
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• pp.811-818
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• 2009

In order to meet increasingly complex and rigorous technical specifications, extensive effort has been devoted to fabricate clad materials with multi-layered metal plates. In this study, novel stainless steel/aluminum/copper (STS/Al/Cu) three-ply clad materials were fabricated by a hot rolling process for cookware applications. The effect of the testing temperature on the mechanical properties of the clad materials and on each component metal was investigated during the tensile tests. The interface properties of the clad materials were also examined by optical microscopy (OM) and an electron probe micro-analyzer (EPMA). The best mechanical and interfacial properties for a warm working process were found in a sample annealed at a temperature of $300^{\circ}C$. For the sample annealed at $400^{\circ}C$, the results of the tensile test indicated that interface delamination occurred only in the region of the Al/Cu interfaces. This was due to the formation of the thick and brittle intermetallic compound of $Al_2Cu$ in the Al/Cu interface. In contrast, no interface delamination was observed in the STS/Al interface, most likely due to its strong bond strength.

• 마이크로전자및패키징학회지
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• 제28권2호
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• pp.95-103
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• 2021

대표적인 저온솔더인 Sn-58Bi에 Ru nanoparticles을 첨가하여 Sn-58Bi-xRu 복합솔더를 제작하고 Cu/OSP 및 ENIG 표면처리된 PCB 기판과 반응시켜 계면반응 및 솔더조인트 신뢰성을 분석하였다. Cu/OSP와의 반응에서 형성된 Cu₆Sn₅ IMC는 Ru 함량에 따른 두께 변화가 거의 없고 100hr aging 후에도 큰 변화없이 고속 전단시험시 솔더 내부로 연성파괴가 발생하였다. ENIG 와의 반응시에는 Ru 함량이 증가함에 따라서 Ni₃Sn₄ IMC 두께가 감소하는 경향을 보였으며 일부 시편에서 ENIG 특유의 취성파괴 현상이 발견되었다. Ru 원소는 계면 부근에서 발견되지 않아서 계면반응에 크게 관여하지 않는 것으로 판단되며 주로 Bi phase와 함께 존재하는 것으로 분석되고 있는데 어떠한 형태로 두 원소가 공존하고 있는지에 대해서는 추가적인 연구가 필요하다.

• 해양환경안전학회지
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• 제27권4호
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• pp.521-528
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• 2021

본 연구는 손상된 선박용 절탄기 핀튜브에 대하여 보수를 목적으로 Inconel 625 아크 열용사 코팅기술 적용 후 실링처리를 실시하였다. 모재(Substrate), 열용사 코팅(Thermal Srpay Coating; TSC) 그리고 열용사 코팅+실링처리(TSC+Sealing) 시편에 대하여 내구성을 평가하기 위해 ASTM G76-05에 의거하여 고상입자 침식(Solid Particle Erosion; SPE) 실험을 실시하였다. 표면 손상 형상은 주사전자현미경과 3D 레이져 현미경을 통해 관찰했으며, 무게 감소량과 표면 거칠기 분석을 실시하여 내구성을 평가하였다. 그 결과 내구성은 TSC와 TSC+Sealing에 비해 Substrate가 우수하게 나타났으며, 이는 TSC 층 내에 존재하는 다수의 기공 결함에 기인한 것으로 판단된다. 또한 고상입자 침식 손상 메카니즘은 Substrate의 경우 연성 재질 특성인 소성변형과 피로에 의한 균열 생성이 동반되었으며, TSC와 TSC+Sealing의 경우 취성파괴 경향이 확인되었다.

• 한국건설순환자원학회논문집
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• 제8권4호
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• pp.458-466
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• 2020

이 연구의 목적은 비정질 강섬유와 폴리아미드 섬유를 이용한 고성능 하이브리드 섬유보강 콘크리트의 압축 및 인장 거동을 평가하는 것이다. 이를 위하여 목표 압축강도 40MPa 및 60MPa 각각에 대해서 비정질 강섬유와 폴리아미드 섬유를 총 부피비로 1.0% 혼입한 고성능 하이브리드 섬유보강 콘크리트를 제작한 후 압축강도, 압축인성, 직접인장강도 및 응력-변형률 특징 등의 압축 및 인장 거동을 평가하였다. 그 결과, 고성능 하이브리드 섬유보강 콘크리트의 압축강도는 플레인 콘크리크보다 다소 감소하였으나, 압축인성, 압축인성 비, 직접인장강도는 크게 증가하는 것으로 나타났다. 또한 압축 및 인장 시험시 플레인 콘크리트는 최대응력 이후 취성파괴를 나타냈으나, HPHFRC는 변형연화 현상을 나타내어, 압축 및 인장 거동이 크게 개선되는 것으로 나타났다.

• Geomechanics and Engineering
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• 제24권4호
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• pp.349-358
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• 2021

The Brazilian test has been widely used to determine the indirect tensile strength of rock, concrete and other brittle materials. The basic assumption for the calculation formula of Brazilian tensile strength is that the elastic moduli of rock are the same both in tension and compression. However, the fact is that the elastic moduli in tension and compression of most rocks are different. Thus, the formula of Brazilian tensile strength under the assumption of isotropy is unreasonable. In the present study, we conducted Brazilian tests on flat disk-shaped rock specimens and attached strain gauges at the center of the disc to measure the strains of rock. A tension-compression bi-modular model is proposed to interpret the data of the Brazilian test. The relations between the principal strains, principal stresses and the ratio of the compressive modulus to tensile modulus at the disc center are established. Thus, the tensile and compressive moduli as well as the correct tensile strength can be estimated simultaneously by the new formulas. It is found that the tensile and compressive moduli obtained using these formulas were in well agreement with the values obtained from the direct tension and compression tests. The formulas deduced from the Brazilian test based on the assumption of isotropy overestimated the tensile strength and tensile modulus and underestimated the compressive modulus. This work provides a new methodology to estimate tensile strength and moduli of rock simultaneously considering tension-compression bi-modularity.

• Steel and Composite Structures
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• 제38권5호
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• pp.563-582
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• 2021

The present study experimentally and analytically investigated the push-out behaviour of H-shaped steel section embedded in ultrahigh-performance fibre-reinforced concrete (UHPFRC). The effect of significant parameters such as the concrete types, fibre content, embedded steel length, transverse reinforcement ratio and concrete cover on the bond stress, development of bond stress along the embedded length and failure mechanism has been reported. The test results show that the bond slip behaviour of steel-UHPFRC is different from the bond slip behaviour of steel-normal concrete and steel-high strength concrete. The bond-slip curves of steel-normal concrete and steel-high strength concrete exhibit brittle behaviour, and the bond strength decreases rapidly after reaching the peak load, with a residual bond strength of approximately one-half of the peak bond strength. The bond-slip curves of steel-UHPFRC show an obvious ductility, which exhibits a unique displacement pseudoplastic effect. The residual bond strength can still reach from 80% to 90% of the peak bond strength. Compared to steel-normal concrete, the transverse confinement of stirrups has a limited effect on the bond strength in the steel-UHPFRC substrate, but a higher stirrup ratio can improve cracking resistance. The experimental campaign quantifies the local bond stress development and finds that the strain distribution in steel follows an exponential rule along the steel embedded length. Based on the theory of mean bond and local bond stress, the present study proposes empirical approaches to predict the ultimate and residual bond resistance with satisfactory precision. The research findings serve to explain the interface bond mechanism between UHPFRC and steel, which is significant for the design of steel-UHPFRC composite structures and verify the feasibility of eliminating longitudinal rebars and stirrups by using UHPFRC in composite columns.

• Nuclear Engineering and Technology
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• 제54권7호
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• pp.2376-2385
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• 2022

Two groups of oxide dispersion-strengthened reduced-activation ferritic/martensitic steels (A and B) were prepared by adding Y, Ti, and Zr into steels through vacuum induction melting to investigate the inclusions, microstructures, mechanical properties of the alloys. Results showed that particles with Y, Ti, and Zr easily formed. Massive, Zr-rich inclusions were found in B steel. Density of micron inclusions in A steel was 1.42 × 10¹⁴ m^-3, and density of nanoparticles was 3.61 × 10¹⁶ m^-3. More and finer MX carbides were found in steel tempered at 650 ℃, and yield strengths (YS) of A and B steel were 714±2 and 664±3.5 MPa. Thermomechanical processing (TMP) retained many dislocations, which improved the mechanical properties. YSs of A and B treated by TMP were 725±3 and 683±4 MPa. The existence of massive Zr-rich inclusions in B steels interrupted the continuity of the matrix and produced microcracks (fracture), which caused a reduction in mechanical properties. The presence of fine prior austenite grain size and inclusions was attributed to the low DBTTs of the A steels; DBTTs of A650 and A700 alloy were -79 and -65 ℃. Tempering temperature reduction and TMP are simple, readily useable methods that can lead to a superior balance of strength and impact toughness in industry applications.

• Nuclear Engineering and Technology
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• 제54권10호
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• pp.3631-3640
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• 2022

Uranium mill tailing ponds (UMTPs) are risk source of debris flow and a critical source of environmental U and Rn pollution. The technology of microbial induced calcium carbonate precipitation (MICP) has been extensively studied on reinforcement of UMTs, while little attention has been paid to the effects of MICP on U & Rn release, especially when incorporation of metakaolin and bacillus subtilis (MBS). In this study, the reinforcement and U & Rn immobilization role of MBS -MICP solidification in different grouting cycle for uranium mill tailings (UMTs) was comprehensively investigated. The results showed that under the action of about 166.7 g/L metakaolin and ~50% bacillus subtilis, the solidification cycle of MICP was shortened by 50%, the solidified bodies became brittle, and the axial stress increased by up to 7.9%, and U immobilization rates and Rn exhalation rates decrease by 12.6% and 0.8%, respectively. Therefore, the incorporation of MBS can enhance the triaxial compressive strength and improve the immobilization capacity of U and Rn of the UMTs bodies solidified during MICP, due to the reduction of pore volume and surface area, the formation of more crystals general gypsum and gismondine, as well as the enhancing of coprecipitation and encapsulation capacity.

• Geomechanics and Engineering
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• 제30권1호
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• pp.93-105
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• 2022

With the exploitation of natural resources in China, underground resource extraction and underground space development, as well as other engineering activities are increasing, resulting in the creation of many defective rocks. In this paper, uniaxial compression tests were performed on rocks with double holes and fractures at different angles using particle flow code (PFC2D) numerical simulations and laboratory experiments. The failure behavior and mechanical properties of rock samples with holes and fractures at different angles were analyzed. The failure modes of rock with defects at different angles were identified. The fracture propagation and stress evolution characteristics of rock with fractures at different angles were determined. The results reveal that compared to intact rocks, the peak stress, elastic modulus, peak strain, initiation stress, and damage stress of fractured rocks with different fracture angles around holes are lower. As the fracture angle increases, the gap in mechanical properties between the defective rock and the intact rock gradually decreased. In the force chain diagram, the compressive stress concentration range of the combined defect of cracks and holes starts to decrease, and the model is gradually destroyed as the tensile stress range gradually increases. When the peak stress is reached, the acoustic emission energy is highest and the rock undergoes brittle damage. Through a comparative study using laboratory tests, the results of laboratory real rocks and numerical simulation experiments were verified and the macroscopic failure characteristics of the real and simulated rocks were determined to be similar. This study can help us correctly understand the mechanical properties of rocks with defects and provide theoretical guidance for practical rock engineering.