PURPOSES : The purpose of this study was to investigate the performance of additives that affect internal curing in order to reduce the damage occurring in concrete pavements. METHODS : SAP was used as an additive to reduce internal curing in concrete pavements. SAP is an additive that has a very high absorption rate which prevents concrete wrappers from externally draining water. To evaluate the internal curing performance according to the ratio of SAP, we identified the number of cracks and amount of abrasion reduction. RESULTS : Plastic shrinkage and durability of a concrete mixture with added SAP were evaluated. The following results were obtained: (1) SAP showed a tendency to reduce slumps due to absorption of the concrete mixture. (2) It was possible to verify that concrete condensation did not occur during the penetration resistance test and that the initial curing did not lead to reactions within the mixture. (3) Adding more than 0.6% of SAP for dry curing resulted in greater compressive strength at all ages than OPC, with the highest compression strength of 0.9% after 56 days. (4) Regarding abrasion resistance, it was found that adding SAP was 30~50% better than adding the OPC mixture, and at 0.9% compression strength, abrasion resistance showed the best performance. (5) In the chlorine ion immersion resistance experiment, the passing charge of the OPC mixture was rated “high,” but it was rated “normal” in SAP. The results showed that the addition of SAP improved the water density of concrete due to internal curing effects, and that it showed the greatest chlorine ion penetration resistance for a compressive strength of 0.9%. (6) Regarding plastic shrinkage resistance, cracks did not occur on the surface until the end of the experiment, but the plastic shrinkage rate upon addition of SAP was relatively low compared to that of the OPC mixture. CONCLUSIONS : Recent studies have shown that internal curing techniques can be applied using SAP to prevent shrinkage due to the loss of water and to decrease the effects of hydration. If internal curing effects are expressed using SAP, it is thought that contraction due to a loss of moisture and reduction in sign language reaction can be prevented.

In the manufacturing of bulk graphite, pores produced by vaporization and discharge of volatile materials in binders during carbonization reduce the density of bulk graphite, which adversely affects the electrical conductivity, strength and mechanical properties. Therefore, an impregnation process is introduced to fill the pores and increase the density of bulk graphite. In this study, bulk graphite is prepared by varying the viscosity of the impregnant. The microstructure of bulk graphite is observed. The flexural strength and electrical resistivity are measured. As the viscosity of the impregnants decreases and the number of impregnations increases, it is shown that the number of pores decreases. The density before impregnation is 1.62 g/cm3. The density increases to 1.67 g/cm3 and porosity decreases by 18.6 % after three impregnations using 5.1 cP impregnant, resulting in the best pore-filling effect. After three times of impregnation with a viscosity of 5.1 cP, the flexural strength increases by 55.2 % and the electrical resistivity decreases by 86.76 %. This shows that a slight increase in density due to the pore-filling effect improves the properties of bulk graphite.

Concrete masonry prisms are strengthened with steel fiber-reinforced mortar (SFRM) overlay and tested for compressive and diagonal tension strength. Masonry prisms are produced in poor condition considering standard workmanship for masonry buildings in Korea. Amorphous steel fibers are adopted for SFRM, and appropriate mixing ratios of SFRM are derived considering constructability and strength. Masonry prisms are strengthened with different fiber volume ratios, while numerous strengthened faces and additional reinforcing meshes are produced for compression and diagonal tension tests. Compression and diagonal tension strength are increased by up to 122% and 856%, respectively, and the enhancement effect for diagonal tension strength was superior compared to compression strength. Finally, the test results and strength prediction equations based on existing literature and regression analysis are compared.

In this study, the fatigue properties of press die steel, such as SKD11, and three high-durability die steel for the cold forming of ultra-high-strength steel sheets are evaluated. Specimens for fatigue, tensile, and hardness tests are manufactured through the heat treatment recommended by steelmakers and ultra-high precision processing. The general mechanical properties and fatigue properties are derived from hardness, tensile, and fatigue tests for four die steel. The tensile and fatigue properties of die steel derived through the tests are compared and analyzed. In particular, the correlation between the fatigue limit and the general mechanical properties such as tensile strength and elongation is analyzed, which allows relational expressions to be obtained through regression analysis. Finally, the study confirms that applying high-durability die steel is necessary for improving the die life in the manufacturing of press dies for ultra-high-strength steel sheets.

The bead geometry according to the welding conditions was analyzed through the laser fillet welding experiment of 9% Ni steel, and the relationship between the shear strength and the five bead geometry measured by selecting the main bead geometry of the fillet weld was analyzed. Among the welding conditions, the welding conditions that directly affect the penetration depth are welding speed and laser power, and the working angle and beam position have a great influence on the formation of leg of vertical and horizontal members. The bead shape, which greatly affects the shear strength, is the horizontal member length, neck thickness, and weld length, and has a proportional relationship with the shear strength. As a result of confirming the relationship between shear strength and bead shape through the derivation of the trend line, it was confirmed that the length of the vertical member, whose R2 value was 0.92, was most closely related to the shear strength.

초록 close In this study, the structural analyses were carried out with the old and new wheel models installed at the automotive tires. The stress and deformation at the middle circumference of wheel were seen to be greatest at two models. Also, the stress and deformation were smallest at the edge of wheel. The maximum deformation of the old model B is about five times larger than that of the new model A. The maximum equivalent stress of the old model B can be seen to be twice as large as the new model A. Also, it can be seen that the new model A is more stable than the old model B in terms of strength. It can be seen that the deformation energy of the old model B is 19 times larger than that of the new model A. And it is thought that the new model A is much more durable than the old model B in terms of impact. By utilizing this study result, the stress and deformation are investigated without the strength test of wheel installed at the automotive tire, and the durability can be seen.

The mechanical properties and microstructures of hexagonal boron nitride (h-BN)-reinforced cement composites are experimentally studied for three and seven curing days. Various sizes (5, 10, and 18 μm) and concentrations (0.1%, 0.25%, 0.5%, and 1.0%) of h-BN are dispersed by the tip ultrasonication method in water and incorporated into the cement composite. The compressive strength of the h-BN reinforced cements increases by 40.9%, when 0.5 wt% of 18 μm-sized h-BN is added. However, the compressive strength decreases when the 1.0 wt% cement composite is added, owing to the aggregation of the h-BNs in the cement composite. The microstructural characterization of the h-BN-reinforced cement composite indicates that the h-BNs act as bridges connecting the cracks, resulting in improved mechanical properties for the reinforced cement composite.

There has been increasing interest in UHPC (Ultra-High Performance Concrete) materials in recent years. Owing to the superior mechanical properties and durability, the UHPC has been widely used for the design of various types of structures. In this paper, machine learning based compressive strength prediction methods of the UHPC are proposed. Various regression-based machine learning models were built to train dataset. For train and validation, 110 data samples collected from the literatures were used. Because the proportion between the compressive strength and its composition is a highly nonlinear, more advanced regression models are demanded to obtain better results. The complex relationship between mixture proportion and concrete compressive strength can be predicted by using the selected regression method.

전 세계적인 코로나19의 확산으로 인해, 진단시약 등과 같은 감염병 의료 및 의료기기 신제품이 비약적으로 개발·출시되고 있으며, 이의 빠른 수급을 위해 각 국가들은 수입규제를 완화하거나 신속한 인·허가를 위한 정책을 펼치고 있다(NIDS, 2020). 반면, 신종감염병과 관련 없는 신개발 의료기기의 경우 오히려 시험검사 등 지연 및 취소되는 사례가 발생하고 있는 등 여전히 엄격한 인·허가규제를 통해 시장에 진출하고 있다. 이에 본 연구는 의료기기신제품이 시장에 진출하면서 마주하게 되는 정부소관법률에 특화하여 규제강도 영향요인을 도출하고 규제강도를 분석하여 규제대응 프레임워크를 제안하였다. 연구방법은 문헌연구, Failure Mode and Effects Analysis(FMEA)기법 적용, 전문가인터뷰(1차):아이디어수집, 전문가인터뷰(2차) : 타당성검증의 방법으로 진행하였으며, FMEA기법의 적용프로세스를 통해 우선 규제단계별 영향요인의 발생영향도와 규제사무 유형별 부담영향도를 곱하여 규제요인의 중요도를 구하고, 규제영향 심각도를 곱하는 방식으로 규제강도 정량화방법을 제시하였다. 시사점은 최근 해외 주요 국가들 및 우리나라 정부가 코로나19에 따른 신개발의료기기의 신속한 인·허가를 위한 특별규제정책 및 완화정책을 펼치며 적극적으로 대응하고 있는 시점에 본 연구에서 제안된 프레임워크를 통해 향후 기존 의료기기 신제품의 인·허가 정책 규제프로세스에도 보다 적극적이며 선제적인 대응이 될 수 있도록 규제의 개선방향과 규제대응 방안이 이루어지길 기대한다.

The Z-type spring is a key part of the non-vibration air suspension. The non-vibration air suspension was developed long ago and widely used in developed countries. It has strong durability, provides a smooth ride, protects the vehicle body, and protects luggages from damage in truck. In this study, the structural strength of the Z-type spring was evaluated by computing the maximum displacement and the von Mises maximum stress results from applying the load condition based on the maximum weight of luggage in the rear space of a truck.

본 논문에서는 해양플랜트에 주로 사용되는 알루미늄 사다리의 독자 모델을 개발하기 위하여, 개량형 알루미늄 합금(6082-T6) 을 적용하고 국제 기준에 부합한 구조강도 설계를 하였다. 국제기준은 ISO, NORSOK, Austria Standard를 참고하였으며, 모든 조건이 만족할 수 있도록 하중 조합을 하였다. 설계된 모델은 유한요소법 [Finite elements method]을 근간으로 하는 해양플랜트 전용 해석프로그램인 SACS를 사용하여 구조 안전성을 검증하여 응력 및 처짐이 모두 허용기준 이내에 만족함을 확인하였다. 개발모델은 모든 허용기준을 만족하면서도 가볍고, 생산성이 향상되어 향후 많은 분야에서 사용이 될 것으로 기대해본다.

본 연구는 두경부의 자기공명 역동적(dynamic) 혈관조영술(DCE-MRA)에서 조영제(1.0 mol/ℓ)와 생리식염수의 희석 주입 비율에 따른 신호강도 차이를 비교 분석하였다. 연구 방법으로는 3.0 T MRI 장치에서 자동주입기를 이용하여 조영제와 생리식염수의 희석주입 비율(조영제:생리식염수)을 10:0 (1000 mmol/ℓ, 조영제 원액), 7:3 (700 mmol/ℓ), 5:5 (500 mmol/ℓ)로 3가지 군으로 하였다. 주입 조건으로는 오른팔 정맥으로 주입 속도 3.0 ㎖/sec으로 하였고 용량은 0.1 ㎖/㎏으로 하였다. 신호강도 측정은 동맥혈관 7 부위와 정맥혈관 2 부위로 하였고 역동적 영상에서의 측정 시기는 정맥이 침범하기 직전의 순수 동맥기, 최고신호 동맥기, 최고신호 정맥기로 구분 하였다. 연구 결과 최고신호 동맥기의 횡정맥동을 제외한 모든 측정 시기의 10:0 (1000 mmol/ℓ), 7:3 (700 mmol/ℓ), 5:5 (500 mmol/ℓ) 순서로 신호강도가 증가하였다(p<0.01). 따라서 두경부의 DCE-MRA에서 조영제와 생리식염수의 5:5 희석주입을 통하여 최고의 신호강도를 구현할 수 있고 최적의 영상을 얻을 수 있다.

최근 해상풍력발전기 시장은 에너지 수요 증가, 화석 연료 기반 발전에 대한 의존도 감소와 환경 규제로 인해 향후 5년 내에 빠른 성장이 예상된다. 이러한 상황에 따라서 전 세계적으로 풍력 발전을 가속화하고 있으며, 해상풍력으로 진입하려는 시도가 많아지고 있다. 노르웨이 해상 안전 관리처(PSA: Petroleum Safety Authority)는 운영하는 동안 충돌사고에 대한 충돌에너지가 35 MJ을 견딜 수 있는 안전설계 기준을 요구하고 있다. 따라서 본 연구에서는 북해 해상풍력발전기 설치 단지에 투입되는 해상풍력발전기 설치 선박(WTIV)의 레그 (Leg)와 선박충돌 사고에 대하여 발생 가능한 충돌시나리오에 대해서 비선형 소성붕괴 거동 결과를 바탕으로 레그의 충돌강도평가법을 분석하였다. 분석된 결과로 현재 설계된 기존 선박을 기준으로 요구치인 35 MJ을 만족을 위해서는 200 % 이상의 단면계수 증가가 필 요하고, 이는 현실적인 레그 설계에서는 불가능한 조건으로 판단됐다. 또한, 합리적인 충돌시나리오를 기반으로 한 충돌에너지 기준의 제정이 필요하다.