• Transactions of the Korean Society of Mechanical Engineers A
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• v.34 no.9
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• pp.1137-1143
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• 2010

This paper presents a fatigue design method for plug- and ring-type gas-welded joints, which takes into account the effects of welding residual stress. To develop this method, we simulated the gas-welding process by performing nonlinear finite element analysis (FEA) To validate the FEA results, numerically calculated residual stresses in the gas welds were then compared with experimental results obtained by the hole-drilling method. To evaluate the fatigue strength of plug- and ring-type gas-welded joints influenced by welding residual stresses, the use of stress amplitude $(\sigma_a)_R$, which includes the welding residual stress in gas welds, is proposed $(\sigma_a)_R$ on the basis of a modified Goodman equation that includes the residual stress effects. Using the stress amplitude $(\sigma_a)_R$ at the hot spot point of gas weld, the relations obtained as the fatigue test results for plug and ring type gas welded joints having various dimensions and shapes were systematically rearranged to obtain the $(\sigma_a)_R-N_f$ relationship. It was found that more systematic and accurate evaluation of the fatigue strength of plug- and ring-type gas-welded joints can be achieved by using $(\sigma_a)_R$.

• Korean Journal of Optics and Photonics
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• v.31 no.5
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• pp.205-212
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• 2020

In the performance of a wind turbine system, the blades play a vital role. However, they are susceptible to damage arising from complex and irregular loading (which may even cause catastrophic collapse), and they are expensive to maintain. Therefore, it is very important both to find defects after blade manufacturing is completed and to find damage after the blade is used for a certain period of time. This study provides a new perspective for the detection of internal defects in glass-fiber- and carbon-fiber-reinforced panels, which are used as the main materials in wind turbine blades. A gap or fracture between fiber-reinforced materials, which may occur during blade manufacturing or operation, is simulated by drilling a hole 5 mm in diameter in the middle layer of the laminated material. Then, a digital-image-correlation (DIC) method is used to detect internal defects in the blade. Tensile load is applied to the fabricated specimen using a tensile tester, and the generated changes are recorded and analyzed with the DIC system. In the glass-fiber-reinforced laminated specimen, internal defects were detected from a strain value of 5% until the end of the experiment, while in the case of the carbon-fiber-reinforced laminated specimen, internal defects were detected from 1% onward. It was proved using the DIC system that the defect was detected as a certain level of strain difference developed around the internal defects, according to the material properties.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.62-62
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• 2009

석탄화력발전소의 CO2배출량 감소와 고효율, 대용량화로 인해 초초임계압(USC:Ultra Super Critical) 화력발전소의 건설이 증가하고 있다. USC 발전소는 효율향상을 위한 증기온도와 압력의 상승 때문에 보일러 고온고압부에 기존의 소재에 비해 고온강도와 내산화성의 재료물성이 향상된 신소재 적용이 불가피하다. 특히 사용된 신소재 중에서 보일러 본체를 구성하는 수냉벽관(Water wall), 과열기와, 재열기용 튜브 및 후육부인 헤더와 배관재로 기존의 2.25Cr-1Mo강을 개량한 2.25Cr-1.6W계 내열강이 적용되고 있다. 2.25Cr-1.6W강은 SMI와 MHI가 공동개발한 소재로 1995년 튜브제품이, 1999년에 단조, 파이프재, 플레이트제품이 ASME code case로 등재되었고, 2009년 ASME code case 2199-4로 개정되어 사용 중이다. 이 소재는 2.25Cr-1Mo강에 고온강도 개선을 위해 석출강화효과가 있는 V과 Nb을 첨가하였고, 탄화물의 열적안정성과 고용강화효과 증대를 위해 W을 첨가하였다. 그리고 제작성과 용접성 및 재료의 인성 향상을 위해 B첨가와 C함량을 낮추었다. 합금성분의 첨가와 조정에 의해 고온강도는 개선되었지만, 보일러 설치 및 보수를 위한 용접과정에서 용접금속과 CGHAZ(Coarse Grain HAZ)에서 용접균열이 발생하였다. 대부분의 용접균열은 용접결함이나 고온 혹은 저온균열이 아닌 2.25Cr-1.6W계강의 강도 개선을 위해 첨가한 V과 Nb이 용접후열처리 도중 입내에 MX형태의 미세석출로 입내를 강화시킴으로서 발생한 재열균열 민감성 증대에 기인된 것으로 판단된다. 이에 본 연구에서 용접 및 후열처리 과정에서 용접금속과 HAZ에서 발생하는 용접금속의 응력분포를 전산해석을 통해 확인하고 실제 후육파이프 용접부에서 잔류응력을 측정해 비교하였다. 용접부 응력분포는 SYSWELD 프로그램을 사용해 해석을 수행하였고, 발전소 실배관재의 용접부 응력측정은 수평부 측정이 용이하도록 지그를 부착한 Potable 잔류응력측정기를 사용해 Hole Drilling Method(HDM)를 적용하여 잔류응력을 측정하였다. 해석 결과 CGHAZ부위의 잔류응력이 용접금속과 기타 부위에 비해 높은 응력분포를 나타냈으며, 이는 CGHAZ와 용접용융선 부근에서 균열이 발생하는 실제값과 일치하는 결과를 보였다. 실제 배관재 용접부에서 측정한 잔류응력값은 항복응력의 약 50% 이하 응력값을 나타냈다. 배관 구조에 기인한 시스템응력의 영향을 제거하기 위해 배관재 용접부를 중심으로 양끝단을 절단 후 용접부에서 측정한 응력은 항복응력 대비 25%수준의 낮은값을 보였다. 그러나 배관재가 장기간 고온환경에 노출되었고 용접금속 내부의 균열이 발생한 상태에서 측정하였기 때문에 용접잔류응력은 상당부분 해소되어 상대적으로 낮은 응력값이 얻어진 것으로 판단된다.

• Journal of the Korean Institute of Landscape Architecture
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• v.48 no.5
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• pp.59-66
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• 2020

Recently, cultivation and management technologies have been needed to adapt due to climate change, which is causing abnormal weather conditions. One technique is to increase the utilization of evergreen broad-leaved species with high ornamental value. A total of five treatments were installed (1m×22.5m), including 60g/㎡ and 80g/㎡ using two types mulching material with an overlapping and hole-drilling mulching method and these were compared to un-mulching treatment a total of planted 92㎡ attheWol-aTestSiteForestattheForestforBiomaterialsResearchCenterinJinju-si, Gyeongsangnam-dofor 10monthsusing3-years-oldQuercusglaucaThunb. In comparison with the control site, the 60g/㎡ overlapping method was about 1.9 times higher than the root collar diameter, but there was no statistical significance between the treatments. Healthy seedlings were found to meet these conditions due to high biomass values and below and T/R ratios of 3.0 or lower and H/D ratios of 7.0 or lower. Comparing the values of LWR, SWR, and RWR, which can be evaluated for seedling due to the mulching treatments, as compared to the control, the growth of the ground areas including leaves and stems was enhanced, but the growth of the underground areas containing roots tended to have high control values. Based on this, the SQI value, which can be evaluated for the comprehensive quality of seedlings, was found to be significantly different between the control site and the mulching treatment sites, confirming that the growth and growth improvement effects were achieved with mulching treatments. The chlorophyll content analysis showed that there was a significant difference from the control site, and it was judged that weed generation in the control acted as an environmental stress, causing a decrease in chlorophyll content. It was found that the overlapping 80g/㎡ of polypropylene mulching material generated about 4 times fewer weeds than the control, and the manpower required for the mulching test field and weeding were equal at 3.3 people/100㎡/1 day. Mulching treatments have demonstrated a significant difference in the promotion of growth and quality of the seedlings and are judged as an alternative that can reduce the economic burden incurred by the purchase of the supplies and the manpower required to weed forestry plantations.