• Title/Summary/Keyword: Crack Depth Detectable by NDI

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Peculiar Fatigue Fracture Behavior of Ultrasonic Nanocrystal Surface Modified SCM435 (초음파 나노 결정 표면개질된 SCM435의 특이한 피로 파괴 특성)

  • Kim, Jae-Hoon;Yun, Seo-Hyun;Nam, Ki-Woo
    • Journal of the Korean Society of Industry Convergence
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    • v.25 no.2_2
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    • pp.239-245
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    • 2022
  • Using the fatigue limit (∆σunsm) and residual stress (σr) of the UNSM smooth specimen, the harmless maximum crack depth (ahlm) according to the crack aspect ratio (As) was evaluated. In addition, the relationship between the minimum crack depth (aNDI1, aNDI2) detectable by non-destructive inspection(NDI), the crack depth (a25, a50) that reduces the fatigue limit by 25% and 50%, and ahlm were evaluated. The harmless crack condition was determined by the deepest crack point (point A). Since ahlm is larger than a25 and a50, a25 and a50 can secure the safety and reliability of steel via UNSM. Because aNDI1 and aNDI2 are larger than a25 and a50, cracks in a25 and a50 cannot be detected by non-destructive testing. Therefore it is necessary to apply more precise NDI.

Improving Reliability of SKD61 Using Nondamaging Technology (표면균열무해화 기법에 의한 SKD61의 신뢰성 향상)

  • Lee, Gum-Hwa;Gu, Kyoung-Hee;Kim, Jae-Hoon;Yun, Seo-Hyun;Kwon, Yung-Kug;Nam, Ki-Woo
    • Journal of the Korean Society of Industry Convergence
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    • v.25 no.5
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    • pp.791-797
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    • 2022
  • Using the fatigue limit (△σunsm) and residual stress (σr) of the UNSM smooth specimen, the harmless maximum crack depth (αhml) according to the crack aspect ratio (As) was evaluated. αhml evaluated the reliability in the relationship between the minimum crack depth (αNDI1, αNDI2) detectable by nondestructive inspection(NDI) and the crack depth (α25, α50) that reduces the fatigue limit by 25% and 50%. All αhml was determined by the crack depth. The αhml of 80N UNSM with high σr and high △σunsm was found to be large. σr in the depth direction had a much effect on αhml. Since αhml50(As=0.6-0.1) and αhml25, α25 and α50(some range) can secure the safety and reliability. Since αhml50 (As=1. 0-0. 6), it cannot be made harmless by UNSM. So safety and reliability cannot be secured. αNDI1 and αNDI2 are larger than α25, α25 cannot be detected by NDI. αNDI1 and αNDI2 are smaller than α50, α25 can detected by NDI.

Reliability Improvement of Offshore Structural Steel F690 Using Surface Crack Nondamaging Technology

  • Lee, Weon-Gu;Gu, Kyoung-Hee;Kim, Cheol-Su;Nam, Ki-Woo
    • Journal of Ocean Engineering and Technology
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    • v.35 no.5
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    • pp.327-335
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    • 2021
  • Microcracks can rapidly grow and develop in high-strength steels used in offshore structures. It is important to render these microcracks harmless to ensure the safety and reliability of offshore structures. Here, the dependence of the aspect ratio (As) of the maximum depth of harmless crack (ahlm) was evaluated under three different conditions considering the threshold stress intensity factor (Δkth) and residual stress of offshore structural steel F690. The threshold stress intensity factor and fatigue limit of fatigue crack propagation, dependent on crack dimensions, were evaluated using Ando's equation, which considers the plastic behavior of fatigue and the stress ratio. ahlm by peening was analyzed using the relationship between Δkth obtained by Ando's equation and Δkth obtained by the sum of applied stress and residual stress. The plate specimen had a width 2W = 12 mm and thickness t = 20 mm, and four value of As were considered: 1.0, 0.6, 0.3, and 0.1. The ahlm was larger as the compressive residual stress distribution increased. Additionally, an increase in the values of As and Δkth(l) led to a larger ahlm. With a safety factor (N) of 2.0, the long-term safety and reliability of structures constructed using F690 can be secured with needle peening. It is necessary to apply a more sensitive non-destructive inspection technique as a non-destructive inspection method for crack detection could not be used to observe fatigue cracks that reduced the fatigue limit of smooth specimens by 50% in the three types of residual stresses considered. The usefulness of non-destructive inspection and non-damaging techniques was reviewed based on the relationship between ahlm, aNDI (minimum crack depth detectable in non-destructive inspection), acr N (crack depth that reduces the fatigue limit to 1/N), and As.

Evaluation of Harmless Crack Size of SCM822H Steel by Double Shot Peening (이중 쇼트 피닝에 의한 SCM822H 강의 무해화 균열 크기 평가)

  • Jin-Woo Choi;Seo-Hyun Yun;Yung-Kug Kwon;Gum-Hwa Lee;Ki-Woo, Nam
    • Journal of the Korean Society of Industry Convergence
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    • v.26 no.6_2
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    • pp.1011-1017
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    • 2023
  • In this study, the harmless crack size (ahml) by double shot peening (DSP) using shot balls with different diameters was evaluated on carburized, quenched-tempered SCM822H steel. The minimum crack size (aNDI) detectable by non-destructive inspection was also evaluated. The relationship between the crack size (a25,50) that reduces the fatigue limit by 25% and 50% and ahml was evaluated. The residual stress of DSP was greater in SP(0.6+0.08) than SP(0.8+0.08) and appeared deeper in the depth direction. In addition, the hardness below the surface appeared larger. The fatigue limit of DSP increased 2.07 times and 1.95 times compared to non-SP. All ahml of the DSP specimen was determined at the depth (a). The compressive residual stress distribution affects ahml, and the ahml of SP(0.6+0.08), which has a large compressive residual stress and a high fatigue limit, appeared large. ahml of SP(0.6+0.08) introduced deeper than the residual stress of SP(0.8+0.08) is larger in the range of As=1.0-0.3. Since the residual stress in the thickness direction has a greater effect on ahml than the residual stress at the surface, it is necessary to introduce it more deeply. The relation of ahml, a25,50, and aNDI were evaluated in the point for safety and reliability.