• Title/Summary/Keyword: residual surface stresses

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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.

Study on PWHT embrittlement of weld HAZ in Cr-Mo steel (Cr-Mo 鋼 溶接熱影響部의 溶接後熱處理 脆化에 관한 硏究)

  • 임재규;정세희
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.11 no.2
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    • pp.314-321
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    • 1987
  • Post weld heat treatment (PWHT) of weldment of the low alloy Cr-Mo steel, in general, is carried out not only to remove residual stress and hydrogen existing in weldment but to improve fracture toughness of weld heat affected zone (HAZ). There occur some problems such as toughness decrement and stress relief cracking (SRC) in the coarse grained region of weld HAZ when PWHT is practiced. Especially, embrittlement of structure directly relates to the mode of fracture and is appeared as the difference of fracture surface such as grain boundary failure. Therefore, in this paper, the effect of heating rate on PWHT embrittlement under the various kinds of stresses simulated residual stress in weld HAZ was evaluated by COD fracture toughness test and observation of fracture surface. Fracture toughness of weld HAZ decreased with increment of heating rate under no stress, but it was improved to increment of heating rate under the stress. Grain boundary failure didn't almost appear at the heating rate of 600.deg.C/hr but it appeared from being the applied stress of 294 MPa at 220.deg.C/hr and 196 MPa at 60.deg.C/hr.

Applicability of Hammer-Peening Treatment for Fatigue Life Improvement of Fatigue Damaged Weld Joints (피로손상된 용접이음의 피로수명 향상을 위한 햄머피닝 처리법의 적용)

  • Kim, In Tae;Park, Min Ho;Cheung, Jin Hwan
    • Journal of the Korea institute for structural maintenance and inspection
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    • v.17 no.3
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    • pp.48-55
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    • 2013
  • In this study, fatigue tests were performed on longitudinal out-of-plane gusset fillet welded joints and transverse non-load-carrying cruciform rib fillet welded joints, and then applicability of hammer-peening treatment on improvement of fatigue life for fatigue damaged weld joints were investigated. Fatigue tests were carried out on three types of gusset and rib welded specimens: as-welded specimens, post-weld hammer peened specimens and hammer peened specimens at 50% of as-welded specimen's fatigue life. Before and after hammer peening treatment, the geometry of weld toes and surface stresses near weld toes were measured. As a result of hammer peening treatment, compressive residual stresses of 30-83MPa were introduced near weld toes of the gusset and rib welded joints, and 130% increase in fatigue life and fatigue limit of the welded joints could be realized by hammer peening treatment at 50% fatigue life of as-welded conditions.

Failure Analysis and Countermeasures of SCM435 High-Tension Bolt of Three-Step Injection Mold

  • Yun, Seo-Hyun;Nam, Ki-Woo
    • Journal of the Korean Society of Industry Convergence
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    • v.23 no.4_1
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    • pp.531-539
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    • 2020
  • When injection mold is repeatedly used for mass production, fatigue phenomenon due to cyclic stress may occur. The surface and interior of structure might be damaged due to cyclic stress or strain. The objective of this study was to analyze failure of SCM435 high-tension bolts connecting upper and lower parts of a three-stage injection molding machine. These bolts have to undergo an accurate heat treatment to prevent the formation of chromium carbide and the action of dynamic stresses. Bolts were fractured by cyclic bending stress in the observation of ratchet marks and beach marks. Damaged specimen showed an acicular microstructure. Impurity was observed. Chromium carbide was observed near the crack origin. Both shape parameters of the Vickers hardness were similar. However, the scale parameter of the damaged specimen was about 20% smaller than that of the as-received specimen. Much degradation occurred in the damaged specimen. Bolts should undergo an accurate heat treatment to prevent the formation of chromium carbide. They must prevent the action of dynamic stresses. Bolts need accurate tightening and accuracy of heat treatment and screws need compression residual stress due to peening.

Analysis of an Actual Slope Failure in the Residual Soil by Suction Stress Based Effective Stress (흡수응력에 기반한 유효응력에 의한 실제 잔류토 사면 붕괴의 해석)

  • Oh, Seboong;Lu, Ning;Park, Young Mog;Lee, Junsuk
    • KSCE Journal of Civil and Environmental Engineering Research
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    • v.32 no.3C
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    • pp.113-120
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    • 2012
  • An actual slope failure was analyzed in residual soils at Jinju. Due to rainfall infiltration, the safety factor decreases in the unsaturated layers, since the effective stress and shear strength decrease. In this study, the effective stress is based on suction stress using soil water retention curve. Unsaturated properties were evaluated on soil water retention curve, hydraulic conductivity and shear strength with samples from the site. After infiltration analysis of unsaturated flow under the actual rainfall, the distribution of pore water pressure could be calculated in the slope layers. In the stress field of finite elements, an elastic analysis calculated total stress distribution in the layers and also shear stresses on the slip surface using elastic model. On the slip surface, suction stress and effective stress evaluated the shear strength. As a result, the factor of safety was calculated due to rainfall, which could simulate the actual slope failure. In particular, it was found that the suction stress increases and both the effective stress and the shear strength decrease simultaneously on the slip surface.

A Study on Enhanced of Anti-scratch performance of Nanostructured Polymer Surface (고분자 나노 표면의 내스크래치 특성 향상 연구)

  • Yeo, N.E.;Cho, W.K.;Kim, D.I.;Jeong, M.Y.
    • Journal of the Microelectronics and Packaging Society
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    • v.24 no.3
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    • pp.41-46
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    • 2017
  • In this study, rapid cooling method was proposed to improve the anti-scratch performance of anti-reflection film fabricated by nanoimprint lithography. Effects of cooling time on the mechanical properties and optical properties were evaluated. Pencil hardness measurements showed that anti-scratch performance enhanced as the cooling time increased while characterization on the optical property showed that reflectance on scratch increased as the cooling time increased. Therefore, it was concluded that the anti-scratch performance and optical properties are highly influenced by the cooling time. The observed results explained in terms of residual stress and free volume in polymeric materials.

Study on Electro-Mechanical Characteristics of Array Type Capacitive Pressure Sensors with Stainless Steel Diaphragm and Substrate (스테인리스 강 박막 및 기판을 이용한 배열형 정전용량 압력센서의 전기 기계적 특성연구)

  • Lee, Heung-Shik;Chang, Sung-Pil;Cho, Chong-Du
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.30 no.11 s.254
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    • pp.1369-1375
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    • 2006
  • In this work, mechanical characteristics of stainless steel diaphragm have been studied as a potential robust substrate and a diaphragm material for micromachined devices. Lamination process techniques combined with traditional micromachining processes have been adopted as suitable fabrication technologies. To illustrate these principles, capacitive pressure sensors based on a stainless steel diaphragm have been designed, fabricated and characterized. The fabrication process for stainless steel micromachined devices keeps the membrane and substrate being at the environment of 8.65MPa pressure and $175^{\circ}C$ for a half hour and then subsequently cooled to $25^{\circ}C$. Each sensor uses a stainless steel substrate, a laminated stainless steel film as a suspended movable plate and a fixed, surface micromachined back electrode of electroplated nickel. The finite element method is adopted to investigate residual stresses formed in the process. Besides, out-of-plane deflections are calculated under pressures on the diaphragm. The sensitivity of the device fabricated using these technologies is 9.03 ppm $kPa^{-1}$ with a net capacitance change of 0.14 pF over a range 0$\sim$180 kPa.

Cavity and Interface effect of PI-Film on Charge Accumulation and PD Activity under Bipolar Pulse Voltage

  • Akram, Shakeel;Wu, Guangning;Gao, GuoQiang;Liu, Yang
    • Journal of Electrical Engineering and Technology
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    • v.10 no.5
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    • pp.2089-2098
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    • 2015
  • With the continuous development in insulation of electrical equipment design, the reliability of the system has been enhanced. However, in the manufacturing process and during operation under continues stresses introduce local defects, such as voids between interfaces that can responsible to occurrence of partial discharge (PD), electric field distortion and accumulation of charges. These defects may lead to localize corrosion and material degradation of insulation system, and a serious threat to the equipment. A model of three layers of PI film with air gap is presented to understand the influence of interface and voids on exploitation conditions such as strong electrical field, PD activity and charge movement. The analytical analysis, and experimental results are good agreement and show that the lose contact between interfaces accumulate more residual charges and in consequences increase the electric field intensity and accelerates internal discharges. These residual charges are trapped charges, injected by the electrodes has often same polarity, so the electric field in cavities increases significantly and thus partial discharge inception voltage (PDIV) decreases. Contrary, number of PD discharge quantity increases due to interface. Interfacial polarization effect has opposite impact on electric field and PDIV as compare to void.

A Optimal 3D FE Model for Evaluation of Peening Residual Stress Under Angled Multi-impacts (다중경사충돌시 피닝잔류응력 평가를 위한 최적의 3차원 유한요소모델)

  • Hyun, Hong-Chul;Kim, Tae-Hyung;Lee, Hyung-Yil
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.36 no.2
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    • pp.125-135
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    • 2012
  • The FE model for shot peening often assume that shots impact vertically on the engineering parts to generate compressive residual stresses. However, the shots obliquely impact on the surface in actual peening. In this work, we propose a 3D finite element (FE) model for evaluation of residual stress under angled shot peening. Using the FE model for angled multi-impact, we examine the effects of factors such as impact angle, impact pattern and the number of shots. Plastic deformation of shot is also considered. To validate the model, we then compare the FE solution with experimental result by X-ray diffraction (XRD). The proposed model will be a base of 3D multi-impact FE model with diverse impact angles.

Ferroelectric Domans in $BaTiO_3$ ($BaTiO_3$의 강유전성 분역)

  • 박봉모;정수진
    • Journal of the Korean Ceramic Society
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    • v.33 no.1
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    • pp.56-64
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    • 1996
  • A large amount of papers about the cubic-to-tetragonal phase transition the ferroelectric domain structures of the BaTiO3 were already reported but there exist still some needs to observe the domain behaviors directly. In this study the domain structures of the tinned plates prepared from ta single crystal grown by the TSSG technique were observed using a polarizing microscope TE and X-ray topography. The spatial relation be-tween the orientation states of domains was investigated and the effects of external stresses and electric fields on the behaviors of ferroelectric and ferroelastic domains were studied. All the 90$^{\circ}$walls cut off in the crystal are the wedge shaped lamellar domains and all the straight boundaries in the observed domain patte군 can be interpreted as the head-to-tail 90$^{\circ}$walls. The irregular overlapped boundaries commonly observed by using a polarizing microscope and X-ray topography are complex combinations of well-known 90$^{\circ}$walls and are domain walls were predominant and were stabilized after surface polishing. In the paraelectric phase region the domain walls vanished but the residual surface strain patterns could be seen at the same positions of the stabilized 90$^{\circ}$a-a walls in the tetragonal phase region, These stabilized walls resulted from the surface strain had a memory effect in domain formation during the repeated phase transitions and could notr be affected by an external electtric field.

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