• Title/Summary/Keyword: Crack Reduction

Search Result 427, Processing Time 0.029 seconds

Quantitative Study on Threshold Condition of Critical Non-propagating Crack (임계정류피로크랙의 하한계 전파조건의 정량적 고찰)

  • Kim, Min-Gun
    • Journal of Industrial Technology
    • /
    • v.30 no.B
    • /
    • pp.17-23
    • /
    • 2010
  • Since the propagation of a short fatigue crack is directly related to the large crack which causes the fracture of bulk specimen, the detailed study on the propagation of the short crack is essential to prevent the fatigue fracture. However, a number of recent studies have demonstrated that the short crack can grow at a low applied stress level which are predicted from the threshold condition of large crack. In present study, the threshold condition for the propagation of short fatigue crack is examined with respect to the micro-structure and cyclic loading history. Specimens employed in this study were decarburized eutectoid steels which have various decarburized ferrite volume fraction. Rotating bending fatigue test was carried out on these specimens with the special emphasis on the "critical non-propagating crack length" It is found that the reduction of the endurance limit of their particular micro-structures can be due to the increase of the length of critical non-propagating crack, and the quantitative relationship between the threshold stress ${\sigma}_{wo}$ and the critical non-propagating crack length $L_c$ can be written as ${\sigma}_{wo}{^m}{\cdot}L_c=C$ where m,C is constant. Further experiments were carried out on cyclic loading history on the length of critical non-propagating crack. It shown that the length of critical non-propagating crack is closely related to cyclic loading history.

  • PDF

Modeling of chloride diffusion in concrete considering wedge-shaped single crack and steady-state condition

  • Yang, Keun-Hyeok;Cheon, Ju Hyun;Kwon, Seung-Jun
    • Computers and Concrete
    • /
    • v.19 no.2
    • /
    • pp.211-216
    • /
    • 2017
  • Crack on concrete surface allows more rapid penetration of chlorides. Crack width and depth are dominant parameters for chloride behavior, however their effects on chloride penetration are difficult to quantify. In the present work, the previous anisotropic (1-D) model on chloride diffusion in concrete with single crack is improved considering crack shape and roughness. In the previous model, parallel-piped shape was adopted for crack shape in steady-state condition. The previous model with single crack is improved considering wedge shape of crack profile and roughness. For verifying the proposed model, concrete samples for nuclear power plant are prepared and various crack widths are induced 0.0 to 1.2 mm. The chloride diffusion coefficients in steady-state condition are evaluated and compared with simulation results. The proposed model which can handle crack shape and roughness factor is evaluated to decrease chloride diffusion and can provide more reasonable results due to reduced area of crack profile. The roughness effect on diffusion is evaluated to be 10-20% of reduction in chloride diffusion.

Modeling of stress corrosion crack growth and lifetime of pipe grade high density polyethylene by using crack layer theory (Crack Layer 이론을 이용한 배관용 고밀도 폴리에틸렌의 응력부식균열 진전 및 수명 예측 모델)

  • Wee, Jung-Wook;Choi, Byoung-Ho
    • Transactions of the Korean Society of Pressure Vessels and Piping
    • /
    • v.11 no.2
    • /
    • pp.45-50
    • /
    • 2015
  • In many cases, the field fracture mechanism of the thermoplastic pipe is considered as either brittle or environmental fractures. Thus the estimation of the lifetime by modeling slow crack growth considering such fracture mechanisms is required. In comparison of the some conventional and empirical equations to explain the slow crack growth rate such as the Paris' law, the crack layer theory can be used to simulate the crack and process zone growth behaviors precisely, so the lifetime of thermoplastic pipe can also be accurately estimated. In this study, the modified crack layer theory for the stress corrosion cracking (SCC) of high density polyethylene is introduced with detailed algorithm. The oxidation induction time of the HDPE is also considered for the reduction of specific fracture energy during exposed to chemical environments. Furthermore, the parametric study for an important SCC parameter is conducted to understand the slow crack growth behavior of SCC.

Polygonal finite element modeling of crack propagation via automatic adaptive mesh refinement

  • Shahrezaei, M.;Moslemi, H.
    • Structural Engineering and Mechanics
    • /
    • v.75 no.6
    • /
    • pp.685-699
    • /
    • 2020
  • Polygonal finite element provides a great flexibility in mesh generation of crack propagation problems where the topology of the domain changes significantly. However, the control of the discretization error in such problems is a main concern. In this paper, a polygonal-FEM is presented in modeling of crack propagation problems via an automatic adaptive mesh refinement procedure. The adaptive mesh refinement is accomplished based on the Zienkiewicz-Zhu error estimator in conjunction with a weighted SPR technique. Adaptive mesh refinement is employed in some steps for reduction of the discretization error and not for tracking the crack. In the steps that no adaptive mesh refinement is required, local modifications are applied on the mesh to prevent poor polygonal element shapes. Finally, several numerical examples are analyzed to demonstrate the efficiency, accuracy and robustness of the proposed computational algorithm in crack propagation problems.

3-D fracture analysis of cracked aluminum plates repaired with single and double composite patches using XFEM

  • Jamal-Omidi, Majid;Falah, Mehdi;Taherifar, Davood
    • Structural Engineering and Mechanics
    • /
    • v.50 no.4
    • /
    • pp.525-539
    • /
    • 2014
  • Bonded composite-patch repair has been widely used to restore or extend the service life of damaged structures due to its effectiveness as a mechanical repair technique. In this paper using extended finite element method (XFEM), three-dimensional crack models are developed to examine the fracture behavior of centrally cracked aluminum plates repaired with single and double sided composite patches. Stress intensity factor (SIF) at the crack tip is used as the fracture criterion. In this regard, the effects of the crack lengths, patch materials, orientation of plies, adhesive and patch thickness are examined to estimate the SIF of the repaired plate and the repair performance. The obtained results show that composite patches have significant effect on reduction of the SIF at the crack tip. It is also proved that using double symmetric repair, in comparison to single one, reduces considerably SIF at the crack tip. Hence, the residual strength can be improved significantly as well as fatigue life of the structure. Investigation of ply orientation effects shows SIF increase as the ply orientation is changed from $0^{\circ}$ (perpendicular to the advancing crack) to $90^{\circ}$ (parallel to the crack line). However, the effectiveness of the ply orientation depends on the loading direction and the crack direction.

Leak-Before-Break Behavior and Crack Opening Displacement in Piping Under Bending Load (굽힘하중을 받는 배관의 파단전누설거동 및 균열개구변위)

  • Nam, Ki-Woo
    • Transactions of the Korean Society of Mechanical Engineers A
    • /
    • v.34 no.6
    • /
    • pp.725-730
    • /
    • 2010
  • The leak-before-break behavior and the crack opening displacement were investigated of statically indeterminate piping system and statically determinate piping system after a crack penetration. The reduction in the ultimate strength caused by a crack was relatively small in the statically indeterminate piping system. The leak-before-break in the statically indeterminate piping system had a larger safety margin than that in the statically determinate piping system. The crack opening displacement after crack penetration in a pipe with a nonpenetrating crack was evaluated by using a plastic rotation angle.

A Study on Growth Characteristics of the Surface Fatigue Crack Propagated from a Small Surface Defect in Carbon Steels (탄소강재(炭素鋼材)의 작은 표면결함(表面缺陷)에서 성장(成長)하는 표면피로(表面疲勞)균열의 성장특성(成長特性)에 관한 연구(硏究))

  • Chang-Min,Suh;Yong-Goo,Kang
    • Bulletin of the Society of Naval Architects of Korea
    • /
    • v.21 no.1
    • /
    • pp.35-42
    • /
    • 1984
  • In the present study, rotating bending fatigue tests have been carried out in three kinds of carbon steel specimens; an annealed low carbon steel, an annealed high carbon steel and quenched-tempered high carbon steel; with a small artificial surface defect that might exist in real structures. Fatigue crack lengths have been observed by a method of replication in order to investigate the growth characteristic of fatigue crack in the viewpoints of strength of materials and fracture mechanics. The main results obtained are as follows: 1) The effect of a small surface defect upon the reduction of fatigue limit is considerably large, and the rate of fatigue limit reduction grows in the following order; annealed low carbon steel(mild steel), annealed high carbon steel, quenched-tempered high carbon steel. 2) When the growth rate of surface crack length(2a) was investigated in the viewpoints of fracture mechanics based upon $ ${\Delta}K_{\varepsilon}$, the dependence of stress level and of surface defect size disappear, and there exists a linear relationships between d(2a)/dN and ${\Delta}K_{{\varepsilon}t},\;\Delta_{{\varepsilon}t}\sqrt{{\pi}a}$, on log. plot, i.e, $d(2a)/dN={C{\cdot}{\Delta}K_{\varepsilon}}^3_t$, where ${\Delta}_{{\varepsilon}t}\sqrt{{\pi}a}$ a is the cyclic total strain intensity factor range.

  • PDF

An application of damage detection technique to the railway tunnel lining (철도터널 라이닝에 대한 손상도 파악기법의 현장적용)

  • Bang Choon-seok;Lee Jun S.;Choi Il-Yoon;Lee Hee-Up;Kim Yun Tae
    • Proceedings of the KSR Conference
    • /
    • 2004.06a
    • /
    • pp.1142-1147
    • /
    • 2004
  • In this study, two damage detection techniques are applied to the railway tunnel liner based on the static deformation data. Models based on uniform reduction of stiffness and smeared crack concept are both employed, and the efficiency and relative advantage are compared with each other. Numerical analyses are performed on the idealized tunnel structure and the effect of white noise, common in most measurement data, is also investigated to better understand the suitability of the proposed models. As a result, model 1 based on uniform stiffness reduction method is shown to be relatively insensitive to the noise, while model 2 with the smeared crack concept is proven to be easily applied to the field situation since the effect of stiffness reduction is rather small. Finally, real deformation data of a rail tunnel in which health monitoring system is in operation are introduced to find the possible damage and it is shown that the prediction shows quite satisfactory result.

  • PDF

Effect of process parameters on propagation of edge crack in the cold rolling (냉간 박판압연공정에서 공정변수가 엣지 크랙 성장에 미치는 영향)

  • Cui, Xiang Zi;Lee, S.H.;Lee, S.J.;Lee, J.B.;KIm, B.M.
    • Proceedings of the Korean Society for Technology of Plasticity Conference
    • /
    • 2009.05a
    • /
    • pp.405-408
    • /
    • 2009
  • Edge cracks in cold rolling always influence to the quality of productions, while the "V" shaped cracks were propagated by passing the roll gap. We set up the sizes and shapes of initial cracks in simulation according to the references from real productions. Different to in hot rolling, the cracks in cold rolling couldn't be reduced from propagation automatically after generated, even if these could be reduced by changing the process parameters. In this paper, we described the affections of process parameters on the propagation of edge cracks, such as reduction ratio and tension. We predicted that the dependence of the cracks propagations of changing of process conditions and expected to gain the smaller edge cracks. By raising the reduction ratio, the cracks were propagated increasingly in both transverse and rolling directions. And as tension raise, the cracks became propagated in both directions in which transverse direction was less effectively.

  • PDF

Numerical analysis of the Influence of the presence of disbond region in adhesive layer on the stress intensity factors (SIF) and crack opening displacement (COD) in plates repaired with a composite patch

  • Benchiha, Aicha;Madani, Kouider;Touzain, Sebastien;Feaugas, Xavier;Ratwani, Mohan
    • Steel and Composite Structures
    • /
    • v.20 no.4
    • /
    • pp.951-962
    • /
    • 2016
  • The determination of the stress intensity factor at the crack tip is one of the most widely used methods to predict the fatigue life of aircraft structures. This prediction is more complicated for repaired cracks with bonded composite patch. This study is used to compute the stress intensity factor (SIF) and crack opening displacement (COD) for cracks repaired with single and double-sided composite patches. The effect of the presence of disbond region in adhesive at the crack was taken into consideration. The results show that there is a considerable reduction in the asymptotic value of the stress-intensity factors and the crack opening displacement at the crack tip. The use of a double-sided patch suppresses the bending effect due to the eccentricity of the patch on one side only.