• Title/Summary/Keyword: Crack Tip

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Angle Beam Ultrasonic Testing Models and Their Application to Identification and Sizing of Surface Breaking Vertical Cracks

  • Song, Sung-Jin;Kim, Hak-Joon;Jung, Hee-Jun;Kim, Young-H.
    • Journal of the Korean Society for Nondestructive Testing
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    • v.22 no.6
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    • pp.627-636
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    • 2002
  • Identification and sizing of surface breaking vertical cracks using angle beam ultrasonic testing in practical situation quite often become very difficult tasks due to the presence of non-relevant signals caused by geometric reflectors. The present work introduces effective and systematic approaches to take care of such a difficulty by use oi angle beam ultrasonic testing models that can predict the expected signals from various targets very accurately. Specifically, the model-based TIFD (Technique for Identification of Flaw signals using Deconvolution) is Proposed for the identification of the crack tip signals from the non-relevant geometric reflection signals. In addition, the model-based Size-Amplitude Curve is introduced for the reliable sizing of surface breaking vertical cracks.

Study on Dynamic Fracture Path Prediction According to Load Point using Cast Iron FC200 (주철 FC200을 이용한 하중점에 따른 동적파괴경로 예측 연구)

  • Jik-Su Yu;Kyu-Chun Jo
    • Journal of the Korean Society of Industry Convergence
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    • v.27 no.4_2
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    • pp.973-980
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    • 2024
  • Dependence of dynamic fracture path on loading velocity was observed from experimental results based on the three point bending fracture in cast iron. In this study, 3D and 2D numerical simulations are used to evaluate singular stress fields near crack tip and fracture mechanics parameters. Moving finite element technique, 2D and 3D Delauney automatic mesh generation and contact-noncontact evaluation is introduced into the numerical method. Dynamic fracture thoughness is decreased with increase of impact loading velocity. Fracture mode-ratio corresponds to initial kink angle of fracture path. The numerical result shows that the maximum hoop stress criterion cannot be applied to dynamic fracture of cast iron.

Evaluation of Fracture Toughness in Steel Weldment for Inner Wall of LNG Storage Tank (LNG 저장탱크 내조용 강 용접부의 파괴인성 평가)

  • Jang J.-i.;Ju J.-B.;Yang Y.-c.;Kim W.-s.;Hong S. H.;Kwon D.
    • Journal of the Korean Institute of Gas
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    • v.2 no.1
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    • pp.7-13
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    • 1998
  • In this study, for the safety performance of LNG storage tank, the fracture toughness in X-grooved weld HAZ(heat-affected zone) of $9\%$ Ni steel was evaluated qualitatively and quantitatively, and the relation with the change in microstructure was analyzed. The toughness assessment was peformed through the modified CTOD test proposed for thick weldment with X-groove. Additionally, microstructures of HAZ were evaluated by OM, SEM and XRD. From the results, HAZ toughness of SMA(shielded metal arc)-welded $9\%$ Ni steel decreased as the evaluated region approached the fusion line. The decrease in toughness was apparently caused by the increase in the fraction of coarse-grained zone within HAZ. On the other hand, toughness drop with decreasing test temperature in F.L.(fusion line) ${\~}$F.L.+3mm was larger than that in F.L.+5mm${\~}$F.L.+7mm region due to the fact that in the former regions, retained austenite had poor stability.

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A Constitutive Law for Porous Solids with Pressure-Sensitive Matrices and a Void Nucleation Model (평균수직응력에 민감한 모재를 가진 기공체의 구성식과 기공생성모델)

  • Jeong, Hyeon-Yong
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.24 no.2 s.173
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    • pp.472-480
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    • 2000
  • A macroscopic yield criterion for porous solids with pressure-sensitive matrices modeled by Coulomb's yield criterion was obtained by generalizing Gurson's yield criterion with consideration of the hydrostatic yield stresses for a spherical thick-walled shell and by fitting the finite element results of a voided cube. The macroscopic yield criterion is valid for negative mean normal stresses as well as for positive mean normal stresses. From the yield criterion, a plastic potential function for the porous solids was derived either for plastic normality flow or for plastic non-normality flow of pressure- sensitive matrices. In addition, the elastic relation, an evolution equation of the plastic flow stress of the matrices and an evolution equation of the void volume fraction were presented to complete a set of constitutive relations. The set of constitutive relations was implemented into a finite element code ABAQUS to analyze the material behavior of rubber-toughened epoxies. The cavitation and the deformation behavior were analyzed around a crack tip under three-point bending and around notch tips under four-point bending. In the numerical analyses, the cavitation of rubber particles was considered via a stress-controlled nucleation model. The numerical results indicate that a reasonable cavitation zone can be obtained with void nucleation controlled by the macroscopic mean normal stress, and a plastic zone is smaller around a notch tip under compression than under tension. These numerical results agree well with corresponding experimental results on the cavitation and plastic zones.

A Study on Hot Straining Embrittlement of Coarse Grained HAZ in Steel Weldments (강 용접열영향부 조립역의 열변형취화에 관한 연구)

  • 정세희;김태영;임재규
    • Journal of Welding and Joining
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    • v.3 no.1
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    • pp.22-31
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    • 1985
  • Hot straining embrittlement is one of the most important factors which cause the brittle fracture initiation even in the service temperature in the case of mild steel and high tensile steel. Therefore it is necessary to analyze thoroughly the hot straining embrittlement occurred in weld HAZ of the structural steels. The behaviors of plastic deformation and fracture toughness at the notch tip of the hot strained weld HAZ in structural steels (SB 41 KS, SA 588-Grade A ASTM) have been studied by the recrystallization technique and crack opening displacement (COD) test method. The obtained results are summarized as follows; 1. The plastic zone is formed at the notch tip of weld HAZ owing to nomotonic and cyclic hot stran, and the maximum plastic strain increases with the accumulated hot straining amounts. 2. The distribution of the effective strain at the plastic deformed zone in HAZ can be determined as follows; (.epsilon. over bar $_{p}$ )$_{\chi}$=.epsilon. over bar $_{cr}$ ( $R_{/chi}$/.chi.)$^{m}$ where, .epsilon. over bar $_{cr}$ : (SB 41; .epsilon. over bar $_{cr}$ = 0.2, SA 588; .epsilon. over bar $_{cr}$ = 0.1) 3. The embrittlement of weld HAZ in SB 41 and SA 588 is influenced by hot strain, and the degree of embrittlement becomes deeper with hot straining amounts. 4. The embrittlement of weld HAZ of SB 41 is not influenced by the hot straining amounts until .epsilon. over bar $_{max}$ = 0.36, $R_{\chi}$ = 0.065mm, however the embrittlement of structure in SA 588 is considerably influenced even by a small quantity of the hot straining amounts.s.

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Numerical Computation of Dynamic Stress Intensity Factors in Axisymmetric Problems (축대칭 문제에서의 동적 응력확대계수의 계산)

  • 이성희;심우진
    • Journal of the Computational Structural Engineering Institute of Korea
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    • v.16 no.2
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    • pp.207-216
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    • 2003
  • In this paper, the finite element method for the elastodynamic axisymmetric fracture analysis is presented in matrix form through the application of the Galerkin method to the time integral equations of motion with no inertia forces. Isoparametric quadratic quadrilateral element and triangular crack tip singular elements with one-quarter node are used in the mesh division of the finite element model. To show the validity and accuracy of the proposed method, the infinite elastic medium with the penny shaped crack is solved first and compared with the analytical solution and the numerical results by the finite difference method and the boundary element method existing in the published literatures, and then the dynamic stress intensity factors of solid and hollow cylinders of finite dimensions haying penny-shaped cracks and internal and external circumferential tracks are computed in detail.

Microstructure Generation and Linearly Elastic Characteristic Analysis of Hierarchical Models for Dual-Phase Composite Materials (이종 입자복합재의 미세구조 생성과 계층적 모델의 선형 탄성적 응답특성 해석)

  • Cho, Jin-Rae
    • Journal of the Computational Structural Engineering Institute of Korea
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    • v.31 no.3
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    • pp.133-140
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    • 2018
  • This paper is concerned with the 2-D micostructure generation for $Ni-A{\ell}_2O_3$ dual-phase composite materials and the numerical analysis of mechanical characteristic of hierarchical models of microstructure which are defined in terms of the scale of microstructure. The microstructures of dual-phase composite materials were generated by applying the mathematical RMDF(random morphology description functions) technique to a 2-D RVE of composite materials. And, the hierarchical models of microstructure were defined by the number of Gaussian points. Meanwhile, the volume fractions of metal and ceramic particles were set by adjusting the level of RMD functions. The microstructures which were generated by RMDF technique are definitely random even though the total number of Gaussian points is the same. The randomly generated microstructures were applied to a 2-D beam model, and the variation of normal and shear stresses to the scale of microstructure was numerically investigated. In addition, through the crack analyses, the influence of RMDF randomness and Gauss point number on the crack-tip stress is investigated.

A Study on the Characteristics of Delayed Hydride Cracking in Zr-2.5Nb Pressure Tube with the Heating-up and Heat-treatment (열처리 및 가열방식에 따른 Zr-2.5Nb 압력관의 수소지연균열 특성에 관한 연구)

  • Na, Eun-Young
    • Journal of Ocean Engineering and Technology
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    • v.23 no.2
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    • pp.69-73
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    • 2009
  • The objective of this study was to obtain a better understanding of the delayed hydride cracking (DHC) of Zr-2.5Nb alloy. The DHC model has some defects: first, it cannot explain why the DHC velocity (DHCV) becomes constant regardless of an applied stress intensity factor, even though the stress gradient is affected by the applied stress intensity factor at the notch tip. Second, it cannot explain why the DHCV has a strong dependence on the method of approaching the test temperature by a cool-down or a heating-up, even under the same stress gradient, and third, it cannot predict any hydride size effect on the DHC velocity. The DHC tests were conducted on Zr-2.5Nb compact tension specimens with the test temperatures reached by a heating-up method and a cool-down method. Crack velocities were measured in hydrided specimens, which were cooled from solution-treatment temperatures at different rates by being furnace-cooled, water-quenched, and liquid nitrogen-quenched. The resulting hydride size, morphology, and distributions were examined by optical metallography. It was found that fast cooling rates, which produce very finely dispersed hydrides, result in higher crack growth rates. This different DHC behavior of the Zr-2.5Nb tube with the cooling rate after a homogenization treatment is due to the precipitation of the $\gamma$-hydrides only in the water-quenched Zr-2.5Nb tube. This experiment will provide supporting evidence that the terminal solid solubility of a dissolution (TSSD) of $\gamma$-hydrides is higher than that of $\delta$-hydrides.

Finite Element Simulation of Fracture Toughness Test (파괴인성시험의 유한요소 시뮬레이션)

  • Chu, Seok Jae;Liu, Conghao
    • Transactions of the Korean Society of Mechanical Engineers A
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    • v.37 no.4
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    • pp.491-496
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    • 2013
  • Finite element simulations of tensile tests were performed to determine the equivalent stress - equivalent plastic strain curves, critical equivalent stresses, and critical equivalent plastic strains. Then, the curves were used as inputs to finite element simulations of fracture toughness tests to determine the plane strain fracture toughness. The critical COD was taken as the COD when the equivalent plastic strain at the crack tip reached a critical value, and it was used as a crack growth criterion. The relationship between the critical COD and the critical equivalent plastic strain or the reduction of area was found. The relationship between the plane strain fracture toughness and the product of the critical equivalent stress and the critical equivalent plastic strain was also found.

Parallel Computation of a Nonlinear Structural Problem using Parallel Multifrontal Solver (다중 프런트 해법을 이용한 비선형 구조문제의 병렬계산)

  • Jeong, Sun Wan;Kim, Seung Jo
    • Journal of the Korean Society for Aeronautical & Space Sciences
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    • v.31 no.2
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    • pp.41-50
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    • 2003
  • In this paper, nonlinear parallel structural analyses are introduced by using the parallel multifrontal solver and damage localization for 2D and 3D crack models is presented as the application of nonlinear parallel computation. The parallel algorithms related with nonliear reduce the amount of memory used is carried out because many variables should be utilized for this highly nonlinear damage analysis. Also, Riks' continuation method is parallelized to search the solution when strain softening occurs due to damage evolution. For damage localization problem, several computational models having up to around 1-million degree of freedoms are used. The parallel performance in this nonlinear parallel algorithm is shown through these examples and the local variation of damage at crack tip is compared among the models with different degree of freedoms.