• Structural Engineering and Mechanics
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• v.14 no.2
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• pp.209-221
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• 2002

In this paper a fracture criterion is proposed for cracked cylindrical samples of high-strength prestressing steels of different yield strength. The surface crack is assumed to be semi-elliptical, a geometry very adequate to model sharp defects produced by any subcritical mechanism of cracking: mechanical fatigue, stress-corrosion cracking, hydrogen embrittlement or corrosion fatigue. Two fracture criteria with different meanings are considered: a global (energetic) criterion based on the energy release rate G, and a local (stress) criterion based on the stress intensity factor $K_I$. The advantages and disadvantages of both criteria for engineering design are discussed in this paper on the basis of many experimental results of fracture tests on cracked wires of high-strength prestressing steels of different yield strength and with different degrees of strength anisotropy.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.9 s.252
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• pp.1086-1093
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• 2006

A local failure criterion for the API X65 steel is applied to predict ductile failure of full-scale API X65 pipes with simulated corrosion and gouge defects under internal pressure. The local failure criterion is the stress-modified fracture strain for the API X65 steel as a function of the stress triaxiality (defined by the ratio of the hydrostatic stress to the effective stress). Based on detailed FE analyses with the proposed local failure criteria, burst pressures of defective pipes are estimated and compared with experimental data. The predicted burst pressures are in good agreement with experimental data. Noting that an assessment equation against the gouge defect is not yet available, parametric study is performed, from which a simple equation is proposed to predict burst pressure fur API X65 pipes with gouge defects.

• Structural Engineering and Mechanics
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• v.50 no.6
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• pp.723-740
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• 2014

A fracture criterion based on the strain energy density (SED) over a control volume, which embraces the notch edge, is employed in the present paper to assess the fracture loads of some U-notched Brazilian disk (UNBD) specimens. The specimens are made of commercial graphite and have been tested under pure mode I, pure mode II and mixed mode I/II loading. The results show that the SED criterion allows to successfully assess the fracture loads of graphite specimens for different notch tip radii and various mode mixity conditions with discrepancies that fall inside the scatter band of ${\pm}20%$.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.20 no.4
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• pp.1356-1362
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• 1996

Fracture toughness of heterogeneous brittle materials such as poly crystalline ceramics used to present the size (thickness) effect as well as statistically distributed results. There is belief that both(size effect and scatter) must be associated with each other. However, no generally accepted theory has been established so far. Using statistical approach, a probabilistic modeling for the fracture toughness which describes the thickness effect was attempted in this paper, Weibull distribution of specific fracture energy(SFE)at local areas and Griffith criterion are applied to the model. In addition, the newly developed model was verified with experimental results of alumina.

• Geomechanics and Engineering
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• v.11 no.2
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• pp.253-267
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• 2016

The stability of surrounding rock will be poor when the tunnel is excavated through nearly horizontal stratum. In this paper, the instability mechanism of local nearly horizontal stratum in super-large section and deep buried tunnel is revealed by the analysis of the macro failure and micro fracture. A structural model is proposed to explain the mechanics of surrounding rock collapse under the action of stress redistribution and shed light on the macroscopic analytical approach of the stability of surrounding rock. Then, some highly effective formulas applied in the tunnel engineering are developed according to the theory of mixed-mode micro fracture. And well-documented field case is made to demonstrate the effectiveness and accuracy of the proposed analytical methods of mixed-mode fracture. Meanwhile, in order to make the more accurate judgment about yield failure of rock mass, a series of comprehensive failure criteria are formed. In addition, the relationship between the nonlinear failure criterion and $K_I$ and $K_{II}$ of micro fracture is established to make the surrounding rock failure criterion more comprehensive and accurate. Further, the influence of the parameters related to the tension-shear mixed-mode fracture and compression-shear mixed-mode fracture on the propagation of rock crack is analyzed. Results show that ${\sigma}_3$ changes linearly with the change of ${\sigma}_1$. And the change rate is related to ${\beta}$, angle between the cracks and ${\sigma}_1$. The proposed simple analytical approach is economical and efficient, and suitable for the analysis of local nearly horizontal stratum in super-large section and deep buried tunnel.

• Proceedings of the KSME Conference
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• 2005.11a
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• pp.137.1-137.1
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• 2005

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.12
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• pp.1393-1400
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• 2009

The objective of this study is to validate local failure criteria, which were proposed based on the notched-bar specimen tests combining with finite element (FE) simulations, using the results of real-scale pipe failure tests. This study conducted burst test using wall-thinned pipe specimens, which were made of 4 inch Sch.80 ASTM A106 Gr.B carbon steel pipe, under simple internal pressure at ambient temperature and performed associated FE simulations. Failure pressures were estimated by applying the failure criteria to the results of FE simulations and were compared with experimental failure pressures. It showed that the local stress based criterion, given as true ultimate tensile stress of material, accurately estimated the failure pressure of wall-thinned pipe specimens. However, the local strain based criterion, which is fracture strain of material as a function of stress tri-axiality, could not predict the failure pressure. It was confirmed that the local stress based criterion is reliably applicable to estimation of failure pressure of local wall-thinned piping components.

• Journal of Power System Engineering
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• v.18 no.1
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• pp.76-83
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• 2014

This study was carried out an experimental and finite element analysis on the fracture behavior of straight pipes with local wall thinning under internal pressure. Local wall thinning was machined on the pipes in order to simulate erosion/corrosion metal loss. The configurations of the eroded area has an eroded ratio of d/t=0.80~0.92 and an eroded length of l=25, 50 and 102 mm. Three-dimensional elastic-plastic analyses were also carried out using the finite element method, which is able to accurately simulate failure behaviors. In regards to the relation ship between pressure and eroded ratio, the criterion that can be used safely under operating pressure and design pressure were obtained from this calculation. The results of this calculation were in relatively good agreement with that of the experiment.

• Steel and Composite Structures
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• v.32 no.2
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• pp.199-212
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• 2019

This paper presents an investigation on seismic behavior of out-of-code Q690 circular high-strength concrete-filled thin-walled steel tubular (HCFTST) columns made up of high-strength (HS) steel tubes (yield strength $f_y{\geq}690MPa$). Eight Q690 circular HCFTST columns with various diameter-to-thickness (D/t) ratios, concrete cylinder compressive strengths ($f_c$) and axial compression ratios (n) were tested under the constant axial loading and reversed cyclic lateral loading. The obtained lateral load-displacement hysteretic curves, energy dissipation, skeleton curves and ductility, and stiffness degradation were analyzed in detail to reflect the influences of tested parameters. Subsequently, a simplified shear strength model was derived and validated by the test results. Finally, a finite element analysis (FEA) model incorporating a stress triaxiality dependent fracture criterion was established to simulate the seismic behavior. The systematic investigation indicates the following: compared to the D/t ratio and axial compression ratio, improving the concrete compressive strength (e.g., the HS thin-walled steel tube filled with HS concrete) had a slight influence on the ductility but an obvious enhancement of energy dissipation and peak load; the simplified shear strength model based on truss mechanism accurately predicted the shear-resisting capacity; and the established FEA model incorporating steel fracture criterion simulated well the seismic behavior (e.g., hysteretic curve, local buckling and fracture), which can be applied to the seismic analysis and design of Q690 circular HCFTST columns.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1994.10a
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• pp.229-234
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• 1994

The major objective of the present paper is to estabilish finite element simulation technique in order to further analyze the shearing process. For this the ductile fracture criterion and element kill method are used in the present work. It is postulated that a fracture initiation is based on the magnitude of local effective strain. The features of sheared surface are easily observed by the element kill method. The simulation results are compard with existing experimental results. It is found that the results of the present work are close agreement with the existing results.