• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.1
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• pp.103-111
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• 1990

This paper presents a methodology for predicting stable crack growth and instability of a cracked body under monotonically increasing load. It is based on a model that incremental crack extensions and load increments after fracture initiation occur by turns in sequence and the criterion that the crack grows by an incremebt .delta.a when the opening displacement at the current crack tip increases by a critical value V$_{c}$. It is shown that the value I$_{c}$ = V$_{c}$/ .delta. a is a material constant characterizing ductile crack growth resistance. Along with the fracture initiation toughness value, the constant is used for the calculation of the loads against crack extensions by adding up each increment. The specimen failure is defined to occur when the necessary load increment for crack extension is zero or when the limit load in the current ligament is reached. The predicted failure loads are in good agreement with the avaliable experimental failure loads for the compact and center-cracked tension specimens of 7075-T651, 2024-T351 aluminum alloys and 304 stainless steel.steel.

• Nuclear Engineering and Technology
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• v.19 no.3
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• pp.169-179
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• 1987

The change of material J-R (J-T) curve with crack extension and J-calculation method was investigated to give experimental and analytical method for reliable J-R (J-T) curve, which was adapted recently as a tool for instability analysis of Nuclear Pressure Vessel. Experiments were carried out by Single Specimen Unloading Compliance Method using 1/2"T, Compact-Tension Type fracture mechanic specimens which were the same size and material as domestic nuclear pressure vessel material surveillance specimens. The results revealed that crack extension up to 25~30% of initial uncracked ligament and JD (Deformation theory J) calculation method, currently being used in NUREG-0744, could give rather reliable material J-R (J-T) curve than the small crack extension and JM (Modified J) calculation method. But as JM results more or less higher J at instability, the application of JM should be considered regarding to the problem of power plant availability.lity.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.18 no.1
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• pp.101-109
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• 2008

The stability of a rotating cantilever pipe conveying fluid with a crack and tip mass is investigated by the numerical method. That is, the effects of the rotating angular velocity, mass ratio, crack severity and tip mass on the critical flow velocity for flutter instability of system are studied. The equations of motion of rotating pipe are derived by using the Euler-Bernoulli beam theory and the extended Hamilton's principle. The crack section of pipe is represented by a local flexibility matrix connecting two undamaged pipe segments. Also, the crack is assumed to be in the first mode of fracture and always opened during the vibrations. When the tip mass and crack are constant, the critical flow velocity for flutter is proportional to the rotating angular velocity of pipe. In addition, the stability maps of the rotating pipe system as a rotating angular velocity and mass ratio ${\beta}$ are presented.

• Geomechanics and Engineering
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• v.29 no.4
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• pp.377-390
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• 2022

The stability of the roof rock-coal pillar-floor rock composite structure is of great significance to coal mine safety production. The cracks existing in the composite structure seriously affect the stability of the roof rock-coal pillar-floor rock composite structure. The numerical simulation tests of rock-coal-rock composite structures with different crack characteristics were carried out to reveal the composite structures' mechanical properties and failure mechanisms. The test results show that the rock-coal-rock composite structure's peak stress and elastic modulus are directly proportional to the crack angle and inversely proportional to the crack length. The smaller the crack angle, the more branch cracks produced near the main control crack in the rock-coal-rock composite structure, and the larger the angle between the main control crack and the crack. The smaller the crack length, the larger the width of the crack zone. The impact energy index of the rock-coal-rock composite structure decreases first and then increases with the increase of crack length and increases with the increase of crack angle. The functional relationships between the different crack characteristics, peak stress, and impact energy index are determined based on the sensitivity analysis. The determination of the functional relationship can fully grasp the influence of the crack angle and the crack length on the peak stress and impact energy index of the coal-rock composite structure. The research results can provide a theoretical basis and guidance for preventing the instability and failure of the coal pillar-roof composite structure.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.31 no.2A
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• pp.71-77
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• 2011

In this paper elasto-plastic dynamic behavior of solid is analyzed by using smoothed particle hydrodynamics (SPH) without tensile instability which caused by a clustering of SPH particles. In solid body computations, the instability may corrupt physical behavior by numerical fragmentation which, in some cases of elastic or brittle solids, is so severe that the dynamics of the system is completely wrong. The instability removed by using an artificial stress which introduces negligible errors in long-wavelength modes. Applications to several test problems show that the artificial stress works effectively. These problems include the collision of rubber cylinders, fracture and crack of plate.

• Geomechanics and Engineering
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• v.33 no.6
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• pp.597-609
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• 2023

The instability and failure of engineered rock masses are influenced by crack initiation and propagation. Uniaxial compression and acoustic emission (AE) experiments were conducted on cracked sandstone. The effect of the crack's dip on the crack initiation was investigated using fracture mechanics. The crack propagation was investigated based on stress-strain curves, AE multi-parameter characteristics, and failure modes. The results show that the crack initiation occurs at the tip of the pre-fabricated crack, and the crack initiation angle increases from 0° to 70° as the dip angle increases from 0° to 90°. The fracture strength k_cr is derived varies in a U-shaped pattern as β increased, and the superior crack angle β_m is between 36.2 and 36.6 and is influenced by the properties of the rock and the crack surface. Low-strength, large-scale tensile cracks form during the crack initiation in the cracked sandstone, corresponding to the start of the AE energy, the first decrease in the b-value, and a low r-value. When macroscopic surface cracks form in the cracked sandstone, high-strength, large-scale shear cracks form, resulting in a rapid increase in the AE energy, a second decrease in the b-value and an abrupt increase in the r-value. This research has significant theoretical implications for rock failure mechanisms and establishment of damage indicators in underground engineering.

• Nuclear Engineering and Technology
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• v.31 no.3
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• pp.267-275
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• 1999

This paper deals with the industrial problem of thermal striping damage on the French prototype fast breeder reactor, Phenix and it was studied in coordination with the research program of IAEA. The thermomechanical and fracture mechanics evaluation procedure of thermal striping damage on the tee-junction of the secondary piping using Green's function method and standard FEM is presented. The thermohydraulic(T/H) loading condition used in the present analysis is the random type thermal loads computed by T/H analysis on the turbulent mixing of the two flows with different temperatures. The thermomechanical fatigue damage was evaluated according to ASME code section 111 subsection NH. The results of the fatigue analysis showed that fatigue failure would occur at the welded joint within 90,000 hours of operation. The assessment for the fracture behavior of the welded joint showed that the crack would be initiated at an early stage in the operation. It took 42,698.9 hours for the crack to propagate up to 5 mm along the thickness direction. After then, however, the instability analysis, using tearing modulus, showed that the crack would be arrested, which was in agreement with the actual observation of the crack. An efficient analysis procedure using Green's function approach for the crack propagation problem under random type load was proposed in this study. The analysis results showed good agreement with those of the practical observations.

• Steel and Composite Structures
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• v.22 no.2
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• pp.399-409
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• 2016

The fracture propagation mechanism and fractured rock mass failure mechanism were important research in geotechnical engineering field. Many failures and instability in geotechnical engineering were related on fractures propagation, coalescence and interaction in rock mass under the external force. Most of the current research were limited to two-dimensional for the brittleness and transparency of three-dimensional fracture materials couldn't meet the requirements of the experiment. New materials with good transparent and brittleness were developed by authors. The making method of multi fracture specimens were established and made molds that could be reused. The tension-compression ratio of the material reached above 1/6 in normal temperature. Uniaxial and biaxial loading tests of single and double fracture specimens were carried out. Four new fractures were not found in the experiment of two-dimensional fractures such as the fin shaped crack, wrapping wing crack and petal crack and anti-wing crack. The relationship between stress and strain of the specimens were studied. The specimens with the load had experienced four stages of deformation and the process of the fracture propagation was clearly seen in each stage. The expansion characteristics of the fractured specimens were more obvious than the previous research.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.13 no.5
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• pp.400-408
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• 2003

This paper investigates the vibration characteristics of steam generator (SG) U-tubes with defect. The operating SG shell-side flow field conditions for determining the fluidelastic instability parameters such as added mass are obtained from three-dimensional SG flow calculation. Modal analyses are performed for the U-tubes either with axial or circumferential flaw with different sizes. Special emphases are on the effects of flaw orientation and size on the modal and instability characteristics of tubes, which are expressed in terms of the natural frequency, corresponding mode shape and stability ratio. Also, addressed is the effect of the internal pressure on the vibration characteristics of the tube.

• Geomechanics and Engineering
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• v.12 no.6
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• pp.919-933
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• 2017

A series of Brazilian tests under diameter compression for disc specimens was carried out to investigate the strength and failure behavior by using acoustic emission (AE) and photography monitoring technique. On the basis of experimental results, load-displacement curves, AE counts, real-time crack evolution process, failure modes and strength property of granite specimens containing two pre-existing holes were analyzed in detail. Two typical types of load-displacement curves are identified, i.e., sudden instability (type I) and progressive failure (type II). In accordance with the two types of load-displacement curves, the AE events also have different responses. The present experiments on disc specimens containing two pre-existing holes under Brazilian test reveal four distinct failure modes, including diametrical splitting failure mode (mode I), one crack coalescence failure mode (mode II), two crack coalescences failure mode (mode III) and no crack coalescence failure mode (mode IV). Compared with intact granite specimen, the disc specimen containing two holes fails with lower strength, which is closely related to the bridge angle. The failure strength of pre-holed specimen first decreases and then increases with the bridge angle. Finally, a preliminary interpretation was proposed to explain the strength evolution law of granite specimen containing two holes based on the microscopic observation of fracture plane.