• Smart Structures and Systems
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• v.10 no.1
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• pp.47-65
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• 2012

The presence of crack-like defects in mechanical and structural elements produces failures during their service life that in some cases can be catastrophic. So, the early detection of the fatigue cracks is particularly important because they grow rapidly, with a propagation velocity that increases exponentially, and may lead to long out-of-service periods, heavy damages of machines and severe economic consequences. In this work, a non-destructive method for the detection and identification of elliptical cracks in shafts based on stress wave propagation is proposed. The propagation of a stress wave in a cracked shaft has been numerically analyzed and numerical results have been used to detect and identify the crack through the genetic algorithm optimization method. The results obtained in this work allow the development of an on-line method for damage detection and identification for cracked shaft-like components using an easy and portable dynamic testing device.

• Proceedings of the KSR Conference
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• 2008.11b
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• pp.874-882
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• 2008

In a domestic, HSR-350x which has the maximum speed 350km/h was developed and then next, the next generation high speed train which has the maximum speed 400km/h has still been developing. With developing the next generation high speed railway, there need to be a general plan to make sure of dynamic safety though the a study on the crack and failure of rail by rolling contact fatigue. Therefore, this study investigated occurring stress of rail according to the track quality, train velocity, wheel radius, track stiffness, distance between sleepers, axial force using Eisenmann's equations. For the more, via the finite element method, it investigated shear force on the rail head which could be changed by the early crack length, angle and temperature. As a result, this study confirmed the main elements which effect on the fatigue life cycle of rail.

• Journal of the Korean Society for Precision Engineering
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• v.12 no.12
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• pp.64-71
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• 1995

This investigation evaluates dynamic fracture characteristics of two alloy steels (STD-11 and STS-3) and a gray cast iron (GC-30). The dynamic fracture toughness of crack initiation and some of the dynamic fracturing characteristics were evaluated by using the instrumented Charpy impact testing procedures. It was found from experimental results for three kinds of materials that inertia force is directly proportional to impact velocity. The duration time of inertia force was found to be constant regardless of impact velocities in steel specimens.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.29 no.6
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• pp.583-590
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• 2016

This paper presents a dynamic crack propagation algorithm with Rayleigh damping effect based on the MLS(Moving Least Squares) Difference Method. Dynamic equilibrium equation and constitutive equation are derived by considering Rayliegh damping and governing equations are discretized by the MLS derivative approximation; the proportional damping, which has not been properly treated in the conventional strong formulations, was implemented in both the equilibrium equation and constitutive equation. Dynamic equilibrium equation including time relevant terms is integrated by the Central Difference Method and the discrete equations are simplified by lagging the velocity one step behind. A geometrical feature of crack is modeled by imposing the traction-free condition onto the nodes placed at crack surfaces and the effect of movement and addition of the nodes at every time step due to crack growth is appropriately reflected on the construction of total system. The robustness of the proposed numerical algorithm was proved by simulating single and multiple crack growth problems and the effect of proportional damping on the dynamic crack propagation analysis was effectively demonstrated.

• The Journal of Engineering Geology
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• v.11 no.3
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• pp.315-325
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• 2001

A series of laboratory tests was conducted to observe damage process by stress and to understand characteristics of permeability related with rock damage. Rock specimens which were composed of the Cretaceous medium grained granites were experienced of damage stress between 65% and 95% of the compressive strength. Rock deformation by damage process was identified with the elastic wave velocity test. Relationship between rock damage and permeability change was also analyzed by water injection test in the laboratory. According to the results of the tests, damage tends to be occurred from stress level of 80% of the compressive strength and it reduces elastic wave velocity. The damaged specimens with stress more than 80% of the compressive strength showed crack density more than 0.6 and persistent length with good connectivity of cracks. They also have higher permeability than that of specimens with crack density less than 0.6. Considered with the above results, the rock specimens used in this study were fully damaged from stress level of 80% of the compressive strength. Crack initiation and propagation by damage caused good connectivity of cracks through rock specimen. These damage process, therefore, brought high permeability coefficient through water flow conduit in the rock specimen.

• Geomechanics and Engineering
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• v.20 no.5
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• pp.421-432
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• 2020

Cutting of rocks is very common encountered in tunneling and mining during underground excavations. A deep understanding of rock-tool interaction can promote industrial applications significantly. In this paper, a distinct element method based approach, PFC^3D, is adopted to simulate the rock cutting under different operation conditions (cutting velocity, depth of cut and rake angle) and with various tool geometries (tip angle, tip wear and tip shape). Simulation results showed that the cutting force and accumulated number of cracks increase with increasing cutting velocity, cut depth, tip angle and pick abrasion. The number of cracks and cutting force decrease with increasing negative rake angle and increase with increasing positive rake angle. The numerical approach can offer a better insight into the rock-tool interaction during the rock cutting process. The proposed numerical method can be used to assess the rock cuttability, to estimate the cutting performance, and to design the cutter head.

• Tribology and Lubricants
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• v.15 no.2
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• pp.141-149
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• 1999

Fracture, fatigue and wear characteristics of Al-Si alloy used for compressor are experimentally studied. Plane strain fracture toughness test is carried out using three point bending specimen. Fatigue test is performed under constant loading condition and wear test is carried out as a function of sliding velocity and applied load. To obtain the crack propagation characteristics and wear mechanism of Al-Si alloy, fracture and worn surfaces are investigated using SEM. It is verified that fracture and fatigue strength of Al-Si alloy are improved by the fine microstructure of alloy. The wear behavior and specific wear amount of Al-Si alloy are not dependent on the microstructure but on a function of the silicon content. Anodizing on the surface of Al-Si alloy, surface hardness and wear characteristics are improved.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.5
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• pp.1134-1143
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• 1994

A study of adiabatic shear band formation and propagation of 4340 steel was done using the stepped speciment which was subjected to high velocity impact. The high velocity impact was performed on compression Hopkinson bar impact machine. After the controlled impact, the specimen was prepared for visual inspection. Numerical simulation was also performed with same geometrical dimension using explicit time integration finite element code. Experimental results were then compared with the numerical prediction. It was found that the numerical prediction is quite accurate, average thickness of adiabatic shear band is about $10{\mu}m$, the macro crack around shoulder is due to folding, and the deformation control ring is effective to freeze the propagation of adiabatic shear band.

• Computers and Concrete
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• v.24 no.6
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• pp.499-511
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• 2019

In this paper, wave propagation of double-bonded Cooper-Naghdi micro sandwich cylindrical shells with porous core and carbon nanotube reinforced composite (CNTRC) face sheets are investigated subjected to multi-physical loadings with temperature dependent material properties. The governing equations of motion are derived by Hamilton's principle. Then, the influences of various parameters such as wave number, CNT volume fraction, temperature change, Skempton coefficient, material length scale parameter, porosity coefficient on the phase velocity of double-bonded micro sandwich shell are taken into account. It is seen that by increasing of Skempton coefficient, the phase velocity decreases for higher wave number and the results become approximately the constant. Also, by increasing of the material length scale parameter, the cut of frequency increases, because the stiffness of micro structure increases. The obtained results for this article can be used to detect, locate and quantify crack.

• Transactions of the Korean Society of Automotive Engineers
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• v.12 no.1
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• pp.138-144
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• 2004

Occupant protection in the side impact of a vehicle becomes one of the most important issues today. So, to reduce development time and cost, it needs test equipment which conducts an accurate simulation of the side impact crash. This paper describes a new test method for side impact, which utilizes a standard 12inch-HYGE-type sled facility. If a side impact sled test can simulate vehicle intrusion very well, it will contribute to develop full-scale side impact crash performance. The newly developed sled test method enables simulation for dummy motion, injury, door velocity, trim crack, and vehicle structure to be accurate. Ant also this sled test method can be applied to the development of side air-bag.