• Journal of the Korean Society for Nondestructive Testing
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• v.7 no.2
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• pp.35-41
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• 1988

The optimum test conditions of measuring the crack height were determined for the two-probe ultrasonic diffraction method. The applicability and the accuracy of the two-probe ultrasonic diffraction method on the inclined artificial cracks and the fatigue cracks were evaluated. It us possible to measure the height of the normal and inclined artificial cracks with the maximum error of ${\pm}\;0.5mm$ with the two-probe ultrasonic diffraction method. It was found, however, that the accuracy of this method in meaasuring the height of the fatigue crack depends on the degree of closure of the crack tip. It was desirable to choose a refraction angle as small as possible, but the angle should not be so small that the distortion of the lateral waveform became appreciable.

• Structural Engineering and Mechanics
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• v.29 no.1
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• pp.55-75
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• 2008

In this paper, the behavior of two collinear Mode-I cracks in piezoelectric/piezomagnetic materials subjected to a uniform tension loading was investigated by the generalized Almansi's theorem. Through the Fourier transform, the problem can be solved with the help of two pairs of triple integral equations, in which the unknown variables were the jumps of displacements across the crack surfaces. To solve the triple integral equations, the jumps of displacements across the crack surfaces were directly expanded as a series of Jacobi polynomials to obtain the relations among the electric displacement intensity factors, the magnetic flux intensity factors and the stress intensity factors at the crack tips. The interaction of two collinear cracks was also discussed in the present paper.

• Geomechanics and Engineering
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• v.19 no.1
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• pp.49-60
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• 2019

Crack instability propagation during coal and rock mass failure is the main reason for electromagnetic radiation (EMR) generation. However, original cracks on coal and rock mass are hard to study, making it complex to reveal EMR laws and mechanisms. In this paper, we prefabricated cracks of different inclinations in coal and rock samples as the analogues of the native cracks, carried out uniaxial compression experiments using these coal and rock samples, explored, the effects of the prefabricated cracks on EMR laws, and verified these laws by measuring the surface potential signals. The results show that prefabricated cracks are the main factor leading to the failure of coal and rock samples. When the inclination between the prefabricated crack and axial stress is smaller, the wing cracks occur first from the two tips of the prefabricated crack and expand to shear cracks or coplanar secondary cracks whose advance directions are coplanar or nearly coplanar with the prefabricated crack's direction. The sample failure is mainly due to the composited tensile and shear destructions of the wing cracks. When the inclination becomes bigger, the wing cracks appear at the early stage, extend to the direction of the maximum principal stress, and eventually run through both ends of the sample, resulting in the sample's tensile failure. The effect of prefabricated cracks of different inclinations on electromagnetic (EM) signals is different. For samples with prefabricated cracks of smaller inclination, EMR is mainly generated due to the variable motion of free charges generated due to crushing, friction, and slippage between the crack walls. For samples with larger inclination, EMR is generated due to friction and slippage in between the crack walls as well as the charge separation caused by tensile extension at the cracks' tips before sample failure. These conclusions are further verified by the surface potential distribution during the loading process.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.4 s.97
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• pp.483-491
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• 2005

In this paper a dynamic behavior of a double-cracked cantilever pipe with the tip mass and a moving mass is presented. Based on the Euler-Bernoulli beam theory, the equation of motion is derived by using Lagrange's equation. The influences of the moving mass, the tip mass and double cracks have been studied on the dynamic behavior of a cantilever pipe system by numerical method. The cracks section are represented by the local flexibility matrix connecting two undamaged beam segments. Therefore, the cracks are modelled as a rotational spring. This matrix defines the relationship between the displacements and forces across the crack section and is derived by applying fundamental fracture mechanics theory. We investigated about the effect of the two cracks and a tip mass on the dynamic behavior of a cantilever pipe with a moving mass.

• Structural Monitoring and Maintenance
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• v.9 no.4
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• pp.373-388
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• 2022

A USW based diagnostic procedure is presented for estimating the depth of surface-breaking cracks. The diagnosis is demonstrated on seven lab-scale SFRC beam specimens, which are subjected to the CMOD controlled three-point bending test to create real bending cracks. Then, the recorded multiple ultrasonic signals are examined with the signal processing techniques, including wavelet transform and two-dimensional Fourier transform, to investigate the relationships between the crack depth and two diagnostic indices, namely the attenuation coefficient and dispersion index (DI). Finally, the reliabilities of these indices for depth estimation are verified with the visually measured crack depths as well as the crack features obtained with a digital image processing algorithm. It is found that the DI outperforms the attenuation coefficient in depth estimation, where this index displays good agreement with the visual inspection for 86% of the inspected specimens.

• Structural Engineering and Mechanics
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• v.10 no.1
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• pp.81-92
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• 2000

This study is an investigation of the effect of cracks on the dynamical characteristics of a cantilever beam, having multiple open-edge transverse cracks. The flexibilities due to crack have been identified for several crack depths and locations. In the study the finite element method and component mode synthesis methods are used. Coupling the components is performed by a flexibility matrix taking into account the interaction forces. Each component is modelled by cantilever beam finite elements with two nodes and three degrees of freedom at each node. The results obtained lead to conclusion that, by using the drop in the natural frequencies and the change in the mode shapes, the presence and nature of cracks in a structure can be detected. There is some counter-evidence, however, that the effects due to multiple cracks may interact to make detection more difficult than for isolated cracks.

• Journal of Mechanical Science and Technology
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• v.16 no.2
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• pp.182-191
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• 2002

The reliable stress intensity factor analysis is required for fracture mechanics design or safety evaluation of mechanical joints at which cracks often initiate and grow. It has been reported that cracks in mechanical joints usually nucleate as corner cracks at the faying surface of joints and grow as elliptical arc through cracks. In this paper, three dimensional finite element analyses are performed for elliptical arc through cracks in mechanical joints. Thereafter stress intensity factors along elliptical crack front including two surface points are determined by the virtual crack closure technique. Virtual crack closure technique is a method to calculate stress intensity factor using the finite element analysis and can be applied to non-orthogonal mesh. As a result, the effects of clearance on the stress intensity factor are investigated and crack shape are then predicted.

• Journal of the Korean Society for Precision Engineering
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• v.7 no.4
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• pp.55-64
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• 1990

The fatigue crack growth behavior of physically-short cracks(0.2${\Delta}K$ with $da/dN<1{\times}10^{-7}m/cycle$. The transition crack lengths where similtude with ${\Delta}K$ existed was between 1 and 2mm. The effective stress intensity factor range based on COD measurements gave better correlation between the physically-short and long cracks. Thus it can be considered that the crack closure effect is one of the main factors which causes the differences between these two cracks.

• Proceedings of the KSME Conference
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• 2000.11a
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• pp.141-146
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• 2000

Crack closure and growth behavior of physically short fatigue cracks under random loading are investigated by performing narrow- and wide-band random loading tests for various stress ratios. The importance of the crack closure phenomenon is examined by predicting the growth lives of short cracks using obtained crack opening behavior. Artificially prepared two-dimensional, short through-thickness cracks are used. The crack opening load of short cracks is much lower under random loading than under constant-amplitude loading corresponding to the largest load cycle in a random load history. This result indicates that the largest load cycle in a random load history has an effect to enhance crack opening of short cracks. Most of the life prediction ratios are within the factor of 2 scatter band except several data at very short crack sizes, indicating that crack growth predictions based on the measured crack opening data are excellent. From the results obtained in this study, it can be concluded that crack closure is the primary factor governing fatigue crack growth of short cracks under random loading as well as under constant-amplitude loading.

• Structural Engineering and Mechanics
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• v.67 no.3
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• pp.277-281
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• 2018

The analytical solution of two functionally graded layers with Volterra type screw dislocation is investigated under anti-plane shear impact loading. The energy dissipation of FGM layers is modeled by viscous damping and the properties of the materials are assumed to change exponentially along the thickness of the layers. In this study, the rate of gradual change ofshear moduli, mass density and damping constant are assumed to be same. At first, the stress fields in the interface of the FGM layers are derived by using a single dislocation. Then, by determining a distributed dislocation density on the crack surface and by using the Fourier and Laplace integral transforms, the problem are reduce to a system ofsingular integral equations with simple Cauchy kernel. The dynamic stress intensity factors are determined by numerical Laplace inversion and the distributed dislocation technique. Finally, various examples are provided to investigate the effects of the geometrical parameters, material properties, viscous damping and cracks configuration on the dynamic fracture behavior of the interacting cracks.