• The Journal of Engineering Geology
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• v.21 no.4
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• pp.295-304
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• 2011

The first step in improving our understanding of uncertainties suclt as rock mass strength parameters and deformation modulus in rock masses around high-level radioactive waste disposal repositories, for improved safety, is to study the process of crack development in intact rock. Therefore, in this study, the fracture process and crack development were examined in samples of KURT granite taken from the KAERI Underground Research Tunnel (KURT), based on acoustic emission (AE) and moment tensor analysis. The results show that crack initiation, coalescence, and unstable crack occurred at rock uniaxial compressive strengths of 0.45, 0.73, and 0.84, respectively. In addition, moment tensor analysis indicated that during the early stage of loading, tensile cracks were predominant. With increasing applied stress, the number of shear cracks gradually increased. When the applied stress exceeded the stress level required for crack damage, unstable shear cracks which directly result in failure of the rock were generated along the failure plane.

• Advances in concrete construction
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• v.6 no.5
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• pp.423-463
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• 2018

This paper investigates the behavior of normal and self-compacted steel fiber reinforced concrete (SCC-SFRC) corbels rehabilitated by Basalt Fiber Mesh (BFM) and Basalt Fiber Fabric (BFF) for the first time in literature. The research objective is to study the effectiveness of BFM and BFF in the rehabilitation of damaged reinforced concrete corbels with and without epoxy injection. The experimental program includes two types of concrete: normal concrete, and self-compacted concrete. For normal concrete, 12 corbels were rehabilitated by BFM without injection epoxy in cracks, with two values of compressive strength, three ratios of steel fiber (SF), and two values of shear span. For self-compacted concrete, 48 corbels were rehabilitated with different parameters where 12 corbels were rehabilitated by BFM with and without epoxy injection, 18 heated corbels with three different high-temperature level were rehabilitated by repairing cracks only by epoxy injection, and 18 heated corbels with three different high-temperature level were rehabilitated by repairing cracks by epoxy and wrapping by BFF. All 48 corbels have two values of compressive strength, three values volumetric ratios of SF, and two values of the shear span. Test results indicate that RC corbels rehabilitated by BFM only without injection did not show any increase in the ultimate load capacity. Moreover, For RC corbels that were repaired by epoxy without basalt wrapping, the ultimate load capacities showed an increase depending on the mode of failure of corbels before the rehabilitation. However, the rehabilitation with only crack repairing by epoxy injection is more effective on medium strength corbels as compared to high strength ones. Finally, it can be concluded that use of BFF is an effective and powerful technique for the strengthening of damaged RC corbels.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.12
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• pp.1453-1463
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• 2013

In this study, we investigated the effect of the indenter geometry on the crack characteristics by indentation cracking test and FEA. We conducted various cohesive finite element simulations based on the findings of Lee et al. (2012), who examined the effect of cohesive model parameters on crack size and formulated conditions for crack initiation and propagation. First, we verified the FE model through comparisons with experimental results that were obtained from Berkovich and Vickers indentations. We observed whether nonsymmetrical cracks formed beneath the surface during Berkovich indentation via FEA. Finally, we examined the relation between the crack size and the number of cracks. Based on this relation and the effect of the indenter angle on the crack size, we can predict from the crack size obtained with an indenter of one shape (such as Berkovich or Vickers) the crack size for an indenter of different shape.

• Smart Structures and Systems
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• v.29 no.1
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• pp.29-39
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• 2022

Crack detection plays an important role in the maintenance and protection of steel box girder of bridges. However, since the cracks only occupy an extremely small region of the high-resolution images captured from actual conditions, the existing methods cannot deal with this kind of image effectively. To solve this problem, this paper proposed a novel three-stage method based on deep learning technology and morphology operations. The training set and test set used in this paper are composed of 360 images (4928 × 3264 pixels) in steel girder box. The first stage of the proposed model converted high-resolution images into sub-images by using patch-based method and located the region of cracks by CBAM ResNet-50 model. The Recall reaches 0.95 on the test set. The second stage of our method uses the Attention U-Net model to get the accurate geometric edges of cracks based on results in the first stage. The IoU of the segmentation model implemented in this stage attains 0.48. In the third stage of the model, we remove the wrong-predicted isolated points in the predicted results through dilate operation and outlier elimination algorithm. The IoU of test set ascends to 0.70 after this stage. Ablation experiments are conducted to optimize the parameters and further promote the accuracy of the proposed method. The result shows that: (1) the best patch size of sub-images is 1024 × 1024. (2) the CBAM ResNet-50 and the Attention U-Net achieved the best results in the first and the second stage, respectively. (3) Pre-training the model of the first two stages can improve the IoU by 2.9%. In general, our method is of great significance for crack detection.

• Steel and Composite Structures
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• v.53 no.2
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• pp.169-194
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• 2024

Although the excellent characteristics of functionally graded materials (FGMs) cracks could be found due to manufacturing defects or extreme working conditions. The existence of these cracks may threaten the material or structural strength, reliability, and lifetime. Due to high cost and restrictions offered by practical operational features these cracked components couldn't be replaced immediately. Such circumstances lead to the requirement of assessing the dynamic performance of cracked functionally graded structural components especially under moving objects. The present study aims to comprehensively investigate the dynamic behavior of functionally graded cracked Timoshenko beams (FGCTBs) resting on Winkler foundation and subjected to moving load through shear locking free finite elements methodology. The through thickness material distribution is simulated by the exponential gradation law. The geometric discontinuity due to cracks is represented using the massless rotational spring approach. The shear locking phenomena is avoided by using the different interpolation functions orders for both deflections and rotations. Based on Timoshenko beam element, a shear locking free finite elements methodology is developed. The unconditionally stable Newmark procedure is employed to solve the forced vibration problem. Accuracy of the developed procedure is verified by comparing the obtained results with the available results and an excellent agreement is found. Parametric studies are conducted to explore effects of the geometrical, material characteristics, crack geometrical characteristics, the elastic foundation parameter, and the moving load speed on the dynamic behavior for different boundary conditions. Obtained results revealed the significant effect these parameters on the dynamic performance of FGCTBs.

• Journal of Welding and Joining
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• v.32 no.6
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• pp.41-46
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• 2014

In this study, cracking susceptibility of laser cladding was investigated according to the processing parameters such as laser power, scan speed and feeding rate with blended powders of stellite#6 and technolase40s (WC+NiCr). The solidification microstructure of clad was composed of Co-based dendrite structures with ${\gamma}+Cr7C3$ eutectic phases at the dendritic boundaries. The crack propagation showed transgranular fracture along dendritic boundaries due to brittle chrome carbide at the eutectic phases. From results of fractography experiments, the fracture surface was typical cleavage brittle fracture in the clad and substrate. The number of clad cracks, caused by a tensile stress after the solidification, increased with increase of laser power, scan speed and feeding rate. Increase of the laser power caused large pores by facilitating WC decarburizing reaction. And the pores affected increase of crack susceptibility. High scan speed caused increment of clad cracks due to thermal stress and WC particle fractures. Also, increase of the feeding rate accompanied an amount of WC particles causing crack initiation and decarburizing reaction.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.1
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• pp.126-137
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• 2003

Many research works have been performed on the J-T approach for elastic-plastic crack-tip stress fields in a variety of plane strain specimens. To generalize the validity of J-T method, further investigations are however needed fur more practical 3D structures than the idealized plane strain specimens. The present study deals mainly with 3D finite element (FE) modeling of welded plate and straight pipe, and accompanying elastic, elastic-plastic FE analyses. Manual 3D modeling is almost prohibitive, since the models contain semi-elliptical interfacial cracks which require singular elements. To overcome this kind of barrier, we develop a program generating the meshes fur semi-elliptical interfacial cracks. We then compare the detailed 3D FE stress fields to those predicted with J-T two parameters. The validity of J-T approach is thereby extended to 3D yield-strength-mismatched weld joints, and useful information is inferred fur the design or assessment of pipe welds.

• Smart Structures and Systems
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• v.22 no.3
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• pp.259-276
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• 2018

This paper studies the energies and energy release rate (ERR) for the initially rotationally symmetric compressed (or stretched) in the inward (outward) radial direction of the PZT/Elastic/PZT sandwich circular plate with interface penny-shaped cracks. The investigations are made by utilizing the so-called three-dimensional linearized field equations and relations of electro-elasticity for piezoelectric materials. The quantities related to the initial stress state are determined within the scope of the classical linear theory of piezoelectricity. Mathematical formulation of the corresponding problem and determination of the quantities related to the stress-strain state which appear as a result of the action of the uniformly normal additional opening forces acting on the penny-shaped crack's edges are made within the scope of the aforementioned three-dimensional linearized field equations solution which is obtained with the use of the FEM modelling. Numerical results of the energies and ERR and the influence of the problem parameters on these quantities are presented and discussed for the PZT- 5H/Al/PZT-5H, PZT-4/Al/PZT-4, $BaTiO_3/Al/BaTiO_3$ and PZT-5H/StPZT-5H sandwich plates. In particular, it is established that the magnitude of the influence of the piezoelectricity and initial loading on the ERR increases with crack radius length.

• Journal of the Microelectronics and Packaging Society
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• v.21 no.4
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• pp.63-68
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• 2014

For a nanoscale Cu/low-k wafer, inter-layer dielectric (ILD) and metal layers peelings, cracks, chipping, and delamination are the most common dicing defects by traditional diamond blade saw process. Sidewall void in sawing street is one of the key factors to bring about cracks and chipping. The aim of this research is to evaluate laser grooving & mechanical sawing parameters to eliminate sidewall void and avoid top-side chipping as well as peeling. An ultra-fast pico-second (ps) laser is applied to groove/singulate the 28-nanometer node wafer with Cu/low-k dielectric. A series of comprehensive parametric study on the recipes of input laser power, repetition rate, grooving speed, defocus amount and street index has been conducted to improve the quality of dicing process. The effects of the laser kerf geometry, grooving edge quality and defects are evaluated by using scanning electron microscopy (SEM) and focused ion beam (FIB). Experimental results have shown that the laser grooving technique is capable to improve the quality and yield issues on Cu/low-k wafer dicing process.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.7
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• pp.1250-1261
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• 2002

Many research works have validated the J-T approach to elastic-plastic crack-tip stress fields in a variety of plane strain specimens. To generalize the validity of J-T method, further investigations are however needed for more practical 3D structures than the idealized plane strain specimens. In this work, we perform 3D finite element (FE) modeling of welded plate and straight pipe, and accompanying elastic, elastic-plastic FE analyses. Manual 3D modeling is almost prohibitive, since the models contain semi-elliptical interfacial cracks which require singular elements. To overcome this kind of barrier, we develop a program generating the meshes for semi-elliptical interfacial cracks. We then compare the detailed 3D FE stress fields to those predicted with J-T two parameters. Thereby we extend the validity of J-T application to 3D structures and infer some useful informations for the design or assessment of pipe welds.