• 한국안전학회지
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• 제18권3호
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• pp.1-7
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• 2003

In this study, CT specimens were prepared from spring steel(SPS5) processed shot peening. The fatigue crack growth tests were carried out in the environment of the room temperature md low temperature at $25^{\circ}C$, $-30^{\circ}C$, $-50^{\circ}C$, $-70^{\circ}C$ $-100^{\circ}C$ and $-150^{\circ}C$ in the range of stress ratio of 0.05 by means of opening mode displacement. The threshold stress intensity factor range ΔKth in the early stage of fatigue crack growth (Region I) and stress intensity factor range $\Delta$K in the stable of fatigue crack growth (Region II) were decreased in proportion to descend temperature. It was shown that the fatigue resistance characteristics and fracture strength at low temperature are considerable higher than those of mom temperature in the early stage and stable of fatigue crack growth region.

• 한국안전학회지
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• 제19권1호
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• pp.31-37
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• 2004

The mechanical strength and crack propagation of plain woven carbon fiber fabric laminate composites are examined by acoustic emission(AE). AE signals are measured during the tensile test and fracture tests. Thus, the relationship between AE signal and load-displacement curves and crack extension length are shown. Also the fracture mechanisms in terms of AE characteristics are discussed in viewpoint of crack propagation behavior.

• 대한기계학회논문집A
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• 제25권6호
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• pp.907-915
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• 2001

Application of bonded dissimilar materials in many industries are increasing. When these materials are to be used in structures, it needs to evaluate the failure strength applying fracture mechanics. Al/Epoxy bonded dissimilar materials with an interface crack and an interface crack emanating from an edge semicircular hole were prepared, experiment of fracture toughness were carried out. Stress intensity factors of interface cracks in bonded dissimilar materials were computed with boundary element method(BEM) and the fracture criteria of mixed mode crack were analyzed. From the results, the fracture criteria and the method of strength evaluation by the fracture toughness in Al/Epoxy bonded dissimilar materials were proposed.

• Journal of Welding and Joining
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• 제30권3호
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• pp.57-65
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• 2012

The interest of austenitic heat resistant stainless steels containing high Si has increased because they have higher resistance of oxidation and higher strength at high temperature than STS 310. This study carried out Varestraint test for evaluation of solidification cracking sensitivity of 14 different stainless steels. As a result of Varestraint test, all specimens solidified as primary ferrite, and solidification crack sensitivity increased with adding $N_2$ to shielding gas. Nb and W had beneficial effect on solidification crack resistance in case of less than FN 2 containing, but crack sensitivity increased with Nb and W in case of more than FN 2. Ce had beneficial effect on solidification crack resistance but impaired weld bead appearance.

• 대한기계학회논문집A
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• 제21권7호
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• pp.1089-1096
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• 1997

In this paper, Rancho Seco transient which is reported as a typical pressurized thermal shock event is postulated to be occuring in the Kori unit 1 plant, the oldest nuclear power plant in Korea. For the given material properties, transient history such as temperature and pressure, and postulated flaw, the stress distribution is obtained to calculate stress intensities for a wide range of assumed crack sizes. The stress intensities are compared with the fracture toughness, which is determined using the material properties and the distribution of the nil ductility transition temperature, to determine if cracking is expected to occur during the transient. The allowable operating year for the transient is determined and the evaluation results are discussed.

• Steel and Composite Structures
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• 제36권4호
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• pp.463-480
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• 2020

Although detailed shell analysis is suitable to predict the ductile crack initiation life of steel members, such detailed method adds time expense and complexity. In order to simply predict the ductile crack initiation life of stiffened steel bridge piers, a total of 33 cases are simulated to carry out the parametric analyses. In the analysis, the effects of the width-to-thickness ratio, slenderness ratio, plate thickness and so on are considered. Both shell analyses and beam analyses about these 33 cases are conducted. The plastic strain and damage index obtained from shell and beam analyses are compared. The modified factor β_s is determined based on the predicted results obtained from both shell and beam analyses in order to simulate the strain concentration at the base corner of the steel bridge piers. Finally, three experimental results are employed to verify the validity of the proposed method in this study.

• 동력기계공학회지
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• 제10권2호
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• pp.117-123
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• 2006

Woven fabrics composites are used as primary structural components in many applications because of their superior properties that offer high specific strength and stiffness. However, the complexity of the fabric structure makes understanding of their failure behavior very difficult. Also, laminate woven fabrics CFRP have unique failure mechanisms such as fiber bridging, fiber/matrix crack and so on. In particular, the delamination phenomenon of the composite materials is one of the most frequent failure mechanisms. So, we estimated interlaminar fracture and damage in composites using as ENF specimen by a 3 point bending test. And AE characteristics were examined for crack propagation on plain woven CFRP. We obtained the following conclusions from the results of the evaluation of the 3 point bending fracture test and AE characteristic estimation. AE counts of maximum crack length were obtained as $85.97{\times}10^4\;and\;93{\times}10^3\;for\;a_0/L=0.3$ and 0.6, respectively. Also the maximum amplitudes were over 80dB at both $a_0/L=0.3\;and\;0.6$. $G_{IIc}$ at that's $a_0/L$ ratio were obtained with $1.07kJ/m^2\;and\;3.79kJ/m^2$.

• 한국정밀공학회지
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• 제23권4호
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• pp.116-123
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• 2006

During the last two decades, many researchers investigated influences of stress triaxiality on ductile fracture for various specimens and structures. With respect to a transferability issue, the local approach reflecting micro-mechanical specifics is one of effective methods to predict constraint effects. In this paper, the applicability of the local approach was examined through a series of finite element analyses incorporating modified GTN (Gurson-Tvergaard-Needleman) and Rousselier models as well as fracture toughness tests. To achieve this goal, fracture resistance (J-R) curves of several types of compact tension (CT) specimens with various crack length, with various thickness and with/without 20% side- grooves were estimated. Then. the constraint effects were examined by comparing the numerically estimated J-R curves with experimentally determined ones. The assessment results showed that the damage models might be used as useful tool for fracture toughness estimation and both the crack length and thickness effects should be considered for realistic structural integrity evaluation.

• 한국생산제조학회지
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• 제19권4호
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• pp.462-468
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• 2010

Fatigue crack growth caused by surface defects is one of the most important subjects for the evaluation and the assurance of safety in pressure vessels, piping systems, LPG/LNG fuel tank and other various structures. So, this paper aims to investigate the relationship between the surface roughness of fracture CT specimens and the moments on the specimen when doing fatigue test for the evaluation and the assurance of safety of structures from fatigue crack deconstruction. In this experiment, the CT specimens were loaded by a fatigue testing machine with changing loads until they are broken. The surface roughness of the fracture CT specimens was measured using 3D precise shape measuring equipment and digital holography. As a result of this study, It was identified that the average roughnesses are similar at the positions that has a same moments by comparing the results with the moments on the specimen according to the position.

• Computers and Concrete
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• 제32권5호
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• pp.475-486
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• 2023

Nondestructive evaluation (NDE) is an important task of civil engineering structure monitoring and inspection, but minor damage such as small cracks in local structure is difficult to observe. If cracks continued expansion may cause partial or even overall damage to the structure. Therefore, monitoring and detecting the structure in the early stage of crack propagation is important. The crack detection technology based on machine vision has been widely studied, but there are still some problems such as bad recognition effect for small cracks. In this paper, we proposed a deep learning method based on sweep signals to evaluate concrete surface crack with a width less than 1 mm. Two convolutional neural networks (CNNs) are used to analyze the one-dimensional (1D) frequency sweep signal and the two-dimensional (2D) time-frequency image, respectively, and the probability value of average damage (ADPV) is proposed to evaluate the minor damage of structural. Finally, we use the standard deviation of energy ratio change (ERVSD) and infrared thermography (IRT) to compare with ADPV to verify the effectiveness of the method proposed in this paper. The experiment results show that the method proposed in this paper can effectively predict whether the concrete surface is damaged and the severity of damage.