• Nuclear Engineering and Technology
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• v.52 no.9
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• pp.2092-2099
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• 2020

Many damages due to flow-accelerated corrosion and cracking have been observed during recent in-service inspections of nuclear power plants. To determine the operability or repair for damaged pipes, an integrity evaluation related to the damaged piping system should be performed by using already proven code and standards. One of them, the ASME Code Case is most popularly used to integrity assessment in nuclear power plants. However, the recent version of CC N-513 still recommends the simplified method which means a damaged elbow is assumed as an equivalent straight pipe. In addition, to enhance the accuracy integrity assessment in elbow, several previous studies recommend that the SIF and elastic COD values for an elbow with relatively large crack could be predicted by an interpolation technique. However, those estimates for elbow with relatively large crack might be derived to inaccurate results for crack growth analysis, such as for the allowable crack size and life estimation. Therefore, in this paper, the effect of crack length (0.3≤θ₁/π≤0.5) on SIF and elastic COD for elbow is systematically investigated. Then, for large crack in elbow, accurate estimates for SIF and elastic COD, which are widely used to assess the integrity of elbows, are proposed. Those proposed solutions are expected to be the technical basis for revisions of CC N-513-4 through the validation.

• Journal of the Korea Concrete Institute
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• v.12 no.3
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• pp.3-10
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• 2000

In this study, we compared a plain mortar with the CAS-system shrinkage compensation mortar for Ondol that is Korean traditional heating system. The Ondol mortar is necessary to have properties as non-crack, fine flatness and stability for thermal changes. especially, mortar'crack prevention is to be most important property in Ondol mortar. To develope the stable material on the crack-prevention, we used to calcium-sulfo-aluminate(CSA)system in shrinkage compensation mortar. And so, we confirmed the effects of calcium-sulfo-aluminate(CSA) system for mortar's physical properties such as setting time, compressive strength and expansion ratio for crack prevention. The initial and final setting time of the CSA mortar is faster than plain mortar about 2hours. And, Compressive strength increased about 20% that plain mortar. The crack length per unit area, plain mortar is 0.426∼0.481m/m2. The Results of apartment construction field test, the shrinkage compensation mortar is excellent about the crack-reduce effect.

• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.28 no.3
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• pp.286-294
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• 1992

In this paper, the corrosion fatigue crack propagation behavior of structure rolled steel (SWS 41C) was investigated by changing the thickness, and this experiment was done by the three point bending corrosion fatigue tester. The main results obtained are as follows: 1) As the thickness of specimen becomes thicker, the corrosion sensitivity to initial stage crack becomes some sensitive, and that the fatigue life becomes more sensitive. 2) The crack growth rate to initial stage crack (da/dN) was retarded as the thickness of specimen becomes thicker. But after initial stage crack, as the thickness of specimen is more thicker, da/dN is more rapid. 3) As the corrosion fatigue crack length grows, the accelerative factor of thick specimen (t=12mm) is more higher than that of thin specimen (t=6mm). 4) As the corrosion fatigue crack length grows, the corroson potential of both thick specimen and thin specimen becomes more less noble potential, however thick specimen (t=12mm) tends to more less noble potential than that of thin specimen(t=6mm).

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.05a
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• pp.856-860
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• 2002

The main objective of this study is to consider the effect of fatigue crack behavior on the variable depth of micro hole defects in SM20C at the symmetric position. The fatigue crack propagation test is performed by rotary bending fatigue test machine. The relationship between crack length(2a), cycles(N) and crack growth rate(da/dN) are investigated in this study. The result from the rotary bending fatigue test under the applied stress at 250MPa turned out that the fatigue life illustrated almost constant when the depth of symmetric micro hole deflects is both part A and B at the hope depth(h) = 0.5mm.

• Journal of the Society of Naval Architects of Korea
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• v.40 no.3
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• pp.45-53
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• 2003

Ships and ocean structures are subjected to random loads caused by irregular waves. The irregularity of amplitude from random loading affects on fatigue crack growth and fatigue life. However the effects of irregularity of loading on fatigue including random loading have not been explained exactly. Therefore in this paper crack growth tests on DENT specimens under constant-amplitude loading including a single tensile overload are conducted to investigate the effect of overload on crack growth rate. The size of plastic zone and crack growth rate before and after a single tensile overloading are measured using ESPI system. Crack growth retardation model that is characterized by crack growth length and the size of plastic zone was proposed and compared with test result. From the research, the validity of proposed model is examined on crack growth retardation, and consequently fatigue life.

• Journal of the Korean Society for Precision Engineering
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• v.23 no.4 s.181
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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.

• Journal of the Korean Society of Manufacturing Technology Engineers
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• v.25 no.5
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• pp.386-392
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• 2016

The sizes of last stage blades (LSB) in a low-pressure steam turbine have been getting larger for the development of high-capacity power plants. They are also larger than other blades in the same system. As a result, crack propagation in an LSB is caused by the large centrifugal force, low natural frequency, and repeated turbine startups. In this study, the critical crack length for a fracture and vibration characteristics, in accordance with crack propagation, were analyzed using a finite element method to calculate the stress intensity factor (SIF) and the natural frequency that was affected by the stress-stiffening effect. It was calculated that the frequency of the third and fifth modes passed the excited harmonic resonance (5X and 10X) and the observed calculated critical crack length matched that of the real fractured surface.

• Journal of Advanced Marine Engineering and Technology
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• v.8 no.1
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• pp.49-63
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• 1984

The 5086-H116 Al-Alloy plate specimens having an edge through-thickness notch were investigated to find out the characteristics of the corner crack propagation by the plane bending fatigue. The experiments were also carried out in order to clarify the change of the corner crack propagation behaviour due to the various materials and their thicknesses. In addition, the retardation effect of overload on the corner crack propagation was quantatively studied. Main results obtained are as follows; 1. In the case of estimating the crack propagation rate of the corner crack, it is more reasonable to consider the growth rate of fracture surface area than that of crack length. 2. The shape of the corner crack growing in the plane plate under the bending fatigue can be estimated. 3. The crack propagation rate increases with the increasing of the thickness and the decreasing of the Young's modulus of materials. 4. Regardless of a thickness and kind of materials of specimen, the characteristics of the corner crack propagation can be concluded. 5. The retardation effect of overload is distinct in the corner crack propagation.

• Transactions of the Korean Society of Mechanical Engineers
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• v.11 no.2
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• pp.243-252
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• 1987

The propagation and closure behavior of surface crack initiated at a sharply notched specimens were investigated in 5083-H113 aluminium alloy under constant amplitude of tension load by the unloading elastic compliance method. The crack shape (aspect ratio) was found to be approximately semicircular during the crack was being small and to be changed to semi-elliptical during it was being long. The propagation rate of a surface crack initiated from notch root decelerated with increasing crack length when the crack was small and then accelerated when it was large. The effect of stress ratio was large in lower .DELTA.K range, but the effective stress intensity factor range .DELTA.K$_{eff}$ was found to diminish the difference of the crack propagation rate. By considering the increase in crack closure stress with crack length and examining the microphotographs, plasticity-induced and roughness-induced crack closure mechanisms were predominant in the range of this study.y.

• Journal of Ocean Engineering and Technology
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• v.6 no.1
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• pp.96-104
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• 1992

Aircraft structures and engineering structures are always subject to variable amplitude loads. Variable amplitude loads include some kind of loading history; for example, constant amplitude load, single peak overload and block overload etc. Crack growth under variable amplitude loading exhibits retardation effect. In this study, the 4 point bending fatigue test was performed by hydrolic servo fatigue testing machine on 7075-T6 Al-alloy. The retardation effect of overload ratio and numbers of overload cycle was quantitatively studied. 1) Change of retardation effect against increment of overload ratio is more evident when the multiple overload is applied than single overload is done. 2) The number of overload cycle is very important factor about the crack growth retardation effect when the overload ratio is more than 1.75; that is not when the overload ratio is less than 1.75. 3) Overload affected zone size increased gradually by increment of crack growth retardation effect. 4) Crack driving force is more greatly reduced when the crack tip branched off two direction than it sloped to one direction.