• Transactions of the Korean Society of Mechanical Engineers
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• v.10 no.6
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• pp.898-905
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• 1986

In order to obtain the microstructure improving fatigue crack propagation resistance of steels, fatigue crack propagation behavior of martensite-ferrite dual phase steels is investigated in terms of crack deflection and crack closure. The results obtained are as follows; (1) .DELTA.K$_{th}$ and fatigue crack propagation resistance in low .DELTA.K region increases with increasing hardness of second phase. But the difference of this crack propagation resistance decreases with increasing .DELTA.D. (2) In low .DELTA.K region, crack closure increases with increasing hardness of second phase, when the materials have all the sam volume fractionof second phase, or when yield strengths are similar in all materials. (3) These crack closure can be explained by fracture surface roughness due to crack deflection.n.

• Journal of the Korean Society for Precision Engineering
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• v.9 no.1
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• pp.106-117
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• 1992

In this study, the propagation behaviour and the closure phenomena of physically small surface cracks were investigated by the techinque of the Kikukawa-unloading elastic compliance method using a back face strain gage. The surface cracks initiated and propagated from notched specimens under constant amplitude bending load. The crack shape (aspect ratio) with approximately semi-circular at the early stage was changed to semi-elliptical as the cracks grew larger. The crack depth (a) could be expressed uniquenly by the crack length (c). The dependence of the crack propagation rate on the stress ratio R was strongly related in the lower ${\Delta}K$ range. The deceleration of the surface crack propagation rate was prominent in lower R during the crack length was small. When the propagation rate was rearranged with the effective stress intensity factor range ${\Delta}$K_{eff} the dependence of the crack propagation rate on the stress ratio R was found to be diminshed. These were caused by the crack closure phenomena that was most prominent at the lower propagation rate. The mechanism of crack closure phenomena was dominated by the plasticity-induced mechanism.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.7 no.2
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• pp.99-106
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• 1987

Annealing were performed to investigate the behaviors of the residual stress remaining on the member of a steel structure. According to the fatigue test, the welding part has higher fatigue crack growth rate than the base metal part because the hardening of welding part reduce fracture toughness. However, the heat treatment decrease the hardness and increase the resistance to failure. Thus, the fatigue crack growth rate is improved and it reaches the minimum at $650^{\circ}C$. Elber' s equation includes the effect of the crack-close so that this equation provides a lower the fatigue crack growth rate than Paris-Erdogan' s equation, the Elber's curves show no significant difference to indentify the effect of the residual stress. The Pop loading along the crack length increases as the hardness goes higher. The heat treatment not only decrease the hardness, and the fatigue crack growth rate, but increase the absorption energy and fracture toughness on the member of a steel structure. As the result, the heat treatment produces the resistant ability to cracking which can reduce the degree of danger to failure.

• Proceedings of the Korean Nuclear Society Conference
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• 1995.05b
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• pp.709-714
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• 1995

영광 3, 4호기에 사용되고 있는 원전 압력용기용 SA-508 IH 강의 미세구조 및 상온 피로균 열성장 특성을 고찰하였다. 본 강재의 미세구조, 석출물 분포 및 형상을 투과전자현미경을 통하여 관찰하였으며 임계영역에서의 거동 및 균열닫힘에 주목하여 피로균열성장 특성을 연구하였다. 다양한 형태의 (Fe, MR)$_3$C 세멘타이트 및 Mo$_2$C 석출물이 입계, 래스경계면 및 입내에 분포되고 있음을 확인하였다. Paris 영역에서의 피로균열성장 속도는 ASME 기준선과 유사하였으며, 임계 영역에서는 일반적인 저합금강의 경우보다 다소 낮게 나타났다. 파면조사 결과 입내 연성파괴현상이 전 $\Delta$K 영역에서 나타나고 있으며, $\Delta$K$_{th}$ 부터 $\Delta$K가 12 Mpa√m 영역에서는 입계파괴 및 입내 평활면이 관찰되었다.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.11
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• pp.1361-1368
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• 2011

Fatigue crack propagation behavior and the fatigue life in-high performance steel were investigated by means of fatigue crack propagation tests under constant loading conditions of 'R=0.1 and f=0.1 Hz', 'R=0.3 and f=0.3 Hz', and 'R=0.5 and f=0.5 Hz' for the load ratio and frequency, respectively. A modified Forman model was developed to describe the fatigue crack propagation behavior for the conditions. The modified Forman model is applicable to all fatigue crack propagation regions I, II, and III by implementing the threshold stress intensity factor range and the effective stress intensity factor range caused by crack closure. The results show that predicted fatigue lives of Forman and modified Forman models were 8,814 and 12,292 cycles, respectively when the crack propagated approximately 5.0 mm and the load ratio and frequency were both 0.1. Comparison of the test results indicates that the modified Forman model showed much more effective fatigue crack propagation behavior in high-performance steel.

• Journal of Korean Society of Steel Construction
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• v.15 no.5 s.66
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• pp.475-487
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• 2003

This study developed an analysis model of estimating fatigue damage using the linear elastic fracture mechanics method. Stress history occurring to an element when a truck passed over a bridge was defined as block loading and crack closure theory explaining load interaction effect was applied. Stress range frequency analysis considering dead load stress and crack opening was done. Probability of stress range frequency distribution was applied and the probability distribution parameters were estimated. The Monte Carlo simulation of generating the probability various of distribution was performed. The probability distribution of failure block numbers was obtained. With this the fatigue reliability of an element not occurring in failure could be calculated. The failure block number divided by average daily truck traffic remains the life of a day. Fatigue reliability analysis model was carried out for the welding member of cross beam flange and vertical stiffener of steel box bridge using the proposed model. Consequently, a 3.8% difference was observed between the remaining life in the peak analysis method and in the proposed analysis model. The proposed analysis model considered crack closure phase and crack retard.

• Journal of the Korean Society for Nondestructive Testing
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• v.28 no.4
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• pp.352-357
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• 2008

The detection of micro cracks in materials at the early stage of fracture is important in many structural safety assurance problems. The nonlinear ultrasonic technique (NUT) has been considered as a positive method for this, since it is more sensitive to micro crack than conventional linear ultrasonic methods. The basic principle is that the waveform is distorted by nonlinear stress-displacement relationship on the crack interface when the ultrasonic wave transmits through, and resultantly higher order harmonics are generated. This phenomenon is called the contact acoustic nonlinearity (CAN). The purpose of this paper is to prove the applicability of CAN experimentally by detection of micro fatigue crack artificailly initiated in Aluminum specimen. For this, we prepared fatigue specimens of Al6061 material with V-notch to initiate the crack, and the amplitude of second order harmonic was measured by scanning along the crack direction. From the results, we could see that the harmonic amplitude had good correlation with COD and it can be used to detect the crack depth in more accurately than the common 6 dB drop echo method.

• Journal of the Korean Recycled Construction Resources Institute
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• v.6 no.4
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• pp.283-288
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• 2018

Since the crack self-healing materials are activated according to the exposure environmental conditions from the time of crack occurrence, it is very important to clarify the relationship between the healing performance and the exposure environmental conditions of the crack surface. In this paper, the influence of the exposure environmental conditions on the crack healing performance of self-healing repair mortar was investigated through the water permeability test. The influence of temperature and humidity on the crack width of cracked specimens was evaluated. As a result of measuring the change of the crack width, the effect of curing temperature was negligible but it was confirmed that crack-closing occurred due to the change of dry-wet condition. The healing materials produced on the crack surface of the specimens was identified as calcite minerals. Since the minerals with high density are precipitated under the influence of gravity, the healing performance is somewhat different according to the direction of the crack surface, and the healing performance was significantly improved in the wet exposure condition than the air exposure condition.

• Journal of the Korea Concrete Institute
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• v.29 no.1
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• pp.11-21
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• 2017

The paper presents experimental results of self-healing effects of OPC-GGBFS paste with cracked and uncracked specimens. The self-healing of cracked specimens is processes of crack closing with re-hydration of unhydrated in crack surface. The OPC paste with GGBFS replacement ratios of 0%, 10%, 20% and 30% were prepared having a constant water-binder ratios of 0.5. The OPC-GGBFS paste specimens immersed in tap-water and sea-water. The temperature of tap and sea-water was $5^{\circ}C$, $15^{\circ}C$ and $25^{\circ}C$. The cracked specimens after deterioration were immersed for 60 days. The relationship between self-healing effect and age-effect was calculate based upon the experimental results. The self-healing effect was measured in ultrasonic pulse velocity (UPV) before and after loading. When the relative change rate of UPV is increases with the increase in GGBFS replacement ratios. Moreover, the self-healing effect is increased with the temperature of tap-water is increase. But the cracked specimens immersed in sea-water was unclear effects of different temperature. Furemore, most of the healing for OPC-GGBFS specimens immersed in tap-water and sea-water occurred during the first 30 days. The self-healing effect until 30 days is higher than that the age-effect. After 30 days, self-effect and age-effect was largely decreases. SEM/EDS analysis of crack on the surface of the specimens in tap-water were covered with aragonite, and sea-water were covered with brucite.