• 대한기계학회:학술대회논문집
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• 대한기계학회 2001년도 춘계학술대회논문집A
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• pp.573-578
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• 2001

For the corrosion protect ion of the natural gas transmission pipelines, two methods are used, cathodic protection and coating technique. In the case of cathodic protection, defects are embrittled by occurring hydrogen at the crack tip or material surface. It is however very important to evaluate whether cracks in the embrittled area can grow or not, especially in weld metal. In this work, on the basis of elastic plastic fracture mechanics, we performed CTOD test ing with varying test conditions, such as the potential and current density. The CTOD of the base steel and weld metal showed a strong dependence of the test conditions. The CTOD decreased with increasing cathodic potential and current density. The morphology of the fracture surface showed quasi-cleavage. Hydrogen introduced fractures, caused by cathodic overprotection.

• 산업기술연구
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• 제13권
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• pp.49-57
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• 1993

Bending fatigue tests were carried out on the specimenes with two different strength levels in order to investigate small crack growth rate. The main results obtained are as follows : (1) Small crack growth law, $da/dn=C{\sigma}_a{^n}a$ is useful several materials, but is not the rule applicable to all cases generally. (2) When da/dn in several specimens are equal, COD near the crack tip are also nearly equal. (3) Crack tip opening displacement(CTOD) is the main factor to control the small crack growth rate, and da/dn ${\propto}$ CTOD comes into being between the two. Accordingly, $da/dn=C({\sigma}_a{^2}/{\sigma}_s){^n}a^n$ shows the small crack growth rate being reflected crack closure phenomenon.

• 한국산업융합학회 논문집
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• 제26권5호
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• pp.875-884
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• 2023

Hydrogen infiltration into metals has been reported to induce alterations in their mechanical properties under load. In this study, we conducted CTOD (Crack Tip Opening Displacement) tests on steel specimens designed for use in liquid hydrogen storage systems. Electrochemical hydrogen charging was performed using both FCC series austenitic stainless steel and BCC series structural steel specimens, while CTOD testing was carried out using a 500kN-class material testing machine. Results indicate a notable divergence in behavior: SS400 test samples exhibited a higher susceptibility to failure compared to austenitic stainless steel counterparts, whereas SUS 316L test samples displayed minimal changes in displacement and maximum load due to hydrogen charging. However, SEM (Scanning Electron Microscopy) analysis results presented challenges in clearly explaining the mechanical degradation phenomenon in the tested materials. This study's resultant database holds significant promise for enhancing the safety design of liquid hydrogen storage systems, providing invaluable insights into the performance of various steel alloys under the influence of hydrogen embrittlement.

• 대한기계학회논문집
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• 제17권9호
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• pp.2223-2233
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• 1993

An engineering method is suggested to calculate the applied load versus crack extension in the elastic-plastic fracture. The condition for an increment of crack extension is set by a critical increment of crack-up opening displacement(CTOD). The ratio of the CTOD increment to the incremental crack extention is a critical crack-tip opening angle(CTOA), assumed to be constant for a material of a given thickness. The Dugdale model of crack-tip deformation in an infinite plate is applied to the method, and a complete solution for crack extension and crack instability is obtained. For finite-size specimens of arbitrary geometry in general yielding, an approximate generalization of the Dugdale model is suggested so that the approximation approaches the small-scale yielding solution in a low applied load and the finite-element solution in a large applied load. Maximum load is calculated so that an applied load attains either a limit load on an unbroken ligament or a peak load during crack extension. The proposed method was applied to three-point bend specimens of a carbon steel SM45C in various sizes. Reasonable agreements are found between calculated maximum loads and experimental failure loads. Therefore, the method can be a viable alternative to the J-R curve approach in the elastic-plastic fracture analysis.

• Journal of Mechanical Science and Technology
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• 제15권8호
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• pp.1079-1089
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• 2001

This paper proposes J and CTOD estimation schemes applied to fracture toughness testing, covering typical homogeneous and bi-material specimens. Recommendations are based on the plastic limit analysis (either slip line field or finite element limit analyses), assuming the rigid plastic material behavior. The main outcome of the present study is that the J and CTOD estimation schemes (both codified and non-codified), recommended for homogeneous specimens, can be equally used for bi-material specimens with interface cracks. The effect of yield strength mismatch in bi-material specimens on the J-integral CTOD is discussed.

• 한국기계가공학회지
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• 제20권3호
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• pp.24-32
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• 2021

The size of container ships is increasing to increase the cargo loading capacity. However, container ships are limited in terms of the hull longitudinal strength. To overcome this limitation, brittle crack arrest steel can be used. This study was aimed at examining the influence of the heat input on the welding procedures of flux cored arc welding and submerged arc welding. In the experiment, the crack tip opening displacement test, which pertains to a parameter of fracture mechanics, was performed, and a 3-point bending tester was adopted. Based on the results, the crack measurement method was presented, and the stress expansion coefficient value for the pre-fatigue crack length was derived according to the heat input after the pre-cracking length was measured. It was noted that the heat input affected the crack tip opening displacement of brittle crack arrest steel.

• 대한기계학회논문집A
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• 제40권12호
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• pp.1021-1026
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• 2016

본 연구는 점탄성을 가진 HTPB 입자강화 복합재료의 균열저항에 대하여 쐐기분열시험 및 균열선단열림각도를 이용하여 평가하였다. 일반적으로 균열진전 함수로서 균열선단열림각도는 파괴저항을 결정하기 위해 사용되며, 수치적으로 일정한 값을 가진다. 디지털 화상관련법은 임계 균열선단열림각도를 도출하기 위하여 균열선단열림변위 및 균열진전길이를 측정하기 위해 사용되었다. 본 연구의 시험 결과에서 입자강화 복합재료의 임계 균열선단열림각도는 균열초기 진전 후 일정한 값을 가진다. 임계 균열선단열림각도는 $50^{\circ}C$에서 $-40^{\circ}C$로 온도의 감소에 따라 증가한다. 이러한 임계 균열선단열림각도는 입자강화 복합재료의 파괴역학 파라미터로 사용될 수 있다.

• 한국기계가공학회지
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• 제20권7호
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• pp.8-14
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• 2021

Even welds that have passed non-destructive testing in the case of brittle crack arrest steel materials will actually have very fine weld defects. Based on studies showing that these defects adversely affect the structure if subjected to a certain period of load, the following conclusions were obtained by conducting CTOD tests on welding joints of high-strength BCA materials, structures comprising the upper decks of a large container vessel. First of all, the fatigue pre-cracking in the weld metal and heat affected areas was tested and the behavior was identified. Both parts of the welding joint are allowable range for the class regulations. In addition, CTOD results showed that the CTOD value in the heat affected area was more than 0.5 times higher than in the weld metal area.

• 대한기계학회논문집A
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• 제26권3호
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• pp.528-536
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• 2002

As one step for the safety performance of LNG storage tank, the change in fracture toughness within the X-grooved weld heat-affected zone (HAZ) of newly developed 9% Ni steel, which was submerged arc (SA)-welded, was investigated. Both crack initiation fracture toughness and crack arrest fracture toughness were evaluated by the crack tip opening displacement (CTOD) tests and compact crack arrest (CCA) tests. As the evaluated region approached the fusion line, each test result shorted different tendency, that is, crack initiation toughness decreased while crack arrest toughness increased. The results were discussed through the observation of the microstructural change.

• Journal of Welding and Joining
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• 제7권4호
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• pp.38-45
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• 1989

In this study, in order to evaluate the shape and the size of the plastic zone at the notch tip before stable crack growth, a newly developed technique for plastic strain measurement, that is, the recrystallization-etching technique was applied to observe the intense strain zone at the notch tip of weld HAZ. 1) The recrystallized specimens showed that the amount of the intense strain zone, more than 20% plastic zone, was quantitatively observed as the plane strain state during the growth of the plastic zone. 2) The behavior of plastic deformation at midsection are different for parent and weld HAZ. In addition, the micro crack initiation occurs at midsection, parent and weld HAZ when the crack opening displacement(COD) value is .delta.$_{t}$=0.4mm. 3) The plastic zone for parent proceeds in the forward direction at notch tip and for weld HAZ in the right and left direction at the notch tip. 4) The relation between plastic strain energy(Wp) and COD(.delta.$_{t}$) depended on yield stress, gradient and plastic strain size.ize.