• 대한기계학회논문집
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• 제14권3호
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• pp.603-609
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• 1990

본 연구에서는 미소균열의 정의로서 균열의 크기가 재료의 조직의 크기와 order적으로 같은 균열의 특성이라는 것과 균열의 크기가 소성역 크기와 order적으로 같은 균열의 특성에 착안해서 탄소강 평활재와 예균열재(pre-cracked specimen)에 대 해서 응력비 R=-1 및 R=0의 피로한도 특성과 평활재의 미소균열의 검출 및 정류균열의 생성기구를 균열 열림 닫힘에 주목해서 검토하였다.

• Corrosion Science and Technology
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• 제3권1호
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• pp.14-19
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• 2004

Crack growth rate and threshold stress intensity factor for stress corrosion cracking(SCC), $K_{ISCC}$ were measured for type 403 stainless steel in 3,5% NaCl solution at room temperature and SCC was monitored by electrochemical noise technique during $K_{ISCC}$ testing. In rising load test, pits were formed at the tip of pre-crack for the pre-cracked compact tension specimen unlike in smooth round specimen in which only unstable pits were observed and hence immune to SCC. Micro-cracks were found to initiate from the pits in the former specimen, and initiation of micro-crack as well as macro-crack was detected by electrochemical noise technique in rising load $K_{ISCC}$ tests. Crack growth rate increased with increasing either displacement rate or stress intensity factor at crack initiation and was higher in rising load $K_{ISCC}$ test compared to constant load $K_{ISCC}$ test at given stress intensities.

• Structural Engineering and Mechanics
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• 제55권4호
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• pp.885-900
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• 2015

This paper introduces Romberg-Richardson's method as one of the numerical integration tools for computation of stress intensity factor in a pre-cracked specimen subjected to a complex stress field across the crack faces. Also, the computation of stress intensity factor for various stress fields using existing three methods: average stress over interval method, piecewise linear stress method, piecewise quadratic method are modified by using Richardson extrapolation method. The direct integration method is used as reference for constant and linear stress distribution across the crack faces while Gauss-Chebyshev method is used as reference for nonlinear distribution of stress across the crack faces in order to obtain the stress intensity factor. It is found that modified methods (average stress over intervals-Richardson method, piecewise linear stress-Richardson method, piecewise quadratic-Richardson method) yield more accurate results after a few numbers of iterations than those obtained using these methods in their original form. Romberg-Richardson's method is proven to be more efficient and accurate than Gauss-Chebyshev method for complex stress field.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 1998년도 특별강연 및 춘계학술발표 개요집
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• pp.49-51
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• 1998

• 대한기계학회논문집
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• 제19권2호
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• pp.402-407
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• 1995

The effects of room temperature and strain aging treatment are discussed on the critical condition for the onset of growth of non-propagating cracks on 0.15% C low carbon steel, with special emphasis on the length of the critical non-propagating crack and on the crack opening displacement(COD) at the crack tip. It is found from the experimental analysis that room temperature and strain aging of a fatigue pre-cracked specimen introduced the closure of a crack tip of the pre-crack and the reduction of crack opening displacement at the wake of crack, together with an improvement in crack growth resistance of the microstructure. This may cause an increase in the endurance limit of the specimen, through the enhancement of effective stress for the onset of growth of the critical non-propagating crack.

• 한국군사과학기술학회지
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• 제4권2호
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• pp.42-51
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• 2001

The effects of pre-indentation technique are presented for A12024-T3 Alclad alloy using as skin material for aircraft fuselage and wing. Indentations were applied to specimens to be placed on the presumed path of fatigue crack growth before fatigue tests. Tension-tension fatigue tests were conducted on the edge cracked specimens in the L-T orientation. Test results were analyzed to investigate the effectiveness of pre-indentation with the variation of specimen's thickness, position of indentation and applied maximum stress. Fatigue crack retardation by pre-indentation is well recognized in the various conditions.

• 비파괴검사학회지
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• 제32권4호
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• pp.355-361
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• 2012

This paper presents an experimental study on the detection and location of nonlinear scattering source due to the presence of fatigue crack in a laboratory specimen. The proposed technique is based on a combination of nonlinear elastic wave spectroscopy(NEWS) and time reversal(TR) focusing approach. In order to focus on the nonlinear scattering position due to the fatigue crack, we employed only one transmitting transducer and one receiving transducer, taking advantage of long duration of reception signal that includes multiple linear scattering such as mode conversion and boundary reflections. NEWS technique was then used as a pre-treatment of TR for spatial focusing of reemitted second harmonic signal. The robustness of this approach was demonstrated on a cracked specimen and the nonlinear TR focusing behavior is observed on the crack interface from which the second harmonic signal was originated.

• Journal of Mechanical Science and Technology
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• 제20권2호
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• pp.197-204
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• 2006

The purpose of this study is to evaluate the AE characteristics for the basemetal, PWHT (post-weld heat treatment) and weldment specimens of SA-516 steel during fracture testing. Four-point bending and AE tests were conducted simultaneously. AE signals were emitted in the process of plastic deformation. AE signal strength and amplitude of the weldment was the strongest, followed by PWHT specimen and basemetal. More AE signals were emitted from the weldment samples because of the oxides, and discontinuous mechanical properties. AE signal strength and amplitude for the basemetal or PWHT specimen decreased remarkably compared to the weldment because of lower strength. Pre-cracked specimens emitted even lower event counts than the corresponding blunt notched specimens. Dimple fracture from void coalescence mechanism is associated with low-level AE signal strength for the basemetal or PWHT. Tearing mode and dimple formation were shown on the fracture surfaces of the weldment, but only a small fraction produced detectable AE.

• 비파괴검사학회지
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• 제13권3호
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• pp.7-13
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• 1993

AISI 4130 금속재를 사용한 균열 컴팩터 인장시편의 파괴거동이 음향방출(AE) 및 초음파 C-scan을 이용하여 조사되었다. 모든 시편들을 특정 수준의 하중까지 증가시키면서, 크랙 개구변위 (COD)와 더불어 여러가지의 음향방출 인자들을 얻었다. 크랙 선단의 크랙 개구변위와 손상(소성)역을 계산하기 위하여 탄소성 유한요소 해석이 수행되었다. 펄스-반사, 침수형으로한 초음파 C-scan은 손상역 크기와 상사시키기 위하여 행해졌다. 유한요소 해석 결과와 측정된 크랙개구변위는 만족할 정도로 일치하였다. 음향방출 결과에서, 시험시편들은 연성거동을 나타내었다. 총 사상수대 크랙 개구변위의 기울기는 크랙 개시점을 결정하는 데에 유용하였다. 예비 시험적인 C-scan 화상은 손상역의 초음파 진폭변화를 보여주었고, 손상역의 형상 및 크기가 유한요소 결과와 정성적으로 부합되었다. 손상역 크기에 관한 추가적인 연구가 요약되었다.

• 한국압력기기공학회 논문집
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• 제12권1호
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• pp.62-69
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• 2016

Current guidance considers that uniaxially loaded specimen with a deep crack is used for the determination of the ductile-to-brittle transition temperature. However, reactor pressure vessel is under biaxial loading in real and the existence of deep crack is not probable through periodic in-service-inspection. The elastic stress intensity factor and the elastic-plastic J-integral which were used for crack-tip stress field and fracture mechanics assessment parameters. The difference of the loading condition and crack geometry can significantly influence on these parameters. Thus, a constraint effect caused by differences between standard specimens and a real structure can over/underestimate the fracture toughness, and it affects the results of the structural integrity assessment, consequentially. The present paper investigates the constraint effects by evaluating the master curve $T_0$ reference temperature of PCVN (Pre-cracked Charpy V-Notch) and small scale cruciform specimens which was designed to simulate biaxial loading condition with shallow crack through the fracture toughness tests and 3-dimensional elastic-plastic finite element analyses. Based on the finite element analysis results, the fracture toughness values of a small scale cruciform specimen were estimated, and the geometry-dependent factors of the cruciform specimen considered in the present study were determined. Finally, the transferability of the test results of these specimens was discussed.