• 동력기계공학회지
• /
• 제13권4호
• /
• pp.43-48
• /
• 2009

The purpose of this study is to show how to eliminate the noises and to obtain the data related with the relationship between AE signal characteristics and mechanical behaviors for the pressure vessel steel. Various kinds of noises are introduced into the AE data in the course of experiments. Accordingly, real AE data have to be obtained after tests. AE test was carried out under four point bending load. Among AE signals, counts and signal strength are used to find out the differences of AE characteristics between the basemetal and weldment. After tests, this paper shows the procedures of filtering the noises against basemetal of the pressure vessel steel to obtain the real data around crack tip. Relationships between plastic zone size and cumulative AE counts are shown also. AE signals were absent within an elastic region, regardless of the specimens. Most of AE signals are produced in the process of plastic deformation. The deformation and fracture modes of basemetal and weldment are quiet different.

• 한국안전학회지
• /
• 제2권3호
• /
• pp.21-27
• /
• 1987

The effect of plastic deformation in an interfacial crack is considered. Yield zones are assumed to have the form of a strip along the interface. The crack is subjected to mode III loads at infinity and lies along the interface of two semi-infinite planes with different material properties. The size of the yield zones, the relation between the size of the yield zone and CTOD are obtained in a closed form solution. The J integral also can be obtained in a closed form Solution.

• 한국분말재료학회지
• /
• 제5권1호
• /
• pp.22-27
• /
• 1998

The effect of the $\alpha$/$\beta$ phase fraction on the mechanical properties in silicon nitrides was investigated in part 1. In part II, we describe the role of microstructure on the mechanical properties and contact damage of silicon nitrides with coarse/equiaxed and coarse/elongated microstructures. Grain sizes and shapes were controlled by starting powder. Hertzian indentation using spherical indenter was also used to investigate contact damage behavior. Cone cracks from the spherical indentation were suppressed when the silicon nitride contains coarse and elongated grains. Coarse and elongated grains played an important role of cone crack suppression. The size of quasi-plastic zone does not depend on grain size or shape but depends on the fraction of $\alpha$/$\beta$ phase. A quasi-plastic zone was consisting of microcracks by shear stress during indentation.

• 대한기계학회논문집A
• /
• 제28권8호
• /
• pp.1109-1115
• /
• 2004

The objective of this study is to elucidate what governs delayed hydride cracking (DHC) in Zr-2.5Nb tubes by correlating the striation spacings with DHCV(DHC Velocity). To this end, DHC tests were conducted on the compact tension specimens taken from the Zr-2.5Nb tubes at different temperatures ranging from 100 to $300^{\circ}C$ with a 3 to 6 data set at each test conditions. The compact tension specimens were electrolytically charged with 27 to 87 ppm H before DHC tests. After DHC tests, the striation spacings and DHCV were determined with the increasing the test temperature and yield strength. The striation spacing and DHCV increased as a function of yield $strength^2$ and the temperature. Since the plastic zone size ahead of the crack tip can be represented by ${\sim}(K_{IH}/{\sigma}_{Y})^2$, we conclude that the striation spacing is governed by the plastic zone size which in turn determines a gradient of hydrogen concentration at the crack tip. The relationship between the plastic zone size and the striation spacing was validated through a complimentary experiment using double cantilever beam specimens. Two main factors to govern DHCV of Zr-2.5Nb tubes are concluded to be hydrogen diffusion and a hydrogen concentration gradient at the crack tip that are controlled by temperature and yield strength, respectively. The activation energy of DHCV in the Zr-2.5Nb tubes is discussed on the basis of temperature dependency of hydrogen diffusion and the striation spacing.

• Advances in materials Research
• /
• 제4권2호
• /
• pp.87-96
• /
• 2015

In this paper, three-dimensional finite element method is used to analyze the J integral for repaired cracks in plates with bonded composite patch and stiffeners. For elastic the effect of cracks, the thickness of the patch ($e_r$) and properties of the patch are presented for calculating the J integral. For elastic-plastic a several calculations have been realized to extract the plasticized elements around the crack tip of repaired and un-repaired crack. The obtained results show that the presence of the composite patch and stiffener reduces considerably the size of the plastic zone ahead of the crack. The effects of crack size and the inter-distance of repaired cracks were analysed.

• 대한기계학회논문집
• /
• 제10권1호
• /
• pp.25-33
• /
• 1986

본 연구에서는 선단요소 해법의 방법을 토대로 소형컴퓨터를 위한 비선형 유한요소의 개발을 시도한 것이다. 주로 참조한 선단요소 해법의 프로그램은 Hinton과 Owen이 작성한 프로그램이며 원판메모리를 최대로 활용하여, 활동변수를 최소화 시키므로써, 실제 소형컴퓨터인 HP-3000II(512KB) 컴퓨터에서 총 자유도가 1000정도 되는 유한요소까지는, 해석이 가능하도록 만들어지게 되었다. 이와같이 완성된 프로그램의 응용성과 신뢰성을 검토해 보기 위해서 표준 CT 시편의 유한요소 를 작성하여(124 element, 428 node, 941 freedom) 크랙선단에 형성되는 소성역의 형상과 소성변형 크기를 수치적으로 추적하여 본 결과, 실험결과와 매우 잘 일치함을 볼 수 있어서 프로그램의 신뢰성을 확인 할 수 있었다. 이때 실험은 SUS-304스테인 레스강단의 소성역을 형성시킨다음, 재결정 방법에 의해 소성역의 형상과 크기를 가시 화 및 정량화 하여서 계산결과와 비교 하였다.

• 비파괴검사학회지
• /
• 제13권3호
• /
• pp.7-13
• /
• 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.

• 대한기계학회논문집A
• /
• 제21권7호
• /
• pp.1097-1105
• /
• 1997

Because of the existence of stress interaction field made by other defects and propagating cracks, the structure may be weakened. Therefore in this study, the crack behavior in the interaction field made by two different cracks is studied experimentally. In the experiment, vertical distance between two cracks and applied stress are varied to make different stress interacted field. In addition, the effect of plastic zone is used to examine crack propagation path and rate. Three types of crack propagation in the interacted field were found, and crack propagating path and rate of two cracks were significantly changed according to different applied stress as each crack propagates. And the results are attributed to the effect of the size and shape of the plastic zone.

• 한국정밀공학회지
• /
• 제19권4호
• /
• pp.117-123
• /
• 2002

X-ray diffraction observation of fracture surfaces yields useful information to analyze the causes of failure accidents of engineering structures. This experimental technique, named X-ray fractography, has been developed especially in metal and mechanical engineering fields. The distributions of the residual stress and the half value breadth of diffraction profiles beneath the fatigue fracture surface were measured with SNCM 439, HT100 and Ti-6Al-4V alloy. The size of the maximum plastic zone was successfully determined on the basis of the measured distributions. This size was correlated to maximum stress intensity factor. The distributions of the half value breadth of diffraction profiles on the fatigue fracture surfaces were measured with SNCM 439. HT100. The equations of x-ray parameter distribution were possible to estimated fracture parameters of fatigue fracture surfaces.

• 대한기계학회논문집
• /
• 제18권9호
• /
• pp.2331-2338
• /
• 1994

Experimental conditions of the optical shadow methods of caustics for measurement of the stress intensity factor and the J-integral in various materials(polycarbonate, PMMA, Al 5586D) are investigated. The necessary experimental requirements toe determine accurate values of the stress intensity factors and the J-integrals are described. The ratio of $r_o$ (radius of initial curve) to $r_p$ (plastic zone size) is selected as a parameter to verify the experimental limitation of the method of caustics in determination of fracture parameters. In this study, transmission caustics method was used for compact tension specimens made of polycarbonate and PMMA. while reflection caustics method was applied to c-shaped tension specimen made of Al 5586D. The appropriate ranges of $r_o/r_p$ tp determine accurate values of stress intensity factors were found to be 1.5~1.8. Existing experimental results have been obtained mainly by changing $r_p$ with different loads in $r_o/r_p$. However, in this study we could obtain varying $K_{caus}/K_{th}$ over the wide range of $r_o/r_p$ at fixed load conditions with newly designed optical arrangement. Thus, we could find the range in which theoretical and experimental results agree well each other by changing $r_o$ values only. In Al 5586D specimen, experimental caustics were located inside of the plastic zone, and $K_{caus}/K_{th}$ were found to be not unity in this range. It is found that $J_{caus}/J_{th}=1{\;}with{\;}r_o/t{\geq}0.8$ and the experimental plastic zone includes the contours of caustics.