발간논문

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Vol.28, No.6, 514 ~ 521, 1990
Title
In Situ SEM Observation of Fracture Processes at a Blunt Notch Tip
김병천Byung Chun Kim, 류길열Gil Yeol Ryu, 이성학Sung Hak Lee, 장영원Young Won Chang
Abstract
In this study, a direct in-situ observation of fracture and crack tip deformation behavior under a wedge loading has been carried out in a scanning electron microscope (SEM) using an HSLA steel and an AISI 4340 steel. A specially designed wedge loading stage was adapted in an SEM for the observation of crack propagation on the surface of small double cantilever beam (DCB) specimens. The results for the HSLA steel with simulated microstructures of the heat affected zone of welding show that the formation of shear bands is primarily responsible for the initiation of microcrack at notch tip and crack propagation. A fracture mechanism for the local brittle zone (LBZ) phenomena has also been identified as the initiation of microvoids by the interfacial decohesion between the ferrite matrix and martensite islands. For the 4340 steel, voids were first observed to form around the large MnS inclusions and the plastic deformation became localized into intense shear bands between these voids as they grew. The failure eventually occurred along these shear bands, although fine microvoids would form around alloy carbides within the shear hands. This in-situ SEM technique has been found very useful for studying the microstructural aspects of crack initiation and crack growth behavior on the surface
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