Algorithm of Fatigue Life Assessment by a Unified Theorem without Distinguishing between the Crack Initiation and the Propagation Life : Assessment of Fatigue Life for a Crack Emanating from a Notch

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概要

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• In a previous paper, we showed that the logarithmic curve of ΔK_<RP>〜da/dN was linear even in the ranges of stages I and II of a long crack. In other words, threshold phenomenon appears only circumstantially due to applying ΔK or ΔK_<eff> which involve the term not related to the crack extension force such as elastic deformation in addition to the term related to the crack driving force. In this paper, we give results of fatigue crack initiation/propagation tests using CT specimens with a notch of root radius 1mm. Fatigue cracks initiated in the mid-thickness at a notch root as surface cracks. Having considered the change of an aspect ratio of surface cracks before and after coalescence from previous studies for surface crack propagation and a change of measured RPG load (Re-tensile Plastic zone's Generated load), it was confirmed that the logarithmic curve of ΔK_<RP>〜da/dN for a physically small crack of which the length was greater than one microstructural grain size was in good agreement with that for a long crack. Moreover, for a microstructural short crack which is less than one grain size, it is postulated that dislocations do not exceed beyond barriers such as a grain boundary and a pearlite zone in a ferrite-pearlite microstructure. Then, the equivalent ΔK value is obtained from a plastic zone size. Therefore, from the measured RPG load, equivalent ΔK_<RP> for a microstructural short crack and ΔK_<RP> for physically small crack and a long crack, crack propagation curves are calculated for a crack emanating from a notch root without distinguishing between crack initiation and propagation. The calculated crack propagation curves are in very good agreement with experimental ones. This result can be considered as the first step in establishing a unified fatigue life assessment theory to replace the combined S-N curve and crack propagation curve of linear fracture mechanics.

• 財団法人日本海事協会の論文

著者

• NIWA Toshio

  Department of Chemical Biology, Osaka City University Medical School

• Niwa Toshio

  Department Of Naval Architecture And Marine Systems Engineering Kyushu University

• TOYOSADA MASAHIRO

  Department of Marine Systems Engineering, Kyushu University

• Toyosada Masahiro

  Department Of Naval Architecture And Marine Systems Engineering Kyushu University

• Toyosada Masahiro

  Department Of Marine Systems Engineering Kyushu University

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