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| STUDY OF PLASTIC ZONE AND FATIGUE CRACK PROPAGATION MECHANISM |
| by DENG Rongying; LI He; YU Guiqing (Institute of Mechanics; Academia Sinica; Beijing) (Manuscript received 1 March; 1983; revised manuscript 5 August; 1983) |
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Cite this article:
by DENG Rongying; LI He; YU Guiqing (Institute of Mechanics; Academia Sinica; Beijing) (Manuscript received 1 March; 1983; revised manuscript 5 August; 1983). STUDY OF PLASTIC ZONE AND FATIGUE CRACK PROPAGATION MECHANISM. Acta Metall Sin, 1984, 20(2): 76-209.
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Abstract An evaluation was made of the fatigue crack propagation rate at constant amplitude of cyclic load ratio R=0.5 for the three-point bend specimens of 15MnVN steel quenched and tempered at 200℃. The monotonic plastic [zone size of the fatigue crack tip was measured under observation of phase contract microscope with the relief phenomenon out from the smooth [surface of metal during plastic deformation. The monotonic plastic zone size, perpendicular to the direction of crack surface, under plane stress is proportional to the corresponding (K_(max)/σ_s)~2. Its proportional coefficient is 0.21. According to the relationship between Rice's reversed cyclic and the monotonic plastic zone size, an exponential relation is obtained between the reversed cyclic plastic zone size perpendicular to the direction of crack surface and the related fatigue crack propagation rate. In the process of fatigue crack propagation, when the relative crack size α/W is about 0.687, the plastic zone of crack tip varies from a small [to broad scale. At α/W<0.687, the plastic deformation of crack tip spreads out in two sides of angle θ about 30—60°. While α/W>0.687, it spreads out not only in enlarged angle but also in considerably extent near θ=0°. The metaUographic and SEM observations show that the fatigue crack of 15MnVN steel with platelet martensite structure propagates along transgranular cleavage, i. e. a regenerated nuclear propagation mechanism.
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Received: 18 February 1984
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