微細粒のくさび効果による疲労き裂進展抑制

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Restraint of fatigue crack propagation by wedge effect of some fine particles was examined on a JIS A 5083 P-O Al-Mg alloy. For a fatigue test, a 5 mm thick plate specimen with a central EDM notch was used. As the fine particles to be wedge into fatigue cracks, three kinds of particles were prepared, namely, magnetic particles and two kinds of alumina particles of which the mean particle sizes were 47.3 μm and 15.2 μm. Before the fatigue tests, particles of each kind were suspended in an oil, and the suspension was spread on the specimen surface covering the notch part. Fatigue tests were performed by a servohydraulic type fatigue tester with a test frequency of 2 Hz and a load ratio of R=0. First a control fatigue test was done by spreading only the oil on the specimen surface, and it was proved that the wedge effect of the oil itself is negligible. The alumina particles of large size were also not effective to restrain the crack propagation, because the suspension is difficult to be made due to the large particle size and the particles cannot be sent to the crack tip properly. On the other hand, both of the magnetic particles and the alumina particles of small size had evident effects to restrain the crack propagation, especially in the case of the alumina particles the failure lives were extended by the range from 4×10^5 to 7×10^5 cycles. From some crack propagation plots, it was found that the restraint of crack propagation by the particles occurs mainly in the very early stage of crack propagation and the restraint effects become weak when the fatigue crack length exceeds approximately 3 mm. After the fatigue tests, some macro- and microfractographic analyses were performed using a CCD microscope, a SEM and an EPMA, in order to examine the mechanism of fatigue crack restraint by the wedge effects of the fine particles. From those analyses, it was reasoned that the fine alumina particles wedged into a fatigue crack are subjected to cyclic pressures from the crack surfaces, crushed up into finer particles, and then form a kind of alumina coating (in the vicinity of notch root) which presents the free crack closure as a wedge.
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微細粒のくさび効果による疲労き裂進展抑制

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