亜共析黒鉛鋼の疲労き裂伝ぱ特性におよぼす組織学的因子の影響

抄録

Fatigue tests were carried out on hypo-eutectoid graphitic steels by using notched and holed specimens, and the propagation behavior of fatigue cracks was investigated in connection with microstructural factors.
At the root of notched specimens of graphitic steels, fatigue cracks were observed to grow in a zig-zag way through graphites adjacent to the crack tip.
When a cyclic stress was below 60 per cent of the yield stress or 0.2%-proof stress, the fatigue crack propagation rate, dl⁄dN, of holed specimens was related to the stress intensity factor range ΔK, and paris-Erdogan’s relationship, dl⁄dN=C(ΔK)^m, held in all cases. However, the value of m decreased with increase in the square of mean graphite nodule diameter and the reciprocal square root of mean ferrite path between graphite nodules.
When a cyclic stress exceeded the above mentioned stress, the fatigue crack propagation rate was expressed as follows:
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\ oindentwhere D, E and F are constants independent of the microstructure, on the other hand, n and k depend on it. Microstructural effects on the fatigue crack propagation rate were considered to be mainly caused by microstructural effects on n, because variations of the value of n were larger than those of k.
The value of n in graphitic steels examined in this work enlarged, as the value of m decreased.