3%Si鉄における疲労き裂の不安定化と疲労破壊じん性

抄録

Notch tensile and fatigue tests have been made on 3% Si iron with various grain sizes. The plastic zone size associated with cleavage and fatigue crack propagation has been studied by a sensitive etching technique for detecting plastically deformed material. The mechanism of stable to unstable transition of fatigue crack propagation and the effects of grain size on the fracture toughness (K_c) and on the fatigue fracture toughness (K_fc) have been investigated.
The main conclusions are as follows.
(1) When the maximum stress intensity factor (K_max) is less than 0.7K_fc, the sizes of the slip band zone and the heavily deformed zone near a fatigue crack are proportional to the square of the variation of stress intensity factor (ΔK), while the plastic zone size associated with fatigue crack propagation is proportional to the square of K_max.
(2) The thickness of plastic zone formed near an unstable propagating crack does not depend on the stress intensity factor calculated by the compliance technique, but depends on the temperature and plate thickness.
(3) The process of fatigue fracture in a sharp notched specimen of low toughness material is divided into the following three cases:
(a) K_max<0.7K_fc: Stable crack propagation by fatigue.
(b) 0.7K_fc≤K_max<K_fc: A crack propagates stepwise by the interaction of fatigue and local cleavage, and the rate of crack propagation shows a large scatter. Micro-cleavagecracks are frequently observed ahead of a main crack on the plate surface as well as in the midsection.
(c) K_max≥K_fc: A crack propagates unstably.
(4) Fatigue fracture toughness is not influenced by the mean stress, and its dependence on grain size is smaller than that of K_c.