Abstract
Acceleration of crack growth under intermittent overloading was studied at 650°C by using two types of center notched plate specimens of different thickness made of a SUS 304 stainless steel. When the hold time of overload was very short (20 seconds), the crack growth rate was significantly accelerated to be about 20-50 times as large as that of static creep cracks, and the fracture surface morphology was different from that under static loading, indicating extremely ductile transgranular fracture by glide plane decohesion or microvoid coalescence. The acceleration in growth rate could be ascribed to recovery of the material during the low stress period. In the thinner plate specimens, the crack growth rate under intermittent loading was correlated well with modified J-integral J and agreed with the growth rate of static creep cracks in J-da/dt diagram. In the thicker plate specimens, however, the crack growth rate under intermittent loading did not agree with that of the thinner plate specimens in J-da/dt diagram, being about 1/5 times. Transgranular fatigue type crack growth appeared in the low growth rate region, and the growth rate was different from that of creep cracks.
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