The phenomenon of unstability of ductile fracture in the upper shelf region of a forged steel for nuclear reactor pressure vessels A508 Cl. 3 was studied with a large compliance apparatus, whose spring constants were 100, 170 and 230kgf/mm, at the test temperatures of 100, 200 and 300°C and at the loading rates of 2, 20 and 200mm/min in the crosshead speed. The main results obtained are as follows:
(1) The fracture modes of the specimens consisted of (a) stable fracture, (b) unstable fracture which leads to a complete fracture. rapidly and (c) quasiunstable fracture which does not lead to a complete fracture though a rapid extension of ductile crack takes place.
(2) Side groove, high temperature or small spring constant made a ductile crack more unstable.
(3) High temperature or large spring constant made the occurrence of quasiunstable fracture easier.
(4) Quasiunstable ductile fracture took place before the maximum load, that is, at the J integral value of about 10kgf/mm. The initiation of a microscopic ductile crack, therefore, seems to lead to quasiunstable fracture.
(5) The concept that unstable ductile fracture takes place when Tapp exceeds Tmat seems applicable only to the case in which unstable ductile fracture takes place after the maximum load has been exceeded.
