On Brittle Fracture Initiation (Second Report)

Abstract

In the first report, one of the authors succeeded in the brittle fracture initiation at low stress levels from the notch without any additional factors such as the welding residual stress, the impact stress and the metallurgical embrittlement etc. by using the deep notch test.In this paper, firstly, a review of the fundamental theories of fracture initiation and the theoretical extension of the results of deep notch test to the brittle fracture initiation characteristics for an infinite plate were made.Secondly, the following fundamental experiments on brittle fracture initiation were conducted : (1) Classification of zones for fracture initiation. (2) Applicability of the Griffith-Orowan energy condition. (3) Size of yield zone around the tip of notch. (4) Effect of plate thickness.Next, the brittle fracture initiation characteristics for various steels such as the mild steel, various high strength steels including HT 60, HT 70, HT 80, HT100, and various low temperature steels including 2.5% Ni, 3.5% Ni, 9% Ni steels and Al-killed steels quenched and tempered with the yield strength of 33, 37, 58 kg/mm2 were evaluated.Lastly, the correlations between the brittle fracture initiation temperature for a crack length of 10 mm under a stress level of σy/2.56 and the transition temperatures in various large and small size brittle fracture tests were investigated.