The processes of initiation and stable growth of craks in fracture toughness tests were observed in detail on various steels of moderate ductility by using fractographic and sectioning techniques. The effects of test temperature and specimen thickness on these processes were also investigated. The results are summarized as follows.
(1) The stretched zone width increases with temperature for HT80, HT60 and CH1 steels, while it remains approximately constant for S35C steel, in the temperature range where the stable crack growth takes place preceding the unstable crack growth.
(2) The distribution of the stretched zone width along the crack front is fairly flat and is little affected by the test temperature and specimen thickness, while that of the tear dimple region is considerably affected by these factors.
(3) A good correlation is found to exist between the transition zone width and Kc/σY.
