Observations of Crack Initiation and Stable Growth Processes in Fracture Toughness Testing of Several Steels

Abstract

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.

Stretched zone width and blunting line equation for determination of initiation fracture toughness in low carbon highly ductile steels

“…In the context of the literature (Ohji et al 1978;Putatunda and Rigsbee 1985;Shen et al 2001;Yin et al 1983;Kolednik and Stüwe 1986;Amouzouvi and Bassim 1982;Doig et al 1984;Bassim et al 1992;Tarafder et al 2003) it appears that SZW captures the actual blunting process and so, the ASTM blunting line should conform to it. The blunting line procedure has the advantage of not requiring the measurement of SZW.…”

“…Many investigators have questioned the arbitrary offset of 0.2 mm in the prescribed procedure and recommend direct measurement of the stretched zone width and its use for determining J Ic (ESIS P1-92 1992;ESIS P2-92 1992). Thus, determination of J Ic on the basis of direct measurement of the SZW is now recognized as an alternative method for determining the fracture toughness of the material, which is considered as material property (Ohji et al 1978;Putatunda and Rigsbee 1985;Shen et al 2001).…”