Effect of crack depth and specimen width on fracture toughness of a carbonsteel in the ductile-brittle transition region

The effects of crack depth (a/W) and specimen width W on the fracture tough ness and ductile-brittle transition have been investigated using three-poin t bend specimens. Finite element analysis is employed to obtain the stress- strain fields ahead of the crack tip. The results show that both normalized crack depth (a/W) and specimen width (W) affect the fracture toughness and ductile-brittle fracture transition. The measured crack tip opening displa cement decreases and ductile-brittle transition occurs with increasing crac k depth (a/W) from 0.1 to 0.2 and 0.3. At a fixed a/W (0.2 or 0.3). all spe cimens fail by cleavage prior to ductile tearing when specimen width W incr eases from 25 to 40 and 50 mm. The lower bound fracture toughness is not se nsitive to crack depth and specimen width. Finite element analysis shows th at the opening stress in the remaining ligament is elevated with increasing crack depth or specimen width due to the increase of in-plane constraint. The average local cleavage stress is dependent on both crack depth and spec imen width but its lower bound value is not sensitive to constraint level. No fixed distance can be found from the cleavage initiation site to the cra ck tip and this distance increases gradually with decreasing inplane constr aint. (C) 2000 Elsevier Science Ltd. All rights reserved.