Finite element analysis and experimental evaluation of ductile-brittle transition in compact tension specimen

Using finite element analysis, metallographic observations and statistical analysis, the stress field ahead of stationary and growing cracks and the ductile-brittle transition mechanism in compact tension (CT) specimens have been evaluated. Compared to a stationary crack, a growing crack elevates the opening stress on the remaining ligament and this may be partially attributed to the re-sharpening of the crack tip after ductile growth. The area of material covered by the high opening stress of the same magnitude also increases with ductile crack growth. In this study, no significant difference for measured cleavage stress can be found for the specimens fractured with and without ductile crack growth. There is a large scatter for the distance between the cleavage initiation site and the stationary or growing crack tip. Cleavage fracture after some amount of ductile crack growth is attributed to the increase of both the opening stress and the area of material under high opening stresses. Finally, the lower bound toughness is predicted using a small data set statistical model in connection with constraint correction. The predicted values give the same trend as the lower bound of the experimental measurements from the lower-shelf to the temperature at which ductile crack growth occurs.