Influence of dynamic strain ageing on mixed mode I/III fracture toughness of Armco iron

Fracture toughness tests under mode I and mixed mode I/III loading were carried out at different test temperatures ranging from ambient to 673 K. The dynamic strain ageing (DSA) range in Armco iron was identified to be between 383 and 573 K. A marked increase in fracture toughness was observed in the DSA regime and this correlated with the increase in the strain hardening exponent. The magnitude of fracture toughness, however, decreased with increasing loading angle. The extent of decrease was high at temperatures below the DSA regime (≤383 K) which can be understood in terms of the nature of the stress field ahead of a mixed mode I/III as well as the operative fracture mechanism. However, at higher temperatures, the effect of mode III in this respect diminished in the DSA regime (383-573 K) due to DSA causing the opposite effect, that is fracture toughness to increase.