An express technique for the determination of static and dynamic fracture toughness ( KIC, KId) of bcc metals and alloys

Abstract

Despite many investigations carried out during the last four decades on the fracture mechanics, a reliable, easy and economical technique has not been developed for low and middle strength (low carbon low alloyed) steels which have a widespread use in metallic constructions. This prohibited the reliability of such constructions in view of fracture mechanics. The technique described in the ASTM E399 Standard is very troublesome and this hindered the evaluation of fracture toughness of many steels exhibiting similar properties, because the Standard requires very big samples to satisfy the plane strain conditions. This study, which depends on a 20 years investigation, explains an express technique for the determination of fracture toughness of bcc metals and alloys for the temperatures 0⩽ T⩽0.2 T melt [K] and for the deformation rate ε ′⩽10 5 s −1. According to the proposed technique, K IC ( T, ε ′) graph can be constructed by only two experiments: one is a uniaxial tension test at room temperature with a small smooth cylindrical specimen and the other is a fracture toughness test at a low temperature (i.e. 77 K) in accordance with ASTM E399. At this temperature the cracked sample is also very small. The mathematical relations obtained on a realistic theoretical base give results similar to experimentally obtained previous results in a high accuracy. The main assumptions in the theoretical base contain two observations: (1) the fracture toughness exponentially depends on the thermoactivation energy of plastic deformation at the crack tip, (2) a microcrack is formed at a certain distance ahead of the crack tip under a critical stress and this is unified with the main crack to cause the growth.