Dynamic Fracture Toughness K(T, K) Behaviors at Simulated Weld of SM-50 Steel

Abstract

It is known that fracture toughness value changes, within the same material, depending on the ambient temperature and loading rate used for testing. For some material, fracture toughness value obtained at rapid loading rate is considerably lower than the one by static case. Thus the former value has more critical meaning on fracture mechanics standpoint.
We have succeeded for the first time to measure dynamic fracture toughness K(T, K) behaviours by using calibrated piezo-electric transducer for load detection. The signal is amplified by charge amplifier, and is recorded by digital memory. SM 50 steel was chosen for the present investigation because of its expectedly strong loading rate dependence of fracture toughness value. For dynamic testing, extraneous signals due to inertial loading and specimen vibrations were circumvented by lowering initial tup velocity.
K(T, K) was measured at the temperature range between -196°C and room temperature. Strong loading rate dependence was found for simulated H.A.Z. and base metal. On the other hand, simulated weld-bond, which has poor toughness at slow bend test, has small dependence of fracture toughness value on loading rate. The difference in the dependences is very clear at higher temperature range, and they both become less sensitive to loading rate as temperature decreases toward -196°C where the loading rate dependence essentially disappears. Furthermore, temperature dependence of fracture toughness value was found to be large for simulated H.A.Z. and base metal while the dependence is much less for simulated weld-bond.