Rate Dependence of Dynamic Fracture Toughness of 20MnMoNi55 Low-Carbon Steel Using Two Methods on Instrumented Drop Weight Test of PCVN

Abstract

Earlier Charpy V-notch (CVN) specimens were used to determine the nil ductility temperature (NDT) with the help of the impact test. NDT is the measure of embrittlement of material at temperature scale. CVN energy is gross energy supplied to the specimen which includes fracture energy, energy loss due to friction and inertia. So, CVN energy is not a proper characterization of fracture behavior. Thus, the determination of fracture toughness is required. Fracture toughness can be evaluated from quasi-static loading as well as dynamic loading conditions. Fatigue pre-cracked Charpy V-notch (PCVN) specimens are used to find dynamic fracture toughness. Dynamic fracture mechanics differs from quasi-static fracture mechanics in three complex features: rate-dependent material properties, the effect of inertia and reflected stress wave. To overcome the effect of inertia and stress waves, the evaluation of dynamic fracture toughness needs some special technique. In this study, two different methods are used to evaluate the dynamic fracture toughness, one is impact response curve method and another is the J-integral method. For impact response curve method, there is no time constraint present at calculation process but for the J-integral method, there is some condition regarding fracture time to be satisfied to get the valid dynamic fracture toughness data. In this work, Pre-cracked Charpy V-notch (PCVN) specimens of 20MnMoNi55 low-carbon reactor pressure vessel steel are used for impact testing in Pendulum testing machine and Drop weight machine for 7 different velocities ranging from 1 m/s to 9 m/s at cryogenic temperature (−70 °C). Calibration between the test results from the impact test on pendulum type and Drop weight test is done and found to be matching. Dynamic fracture toughness at each velocity is evaluated and compared using two different methods namely impact response curve method and J-integral method. Based on the results found, it is observed that two methods yield the same results if the time constraint referred is maintained for J-integral method.