Assessing the Loading Rate for a Fracture Toughness Test in the Ductile-to-Brittle Transition Region

For fracture toughness tests in the ductile-to-brittle transition region, ASTM E 1921-05 requires specimens to be loaded using a loading rate dK/dt between 0.1 and 2 MPam/s during the initial elastic portion. It has been proposed that the standard allow testing at higher loading rates, including precracked Charpy specimens tested on an instrumented pendulum machine (impact toughness tests). The revised standard would require test results (KJc or To) to be reported along with the relevant loading rate, and should therefore provide guidance on how to assess the value of dK/dt in a relatively simple, but reliable manner. Various options for measuring the loading rate have been investigated in this paper for several fracture toughness tests performed at different loading rates (quasi-static, dynamic and impact). For each loading rate, three different toughness levels have been considered: low, medium and high. Three considerably different materials have been selected: two typical RPV steels (JRQ, JSPS) and a ferritic/martensitic 9% chromium steel (EUROFER97). It is found that the preferable option is given by the ratio between KJc and time at the onset of cleavage, whereas the elastic value Kel/tel or the average dK/dt can be used when partial unloadings are performed.