Assessment of fracture toughness by CT and round bar specimens in a HT780 steel

Abstract

It is known that fracture toughness value of steels is affected by test temperature, specimen thickness and loading rate. In this research, it is attempted to investigate it by changing temperature and loading rate with the rate parameter then being applied. The fracture toughness value is also obtained by round bar specimen with a circular notch. This result has been compared with the results by the CT specimens, and the validity of the fracture toughness value that has been obtained from round bar specimen with a circular notch is examined. The sample material used is HT780 steel. The CT specimens were IT, 2T and 4T-CT defined in ASTM E399, test temperature is varied from 77K to room temperature. The loading rate is also varied for the 1T-CT and the round bar specimen with a circular notch from static to 1 m/s. The fracture toughness value decreases as the test temperature decreases. It decreases with loading rate and specimen thickness. The fracture toughness test result is arranged using the stress intensity factor rate to obtain the rate parameter. Moreover, the fracture toughness value which has been obtained from the round bar specimen with a circular notch is lower than those by 1T-CT specimen, closer to 2T-CT specimen. The test temperature and the loading rate dependency of the fracture toughness value can be arranged by the rate parameter that is the function of temperature and stress intensity factor rate. Availability of the round bar specimen with a circular notch test specimen is discussed to obtain valid fracture toughness values.