Warm pre-stress based pre-cracking criteria for fracture toughness testing

There are presently a magnitude of different fracture toughness testing standards that have different criteria for fatigue pre-cracking specimens prior to testing. The reason for the criteria is that too high pre-fatigue load may influence the subsequently measured fracture toughness value. The criteria have to a large extent been developed specifically for each standard in question and this has lead to the considerable variability in the criteria. The basic reason for the pre-fatigue having an effect on the fracture toughness is the warm pre-stress (WPS) effect. Here, existing data relating to pre-fatigue load levels are examined with the help of a newly developed simple WPS correction and a criteria and correction procedure for too high pre-fatigue loads are proposed. The new criteria focuses on brittle fracture, but is equally applicable for ductile fracture, thus enabling a unification of pre-fatigue criteria in different fracture toughness testing standards.