Safety Assessment of Steels under Cyclic Loading at Low Temperatures

Abstract

In order to investigate the mechanisms and the factors to govern the brittle fracture initiation during the fatigue crack propagation at low temperatures, (1) Fracture toughness tests under wide range of loading rate(2) Fatigue tests at low temperature(3) Fracture toughness tests suffered from pre-loading were performed. And detailed observations and analyses were carried out on the load, the load-line-displacement and the strain near crack tip at around the fracture initiation, and on the fractography by means of the scanning electron microscope.The main results obtained were as follows, (1) The fracture load was always lower than the maximum load of cyclic loading, therefore, the loading rate was non-zero value at the brittle fracture initiation.(2) Since the residual stress due to the cyclic loading was less effective on the brittle fracture initiation if it were compared with the loading rate effect, the fatigue fracture toughness Kfc was estimated as equivalent value as the fracture toughness Kc under the monotonic tensile loading. If the Kfc and the Kc were analyzed as a function of the K (=dK/dt), there is no significant difference between them.(3) The residual stress due to the cyclic loading, though, showed large effect on the fatigue crack extension as usual manner.The calculated fatigue life based on the Paris' formula with taking into consideration of the residual stress showed good one-by-one correspondence with the observed fatigue life up to the brittle fracture initiation.Therefore, a design curve to determine the fatigue life at low temperature was preliminarily proposed.(4) Comparing with the results obtained from the fracture toughness tests after having been given monotonic tensile pre-load, the pre-loading effect of cyclic loading at low temperature was shown to be explained by the ρ+ theory.