Probability distribution on cleavage fracture in function of Jc for reactor ferritic steel in transition temperature region

This paper presents the results and methods used for determining of probability of cleavage fracture toughness of ferritic reactor steel in transition temperature area. The steel tested in this study is steel know as 20MnMoNi 55 by DIN standard, typically used for structures working at low temperatures. In addition, the effect of test specimen geometry, temperature and fatigue crack initiation rate on fracture toughness was investigated in order to predict the fracture behaviour and probability of cleavage fracture. Two main studies joined in on one are presented in this paper. Testing in both studies were performed according to ASTM 1820 standard and the Weibull distribution with regression analysis, which includes fitting the experimental results in a logarithmic coordinate system, was used to predict the Jc values. Cleavage fracture probabilities in the function of Jc for large test specimen, C(T)100 and C(T)200 were determined based on the results obtained by testing of small C(T)50 specimens, for the purpose of direct savings and decreased costs of specimen testing. Cleavage fracture probability, represented by using Weibull distribution of experimental data will provide a clear insight into material behaviour at different temperatures. The influence of Delta K values during fatigue pre-cracking procedure on the fracture toughness scatter was also presented and discussed.