“…This failure criterion consists of the criterion for inherent failure and the criterion for failure along a weak plane as given in equations and , respectively where S 0 is the inherent cohesion, ϕ is the corresponding friction angle, η is a free parameter which can be determined experimentally, and S 0 w and ϕ w are the cohesion and corresponding friction angle of the weak planes, respectively. Parameters determined by Fjær and Nes [] in laboratory are adopted, and the UCS‐normalized major principal stress is given in Figure b. Different from the bimodal curve of tangential stress, four crests can be identified on the normalized maximum principle stress, in which direction wellbore breakouts are more likely to initiate.…”