Effects of biaxial mean stress on the critical plane orientation under biaxial tension/compression fatigue loading conditions

• Effects of biaxial mean stress on critical plane orientation are studied.
• Analytical critical plane orientations are derived and validated by FEA solutions.
• Critical plane orientation depends on the dominant direction of maximum stress.
• Critical plane orientation is used to explain the observed thermal fatigue crazing.
• Effects of stress amplitude and material properties on critical plane are studied.

In this paper, the effects of biaxial mean stress on the orientation of the critical plane defined by maximum damage under biaxial tension/compression fatigue loading conditions are investigated by analytical and computational approaches. The analytical solutions of the critical plane orientation using the Matake’s and Fatemi–Socie’s criteria are first derived and validated by computational results. For the linear loading path of the applied biaxial stress in this study, the critical plane orientation strongly depends on the dominant direction defined by the larger maximum stress. The critical plane, or the crack initiation plane, is parallel to the less dominant direction. For the special case of equal biaxial stress, many possible critical planes or crack initiation planes exist. The critical plane orientations using the Matake’s and Fatemi–Socie’s criteria also depend on the ratio of the mean stress to the stress amplitude and the maximum stress in the dominant direction, respectively. The critical plane orientations are used to partially explain the observed cracking directions of the thermal fatigue crazing in the old residual heat removal system (RHRS) of the nuclear power plant. A Monte-Carlo study on the cracking directions of the thermal fatigue crazing is conducted in a plate which represents the inner surface of the pipe wall. Finally, the effects of material fatigue properties on the critical plane orientation are discussed.