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Failure Trajectories

The Failure Trajectories option in the Display Options display options icon dialog, displays the potential failure plane orientations at each node of the finite element mesh. The trajectory orientations are computed from the principal stress orientations and the material failure envelope, as follows.

The program first computes the orientation (principal angle alpha) of the major principal stress for the current stress state (tensor) at a location. For the minor principal stress sigma3 of the current stress state, a value of major principal stress that would cause failure is then calculated. For this point on the failure envelope, the program computes an instantaneous friction angle phi. The calculation of instantaneous friction angle only occurs for non-linear failure envelopes such as Hoek-Brown materials. For Mohr-Coulomb materials, phi is simply the friction angle of the material. The conjugate failure planes are then oriented at alpha plus/minus (45 - phi/2).

Two different symbols are used for the display of failure trajectories, indicating the mode of potential failure - shear or tension.

  • Shear failure is indicated by a pair of intersecting lines, oriented with respect to the principal stress directions as described above.
  • Tensile failure is indicated by a single line, displayed perpendicular to the direction of maximum tensile stress


  • Since the failure trajectories are displayed at all nodes of the mesh, they do not necessarily indicate that failure has actually occurred. They simply indicate the potential failure planes for the current stress state. To view the locations of yielded finite elements select the Yielded Elements toolbar button rather than the Failure Trajectories option.
  • Failure trajectories are only displayed for Mohr-Coulomb or Hoek-Brown materials. They are not currently displayed for other material types (e.g. Drucker-Prager or Cam Clay)
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