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AltPropagate Command

The AltPropagate command is used to define a competing crack propagation criteria within a single material:

AltPropagate (crit),<(dir)>,<(tract)>

where (crit) and optionally (dir) are the alternate crack propagation criterion and the method to select the alternate crack propagation direction. The optional (tract) can define a material ID for a previously defined traction law material. These parameters function the same as the corresponding parameters in the Propagate Command except (crit) cannot be "steady state" (2) or "energy balance" (3).

The AltPropagate command lets you set a second propagation criterion to use in certain specific models that involve competing fracture modes. When an alternate criterion is used, the following steps take place:

  1. At each time step, the first criterion is checked. If the situation is critical, the crack will propagate by the first criterion and the second criterion will subsequently be turned off for that crack tip.
  2. As long as the first criterion has not become critical, the second or alternate criterion will be checked. The crack will propagate by that criterion if it is critical.
  3. When propagation information is printed to the output file, it will include "(alt)" when propagating by the second criterion and will omit that wording once the primary criterion is activated.

This option was added to allow simulations of competing fracture modes such as crack growth along a weak interface vs. propagation away from the interface into the bulk material. For this problem, the first criterion is set to the bulk fracture properties and the second criterion is the interface fracture criterion. The later is usually set to be by self-similar crack growth or crack growth in the initial crack direction so it will be along the interface. In this scheme, the crack will propagate along the interface until turning into the bulk becomes critical. Once it turns into the bulk, the interface fracture mechanics is turned off.

Notes

  1. Alternate crack propagation and direction criteria are based on material properties that must be set in the analysis.
  2. Alternate crack propagation occurs in the material defined by the tip material parameter #3 when defining cracks.
  3. The AltPropaget command defines the default alternate propagation, direction, and traction law material for all materials. If desired, a specific material can override these defaults using the altcriterion, altdirection, and alttraction material property settings.
  4. The AltProagate command is only allowed for MPM analyses.