Erosion is a numerical mechanism for the automatic removal (deletion) of elements during a simulation. The primary reason for using erosion is to remove very distorted elements from a simulation before the elements become inverted (degenerate). This ensures that the stability timestep remains at a reasonable level and solutions can continue to the desired termination time. Erosion can also be used to allow the simulation of material fracture, cutting and penetration.
There are a number of mechanisms available to initiate erosion of elements. The erosion options can be used in any combination. Elements will erode if any of the criteria are met.
Geometric Strain
Geometric strain is a measure of the distortion of an element and is calculated from the global strain components as
 | (10–28) |
This erosion option allows removal of elements when the local element geometric strain exceeds the specified value. Typical values range from 0.5 to 2.0. The default value of 1.5 can be used in most cases.
Custom result EFF_STN can be used to review the distribution of effective strain in the model.
Timestep
This erosion option allows removal of elements when the local element timestep, multiplied by the time step safety factor falls below the specified value.
Custom result TIMESTEP can be used to review the time step for each element.
Material Failure
Using this option, elements will automatically erode if a material failure property is defined in the material used in the elements, and the failure criteria has been reached. Elements with materials including a damage model will also erode if damage reaches a value of 1.0.
Other Mechanisms of Erosion
Aside from Geometric Strain, Timestep and Material Failure, the Explicit solver may erode elements for the following reasons:
If Element Self Contact is set to Yes (see Element Self Contact in the Explicit Dynamics Analysis Guide) and an element deforms such that one of its nodes comes within a specified distance of one of its faces, the element is eroded to prevent it becoming degenerate.
In 2d analyses, elements that become degenerate are automatically eroded irrespective of whether Element Self Contact is set to Yes or No.
Retained Inertia
If all elements that are connected to a node in the mesh are eroded, the inertia of the resulting free node can be retained. The mass and momentum of the free node is retained and can be involved in subsequent impact events to transfer momentum in the system. If this option is set to No, all free nodes will be automatically removed from the simulation.
Note:
Erosion is not a physical process and should be used with caution.
The internal energy of elements which are eroded is always removed from the system. This energy is accumulated in the work done term for global energy conservation purposes.