Background
Materials are not able to withstand tensile stresses which exceed the material's local tensile strength. The computation of the dynamic motion of materials assuming that they always remain continuous, even if the predicted local stresses reach very large values, will lead to unphysical solutions.
A model has to be constructed to recognize when tensile limits are reached to modify the computation to deal with this and to describe the properties of the material after this formulation has been applied.
Several different modes of failure initiation can be represented in the explicit dynamics system.
Element failure in the explicit dynamics system has two components:
Failure initiation
A number of mechanisms are available to initiate failure in a material (see properties Plastic Strain Failure, Principal Stress Failure, Principal Strain Failure, Tensile Pressure Failure, Johnson-Cook Failure, Grady Spall Failure). When specified criteria are met within an element, a post failure response is activated.
Failure initiation can be identified in the model via the custom result MAT_STATUS. The following key is used.
| MAT_STATUS | Meaning |
|---|---|
| 1 | Material is currently undergoing elastic deformation, or no deformation |
| 2 | The plastic strain in the material increased during the last time increment |
| 3 | The material has failed due to isotropic (bulk) criteria |
| 4 | The material has failed due to isotropic (bulk) criteria |
| 5 | The material has failed in tension due to principal value 1 |
| 6 | The material has failed in tension due to principal value 2 |
| 7 | The material has failed in tension due to principal value 3 |
Post failure response
After failure initiation in an element, the subsequent strength characteristics of the element will change depending on the type of failure model
Instantaneous Failure
Upon failure initiation, the element deviatoric stress will be immediately set to zero and retained at this level. Subsequently, the element will only be able to support compressive pressures.
Gradual Failure (Damage)
After failure initiation, the element stress is limited by a damage evolution law. Usually this results in a gradual reduction in an elements capability to carry deviatoric and/or pressure stresses.
By default, tensile failure models will produce an instantaneous post failure response. Inserting the crack softening failure property, in addition to other failure initiation properties results in a gradual failure response.
The following Failure models are discussed in this section: