FLAC3D FAQs

FAQs: 

Structural Element Links - Compression vs. Tension Direction

The Normal-Yield model properties can be somewhat confusing. In the context of the link model, the tension limit refers to the limiting positive value of the link force, and the compression limit refers to the limiting negative value of the link force. Positive link forces are generated by positive relative incremental displacement, which is defined as the difference between the target incremental displacement and the source incremental displacement (target minus source). This distinction occurs in the node-local coordinate system. The result is that, depending on the local system and the model configuration, the "compression" limit might not refer to what one would instinctively think of as compression when viewed from the element perspective.

For example, in general liner elements assign a nodal system with the z-direction pointing away from the zones. This means that the liner structure liner property coupling-yield-normal property will map to the structure link property yield-compression property in the normal-yield deformable model. The following figure illustrates the system.

How do I manually set structural element link properties?

By default, deformable link model properties are set automatically by FLAC3D during a geometry update, which happens during the validation stage of a cycle on the first cycle taken and if in large strain mode at every update interval. Properties that have been set manually may be overridden at this time. It is currently not possible to keep this from happening. You can, however, use FISH to restore your intended manual properties after each global update has occurred.

The fish callback command can be used to insert a FISH call during each cycle, in this case the callback should be after time index -2 but before -1 (see program list cycle-sequence in the FLAC3D 6.0 documentation for a full list). The struct.was.updated FISH intrinsic can be used to determine if an actual full geometry update occurred at the start of that cycle.