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In like manner, when elements are "born," they are not actually added to the model; they are simply reactivated. You must create all elements, including those to be born in later stages of your analysis, while in PREP7. You cannot create new elements in SOLUTION. To "add" an element, you first deactivate it, then reactivate it at the proper load step.

When an element is reactivated, its stiffness, mass, element loads, etc. return to their full original values. Elements are reactivated with no record of strain history (or heat storage, etc.); that is, a reactivated element is generally strain-free. Initial strain defined as a real constant, however, is not be affected by birth and death operations.

Unless large-deformation effects are activated (NLGEOM,ON), some element types will be reactivated in their originally specified geometric configuration. (Large-deformation effects should be included to obtain meaningful results.)

Thermal strains are computed for newly-activated elements based on the current load step temperature and the reference temperature. Thus, newborn elements with thermal loads may not be stress-free as intended. The material property REFT can be used instead of the global TREF to specify material-depend- ent reference temperatures, allowing you to specify the activation temperature as a stress-free temperature.

13.3. Element Birth and Death Usage Hints

The following guidelines apply to analyses using the element birth and death capability:

•Constraint equations (CE, CEINTF, etc.) cannot be applied to inactive DOFs. Inactive DOFs occur when a node has no active ("alive") elements attached to it.

•You can model stress-relieving operations (such as annealing) by deactivating and then reactivating elements.

•In nonlinear analyses, be careful not to deactivate or reactivate elements in such a way as to create singularities (such as sharp re-entrant corners in a structural analysis) or sudden large changes in stiffness. Such situations are likely to cause convergence difficulties.

•If the model is entirely linear--that is, if the model has no contact or other nonlinear element present and the material is linear--except for birth and death elements, the program does not activate optimized defaults (SOLCONTROL,ON).

•Killing contact/target elements or their underlying elements will cause the status of the contact pair to change to far field contact (open and not near contact), even for bonded contact. You may need to kill both the contact/target elements and their underlying elements to reestablish the pre-death contact status when the elements are later reactivated.

•The full Newton-Raphson option with adaptive descent activated (NROPT,FULL,,ON) often yields good results in analyses employing element birth and death.

•You can retrieve a parameter whose value will indicate the status (active or inactive) of an element (*GET,Par,ELEM, n, ATTR, LIVE) This parameter could be used in APDL logical branching (*IF, etc.) or in other applications for which you need to monitor the birth-and-death status of an element.

•Since a Multiframe restart will recreate the database using the *.rdb file, the elements selected in /POST1 can not be killed in a multiframe restart.