Element Birth & Death Chapter Ten Material based on that from 5.5 Nonlinearities Seminar

Element Birth and Death Chapter Overview In this chapter, we will discuss element birth and death capabilities in ANSYS Element birth and death is useful to (re)activate or deactivate certain elements during the course of the analysis Typical examples include manufacturing/assembly processes or analyses involving failure/removal of material: Excavation (as in tunneling or slurry wall excavation) Staged construction (as in unshored bridge erection) Sequential assembly (as in fabrication of layered computer chips, epoxy curing) Weld deposition Annealing September 30, 2001 Inventory #001491 10-2

Element Birth and Death ... Chapter Overview In this chapter, we will cover the following topics: A. Background on Element Birth and Death B. Element Birth and Death Procedure in ANSYS C. Additional Considerations for Birth and Death D. Postprocessing Analyses with Active and Deactivated Elements September 30, 2001 Inventory #001491 10-3

Element Birth and Death A. Background on Birth and Death Definition of Birth and Death: Element birth and death allows the user to (re)activate or deactivate specific elements during the course of an analysis. Elements can be “born” in a later time during the load history. This means that the elements are initially deactivated but are later reactivated in the analysis. Elements can be “killed” during the load history. This means that the elements cease to provide any significant structural response. These changes (activation status) occur at the beginning of a load step and are maintained throughout that load step. Element birth and death is a changing status nonlinearity (similar to contact status). They provide a stepped, not ramped, change in the element status (a ‘sudden’ change in status). September 30, 2001 Inventory #001491 10-4

Element Birth and Death ... Background on Birth and Death Implementation of Birth and Death: When an element is ‘killed,’ it is not removed from the stiffness matrix, but its stiffness is dropped to a low value. Deactivated/killed elements have their stiffness multiplied by a severe reduction factor (default is 1e-6). The stiffness is not set to zero in order to prevent a singular matrix. Element load vectors (e.g., pressure, temperature) associated with killed elements are zeroed out. Mass, Damping, and stress-stiffness matrices are set to zero for deactivated elements. Element stresses and strains are reset to zero as soon as the element is killed. The stiffness matrix size is always preserved since the deactivated elements are not removed. September 30, 2001 Inventory #001491 10-5

Element Birth and Death ... Background on Birth and Death Implementation of Birth and Death (cont’d): When an element is ‘born,’ its stiffness reduction factor is removed. All elements, including those which are initial deactivated, must be present before solution. This is because the size of the stiffness matrix must not change during the course of the analysis. When an element is ‘born,’ its stiffness, mass, and damping matrices return to their original values. Element load vectors are also returned to their original/actual values. After an element becomes ‘alive,’ they do not have any strain history record. They are ‘annealed’ by birth and death operations, being born with all strains and all stresses equal to zero. Note to instructors: Deactivated elements’ strain history is set to zero when element is killed [EKILL]. Hence, when elements are reborn, they are born with zero strain. Initial and thermal strains, however, should appear when the element is reborn. Initial strain is for some elements like LINK1 or BEAM4 which is input via a real constant. Initial strain is not affected by element birth and death. Thermal strain (alpha*delta(T)) will be included during the first solution after an element is reborn, if thermal loads are present. September 30, 2001 Inventory #001491 10-6

Element Birth and Death B. Birth and Death Procedure Although not all elements support birth and death, most of the commonly-used elements do support this feature. A listing of supported elements is provided in the ANSYS Advanced Analysis Guide, Chapter 7. Some notable elements are listed below: 18x elements: LINK180, SHELL181, PLANE182-183, SOLID185-187, BEAM188-189 Core elements: PLANE42, SOLID45, PLANE82, SOLID92, SOLID95 Surface contact elements: TARGE169-170, CONTA171-174 Other structural elements: all LINK, BEAM, PLANE, SURF, and 3D SHELL elements September 30, 2001 Inventory #001491 10-7

Element Birth and Death ... Birth and Death Procedure When building the model, ensure that all elements are generated before solution is performed. Only supported elements, as noted in the previous slide, can be used for birth and death All elements need to be generated before solving the model since the size of the matrices need to be preserved (restarts cannot be performed if the matrices change dimensions). This means that even elements which are activated later in the load history need to be created before solution begins. Do not use select/unselect logic to activate/deactivate elements. We will use specific commands described later. September 30, 2001 Inventory #001491 10-8

Element Birth and Death ... Birth and Death Procedure The solution of models containing birth and death is similar to other nonlinear problems, but there are some additional considerations which must be taken. Main Menu > Solution > -Analysis Type- Sol’n Control… Solution Controls > -Basic Tab- Analysis Options Specify large displacement solution (NLGEOM,ON); otherwise, some element types will be reactivated in their originally specified geometric configuration (i.e., large-deformation effects should be included to obtain meaningful results). September 30, 2001 Inventory #001491 10-9

Element Birth and Death ... Birth and Death Procedure If no other nonlinearities exist, then the Full Newton-Raphson option should be explicitly specified. Main Menu > Preprocessor > Loads > Analysis Options… Specify “Full N-R” for “Newton-Raphson option” (NROPT,FULL). For all birth and death applications, if no other nonlinearities exist, you must be sure to set the Newton-Raphson option explicitly in the first load step, as the program cannot predict the presence of an EKILL command in a subsequent load step. Note to instructors: Adaptive descent can be turned “ON.” It yields good results in birth and death analyses. September 30, 2001 Inventory #001491 10-10

Element Birth and Death ... Birth and Death Procedure The activation/deactivation of elements is done through special commands. Main Menu > Solution > -Load Step Opts- Other > -Birth & Death- “Activate Elem” (EALIVE) allows you to select specific elements to be reactivated for the current load step. “Kill Elements” (EKILL) allows you to specify which elements are to be deactivated for the current load step. “StiffnessMult” (ESTIF) determines the reduction factor to apply to killed elements’ stiffness terms. By default, this value is 1e-6. If the default value is inappropriate for your analysis, it can be specified to a different value. Note that if elements are to be “added” in later load steps, they should be killed in the first load step. September 30, 2001 Inventory #001491 10-11

Element Birth and Death C. Additional Considerations Considerations for Applied Loads: Element load vectors (pressure, temperature) are automatically zeroed out for deactivated elements. Mass is zeroed out, so acceleration loads also do not affect deactivated elements. Concentrated nodal forces are not automatically removed from deactivated elements’ DOF. The user must manually delete concentrated loads from inactive nodes. Similarly, these nodal loads must be reapplied when elements are reactivated. All element and inertial loads are restored for reactivated elements (pressure, temperature, acceleration). Nodal forces, as noted above, are not affected by element birth and death. September 30, 2001 Inventory #001491 10-12

Element Birth and Death ... Additional Considerations Notes about boundary conditions: Constraining nodes of deactivated elements may be important if you want to preserve the shape of the elements when they will be reactivated. Be sure to delete these artificial constriants when reactivating elements. Nodes that are not connected to any elements may “float”. In some cases, you might want to constrain inactive DOFs to reduce the number of equations to be solved or to avoid ill-conditioning (recall that a reduction factor ESTIF generates small terms in the stiffness matrix, which may cause ill-conditioning). Note that constraint equations (CE or CEINTF) cannot be applied to inactive DOFs. September 30, 2001 Inventory #001491 10-13

Element Birth and Death ... Additional Considerations Solution option tips: The LSWRITE and LSSOLVE commands cannot be used with the birth-death option. Multiple load steps need to be performed using a series of explicit SOLVE commands. Adaptive descent may be used with element birth and death, often providing good results. You can model stress-relieving operations (such as annealing) by deactivating and then reactivating elements. September 30, 2001 Inventory #001491 10-14

Element Birth and Death ... Additional Considerations Tips on obtaining convergence: 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. Deactivating and reactivating elements cause sudden changes in the system’s stiffness (a step change). Hence, if these changes are too severe, convergence difficulties may arise. You may need to limit the number of elements which are killed or made alive in a given load step if convergence difficulties are met. Jagged edges (re-entrant corners) should be avoided when deactivating elements. September 30, 2001 Inventory #001491 10-15

Element Birth and Death ... Additional Considerations Alternatives to element birth and death: If the strain history of the killed elements needs to be maintained, you can deactivate elements by changing their material properties in solution: Solution > Load Step Opts > Other > Change Mat Props However, this option does not remove element forces, strains, mass, specific heat, etc. Convergence problems can result from careless use of changing material properties in solution. For example if an element’s stiffness was reduced to zero, but it retained its mass, a singularity could result from an acceleration load. September 30, 2001 Inventory #001491 10-16

Element Birth and Death D. Reviewing Results For the most part, postprocessing a birth and death analysis follows standard procedures. Realize that “killed” elements are still present in your model, and will be included in element displays, output listings, etc. Use selection logic to remove deactivated elements from element displays and other postprocessing operations. If deactivated elements are not unselected when post-processing, they will appear with zero stress, strain, and displacement. This may also tend to produce “smeared” results when using nodal contour plots (element contour plots will not smear results). September 30, 2001 Inventory #001491 10-17

Element Birth and Death ... Reviewing Results Tips on postprocessing different load steps: If your model has multiple load steps where different elements are deactivated in each load step, one way to differentiate between “killed” and “alive” elements is to use components prior to solution. Keep “killed” elements in components to make it easier to unselect these from view when postprocessing. The workshop example for this chapter will use this technique. Another alternative is to save the database with the birth and death status for all elements that matches the birth and death status for that load step. (Save a copy of the database for each load step which changes the birth and death status.) Because this may result in more work in file management, this may not be the preferred method. September 30, 2001 Inventory #001491 10-18

Element Birth and Death ... Reviewing Results Controlling element birth and death with ANSYS results: In certain problems you will need to activate or deactivate elements based on their results. For example if you wanted to kill melted elements in a thermal analysis, the elements could be identified on the basis of their calculated temperatures. The elements can be identified by storing results in the element table and selecting the critical elements via the element table results. The solution can then be restarted with the critical elements deactivated. Refer to the ANSYS Advanced Analysis Techniques for more detail. A sample input file is shown below: … ! Previous Solution Procedure /POST1 ! Enter POST1 SET,… ! Read in Results ETABLE,… ! Store criteria in ETABLE ESEL,S,… ! Select elements based on ETABLE item FINISH /SOLU ! Re-enter SOLUTION ANTYPE,,REST ! Restart the solution EKILL,ALL ! Deactivate selected elements ESEL,ALL ! Restore full element set ... ! Continue with solution September 30, 2001 Inventory #001491 10-19

Element Birth and Death ... Workshop Exercise Please refer to your Workshop Supplement: Workshop 14: Element Birth and Death September 30, 2001 Inventory #001491 10-20