MODCHG

Format

Example

Definitions

Comments

1. The MODCHG Bulk Data Entry is selected by the Subcase Information command MODCHG=option. It can only be referenced by nonlinear subcase(s).

   TYPE=CONTACT: Supported for both Nonlinear Static and Nonlinear Transient Analysis. Supported also for optimization.

   TYPE=ELMSET/RIGID: Supported only for Nonlinear Static Analysis. It is not supported for optimization.

   MODCHG is not supported for CGAP/CGAPG elements and N2S contact in Small Displacement Nonlinear Static Analysis. (MODCHG is supported for N2S in Small Displacement Nonlinear Static Analysis only for cases where the model also contains LGDISP subcase).

2. PARAM,HASHASSM,YES should be specified.
3. For TYPE=CONTACT:
   • When a contact interface is removed from a nonlinear subcase, the contact constraint is inactive for the subcase. However, if the contact interface was active in the preceding subcase (the one referenced with CNTNLSUB by the current subcase), the corresponding contact nodal forces at the end of the preceding subcase are linearly ramped down to zero from the start to the end of the current subcase.
   • When a contact interface is reactivated in a nonlinear subcase, the contact constraint is active for the subcase. In large-displacement nonlinear analysis, contact elements are recreated with the deformed configuration when the contact is newly activated; in small-displacement nonlinear analysis, contact elements are not recreated when the contact is newly activated.
   • Contact constraint is effective immediately when the contact is activated. For FREEZE and TIE contacts, the reference configuration for the zero relative motion condition is the configuration when the contact is newly activated. For non-freeze contact, contact opening/penetration is calculated based on the deformed configuration.
   • A contact interface is neither removed nor reactivated in a nonlinear subcase,
     • if the subcase does not have a preceding subcase, the contact constraint is active;
     • if the subcase has a preceding subcase, the contact constraint may be active (depending on whether it is active in the preceding subcase).
   • The MUMPS solver should be specified.
4. For TYPE=ELMSET:
   • Removal/reactivation of Solids, Shells, CBUSH, CONM2, JOINTG, CBAR, CBEAM, CROD, JOINTG, and Gasket elements (CGASK8, CGASK16, CGASK6, and CGASK12) is supported.
   • Removal/Reactivation of element sets is not allowed in nonlinear transient analysis.
   • Removal of all of the elements referencing a grid will also remove the grid.
   • Both strain-free reactivation (WOSTRN) and strain-recalculated reactivation (WISTRN) of elements is supported for small and large displacement nonlinear static analysis.
   • At the beginning of the removal step, OptiStruct calculates the forces that the elements to be removed exert on the remaining elements at the interface nodes between them. The forces are then treated as applied loads for the remaining elements and are gradually ramped down to zero at the end of the removal step. This means that the removal is not completed until the end of the step. The removed element sets are inactive in the succeeding subcase until they are reactivated.
   • Only the element sets removed in the preceding subcase can be reactivated. Additionally, only active element sets can be removed. Removal and reactivation of the same element set in a single subcase is not allowed.
   • The difference between using OPT=WOSTRN and OPT=WISTRN is illustrated below. Consider example elements A and B, with both the initial and current configuration, as shown. Element A is inactive in the preceding subcase and is reactivated in the current subcase.

     
     
     Figure 1. Reactivated element (A) with OPT=WORSTRN/WISTRN

     Element A

     Reactivated in the current subcase while inactive in the preceding subcase.

     Element B

     Active in both the current and preceding subcases.

     Grids 100 and 101

     Grids shared by elements A and B.

     

     Locations of grids at the start of the current subcase.

     

     Initial locations of grids.

     • For element reactivation with OPT=WOSTRN:

       

       Element A will be use as the zero-stress reference configuration after it is reactivated. So, element A does not take any initial strain when it is reactivated.

     • For element reactivation with OPT=WISTRN:

       

       Element A will be use as the zero-stress reference configuration after it is reactivated.

       

       As the locations of grids are at the start of the current subcase, element A may take an initial strain when it is reactivated. This initial strain is not immediately but gradually applied to element A in this subcase.

5. For TYPE = RIGID:
   • Removal/reactivation of RBE2 and RBE3 rigids is supported. The SIDi field(s) should reference SET Bulk Data Entry(ies) of TYPE=RIGID.
6. PARAM, MCHGRMV, 1 can be used to allow elements removed by MODCHG to still participate in the analysis, but with very small stiffness. It can be used to avoid mesh distortion when elements are re-activated after a large deformation of the structure in the preceding subcase.
7. This card is represented as a property in HyperMesh.