Because of the solid modeling cross-reference checking that the Mechanical APDL program performs, you cannot delete meshed solid model entities, nor can you use EDELE or NDELE to delete elements and nodes that are associated with solid model entities. In order to revise your model, you generally need to clear solid model entities of their meshes by using the mesh clearing commands. These clearing commands can be thought of as the inverse of the meshing commands. After clearing your model, you can proceed to modify your solid model as desired.
The mesh clearing commands delete the nodes and elements associated with the corresponding solid model entity. When you clear a higher level entity, all lower level entities will be automatically cleared, unless those lower entities are themselves meshed with elements. Nodes on the boundary of an entity shared by an adjoining meshed entity are not deleted as a result of clearing.
To delete nodes and point elements associated with selected keypoints, use the KCLEAR command.
To delete nodes and line elements associated with selected lines, use the LCLEAR command.
To delete nodes and area elements associated with selected areas, use the ACLEAR command.
To delete nodes and volume elements associated with selected volumes, use the VCLEAR command.
The program will report how many of each kind of entity have been cleared after a mesh clearing operation. An entity is considered to have been "cleared" if either its elements or its nodes have been cleared.
If the elements/nodes being cleared are at the end of the element/node lists, then the next available element/node ID is reset accordingly. (You can suppress this resetting with a MOPT,CLEAR,OFF command.)
As was discussed earlier, element attributes that were assigned to the solid model by TYPE, REAL, MAT, and ESYS commands followed by a meshing command (AMESH, VMESH, etc.) will be cleared by a mesh clearing command. These "clearable" attributes are designated by negative attribute numbers in the output from listing commands (ALIST, VLIST, etc.). The mesh clearing commands do not affect attributes that were assigned by an attribute association command (AATT, VATT, etc.). In either case, issuing new attribute association commands will overwrite whatever element attributes were previously associated with the cleared solid model.
There are several reasons why you might want to modify element attributes after meshing: you might have simply committed an error in assigning attributes, you might need to change your design, or you might be converting your model from one analysis discipline to another (such as in a sequential thermal-stress analysis). The techniques available for modifying element attributes include the following:
You can clear your mesh using mesh clearing commands and set new attributes using either attribute association commands or commands such as TYPE, REAL, etc. You can then remesh using meshing commands. Because remeshing can sometimes be expensive, this approach should be avoided if the mesh itself is acceptable.
Note what happens when a mesh clearing command is executed: solid model attributes set by a meshing command (identified by negative attribute numbers in listings produced by ALIST, VLIST, etc.) will be deleted. Solid model attributes set by an attribute association command (AATT, VATT, etc.) will not be changed. Thus, because the attribute association commands override the TYPE, REAL, MAT, and ESYS commands, you will not be able to reassign solid model attributes with the TYPE, REAL, MAT, and ESYS commands if you initially assigned attributes by using attribute association commands. (You will need to issue a new attribute association command.) Upon remeshing, the attributes associated with the solid model entities will be assigned to the elements generated on those entities.
Direct Element Modification: Element attributes can also be modified without the expense of remeshing. You can select the elements to be modified, reset attributes (using the TYPE, REAL, MAT, and ESYS commands), and execute EMODIF,ALL. This procedure modifies the element attributes directly, without affecting the corresponding solid model attributes. This procedure, although convenient, can be dangerous, because the element attributes in your finite element model will no longer match the element attributes in your solid model. Also, you could conceivably change element attributes to inappropriate values without receiving any kind of warning. For these reasons, you must proceed cautiously if you decide to attempt to change element attributes by direct element modification.
Another way of directly modifying the material number of specified elements is by using the MPCHG command. Unlike other element-modification commands, which are valid only within PREP7, MPCHG is valid within both PREP7 and SOLUTION. Thus, this command can be used to change element properties between solutions.
Attribute Table Modification: Another possibility would be to change entries in the attribute tables after meshing, but before entering SOLUTION. A warning will be issued if the REAL set or the MAT set contain unused entries (such as could happen if a REAL property set for a beam were assigned to a spar element). No remeshing is required with this procedure.
A Note About Adding and Deleting Midside Nodes: For any of these procedures, if you change the element TYPE attribute to substitute midside-node elements for non-midside-node elements, you will also need to use the EMID command to add the extra midside nodes as required. EMID must be preceded by execution of a MODMSH,DETACH command. Also, in order to delete midside nodes, you must first remove them from the midside-node elements by issuing EMID,-1.
You can delete solid model entities with the entity deletion commands described below. Lower level entities cannot be independently deleted if they are attached to a higher level entity. Thus, if you have created a block using a geometric primitive command, you cannot selectively delete a keypoint that is associated with that block, unless you first delete, in descending hierarchical order, all the higher level entities (lines, areas, and volumes) that are attached to that keypoint.
Conversely, by activating the "sweep" option (that is, setting
KSWP = 1) on the LDELE,
ADELE, or VDELE commands, you can direct
the program to delete all the associated lower level entities automatically. (Such
lower level entities will not be deleted if they are attached to another higher
level entity, however.) For example, if you decide to delete an unmeshed sphere
volume, you can issue a single VDELE command, with
KSWP set to 1, to delete the volume and all its
associated areas, lines, and keypoints.
You can modify the geometry of a solid model by changing the position of its keypoints with the KMODIF command. Any meshed regions attached to modified keypoints will be automatically cleared of nodes and elements. All lines, areas, and volumes attached to the modified keypoint will then be automatically redefined using the active coordinate system.
Unmeshed solid model entities may also be redefined by reissuing the commands that originally defined them. For example, consider the following sequence, in which a second K command is used to modify a keypoint:
CSYS,0 K,1,5.0,6.0,7.0 ! Create KP 1 at X=5.0, Y=6.0, Z=7.0 CSYS,1 K,1,5.0,6.0,7.0 ! Redefine KP 1 at R=5.0, θ=6.0, Z=7.0
Keypoint 1 could only be redefined in this way if it was not attached to any higher level entities. Lines, areas, and volumes can be similarly redefined, but only if they are not attached to any higher level entities.
You can modify unmeshed lines using the operations described below. These operations will also update attached unmeshed areas, even if these areas are attached to volumes.