Suppose you want to use the DEFORM entry to enforce a strain on the three-member structure shown inFigure 8-16. The goal is to impose an initial strain equal to 100 μcm/cm in the center member.
Figure 8-16. Three-Bar Structure with Initial Strain
Consider: (1) the DEFORM entry can only impose a force, (2) from static equilibrium, if no forces are imposed on grid points 2 and 3, the force in all three members must be the same. With this in mind, the following procedure is used.
1. Determine the force required in member 11 that produces the 100 mcm/cm strain.
2. Since there are no external forces acting on the elements, the force of 1065 N is same in all the members. Using the 1065 N force, the stresses computed in the other members.
3. From the stresses found in part 2, the element strains are computed for elements 10 and 12. Multiplying the strain within each element by its length yields the extensional change of the element. These extensional changes are the deform values required to produce a 100 μcm/cm strain in element 11.
Therefore,
These values are entered on the DEFORM Bulk Data entry as shown inListing 8-12. To help understand the implementation of the DEFORM entry, two configurations are being analyzed.
The first configuration (Subcase 1) specifies the boundary conditions as shown inFigure 8-17(a).
For this case, the structure is free to expand without inducing any element forces. The results of this subcase should be the extension of the members as entered on the DEFORM entry. As mentioned previously, a force is applied to the element that produces the specified extension if the element is free to expand.
The second subcase, shown inFigure 8-17(b), is the constrained configuration that is of interest.
If the calculations are correct, the strain in the center element should be the required 100 μcm/cm.
Figure 8-17. Three-Bar Structures with Initial Strain
$ FILENAME - STRAIN.DAT
DEFORM 1 10 1.5E-3 11 1.E-3 12 .75E-3
ENDDATA
Listing 8-12. Initial Strain Input File
The stresses and displacements for the two subcases are shown inFigure 8-18. As can be seen for the free end configuration, the extension of all of the members is consistent with the DEFORM entry.
Note that the extensions for the elements aren’t the same as the values specified on the DEFORM entry. The stress in member 11 is 710 N/cm2. Since E = 7.1 x 106, the strain in element 11 is equal to = E/σ = 100 μcm/cm strain. The stress output is requested for this example because strain output is not available for one-dimensional elements.
Figure 8-18. Stress and Displacement Output of the Three Bar Structure
8.9 Loads Due to Enforced Motion
Statics
You can specify enforced zero displacements on GRID, SPC, or SPC1 entries.
• If you use GRID entries, the constraints become part of the structural model and modifications cannot be made at the subcase level.
• SPC or SPC1 entries are usually used for defining zero displacements, which result in nonzero forces of constraint.
You can specify nonzero enforced displacements on SPC or SPCD entries.
• The SPC entry lets you specify both the component to be constrained and the magnitude of the enforced displacement.
• The SPCD entry lets you specify only the magnitude of the enforced grid point displacement.
When you use an SPCD entry, you must specify the component to be constrained on either
an SPC or SPC1 entry. Using SPCD avoids the decomposition of the stiffness matrix when changes are only made in the magnitudes of the enforced displacements.
NX Nastran calculates the equivalent loads resulting from enforced displacements of grid points and adds them to the other applied loads. You specify the magnitudes of the enforced displacements on SPC entries (SPCAX in the case of conical shell problems) in the global coordinate system. NX Nastran automatically applies the load when you select the associated SPC set in the Case Control section.
See also
• “SPC” in the NX Nastran Quick Reference Guide
• “SPCD” in the NX Nastran Quick Reference Guide
• “SPCAX” in the NX Nastran Quick Reference Guide
Dynamics
In NX Nastran, you can use the:
• TLOAD1 and TLOAD2 entries to define a time-dependent dynamic load or enforced motion for use in a transient response analysis
• RLOAD1 and RLOAD2 entries to define a frequency-dependent dynamic load for use in a frequency response analysis
You can use the TYPE field (field5 on TLOAD1/TLOAD2 and field 8 on RLOAD1 and RLOAD2) to specify the type of dynamic excitation you’re creating.
• If the TYPE field on the TLOAD1/2 entries indicates an enforced motion, the software first assumes that the EXCITEID field points to SPC-type data. If not present, NX Nastran then assumes the excitation is enforced motion using large masses and will then look for DAREA and various static loading data, just as in the case of applied loads.
• With RLOAD1 and RLOAD2 entries, the software only look for SPC-type data in the case of enforced motion.
For all entries (TLOAD1/2, RLOAD1/2), if the TYPE field indicates an applied load, the program will search only for static loading data.
See also
• “TLOAD1” in the NX Nastran Quick Reference Guide
• “TLOAD2” in the NX Nastran Quick Reference Guide
• “RLOAD1” in the NX Nastran Quick Reference Guide
• “RLOAD2” in the NX Nastran Quick Reference Guide
Specifying Enforced Motion Data
In NX Nastran, you specify enforced motion data on the SPC/SPC1/SPCD Bulk Data entries.
(Note that components specified in SPCD data must be also be referenced on SPC or SPC1 entries.)
You can define enforced motion with SPC data (without the use of SPCDs) alone. However, such usage is discouraged in favor of SPC/SPC1 data in connection with SPCDs. In this way, you select SPCD entries via the EXCITEID field in the dynamic load data, while the software uses the SPC/SPC1 sets to identify the constrained displacement set in the Case Control section.
With SPC data alone, enforced motion is applied solely via the Case Control section. The entries referenced by the EXCITEID field on the TLOAD1/2 or RLOAD1/2 entries do not have to exist. This method of specifying enforced motion is less elegant and somewhat more difficult to interpret.
See also
• “SPC” in the NX Nastran Quick Reference Guide
• “SPC1” in the NX Nastran Quick Reference Guide
• “SPCD” in the NX Nastran Quick Reference Guide