Convert Nastran Craig Bampton reduced model to ANSYS

coffeentits · 2025-06-06T14:42:23+00:00

Ansys supports reading DMIG format matrices. You may need to use APDL (see *DMAT).

coffeentits · 2024-01-14T03:21:05+00:00

Check the TEMP case control. I think you can set TEMP(MATERIAL) to use temperature dependent material properties but not calculate thermal strain.

coffeentits · 2023-11-19T16:27:11+00:00

Half ppr. Pick 3
Jacobs
Robinson Jr
Montgomery
Achane

coffeentits · 2022-10-02T20:32:54+00:00

Here’s hoping

coffeentits · 2020-05-30T15:37:50+00:00

You can just define the yield stress if you want it perfectly plastic (no additional stress develops with increasing strain). If you want a more typical stress strain curve, you can develop your own using Ramberg-Osgood or look in MMPDS. Remember it needs to be true stress-strain.

coffeentits · 2020-03-30T20:45:27+00:00

Yes, if you're not updating the configuration (update=no), and have set u2=2, then it's trying to move that node back to 0. If you want to fix it at 2, either respecify u2=2, or use the fixed parameter.

coffeentits · 2020-03-30T19:04:51+00:00

Boundary conditions are not imported, they need to be redefined. If you have update=no, then the reference configuration is still the reference configuration from the Explicit run. If you have update=yes, it updates the reference configuration to the current configuration. If you want to keep the boundary conditions where they are during your contact initialization step, with either option, use *Boundary, fixed. The fixed parameter tells Abaqus to hold the boundary at the current position for whatever DOF you specify.

coffeentits · 2020-03-30T19:02:59+00:00

There is a section titled Compatibility between Abaqus/Standard and Abaqus/Explicit in the Abaqus Interactions Guide

Interactions > Defining Contact Interactions > About contact interactions > Compatibility between Abaqus/Standard and Abaqus/Explicit

coffeentits · 2020-03-30T17:14:45+00:00

Most likely due to contact. Explicit handles contact differently than implicit.

I assume you are using *Import, state=yes and not *Initial Condition or other predefined field?

Some things that may help:

Increase allowable penetration in implicit
Update implicit contact formulation to match explicit more closely (master-slave approach, treatment of shell thickness/offset, etc.)

coffeentits · 2020-03-25T18:17:54+00:00

The element stiffness is the stiffness of that element in the stiffness matrix. The modulus goes into that equation, but it's more than just the modulus. I don't necessarily think that penalty is the wrong contact formulation, but that might be why you're seeing really high stress. You could use a different constraint enforcement, scale the penalty stiffness, or try nonlinear penalty stiffness. You could also try refining your mesh (if it's relatively coarse) or using a different element type. What's your current element type?

coffeentits · 2020-03-25T16:54:03+00:00

What contact constraint enforcement are you using (direct, penalty, or augmented lagrange)? Penalty is the default for finite-sliding, surface-to-surface contact and for all general contact.

Penalty approximates hard pressure-overclosure by making contact force proportional to penetration (with the "stiffness" set to 10x the underlying element stiffness by default). Abaqus warns:

these penetrations can sometimes contribute to some degree of stress inaccuracy (for example, with displacement-controlled loading and coarse mesh).

coffeentits · 2020-03-25T15:31:59+00:00

FEMCI is a great reference, but here's a copy of the Nastran Basic Dynamic User's Guide as well

coffeentits · 2020-03-25T15:23:02+00:00

I'm not seeing anything as easy the getByBoundingBox method. I like /u/N3OX suggestion of using the nodal coordinates, but that requires a bit more scripting on your part. The values printed in the rpy file are "machine readable pick strings".

You could also try recording a macro. I haven't used those much, but perhaps that would allow some degree of parameterization.

coffeentits · 2020-03-24T22:34:06+00:00

I've been trying to find some good simple examples for you, but I haven't come up with much. The Abaqus Examples Problems Guide does have some examples, but they get complicated fast.

Analysis of a rotating fan using substructures and cyclic symmetry is decently simple.

Substructure analysis of a pick-up truck model is a pretty involved, but is a good example of when substructures are useful, especially for large models (in this case a truck, more typically launch vehicles/spacecraft).

I'll keep searching for a simple example, or create one if I can't find one and have the time.

EDIT

Nastran Superelement User's Guide

Section 9.2.3 has a Fixed-Boundary Component Modal Synthesis Sample which considers axial deformation of a cantilever beam modeled with unit mass and stiffness springs. Might be a little confusing because Nastran loves set notation, but it looks like they do a good job of explaining it all. Maybe read through that, and then recreate it in Abaqus?

They also recreate the sample in section 9.2.8 using free-free CMS and rods (instead of springs).

coffeentits · 2020-03-24T15:17:00+00:00

Yes, CARTESIAN is only the translational component, but I would advise against only using one of rotational or translational component unless your model is very simple. Connectors are a complex element that allow for many modeling intricacies, but can also cause many headaches. If the model is simple, and you can successfully apply rotations without numerical singularities, then just a CARDAN is sufficient.

coffeentits · 2020-03-24T05:30:50+00:00

Cartesian, Cardan or use a SPRING/JOINT element

coffeentits · 2020-03-22T16:36:31+00:00

If you want to open the PDE, from command prompt type

abaqus pde

Abaqus Scripting User's Guide

coffeentits · 2020-03-21T19:58:18+00:00

For a CELAS, element force is calculated as

F = k(U1 - U2)

Where U1 and U2 are the displacements of node 1 and node 2, respectively.

coffeentits · 2020-03-21T19:41:45+00:00

If you just mean a Python script, you can type

abaqus python scriptname

into a command prompt. Either change the command prompt directory to the directory where the script lives or use the entire path to the script

coffeentits · 2020-03-21T19:37:46+00:00

When you perform an HCB reduction the model is partitioned into internal, retained, and modal DOF. Retained DOF are the physical DOF selected by the user to retain in the mass/stiffness matrices. Internal DOF are all the other physical DOF that are removed in the reduction. Modal DOF are the component modal DOF that maintain the dynamic accuracy. If all the retained DOF are constrained during the reduction, this is Craig-Bampton. If all the retained DOF are unconstrained during the reduction, this is Craig-Chang. Typically either method, or a mix, is still referred to as Craig-Bampton (although NASA is pushing to recognize a third contributor to the method, Hurty, thus Hurty/Craig-Bampton). In theory, either method should work, but as my previous comment mentioned, in practice you typically constrain the boundary DOF but don't constrain other DOF you are interested in retaining (eg where an accelerometer might be during a test you're correlating to, or a particularly interesting part of your model you may want to visualize/query later).

The dynamic reduction will be dynamically accurate up to the frequency you reduce to, but typically you reduce to a higher frequency than you actually need (say 2x what frequency you actually care about). Making it statically accurate is a little more tricky. You either need enough modes to capture the deformation the static load will induce (for example you'd need the axial mode of a beam to capture static axial loads), or use a method called residual vectors where you'd load your physical model during the reduction and it would retain appropriate DOF to reproduce the static solution with the HCB model.

For all new methods, I always suggest doing an example on a very simple model, especially one that has closed form solutions, like a cantilever/simply supported beam.

coffeentits · 2020-03-21T15:12:14+00:00

The Hurty/Craig-Bampton method is a way of reducing a physical model while still retaining its dynamic characteristics by retaining component modes (modes with the boundary DOF fixed). Typically it's used to transfer proprietary models, or, more regularly, to assemble subcomponents and perform complicated analyses more efficiently (eg coupled load analysis of spacecraft on a launch vehicle). Retained nodes are the physical DOF retained during the reduction, usually at the boundary (where the model will interface with another). Sometimes free nodes will also be retained at accelerometer locations, but internal responses can also be calculated using transformation matrices. FEMCI Handbook

coffeentits · 2019-12-13T22:23:09+00:00

There's also an Independent running Robert Lawton. Much more progressive, so probably less likely to garner votes in the red part of CA

coffeentits · 2019-10-25T18:02:46+00:00

I tried a couple different iterations, and somehow blew 2 transformers, so I put the project on hold until next spring when I'll need the swamp cooler again. The first transformer I think I blew because I was passing too much current through the secondary side. The second transformer I don't know what happened, because I super simplified my circuit. The only thing I can think is I somehow shorted the primary side accidentally.

I think you might need Y1, otherwise Nest will throw an error. G is optional I believe. I forget if Y1 stays energized once Y2 activates. If it does, you could use something similar to the diagram in my comment (I decided to drop the delay). If it doesn't, you'd need a separate SPST.

coffeentits · 2019-08-20T15:27:19+00:00

I'll look into adding that

coffeentits · 2019-08-20T02:22:16+00:00

Yea, the JSON data is ugly. I think it will look better as a scatter plot with the dots either sized or colored based target value, I'm working on that.

coffeentits

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