Soft spring and semi-fixed constraint

[leonvanrijn]

I have two questions about Inventor stress analysis.

1: When a model is lacking a contact, Inventor automatically applies a Soft-spring contact. How can I let Inventor show me where it was added? I want to choose for myself what type of contact is needen and add a manual contact, but I can't find the missing contact.

2: Is it poosible to constrain a certain surface in 1 direction only? I know I can choose to constrain in X, Y of Z axis, but I want to constrain a part in the negative Y-axis only. Is this possible? (For instance a steel fgrame that should be suported by concrete,  so the frame cant bend down but can bend up?)

Can't include a model for this because of confidential info. If my questions aren't clear, please let me knwo so that I can build a simplified model that shows the problem.

thanks in advance

[raviburla]

Hi,

1: Stress analysis solver adds soft spring when the stiffness matrix becomes singular and its done automatically inside the solver and hence modifies the stiffness matrix diagonal. So its not possible to show where the soft spring is added. One way to check for disconnected components is to run Modal Analysis. The disconnected components would be moving away from one-another. Make sure to apply fixed constraints to disable trivial rigid body modes.

2: Constraints by nature fix some or all DOFs, so constraints cannot be used for applying directional deformations (like in your example). However, there is one way to approximate that behavior by using contacts. Be careful when you are performing this workaround as it could destablize the system and/or lead to long solution times. Here is the workaround:

   a. Model the concrete block as a near rigid body - with elastic modulus much larger than the other parts in the model. Ensure that the block size is as small as possible as this will add DOF to the FEA system.

   b. Model the contact between the part you want and this concrete block as separation contacts. This will allow the part to move away from concrete block, but will behave as bonded when force on the part make it to move "into" concrete block.

Please let us know if you have more questions.

Thanks,

Ravi Burla (Autodesk)

[leonvanrijn]

Seperation constraint doesn't seem to work like I want it and you explained it, so I probably did something wrong.

I made a simplified model of similar assembly (attachment).

When applying a pulling force, the part moves away from the concrete like you explained, but when I reverse the load the part it doens't behave as bonded but moves through the concrete.

What am I doing wrong?

Images of the resultst in the attachements as well.

Leon

[raviburla]

Hi,

I looked into the model and have the following observations:

1. Separation contact is not necessary between the regions that are not in contact with the concrete. So Separation:1 needs to be moved to bonded.

2. By default, the deformations are "exaggerated", so even if parts don't penetrate, it gives an illusion of parts being penetrated. Try using actual deformation and probe values at the necessary points to check for actual deformation values.

3. The concrete material needs to be modified to have much larger Young's modulus to make it behave like near-rigid.

Please let me know if you have more questions.

Thanks,
Ravi

[leonvanrijn]

Modifying the Young's modulus and using actual deformations does the trick I guess.

Moving seperation:1 to bonded prevents from creating a gap between the two parts under the load, increasing the Stress in th bend of Steel profile:1, so this is not the right thing to do.

But the other adjustments give the wanted result as far as I can see right now. Thanks!

[raviburla]

Hi,

I am glad that the work-arounds provide you with the solution you were looking for. With respect to the moving separation:1 to bonded, you are right, if the separation contact is providing the physics you wanted, then thats the right contact type to use.

Thanks,

Ravi Burla