Offset bonding between solid element and shell element, increase the stiffness of all structure.

[Anonymous]

Hi, I am trying to simulate a wing under the bending load case. I am using Shell elements to model the skin and solid elements to model the frame (Spars, stringers, ribs). I am modelling the simulation in a assembly, thus the skin and the frame are two separate parts. I am defining the contact between the skin and the frame as offset bonded. By doing this the whole structure becomes stiffer (I can see that from the resultant displacements and stresses are much lower than first analytical estimations). 

Does someone encountered this problem when the stiffness of the whole structure increases due to the offset bonding contact between shells and solids? Could you please help me by answering  what is the reason of this behavior.

Thank you!

[John_Holtz · Accepted solution]

Hi @Anonymous 

My guess is the maximum activation distance is too large, and therefore too much surface area is "welded" together. When using offset bonded contact, the maximum activation distance is interpreted as follows:

• If using "Contacts > Manual" (or "Contacts > Auto" which creates the manual contacts), the maximum activation distance should be no larger than 1.15 times the mesh size. 
• If using "Contacts > Solver", the maximum activation distance should be slightly larger than the gap between the pieces (and larger than 0). 

Let us know if that changes the results.

[Anonymous · Accepted solution]

Thank you for your answer, indeed that might be the problem, I set a higher
activation distance because the gap between the parts varies from 0.2mm to
7 mm. and I set the contact activation distance to 7 mm.