Different displacement with compression GAP and tension GAP 1

[TheMesh]

Hello!

I have some issues with GAPs in my test model.

I test the behavior of Gap elemens in tensile and compressive loading. So, there is the fixed beam with the force 100N on the free end. Also that end is supported with the GAP element.

First I have modeled the beam without the gaps to get the reference displacements in -100N and then in +100N.

Second step was the gap addition. Bulk Data option "Gaps as Contact" was turned on. Firstly GAP element had only Compressive stiffness 100N/m and tensile/compressive load was applied. Secondly GAP element had only Tension stiffness 100N/m and tensile/compressive load was applied. This time everyting is fine - gaps compress with -100N (as contact elements) and open with +100N.

On the Third step I changed only the Compression stiffness to 1000N/m and repeated the loading. This time something was wrong. The displacement of the beam supported by GAP with Tension stiffness didn`t matched the displacement of the same beam from the previous loading (when GAP Compression stiffness was 100N/m). So, how could it be as I didn`t changed the Tension stiffness at all ?



Best regards, TheMesh

 

[TG_eng]

Can you set the initial gap of both the compression and tension properties and then rerun. In your example model it looks like the initial gap is 0.1.

 

[TheMesh]

Hi, thanks for your reply!

Actually I don`t need the initial gap. The idea was to model a support which should be more stiffer in one direction than in another one. If I use the 0.1 gap it seems like there is no contact because the diflection of the beam is 0.02021 - much fewer than initial gap. So I need to deal with no initial gap support.

And how could it be - I change ONLY the GAP property with Copression Stiffness but the diflection of the beam with Tension GAP property changes too.

GAPS with Compression and Tension Stiffness = 100

GAP with Compression Stiffness = 1000 and GAP with Tension Stiffness = 100 (remained the same)


Best regards, TheMesh

 

[TG_eng]

In your model with "No Gap", did you make the nodes coincident and use a coordinate system for each of the gap element's orientation?

 

[TG_eng]

From the QRG for PGAP it looks like the different stiffness comes from whether it's an open gap or closed gap.

Femap exports compression stiffness as KA and tension stiffness as KB.

 

[TheMesh]

In my "No GAP" models there were no GAPs at all, two topside beams were made for displacement checking. For other 4 beams ("GAP Compression Stiffness" and "GAP Tension Stiffness") I turned the GAPs off but they were considered.

According to QRG for PGAP KB should be used for contact stiffness if I didn`t use the KA (Lower-right beam).

So on my first picture I used KA = 100 for middle-left beam and KB = 100 for lower-right one. And beam displacements were the same with account for PGAP remarks.
On my second picture I changed only KA for middle-left beam (KA = 1000). After that the displacement of the lower-right beam should remained unchanged because I didn`t change KB for that beam (KB = 100). But it DID change.