×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Simple Tension test û why is my model too stiff?

Simple Tension test û why is my model too stiff?

Simple Tension test û why is my model too stiff?

(OP)
Hi, I’m trying to validate my FEA models by using experimental stress-strain data from a composite tension test. I’m using NEI Nastran but for some reason the FEA results are way too stiff. I thought initially it was a material model issue so then I changed to an isotropic material – aluminium.

I have now done one of the simplest tests possible: 2D rectangle (with quad elements), fixed BC along curve at one end, force load along curve at the other end (low value so the model is always in a linear regime). The tensile modulus is 70.3 GPa, so logically after outputting the stress and strain results I expected to get the same answer back. However my results were once again too stiff and I had a predicted stiffness of 88GPa.

I really can't understand what is going on. I thought this was such a simple test.

Thanks for any help

RE: Simple Tension test û why is my model too stiff?

you're plotting elastic strain ... looks like something like 0.003".

are you accounting for poisson effects in your hand calc ?

Quando Omni Flunkus Moritati

RE: Simple Tension test û why is my model too stiff?

So perhaps you could post ALL of the details of your hand calls from the FE results

RE: Simple Tension test û why is my model too stiff?

(OP)
Hi guys, I got a reply from my Nastran reseller. Basically the problem was related to how Nastran how output results. Apparently it can't get stress and strain to output in the same analysis (well it can, but it generates "effective strain", i.e. von mises strain, which is not correct for a uniaxial test) so I have to run the model twice: once for stress, the other strain. This seems ridiculous to me that I can't get the right results in a single analysis, but on the other hand I do now get correct stiffness results. Has anyone else found this peculiarity with Nastran?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources