Dear Holly,

Your issue might be related to the way your USDFLD subroutine updates the material properties during the softening phase. Here are a few key aspects to check:

1. Stress Update Algorithm: Ensure that your stress update logic correctly captures the strain-softening behavior. A slight delay in reducing stress could cause an overshoot.

2. Damage Evolution: If you’re using a damage model, verify that the damage variable evolves correctly post-peak and reduces stress accordingly.

3. Strain Rate Effects: If implicit integration is used, numerical stability issues can sometimes result in slight overshoots. Adjusting the step size or using a different integration scheme might help.

4. Confinement Model Parameters: Double-check that the confinement effect is applied consistently and does not artificially enhance stress beyond the expected peak.

A practical approach would be to print intermediate stress values in your subroutine to trace where the deviation starts. Comparing results with a simpler analytical model may also provide insights.

Hope this helps!

Best regards,

Hasan

Dear Hasan,

Thank you for your suggestions. They are really helpful.

In regards to point 1, how can I check that the strain softening behaviour is good?

Also, for the rate of confinement increment, I want to make sure that the input material data is correct. I want to model FRP-confined concrete, but in ABAQUS, I put the data for active confinement in the Properties tab. For actively-confined concrete, the rate of confinement increment is 0. For FRP-confined concrete, the rate of confinement increment is a fixed value. Therefore, should the rate of confinement increment all be 0 for every row in the table?

Holly