• Registered: 2019-12-29
  • Posts: 17

Topic: FRP rupture

Hello,

I am doing a research on simulating the behavior concrete member reinforced with FRP.
I behavior of FRP material was defined by multilinear curve consisted of three points: (0,0), (Strain at rupture, stress at rupture), (Strain at rupture + 0.0001, 0).

After running the model, I found that some strain readings in FRP exceeded the strain at rupture, while the stress value did not exceed rupture stress.
I expected that the reinforcement bars will rupture when the rupture strain is reached, according, strain readings should not exceed rupture strain. However, this is not happening.

I am missing something or doing something wrong?

Thank you.

Quote Post 1

  • From: Finland
  • Registered: 2019-03-28
  • Posts: 138

Re: FRP rupture

Firstly, it is worth to check whether convergence criteria are satisfied in your problem.
Secondly, you would better check all previous steps. And the easiest way is to define strain and stress monitors for the rebar you investigate. Then you can compare observed stress- strain curve with the defined curve.
Thirdly, you might need to modify the softening branch to avoid convergence problems. Also you can define smaller steps prior to failure to catch the failure point.

Do you apply a load or displacement to your model? Displacement is always better. In case of applying a load it is recommended to use arch length method prior to failure.

Quote Post 2

  • Registered: 2019-12-29
  • Posts: 17

Re: FRP rupture

pavlo wrote:

Firstly, it is worth to check whether convergence criteria are satisfied in your problem.
Secondly, you would better check all previous steps. And the easiest way is to define strain and stress monitors for the rebar you investigate. Then you can compare observed stress- strain curve with the defined curve.
Thirdly, you might need to modify the softening branch to avoid convergence problems. Also you can define smaller steps prior to failure to catch the failure point.

Do you apply a load or displacement to your model? Displacement is always better. In case of applying a load it is recommended to use arch length method prior to failure.

Hello pavlo,

1- How would I check the convergence criteria ? and how that would effect the strain readings or the model behavior?
2- I was checking the strain using iso-areas display and I checked the strain at several steps. I found out that the strain exceeded the rapture strain at several steps but the stress did not. The problem is not for one bar, it is for multiple bars, plus I do not know the exact location of maximum strain along the bar. So, the monitoring point would not read the maximum strain or stress in the bar.
3- what kind of modification for the softening branch do recommend? keep in mind that post peak branch almost vertical line used to assign zero strength to the bar after peak (rupture).  Please see the snapshot in the following link
https://drive.google.com/file/d/1rDuz1g … sp=sharing

4- yes, it is displacement controlled

Thank you.

Quote Post 3

  • dpryl
  • CC software developer
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  • Registered: 2007-07-13
  • Posts: 1,107

Re: FRP rupture

Dear msmeleven,
ad 1.: please se ATENA Troubleshooting, 2.1.19 Problems reaching convergence and understanding ATENA convergence parameters and 2.4.2 I get the message "The execution is killed due to violation of stop iteration criteria", what does it mean? for basic related information. You may also be interested in 2.2.22 How to detect a structural collapse or ultimate load capacity.

Ad 2. Have you tried the Maximum/Minimum at integ. points Monitor type for all or selected (listed) reinforcement bars (see also ATENA 3D User's, 3.7.5.3 Monitoring)?

Just to avoid a possible misunderstanding: in a converged step, the max. stress in a bar can NOT exceed the reinforcement material strength. Simply said, if the stress is to be exceeded, the bar breaks. Then, the stress drops and the strain continues to increase.

Ad 3.: We generally recommend 1/100 or 1/1000 of the peak value instead of 0 for the "broken" strength (see also 2.2.16 How can I model a compression-only support with nonlinear springs?).

Regards.

dpryl's Website

Quote Post 4

  • Registered: 2019-12-29
  • Posts: 17

Re: FRP rupture

Dear pavlo,

3- I changed the stress value at break to be 1/1000 of the rupture strength. That provided a reasonable results.

Found out something and I hope if you could confirm if that is right. I am using prefect bond between the FRP and the concrete. Suppose that rupture strength is reached at a certain point named 'X" along bar. After rupture, the strain at X might keep increasing but the stress at X will be zero or minimal. While the rest of the bar is functional and can hold stresses. Is this is right ?

Thank you.

Quote Post 5

  • dpryl
  • CC software developer
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  • Registered: 2007-07-13
  • Posts: 1,107

Re: FRP rupture

Dear msmeleven, yes.
Regards.

dpryl's Website

Quote Post 6

  • Registered: 2019-12-29
  • Posts: 17

Re: FRP rupture

dpryl wrote:

Dear msmeleven, yes.
Regards.

Dear Dpryl,

I have another question. What would happen if only two points (at zero and at rupture) for the material curve were defined? Which means there is no descending branch.

Thanks for your reply and help.

Regards.

Quote Post 7

  • dpryl
  • CC software developer
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  • Registered: 2007-07-13
  • Posts: 1,107

Re: FRP rupture

Dear msmeleven,
a "rupture" point is automatically added (at strain 1% over the last defined point). Please see the explanation text at the Reinf Function tab of the Reinforcement material dialog (if you are using ATENA Science+GiD; the behaviour should be the same in ATENA Egr). However, please note in some old ATENA versions (5.1?) this may differ - to make sure, check the reinforcement material definition in the ATENA Input File (.inp) if you are not using the current 5.6.1 or the 5.7.0 rc.

Regards.

dpryl's Website

Quote Post 8

  • Registered: 2019-12-29
  • Posts: 17

Re: FRP rupture

Dear dpryl,

Thanks for your help.

Regards.

Quote Post 9