## About openses uniaxial material stress-strain curve

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Linbetter
Posts: 5
Joined: Sat Jun 08, 2019 3:32 am
Location: Beijing University of Civil Engineering and Architecture

### About openses uniaxial material stress-strain curve

Hello engineers,

I want to extract the stress-strain curve of opensees uniaxialmaterial.I designed a prismatic test block to simulate.
Below is the code：

wipe
model BasicBuilder -ndm 2 -ndf 3
node 1 0.0 0.0
node 2 0.0 0.3
fix 1 1 1 1
uniaxialMaterial Concrete01 1 -36800 -0.002 -6000 -0.008
section Fiber 1 {
patch rect 1 10 10 -0.075 -0.075 0.075 0.075
}
geomTransf Linear 1
element nonlinearBeamColumn 1 1 2 5 1 1
recorder Node -file C2.out -time -node 2 -dof 1 2 3 disp
pattern Plain 1 Linear {
}
constraints Plain
numberer Plain
system BandGeneral
test NormDispIncr 1.0e-6 100 2
algorithm Newton
integrator DisplacementControl 2 2 0.000001
analysis Static
analyze 1000

I get the following error :
ForceBeamColumn2d::update() -- could not invert flexibility

Is the method I used wrong?
Can anyone help me by that error ??

selimgunay
Posts: 889
Joined: Mon Sep 09, 2013 8:50 pm
Location: University of California, Berkeley

### Re: About openses uniaxial material stress-strain curve

uniaxialMaterial Concrete01 1 -36800 -0.002 -6000 -0.008

Can you first replace this with an elastic material to see if the problem is due to this?

Linbetter
Posts: 5
Joined: Sat Jun 08, 2019 3:32 am
Location: Beijing University of Civil Engineering and Architecture

### Re: About openses uniaxial material stress-strain curve

selimgunay wrote:
Mon Oct 05, 2020 12:21 am
uniaxialMaterial Concrete01 1 -36800 -0.002 -6000 -0.008

Can you first replace this with an elastic material to see if the problem is due to this?
Thank you so much for your help！The program runs successfully after replacing concrete01 with elastic materials.But I want to get the stress-strain curve of other uniaxial materials. Is there a problem with the element I selected？Or my whole method is wrong？

mhscott
Posts: 606
Joined: Tue Jul 06, 2004 3:38 pm
Location: Corvallis, Oregon USA
Contact:

### Re: About openses uniaxial material stress-strain curve

You are putting the beam into flexure and there is no r/f steel so no tension force to balance compression force.

If all you want is stress-strain from a material, use a truss element, not a beam-column element.

selimgunay
Posts: 889
Joined: Mon Sep 09, 2013 8:50 pm
Location: University of California, Berkeley

### Re: About openses uniaxial material stress-strain curve

Another option is to apply axial shortening to your column.

This OpenSees based application allows visualizing stress-strain relations of common steel and concrete materials used in OpenSees

https://apps.peer.berkeley.edu/bridgepbee/

fmk
Posts: 5866
Joined: Fri Jun 11, 2004 2:33 pm
Location: UC Berkeley
Contact:

### Re: About openses uniaxial material stress-strain curve

you could try another modelbuilder provided just for testing material .. here is an example i used awhile back

model UniaxialMaterialTest
uniaxialMaterial Elastic 1 3000
uniaxialMaterial MinMax 2 1 -min -0.001
uniaxialMaterial MinMax 3 1 -max 0.001
uniaxialMaterial MinMax 4 1 -min -0.001 -max 0.001

set strains {0.00 0.0005 0.001 0.002 0.001 0.0005 0.0 -0.0005 -0.0001
-0.002 0.0}

proc doStrains {matTag strains} {
puts "\n Testing \$matTag"
uniaxialTest \$matTag
foreach strain \$strains {
strainUniaxialTest \$strain
set stress [stressUniaxialTest]
set tangent [tangUniaxialTest]
puts "strain: \$strain stress: \$stress tangent: \$tangent"
}
}
doStrains 2 \$strains
doStrains 3 \$strains
doStrains 4 \$strains

Linbetter
Posts: 5
Joined: Sat Jun 08, 2019 3:32 am
Location: Beijing University of Civil Engineering and Architecture

### Re: About openses uniaxial material stress-strain curve

mhscott wrote:
Mon Oct 05, 2020 6:31 am
You are putting the beam into flexure and there is no r/f steel so no tension force to balance compression force.

If all you want is stress-strain from a material, use a truss element, not a beam-column element.
Thanks professor，I tried to use the truss element, but the program still fails.Then, I solved the problem by using zeroLength element and changing the dimensions.Thanks for your help!

Linbetter
Posts: 5
Joined: Sat Jun 08, 2019 3:32 am
Location: Beijing University of Civil Engineering and Architecture

### Re: About openses uniaxial material stress-strain curve

selimgunay wrote:
Mon Oct 05, 2020 11:59 am
Another option is to apply axial shortening to your column.

This OpenSees based application allows visualizing stress-strain relations of common steel and concrete materials used in OpenSees

https://apps.peer.berkeley.edu/bridgepbee/
I solved the problem by using zeroLength element and changing the dimensions.Thanks for your help!

Linbetter
Posts: 5
Joined: Sat Jun 08, 2019 3:32 am
Location: Beijing University of Civil Engineering and Architecture

### Re: About openses uniaxial material stress-strain curve

fmk wrote:
Mon Oct 05, 2020 7:47 pm
you could try another modelbuilder provided just for testing material .. here is an example i used awhile back

model UniaxialMaterialTest
uniaxialMaterial Elastic 1 3000
uniaxialMaterial MinMax 2 1 -min -0.001
uniaxialMaterial MinMax 3 1 -max 0.001
uniaxialMaterial MinMax 4 1 -min -0.001 -max 0.001
My problem has been solved, but still thank you for the new solution, I will try to solve it with your method.Thanks a lot!