Hi,

now I have a good single element model but the results for the biaxial stress state are nearly 8% above the analytical solution of the willam-menetrey failure law.

In a second model I modelled 1/4 symmetry but I think there is no chance to simulate a biaxial failure with it because of the confinement effect.

It would be great if I could send you my GiD model. Should I send it to the cervenka contact mail-adresse on your homepage or can I send it directly to you?

Yes, a single-element model is a good approach for this. The typical (and recommended) modelling is 1/4 symmetry. Please do not forget the effects of non-zero Poisson's number when preparing your boundary conditions. Please also note the confinement effect is very sensitive to even very small tensile or compressive stresses in the 3rd (or 2nd+3rd) dimension(s). As these stresses are easily smaller than 1% of the max. compressive stress, it can make sense to tighten the convergence tolerances in the solution settings.

Hi,

one of the nodes is fixed in each direction so rigid body rotations are blocked.

Maybe I will explain you my intention:
I want to do a patch test on a single solid element to evaluate the bi- and triaxial compressive strength. Then I want to change the eccentricity factor e (from the Willam-Menetrey failure surface) to evaluate its influence on the bi- and triaxial strength.

Is this possible with just one element? I think of a displacement on three sides of the element (for biaxial failure) and on five sides for triaxial failure. In the bi-axial state the compressive strength has to be around 1.2 times of the uniaxial strength (e=0.52), this is what I want to check with the patch-test.

Hello,
1. if you like us to check your model, please send it along with your ATENA User ID (WR) and a description and/or sketch.
2. for a single element in uniaxial compression, the rigid body rotations have to be blocked. Your description of the conditions sounds like allowing rotation around the vertical axis. Normally, one would model 1/4 of the sample and apply symmetry conditions.

Hi,
thank you for your fast reply.

My problem is that if I take the CC3DNonLinCementitious2 material with a concrete strength of fc=-50 MPa and I just have one load direction (uniaxial) the finite element has to yield at the minimum principal stress of -50 MPa (its kind of a very simplified patch-test).

I am taking a 8 node solid element. 4 nodes are loaded with a uniaxial displacement till the element will yield. The other 4 nodes are fixed with boundaries in load direction. One of these nodes is fixed in the other two directions too (for numerical forces in these directions).

Dear Tii, without seeing your model, I can't check if the BCs are right. However, I don't know a way to correctly simulate the classic cube test, where friction at the top and bottom of the sample is NOT eliminated, using a single finite element! Moreover, the stress in the classic experiment is not measured, but a calculated average (force/reference cross section area).

Please also read ATENA Troubleshoting, 2.1.21 The tensile (or compressive) stresses exceed the tensile (or compressive) strength or yield stress – what is wrong?