Dear All users for Riks:
It is such a gospel to have Riks as a numerical tool to model structure buckling behaviour.
However, there is a puzzle for me, when I model a column with solid square cross-section under compression. I used displacement control as loading method.
the model can nicely capture the ultimate critical buckling load, but after the peak load, the displacement applied cannot progress, as I extract the data to make load- compression displacement curve.
To trouble shooting, I also drawn the load - lateral displacement curve of the center height of the specimen. From the curve, I can clearly see the large lateral displacement owing to the buckling.
As shown in attached illustration.
I found that after the specimen reached critical buckling load, the load kept same level and slightly decreased when the lateral deflection kept increasing. This make sense, but the axial displacement only grows very minimal after the critical buckling load occurred, THE ACTUAL AXIAL DISPLACEMENT CANNOT PROGRESS TO THE VALUE I ASSIGNED AT THE BOUNDARY CONDITION SETTING AS DISPLACEMENT CONTROLLED LOADING . As a result, the post peak load decreasing happened just within very limited axial deformation, which results in a non-ductile behaviour.
Can anyone give me some idea why this is happening, how I can make the designated displacement fully reached during the modelling process.
Thanks everyone!
Regards,
Mengli
It is more about material definition, especially the post peak material constitutive relationship.
It might be related to material properties, B.C or Riks analysis setting.
In studies on buckling pipes I have used an ideal elastic-plastic material and it worked. It is more a numerical problem. The problem is the large stiffness change after reaching the maximum load and the buckling. After my experience you should use two steps. At first a 'normal' static step until just before buckling (test different values), then continue with RIKS. For RIKS a larger begin step size is often better. A greater imperfection also often helps.
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