Material Nonlinear analysis using beam elements
I'm undertaking some checks on a simple model to understand how the nonlinear material analysis works, I'm using the nonlinear material function in conjunction with multi-linear elastic links to model the behaviour of a pile under seismic loads where liquefaction occurs .
With reference to the attached file, I initially undertook an analysis of a simply supported steel beam with an elastic material for the steel beam. From this analysis I had a maximum combined stress at mid span of 493 N/mm2. I then assigned a plastic material with an Initial Uniaxial Yield Stress of 450N/mm2 to the steel, so trying to start the beam to yield and redistribute some load into the elastic links.
I ran a non-linear analysis with material nonlinearity but there was no difference to the level of max combined stress in the beam, which I would have expected as a plastic hinge is formed in the beam, so there is greater rotation, greater deflections. I tried running without the geometric nonlinear component, but this didn't change anything.
Once I changed the Initial Uniaxial Yield Stress of the steel to 425N/mm2, I did see a change in line with the expected behaviour, but the the max stress is significantly higher than the Initial Uniaxial Yield Stress (480N/mm2 vs 425N/mm2)?Kindly explain the above discrepancy.