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Effect of load substeps in nonlinear solution accuracy

Effect of load substeps in nonlinear solution accuracy

Effect of load substeps in nonlinear solution accuracy

(OP)
Hi Everyone,
I am doing nonlinear analysis of a box girder modelled with shell181 element. I have
1. Material nonlinearity
2. Geometric nonlinearity
3. Rigid to flexible contact and flexible to flexible contact.
As I dont know the ultimate load carrying capacity, I am trying to find that using my model. I am applying arbitary vertical displacement on the beam and draw the mid span load deflection curve. I assumed that when the reaction load decreased with the increase of deflection,that load is the ultimate load capacity. If the solution does not converge for a small increment of load after sometimes I am considering the last converged solution as the ultimate capacity. I am wondering that the solution varies a lot with the number of substeps. I tried as below

1. Convergence criteria: F and M with tolerence 0.005 and MIREF=0.01.Solution control= On
2. Number of Substeps:50, Maxm=48000, Minimum=30, Time=1, Arbitary applied load=0.7in.
Capacity of the last converged solution=9439.61lb and maximum vertical deflection=0.67
With everything keep same, only changing the substeps as below:
1. Substeps=100, Maxm=48000, Minimum=50, Time=1.
Load Capacity=7700lb and maximum vertical deflection= 0.518in.
I am wondering why the result varies so much? Which result I should consider accurate? By the way both the solutions are the last converged solution. After that it can not converge any more.
Is there anyone who can help me in this regard? I will be really gratefull.
Thanks
Mon

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