I'm trying to figure out element distortion problems in ANSYS. I generally study on fiber reinforced composite materials but for a case study, I did some testing on aluminum pressure vessel. I did burst testing on Al vessel and also, I extracted tensile test coupons from the vessel and done some tensile testing according to ASTM E8 standard.
So I want to model these two cases (internally pressurizing the pressure vessel and tensile test). I started with the simpler case, Al tensile testing. I had the experimental data, so I know the yield Str. and post yield behavior. I used both bilinear and multilinear isotropic hardening models. For multilinear hardening, I entered tabular data which I calculated true stress and strain from experimental engineering stress and strain up to max. eng stress (after that necking begins and conversion equations are not valid). I removed the grips and applied boundary conditions and loads to end faces.
You can see the results in the figure. I'm using video extension-meter and it measures displacement between two marked regions. I have marked same regions in ANSYS and plotted with external program for comparison.
Multilinear hardening fits well if the experimental data were provided. But I realized that, if I apply load as displacement, I can see necking and post-necking behavior of the metal (error margin increases but at least behavior is OK as you can see in the figure). On the contrary, if I apply force rather than a displacement, I get convergence errors after max. stress point (specifically element violations) and so, I cant see any necking or post-necking behavior (red curve in the figure).
I have done some search about giving force as a boundary condition and convergence and I see that it is about calculation method so that force (or stress) should monotonically increase. If there is any decrease, it will stop calculation and give a convergence error.
Can someone give any clarification to this behavior or any work around? (Such as arc length method rather than Newton-Rhapson)
I suspect that similar I'm getting similar convergence problem for determination of burst pressure of Al pressure vessel. Because I'm getting convergence errors after applying internal pressure of 19.7 MPa. But in experiments, burst pressures were around 23.5 MPa. Unfortunately, I can't give displacements as load in pressure vessels.
I also tried to give multilinear tabular data including post necking true stress and strain using some kind of power law but no change about convergence problem.
I'm hoping that overcoming this post-necking convergence error will increase my accuracy for determination of burst pressure of metal vessels.