I was solving a larger shell model using a rigid-body constraint and could not solve the problem successfully when I used the NLGEOM setting. I was able to determine that the rigid-body constraint was causing convergence problems. I was able to reduce the larger model to a model of only three shell quadrilateral elements for the comparison with the solid approach. It turns out that the equivalent solid model does not have any convergence problems. So, it might be that my approach to shell modelling is somehow wrong or that the rigid constraint works differently for shell and solid models. It makes the use of rigid body constraint almost unusable for the shell models and NLGEOM.
The solid model (Solid.inp) is a plate of dimensions x: 30; y: 10; z: 1; meshed with three C3D20 elements. The material has elastic properties of E = 200000 and a Poisson’s ratio of 0. The node-set (a) is fixed (d), the node-set (b) has a prescribed displacement of 0.01 in the displayed direction (e), and the node-set © is used in the rigid body constraint (f). The constraint uses the reference nodes “Ref node” and “Rot node” positioned on the centre of the rightmost face of the model (x, y, z: 5, 30, 0.5). This solid model can be solved using NLGEOM in 3 iterations. Moving the reference nodes around does not affect the solution.
The shell model (g) (Shell.inp) is a plate of dimensions x: 30; y: 10; z: 0; meshed with three S8 elements. Everything else is the same. The reference nodes are positioned at x, y, z: 5, 30, 0. With the default settings for the NLGEOM incrementation, the problem cannot be solved. If the reference nodes are moved around, this affects the convergence of the solution (it should not!). At the reference nodes positioned at x, y, z: 5, 24.5, 0 the solution converges with the default incrementation but uses much larger number of increments.
So what is wrong with the rigid-body constraint? Any suggestions, ideas?
The .inp files: