Non-linear convergence issues
Hi all, i am new to modeling, so apologies if i missed any essential information.
I’m implementing geometrically nonlinear static analysis for a large aircraft wing FEM model in a in-house tool and I’m running into a very strange, repeatable convergence issue.
The model is a composite wing structure, mostly quadrilateral shell elements, with RBE2/RBE3-type couplings and distributed kinematic constraints. Loads are applied through reference points along the wing.
I’m using conventional Newton-Raphson with load control.
the solver approaches a stage/load parameter of approximately: 0.00221323 Eventually the increments become essentially microscopic and the analysis makes no meaningful progress.
What makes this strange: The deformation at this point is extremely small, so I do not expect this to represent a buckling event. Artificial damping over several magnitudes also did nothing. path-following continuation can move around the region, but tends to follow a backwards/looping branch. Zero load converges perfectly. My current interpretation is therefore that the nonlinear tangent stiffness is approaching a near-null direction. What diagnostics would you use next?