Strain Stiffening Instability from Off Axis Terms for highly elastic structures.

[kevmoor]

Strain stiffening of off-diagonal terms in nonlinear timoshenko beam element appears to cause instability for relatively elastic structures despite the linear solution having adequate factors of safety for failure and low deformation. The terms are negative, but interestingly they are orders of magnitude lower than the on-axis. Commenting them out resolves the stability issue. The derivation of the terms are not defined in Owens' dissertation, while the on-axis terms are. Brian gave a source for them (“Nonlinear Finite Element Analysis” by JN Reddy.) But, reviewing this source does not point to a clear definition/implementation and will require further assistance. GXBeam integration to OWENS does not appear to have this issue, need to run comparisons