torsional rigid spring

[Britton229]

We expect to have a torsional rigid spring function in FEBio 3.8.

[gateshian]

I see several difficulties with implementing a torsional spring in a 3D finite deformation code such as FEBio: In the real world most torsional springs have a fixed axis of rotation and cannot rotate excessively. However, in a 3D finite deformation framework the axis of rotation is not necessarily fixed and the rotation could theoretically exceed 2*PI. I haven't been able to figure out a correct formula for dealing with these two challenges. If you know of a paper or website that provides a valid set of equations for a torsional spring in 3D which can undergo large rotations, please let me know and I will implement it.