Appendix 7—figure 1. The geometry of two short rigid straight fibers connected at a bending joint.

The separation is exaggerated to clearly show the geometry. ${\mathit{p}}_1$ and ${\mathit{p}}_2$ are the orientation norm vectors of the two segments. ${\kappa }_E$ and ${\kappa }_B$ are the stiffness of the spring for extension and the spring for bending. $d_B$ is the displacement distance from the joint rotation center. In the more detailed view of the deformed geometry of the two springs, $a,b$ are the lengths of the two edges of the triangle. ${\displaystyle {\ell }_E=\sqrt{a^2+b^2+2ab\cos \alpha }}$. ${\displaystyle {\ell }_B=\sqrt{(a+d_B{)}^2+(b+d_B{)}^2+2(a+d_B)(b+d_B)\cos \alpha }}$.