"koehlerl" schreef
However, my question still remains quite simple: the mathematical
relation between fiber angle and bending stiffness of a fiber matrix
composite.

***
Well, the only thing mildly interesting from a botanical point of view is
"microfibril angle in cell walls". However, this is not all that likely to
affect the properties of a "fiber matrix composite", unless perhaps the
latter is a sheet of paper. So you are asking the wrong group.
PvR