$($a$)$ The leftmost image $($photo$)$ above shows the arrangement of osteons in cortical bone. The two sketches at right show a

common approximation of the resulting anisotropy. Name and briefly describe the mechanical characteristics of this specific

anisotropy and indicate on the sketches above the principal axes that define the material symmetry.

$($b$)$ Consider the typical failure of bone subjected to torsion depicted in the sketch below. Using a stress element, sketch the stresses that occur on the failure surface. Considering the stresses, explain why the bone fails along this surface? $($a$)$ The leftmost image $($photo$)$ above shows the arrangement of osteons in cortical bone. The two sketches at right show a common approximation of the resulting anisotropy. Name and briefly describe the mechanical characteristics of this specific anisotropy and indicate on the sketches above the principal axes that define the material symmetry.

$($b$)$ Consider the typical failure of bone subjected to torsion depicted in the sketch below. Using a stress element, sketch the stresses that occur on the failure surface. Considering the stresses, explain why the bone fails along this surface?