The Stiffest Wire

We consider a polycrystal constituted from orthorhombic single crystals for which one particular principal axis of the crystallites is always oriented parallel to a particular direction; in the plane perpendicular to this direction the crystallites are randomly oriented. Bounds are found for the Young’s modulus in the axial direction. The lower bound on the Young’s modulus, which is realizable, is found to be that of the individual crystallite in the aligned direction. The upper bound determined is necessarily realizable when the single crystal elastic constants satisfy a certain condition. When this condition is not satisfied a bound is found; whether or not this bound is realizable must be examined using the specific elastic constants of the crystal being considered. For all physical examples considered the upper bound was indeed found to be realizable. Thus, generally speaking, a wire constituted as above, with the stiffest direction of the individual crystallites being along the wire, will have a higher Young’s modulus than the maximum modulus of the individual crystallites of which it is composed.