Kinematics and elasticity framework for materials with two fiber families

A kinematics framework is developed for materials with two fiber families that are not necessarily orthogonal or mechanically equivalent. These two latter conditions represent important subclasses that are analyzed. To succinctly define the strain, six scalar strain attributes are developed that have direct physical interpretation. In the hyperelastic limit, this approach allows the Cauchy stress t to be expressed as the sum of six response terms, almost all of which are mutually orthogonal (i.e. 14 of the 15 inner products vanish). For small deformations, the response terms are entirely orthogonal (i.e. all 15 inner products vanish). Experimental advantage is demonstrated for finite strain hyperelastic materials by showing that common tests, for the first time, can directly determine terms in the strain energy function of two fiber composites.