The first parameter in the Hill criterion that needs to be calibrated is the orientation sensitivity parameter . We determine so that it minimizes the distance between the equivalent stress-strain curves in the two testing configurations, that is

(3–79)

where the parallel and perpendicular symbols denote quantities belonging to the 0° and 90° specimen configurations respectively. Specifically, for the calculation of the equivalent stress and plastic strains, we use the following yield ratios

(3–80)

where and are the principal values of the average orientation tensor in the loading direction for the 0° and 90° specimens.

Determining the orientation sensitivity parameter is enough to fully define the Phenomenological yield model. For the Orientation Averaged model, the yield stresses , , and of the unidirectional composite need to be specified. They are determined with an optimization procedure so that the corresponding orientation averaged yield strength tensor Equation 3–70 matches the experimental yield strengths in the 0° and 90° configurations.