Like others, I'm very sorry to learn of Jerry Marsden's passing. He kindly helped all who asked (including me) despite a very busy life and recent illness. It seems fitting to mention this before citing him below.
According to Marsden/Hughes (Mathematical Foundations of Elasticity), stretch & shear (from homogeneous elastic deformation) are determined soley by U in the polar decomposition of the deformation gradient F=RU where R = rot'n & U = right stretch.
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