Hi Dr. Jorgen,

Im a PhD. student in CU Boulders Mechanical Engineering department. I recently wrote some code for calculating the internal forces in a nearly incompressible hyperelastic plane stress element, but Im unsure about how I should calculate the out of plane component of the deformation gradient. The abaqus theory manual starts from a linear elastic assumption in order to derive an equation for the thickness change, but they dont explain when and why that is valid. Would it be possible for you to point me towards some resources that explain methods for calculating the out of plane component of the deformation gradient for hyperelastic plane stress elements?

Thanks,

Chris DeLuca