 The following definitions use the notations shown in the figure.In the reference configuration formula_7, the outward normal to a surface element formula_8 is formula_9 and the traction acting on that surface is formula_10 leading to a force vector formula_11.In the deformed configuration formula_12, the surface element changes to formula_13 with outward normal formula_14 and traction vector formula_15 leading to a force formula_16.