where a„ and crvv are the direct stresses parallel to x and y axes, respectively, oxy is the shear stress in the xy plane, and [D] is known as the elasticity matrix.

Following the same arguments used in the rod and beam formulations, namely, taking the expression for virtual strain to have a similar form to the real strain, eqn. (9.90), and substituting this and the expression for real stress, eqn. (9.92), into the equation of the principle of virtual work, (9.10), gives the element stiffness matrix as

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