Here ε is the engineering strain,
εGL is the Green-Lagrange strain and
σ1 is the stress caused by the unit load. In terms of stiffness matrices, this corresponds to
where KL is the linear stiffness matrix, and
KNL is the nonlinear contribution to the full stiffness matrix. The symbolic linearization point
u0 is the displacement vector caused by the unit load.
By using the term (λ-1), the effect of using the Green-Lagrange strain tensor in the first term is to a large extent removed. Unless the unit load is significantly larger than the buckling load, the result will be the same as the intended, even if geometric nonlinearity was inadvertently selected in the eigenvalue study step.