Abstract
We study the nonlinear interactions in the TGBA phase by using a rotationally invariant elastic free energy. By deforming a single grain boundary so that the smectic layers undergo their rotation within a finite interval, we construct a consistent three-dimensional structure. With this structure we study the energetics and predict the ratio between the intraboundary and interboundary defect spacing, and compare our results with those from linear elasticity and experiment.