dear Karthik, the engineering stress strain curve is only decreasing after the yield point because the measured forces are related to the initial cross-secional area of the specimens. In opposite to this the true stress curve is calculated considering the necking behavior of the specimens, which means the sudden, local reduction of the true cross-sectional area, is considered. This is / was fundamentally important for the numerical calculations where the necking phenomenon could not have been simulated correctly without the true stress strain curve relation.