  Evolution of stellar bars in disk galaxies is accompanied by dynamical
instabilities and secular changes. Following the vertical buckling instability,
the bars are known to weaken dramatically and develop a pronounced boxy/peanut
shape when observed edge-on. Using high-resolution N-body simulations of
stellar disks embedded in live axisymmetric dark matter halos, we have
investigated the long-term changes in the bar morphology, specifically the
evolution of the bar size, its vertical structure and exchange of angular
momentum. We find that following the initial buckling, the bar resumes its
growth from deep inside the corotation radius and follows the Ultra-Harmonic
resonance thereafter. We also find that this secular bar growth triggers a
spectacular secondary vertical buckling instability which leads to the
appearance of characteristic boxy/peanut/X-shaped bulges. The secular bar
growth is crucial for the recurrent buckling to develop. Furthermore, the
secondary buckling is milder, persists over a substantial period of time, ~3
Gyr, and can have observational counterparts. Overall, the stellar bars show
recurrent behavior in their properties and evolve by increasing their linear
and vertical extents, both dynamically and secularly. We also demonstrate
explicitly that the prolonged growth of the bar is mediated by continuous
angular momentum transfer from the disk to the surrounding halo, and that this
angular momentum redistribution is resonant in nature -- a large number of
lower resonances trap disk and halo particles and this trapping is robust, in a
broad agreement with the earlier results in the literature.
