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 , \sim 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 .