Abstract: The ultra-strong magnetic fields of magnetars have a profound implication of radiative and timing phenomena. We studied the dynamics of strong magnetic fields inside and outside magnetars. Inside the magnetar, the strong magnetic stress can break the crust by triggering plastic failures. We studied the interaction of magnetic fields and plastic failures in two scenarios. 1. We propose that magnetar outbursts are consequences of plastic flow triggered by Hall waves generated from the core-crust interface. 2. Alfven waves produced by giant flares can also initiate crustal plastic flow and give rise to the delayed thermal afterglow. The crustal absorption of Alfven is competed by magnetospheric dissipations outside the magnetar. Using a high order simulation of Force-Free Electrodynamics (FFE), we found that the magnetospheric dissipation of Alfven waves is generally weak and most wave energy will dissipate inside the magnetar. If time permits, I will also discuss the implication of plastic failures on magnetar heating and timing anomalies.