  We track the angular momentum transfer in n-body simulations of barred
galaxies by measuring torques to understand the dynamical mechanisms
responsible for the evolution of the bar-disc-dark matter halo system. We find
evidence for three distinct phases of barred galaxy evolution: assembly,
secular growth, and steady-state equilibrium. Using a decomposition of the disc
into orbital families, we track bar mass and angular momentum through time and
correlate the quantities with the phases of evolution. We follow the angular
momentum transfer between particles and identify the dominant torque channels.
We find that the halo model mediates the assembly and growth of the bar for a
high central density halo, and the outer disc mediates the assembly and growth
of the bar in a low central density halo model. Both galaxies exhibit a
steady-state equilibrium phase where the bar is neither lengthening nor
slowing. The steady-state equilibrium results from the balance of torque
between particles that are gaining and losing angular momentum. We propose
observational metrics for barred galaxies that can be used to help determine
the evolutionary phase of a barred galaxy, and discuss the implications of the
phases for galaxy evolution as a whole.
