A mixed mode bending experimental system is developed for quantifying the subcritical debond growth behavior and establishing the fatigue crack growth rate models for materials interfaces of interest under a full spectrum of loading mode mixity. In the fatigue debond growth experiment, the control program calculates and adjusts the applied bending forces to maintain a constant phase angle at the prescribed value. The relationship between crack growth rate and the applied strain energy release rate is obtained by post-processing the experimental results with analytically derived equations. Fatigue growth characteristics of the Al-epoxy interface under various mode mixities were investigated. The steady-state cyclic fatigue delamination growth is found to exhibit a power-law dependence on the applied strain energy release rate range. A phase-Angle dependent fatigue crack growth model was constructed from fitting the steady-state cyclic fatigue delamination growth responses. The subcritical debond growth model can be combined with delamination driving forces obtained for real structures containing the interface of interest to predict the fatigue crack growth behavior and the corresponding structural reliability.