Grain bridging degradation behind a crack tip is the main cyclic fatigue mechanism in nontransforming ceramics. In this work, a compliance function is used to quantify the shielding capacity of grain bridges during cyclic loading of alumina ceramics with different grain sizes. This allows to identify the different stages occurring during cyclic loading. Significant degradation is observed in the coarse grain material and a marked sensitivity to the loading level is outlined. At moderate loads, bridging degradation occurs prior to fatigue crack growth during an incubation period which can reach several million cycles. At low cyclic loads, the shielding capacity can be entirely degraded, leading to a cyclic fatigue threshold equivalent to that of the fine grain material.