Effect of grain disorientation on early fatigue crack propagation in face-centred-cubic polycristals: A three-dimensional dislocation dynamics investigation

Three-dimensional dislocation dynamics simulations are used to study micro-crack interaction with the first micro-structural barrier in face-centred cubic bi-crystals loaded in high cycle fatigue conditions. In the examined configuration, we assumed that micro-crack transmission occurs due to surface relief growth in the secondary grain ahead of the primary crack. This indirect transmission mechanism is shown to strongly depend on grain-1/grain-2 disorientation. For instance, small grain disorientation induces plastic strain localisation ahead of the crack and faster transmission through the first barrier. Conversely, large grain-1/grain-2 disorientation induces plastic strain spreading similar to crack tip blunting yielding slower indirect transmission. A semi-analytical micro-model is developed based on the present simulation results and complementary experimental observations highlighting the original notion of first-barrier compliance. The model captures well known experimental trends including effects of: grain-size, grain disorientation and micro-crack retardation at the first barrier.