In Situ Investigation of Crystallographic Effects on Short Fatigue Crack Propagation From Surface Notches in a High‐Strength Alloy Structural Steel

Abstract

Short fatigue cracks (SFCs) propagation in high‐strength alloy structural steel 34CrNi3Mo was investigated using in situ scanning electron microscopy and electron backscatter diffraction. Prefabricated notches with three initial nominal stress intensity factor range (Δ K ) were fabricated via focused ion beam. SFC behavior was governed primarily by local microstructure rather than initial Δ K . In the microstructurally short crack (MSC) stage, crack‐growth plateaus arose from grain‐boundary interactions and slip‐unfavorable crystallographic orientations. Relatively low‐Taylor‐factor grains preferentially developed slip traces, providing favorable paths for discontinuous crack advance and local acceleration. In the physically short crack stage, the fatigue crack propagation fluctuated without distinct plateaus, indicating a transition from predominantly microstructural control to the combined influence of microstructure and local stress fields. In addition, the effects of plasticity‐induced closure and roughness‐induced closure on SFC behavior are discussed. A relationship between FCP rate and nominal Δ K was established, covering the MSC to long crack stage.