Transition Mechanisms of Very‐High Cycle Fatigue Failure in High‐Strength Al Alloy: Effects of Processing States on <i>S‐N</i> Behavior

ABSTRACT Uniaxial fatigue testing at room temperature was employed to investigate the fatigue characteristics of 7075 high‐strength aluminum alloy under very high cycle fatigue (VHCF) conditions across different processing states. Specimens subjected to surface mechanical rolling treatment (SMRT) exhibited smooth and continuous S‐N curves, demonstrating three distinct failure modes depending on applied stress amplitudes: surface‐induced failure (S‐mode), internal crack initiation without fine granular area (FGA) formation (I‐mode), and internal failure accompanied by FGA (IFGA‐mode). In contrast, welded SMRT specimens and untreated 7075‐T651 alloy counterparts displayed only two failure modes (S‐mode and IFGA‐mode). Notably, fatigue lives in IFGA‐mode failure showed processing‐independent characteristics across different material conditions. The stress intensity factor range (Δ K ) was systematically applied to elucidate the mechanisms underlying these failure mode transitions and the characteristic trends observed in S‐N curves.