Crack‐Tip Stress Distribution and Fractographic Analysis of Brittle Fracture in AA7075‐T6 Aluminium Alloy

Understanding how stresses are distributed around crack tips in high‐strength aluminium alloys is essential for predicting fracture initiation sites, crack deflection and the transition from stable cracking to unstable fracture. This paper analyses the Mode I crack‐tip stress field in fatigue pre‐cracked single‐edge‐notched tension (SENT) specimens of AA7075‐T6 (L‐T orientation) using the singular term of Williams’ asymptotic solution in Cartesian and polar coordinates, a two‐dimensional finite element (FE) model in Ansys Mechanical APDL 2026 R1, and scanning electron microscopy (SEM) with quantitative image analysis. At the maximum test load, the stress intensity factor was K _I  = 13.08 MPa√m ( a ₀  = 3.5 mm, σ  = 94.85 MPa), giving a conditional fracture toughness K _Q  = 13.1 ± 0.2 MPa√m for the three specimens. The FE model, built with a symmetric spider‐web mesh and quarter‐point crack‐tip elements, converged to within 0.2% of this value. The plane‐stress plastic zone extends 0.11 mm ahead of the crack ( r _p / W ≈0.5%), so small‐scale yielding applies, and the ASTM E399 size requirement of 1.70 mm is met; the toughness nevertheless remains conditional because the 5% secant‐offset load could not be evaluated. The FE and Williams opening stresses agree to within 3% at 0.3 mm ahead of the tip, but differ by 10%–31% between 1 and 3 mm, which limits the K‐dominated zone in this geometry to about 0.1a. SEM of the untested surface revealed elongated cavities up to 77 μm long and pits of about 670 μm ² , interpreted as sites of torn‐out coarse constituent particles that can act as crack initiation sites. The overload fracture surface was flat and faceted with few large dimples, consistent with the low ductility of the T6 temper; higher‐resolution fractography and metallographic sectioning are identified as necessary to establish the fracture micromechanism.