Mixed-mode fracture of notched AW 2099-T83 Al-Li Alloy: Effects of inclination angle and stress triaxiality

Third-generation aluminum-lithium alloys such as AW 2099-T83 offer attractive properties for aerospace applications, including low density, high stiffness, and excellent fracture toughness. This study investigates the quasi-static fracture behavior of notched AW 2099-T83 extrusion specimens under mode I and mixed-mode I/II loading conditions. Straight U-notched with radii between 1.5 and 6.0 mm and V-notched with angles 35°, 60° and 90°, and radii of 0.4 and 0.8 mm specimens were tested to characterize mode I fracture, while V-notched specimens with notch angle of 35° and notch radius of 0.8 mm were machined at inclination angles of 0°, 10°, 20°, 30°, 40°, and 50° to examine mixed-mode fracture. Transverse strain measurements were performed to assess stress triaxiality effects on tensile and U-notched specimen with radii of 5 and 6 mm. Straight notched specimens exhibited pronounced notch-strengthening behavior confirming that triaxial constraint governs fracture resistance. Inclined V-notched specimens revealed a counterintuitive relationship: fracture load increased 22 % from 0° (13.80 kN) to 30° (16.89 kN), then declined sharply by 25 % at 50° (12.70 kN). This behavior reflects a transition from constraint-induced strengthening at low angles to shear-dominated failure at high angles, where resolved shear stress aligns with weak microstructural interfaces. Fracture morphology analysis confirmed the evolution from ductile dimple rupture to shear-dominated failure with extensive delamination. The point method of the theory of critical distances (TCD) successfully predicted fracture stresses with 7.24 % average absolute discrepancy.