Investigation of microstructural cracking of riveted and heat-treated AL 7075 alloys under constant Fatigue loading

Abstract

The unique properties of the strength-to-weight ratio of aluminium make them the preferred material aircraft design with safety margins and improved payload. Fatigue cracking of Al7075 poses major issues in administering ageing aircraft structures. Understanding the effect of fatigue on rivet hole geometry and heat treatment on aircraft structures helps identify crack mitigation measures, making aluminium continue operation with high assurance levels. The main objective was to investigate microstructural cracking of riveted and heat-treated Al7075 used in aerospace stringers and frames under constant fatigue loading and develop crack mitigation measures. The specific objectives were to evaluate crack growth and propagation under constant fatigue loading of Al7075-O/T6/T7; to determine crack propagation under constant fatigue loading in 100o Countersunk rivet hole and perpendicular rivet hole geometry of Al7075-O/T6/T7; to characterize cracking of Al7075 under (a) varied heat treatment conditions in terms of crack-path and crack surface morphology, and (b) under 100o countersunk rivet hole and perpendicular rivet hole geometry in terms crack surface morphology; to identify mitigating actions to microstructural cracking Al7075. Al7075-O/T6 were procured from Smiths Advanced Metals U.K., and Al7075-T6 was converted to Al7075-T7 as per Boeing standard BAC562 at Kenya Airways mechanical workshop, Nairobi. High-cycle-fatigue testing was performed at the University of Nairobi Mechanical Engineering workshop. The crack surface morphology was observed via the Tescan Vega-3 scanning electron microscope. The sample size for heat treatment was 6, for hole orientation 12, and for scanning electron microscope analysis 72. Middle-tension specimen geometry was utilized for crack growth rates as per ASTM E647-13. Crack initiation samples for different rivet hole orientation were prepared as per (ASTM E8, 2010). Paris-region material parameters were Paris exponent; 10.069, 10.869, 9.663 and Paris constants; 3E-07, 4E-07, 1E-06 for Al 7075-T7, Al 7075-T6, Al 7075-O respectively. Crack propagation curves for 100o countersunk and perpendicular rivet holes were parallel for the same heat-treated condition. Al7075-O had trans-granular and deflecting angles of about 30o, 45o, and 70o crack paths. Al7075-T6 and Al7075-T7 exhibited trans-granular, minimal deflection crack paths. Internal tissue flaws or stress concentration initiated fatigue cracks. The fatigue crack propagation comprises two phases: crack initiation, occurring along the primary slip plane to inside metal, and crack propagation, displaying fatigue strips with widths 0.28μm, 0.36μm and 0.68μm for 7075-T7, 7075-T6, and 7075-O respectively. The final fracture surfaces were coarse with mixed ductile-brittle fractures of tearing ridges. The dimple size increased with heat treatment from 7075-O to 7075-T6 to Al 7075-T7. The study concludes that fatigue strength increases with heat treatment of Al 7075. The countersunk and perpendicular rivet holes exhibit similar fatigue cracking for the same heat-treated condition. Micro-cracks inducing fracturing start from zones where inclusions, coarse, secondary-stage particulates, and micro-structural flaws are present. The zone of quasi-cleavage planes and fatigue strip widths declines with increasing heat treatment. The final fracture area is attributed to dimples whose dimensions become larger. From the study, it can be concluded that Microstructural impurities majorly cause microstructural cracking. To mitigate against fatigue cracking of aircraft stingers and frames, the study recommends using high-purity Al7075, and should be heat-treated to reduce stress concentrations.