dc.description.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 100 o
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 100 o 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 100 o countersunk and perpendicular rivet holes were parallel for the same
heat-treated condition. Al7075-O had trans-granular and deflecting angles of about 30 o , 45 o , and
70 o 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. |
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