Interaction of fatigue crack growth and intergranular corrosion in maritime aircraft

The objective of this PhD Thesis is to further the understanding of the effect of environmentally assisted degradation on aircraft structures and in particular its effect on ageing aircraft. Particular attention is paid to intergranular corrosion (IGC) coupons, specifically “dome nut hole” coupons that are representative of a critical location in the RAAF AP-3C Orion wing. In circumstances where the airframe is inadequately protected by surface protection systems and corrosion is present, corrosion have the potential to nucleate and grow fatigue cracks. Thus the management of ageing aircraft fleets requires an accurate assessment of fatigue performance including the influence of incipient corrosion. Since the majority of the life of a fatigue crack is spent in the nucleation and propagation of small cracks, the predictive capabilities of crack growth equations must ideally capture small, intermediate and large crack growth behaviours. To this end studies are conducted on fatigue crack growth in a various types of coupons, including coupons that contain incipient corrosion pits and co-located IGC of up to approximately 2 mm deep. The impact of IGC on fatigue crack growth is investigated in order to develop models for crack growth from small cracks to large cracks. The examination via quantitative fractography of the failure surfaces of cracks in these coupons revealed that fatigue cracks often nucleated from corrosion sites and can be strongly affected by the morphologies of the corrosion pit and/or co-located IGC. The results showed that the depths of IGC produced using a corrosion protocol and the influence of IGC on the crack growth can be inconsistent. In other words, variable interactions between crack growth and IGC have been found: from minimal crack interaction with the IGC, to cases where multiple cracks being partitioned by the IGC fissure. In general, the multiple fatigue cracks coalesced beyond the maximum depth of the IGC. The cracks usually nucleated at material discontinuities which in the corroded specimens were corrosion pits that caused a rise in local stress concentration. Cracks have been observed to nucleate at micro-pits as well as at ‘jut-in’ features at the surfaces of a coarsely-formed pit. These features can also cause the deviations in the growth path of micro-cracks. Moreover, in instances where multiple cracks grew from the corrosion pits interference between cracks can occur. The resultant crack growth surface is often tortuous and crack fronts also appeared ill-defined. As a consequence the growth histories of cracks that grew from surface corrosion show some degree of variability. The corroded specimens exhibited crack growth lives that were significantly shorter than the polished (non-corroded) specimens. The average knockdown factor of the corroded DNH coupons in contrast to the polished DNH coupons was approximately between 2 to 5. The scatter in the crack growth lives of the corroded DNH specimens varied considerably particularly at lower peak stresses. The polished coupons tested also exhibited substantial scatter in the total crack growth lives. The cracks in the polished DNH coupons nucleated at discontinuities that arose from machining blemishes at the bore of the fastener holes. The sizes of machining discontinuities, e.g. burrs, micro-notches etc. are on average considerably less than the depth of corrosion pits. The ability of the Supersonic Particle Deposition (SPD) repair technique to enhance the fatigue crack growth life of DNH coupons containing IGC is also investigated. The average fatigue crack growth life of the SPD coupons was approximately double the life of the non-repaired coupons. The results also revealed that the SPD was able to restore the fatigue lives of the DNH specimens to that comparable to a polished specimen particularly in tests at peak stresses of 150 MPa. The nature of the crack growth in the SPD specimens was similar of the crack growth observed in specimens without SPD. The study demonstrate the capability of SPD repairs to complex geometries to enhance structural fatigue life. The test data obtained enabled the interaction between crack growth, pitting and intergranular corrosion in 7075 series aluminium alloy to be assessed and characterised by an empirical crack growth equation that is commonly used in the aerospace industry (i.e. NASGRO). The studies on small cracks that grow from small initial discontinuities show that the Hartman-Schijve variant of the NASGRO formulation can closely capture the crack growth histories by allowing for the appropriate value of the Equivalent Initial Flaw Size (EIFS) and cyclic stress intensity fatigue threshold ∆Kthr. The rate of small crack growth can vary considerably because of the non-homogeneity in the microstructure. This behaviour can usually be captured by assuming minimal crack closure behaviour and allowing for small variations in the cyclic stress intensity threshold ∆Kthr. The experimental crack growth measurements and predictive techniques investigated herein confirms that the Hartman-Schijve variant of the NASGRO equation can accurately capture crack growth, in the presence of incipient corrosion, from small sub-mm sizes through to long cracks of the order of several mm’s. As corrosion introduces complexities in detecting fatigue cracks in ageing aircrafts, the current work is thought to contribute to the understanding and assessment of the fatigue crack growths in environmentally degraded structures.