Gary H. Bray

Effect of artificial aging on the fatigue crack propagation resistance of 2000 series aluminum alloys

Artificial aging performed to impart higher strength generally degrades the fatigue crack propagation (FCP) resistance of naturally aged 2xxx-series aluminum alloys. This behavior is examined for commercial AA2024-T351 and a naturally aged Al–Cu–Mg–Li alloy stressed in high-humidity air. Environmental fatigue crack growth rate decreases with initial-artificial aging, then increases monotonically with increasing aging time. Overaging does not improve cracking resistance. This effect of microstructure is pronounced at low stress intensity factor range and persists for high stress ratio conditions that minimize crack closure. For both alloys, the highest resistance to fatigue crack growth correlates with the presence of artificial aging intensified solute clusters, and absence of distinct precipitates, as evidenced by electron microscopy and small-angle neutron scattering (SANS). Increased crack growth rates correlate with the dissolution of clusters and/or the formation of an increasing amount of S precipitates for AA2024 and T1 precipitates for the Li-bearing alloy. The fundamental effects of very fine-scale clusters and precipitates on cyclic-slip mode and environment-sensitive crack tip damage are unresolved.  2001 Elsevier Science Ltd. All rights reserved.

Effect of Prior Corrosion on the S/N Fatigue Performance of Aluminum Sheet Alloys 2024-T3 and 2524-T3

Aviation industry demand for continuous safety improvement in the face of trends toward increasing service life of aircraft and cost control necessitates stronger prevention and control measures to avoid the likelihood ofstructural failures linked to widespread damage involving corrosion and fatigue. New materials with improved damage tolerance attributes can improve the margin of safety in the presence of widespread damage. An excellent example of one such material is new aluminum alloy 2524 (formerly C188) which has improved fracture toughness and fatigue crack growth resistance relative to incumbent alloy 2024. In this study, the effect of prior corrosion on the S/N fatigue performance of 1.60 and 3.17-mm thick 2524-T3 and 2024-T3 bare sheet was evaluated. The fatigue strength of 2524 was approximately 10% greater and the lifetime to failure 30 to 45% longer than that of the 2024. Two main factors are believed to have contributed to the better performance of 2524: a less damaging configuration of corrosion pits and its better fatigue crack growth resistance.

Al-Li-Cu-Mg-(Ag) Products for Lower Wing Skin Applications

Al-Li-Cu-Mg alloy products, with and without Ag additions provide substantial performance advantages over conventional 2xxx products. For lower wing applications, the combination of specific ultimate tensile strength and damage tolerance is of particular importance and this is an area in which the Al-Li alloys can excel. Since Al-Li products have historically suffered with issues surrounding high property gradients through the plate thickness and high degrees of tensile in-plane anisotropy, a great deal of attention has been paid to the thermo-mechanical processing routes used in the fabrication of the current generation of alloy products. In addition, corrosion resistance is an area that has received greater attention recently since it can impact inspection intervals. In this presentation, the microstructures and properties of two new alloy products aimed for lower wing applications, 2199-T86 and 2060-T8E86, will be reviewed and compared with non-Li 2xxx products. It is concluded that the performance improvements of Al-Li alloys/products in addition to their lower density will enable significant weight savings in modern aircraft.

The Evolution of Plate and Extruded Products with High Strength and Fracture Toughness

From the first use of 2017-T74 on the Junkers F13, improvements have been made to plate and extruded products for applications requiring the highest attainable strength and adequate fracture toughness. One such application is the upper wing of large aircraft. The progression of these product improvements achieved through the development of alloys that include 7075-(T6 & T76), 7150-(T6 & T77) and 7055-(T77 & T79) and most recently 7255-(T77 & T79) is reviewed. The most current advancements include aluminum-copper-lithium, alloy 2055 plate and extruded products that can attain strength equivalent to that of 7055-T77 with higher modulus, similar fracture toughness and improved fatigue, fatigue crack growth and corrosion performance. The achievement of these properties is explained in terms of the several alloy design principles. The highly desired and balanced characteristics make these products ideal for upper wing applications.

Toughness after Interrupted Quench

We discuss data from a range of heat-treatable aluminum alloys, showing both yield strength and fracture toughness vs time at temperature of interrupted quench. Drop in toughness occurs at much shorter hold time than drop in strength. Concurrently the fracture becomes more intergranular. When later the yield strength falls, fracture becomes more transgranular, and toughness may rise. We attribute this pattern to two mechanisms: 1) Early quench precipitates nucleated on grain and/or subgrain boundaries grow to size sufficient to initiate fracture under tension, long before they withdraw significant solute from subsequent age-hardening. 2) Later quench precipitates nucleated on dispersoids and/or dislocations withdraw solute relatively uniformly, reducing matrix yield strength while increasing matrix ductility. We propose that quantitative modeling of change in strength and toughness with change in quench, requires multiple C-curves for multiple types of quench precipitates, and nonlinear relation of toughness to amount of boundary quench precipitate.

The Metallurgy of High Fracture Toughness Aluminum-Based Plate Products for Aircraft Internal Structure

A significant volume of “thick” aluminum plate products is used in the manufacture of an aircraft’s internal structure in applications such as ribs, spars, frames, bulkheads, etc. With the recent launch of more fuel efficient and primarily metallic single aisle aircraft as well as the introduction of composite-intensive twin-aisle aircraft, a number of opportunities exist for upgrading alloys developed more than 30 years ago with a new generation of thick plate products. These include 7xxx aluminum alloys that show significant improvements in both strength and toughness along with Al-Li alloys that show high strength, low density and very high corrosion resistance with significantly improved toughness over previous generation Al-Li. This paper will review these improvements and provide insights into the metallurgy behind better fracture toughness, particularly in the short transverse direction, by considering the impact of composition and processing on quench sensitivity.

Prediction of S-N Fatigue Curves Using Various Long-Crack-Derived Δ K eff Fatigue Crack Growth Curves and a Small Crack Life Prediction Model

In this study, stress-life (S-N) fatigue curves are predicted for high-strength aluminum alloy 7055 for open-hole specimens at two stress ratios, R = 0.1 and 0.5, and smooth specimens at R = 0.1 using the small-crack growth model of Brockenbrough et al. [1,2] and closure-free FCG curves obtained from long-crack tests by the following methods: (1) high R testing at R = 0.7; (2) constant K m a x testing at K m a x of 11 and 24.7 MPa√m; (3) a ΔK e f f curve obtained at the appropriate stress ratio (R = 0.1 or 0.5) by the ASTM method; and (4) a ΔK e f f curve obtained at the appropriate stress ratio by the adjusted compliance ratio (ACR) method. The predictions were compared to experimental S-N fatigue data. The objective of the study was to determine which method of obtaining closure-free FCG curves from long-crack tests provided the best estimates of fatigue life for the three combinations of specimen type and stress ratio in conjunction with the small-crack growth model employed. The ΔK e f f curves obtained by the ACR method yielded the closest and most consistent fatigue predictions for all three conditions. This was attributed to this method being able to account for K m a x sensitivity of fatigue crack growth rates in aluminum alloys that could not be accounted for by the other methods.