Constant-Amplitude Fatigue Crack Growth Behavior of 7XXX Aluminum Alloys

Abstract

Tests were conducted on several variants of 7075 and 7050 high-strength aluminum alloys to determine the role of alloy composition and temper on fatigue crack growth (FCG) resistance. These tests, which were conducted in high-humidity air, covered a crack growth rate range from very slow (near-threshold) to rapid FCG rates. Results of constant-load-amplitude tests showed that alloy ranking was sensitive to the stress-intensity factor range (ΔK) applied. In particular, overaging to a T7 temper reduced low-ΔK FCG resistance greatly, whereas intermediate- and high-ΔK FCG resistance was increased by overaging. The influence of other microstructural variables (purity, copper content, and dispersoid type) on FCG response also depended on the applied ΔK-level. Variations in constant amplitude FCG performance among the alloys were greatest in the near-threshold region. These differences at near-threshold growth rates are important, because this regime often comprises the major portion of component fatigue life; and yet, few data have been presented to date for aluminum alloys in this region. These results illustrate significant potential for improved fatigue performance when alloy selection includes consideration of near-threshold behavior. Fracture surface topography was examined in an effort to relate fracture characteristics to FCG behavior, and is discussed.