Effect of Microstructure on 7XXX Aluminum Alloy Fatigue Crack Growth Behavior Down to Near-Threshold Rates

摘要

This investigation was undertaken to clarify the role of microstructure on steady state (constant load amplitude) and transient (single periodic overload) fatigue crack growth behavior in 7XXX aluminum alloys at low stress intensity factor ranges (delta K). These results were consolidated with data in the literature to obtain an understanding of microstructural effects on crack growth response over a broad delta K range. Constant load amplitude data show that there are significant differences in near-threshold FCG resistance among 7XXX alloys. Overaging to a T7 temper reduces low delta K FCG resistance greatly, whereas intermediate and high delta K FCG resistance is increased by overaging. In the presence of moisture at intermediate delta K levels, increasing Cu content from 1.0 to 2.3% increases crack growth resistance. However, there is no consistent effect of Cu content on near-threshold fatigue performance. Similarly, no clear influence of dispersoid type (Cr vs. Zr) on low delta K FCG resistance was detected. Increasing alloy purity (lowering Fe, Si content) enhances crack growth resistance at high delta K, but has no effect at near-threshold stress intensities. The importance of near-threshold FCG performance to alloy design and materials selection for fatigue resistance is discussed. Potential tradeoffs between design philosophies which emphasize damage tolerance characteristics vs. those which focus on improved life cycle economics are examined.