Effect of environment on fatigue crack growth in ultrafine grain Al–Mg

• Published in: International journal of fatigue Vol. 31; no. 11; pp. 1678 - 1683
• Main Authors: Pao, P.S., Holtz, R.L., Jones, H.N., Feng, C.R.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Kidlington Elsevier Ltd 01.11.2009; Elsevier
• Subjects: Aluminum; Applied sciences; Environmental effect; Exact sciences and technology; Fatigue; Fatigue crack growth; Fatigue crack growth threshold; Mechanical properties and methods of testing. Rheology. Fracture mechanics. Tribology; Metals. Metallurgy; Microstructure; Ultrafine grained material; Fatigue crack growth; Environmental effect; Microstructure; Aluminum; Fatigue crack growth threshold; Ultrafine grained material; Aluminium base alloys; Mechanical properties; Fatigue; Fatigue crack; Crack propagation; Medium effect; Magnesium alloy; Fine grain structure; Fatigue threshold
• ISSN: 0142-1123; 1879-3452
• DOI: 10.1016/j.ijfatigue.2009.03.018

The fatigue crack growth rates, obtained in high vacuum and in ambient air, of ultrafine grain (UFG) Al–7.5Mg (grain size ∼ 250 nm) at various load ratios were compared to those of powder-metallurgy (P/M) Al–7Mg (grain size ∼ 2 μm) and ingot-metallurgy (I/M) Al–7Mg (grain size ∼ 100 μm). In both vacuum and ambient air, fatigue crack growth rates at all stress ratios decrease with increasing grain size. The fatigue crack growth threshold (Δ K th) follows the reverse order, increasing with increasing grain size. These trends are interpreted in terms of fracture surface roughness effects that are correlated with grain size. In vacuum, the thresholds of all three materials exhibit no load ratio dependency at load ratios from 0.1 to 0.5. In air, the threshold of UFG Al–7.5Mg exhibits weak load ratio dependency, while P/M and I/M Al–7Mg exhibit modest load ratio dependency. The environmental effect on the fatigue crack growth rates is assessed by determining the difference in crack growth driving force (Δ K) between air and vacuum. It was found that the environmental contribution to the driving force of all three materials is similar, nearly independent of grain size.