Initiation and early growth of fatigue cracks in an aerospace aluminium alloy

TLDR

Material imperfections, especially inclusions and etch‑pitting of grain boundaries, are key initiators of fatigue cracks in aerospace aluminium alloys. This article observes fatigue‑crack‑initiating flaws and early crack growth in 7050‑T7451 aluminium alloy specimens and full‑scale test articles with production surface finish. The authors refined equivalent initial flaw size (EIFS) approaches by using quantitative fractography to obtain detailed early crack‑growth data, which were then incorporated into a simple crack‑growth model for service‑crack analysis. Observed crack‑growth data reduce EIFS variability and reveal that surface‑treatment processes significantly influence the early fatigue‑growth phase.

Abstract

Abstract Material imperfections usually play a substantial role in the early stages of fatigue cracking. This article presents some observations concerning fatigue crack initiating flaws and early crack growth in 7050‐T7451 aluminium alloy specimens and in full‐scale fatigue test articles with a production surface finish. Equivalent initial flaw size (EIFS) approaches used to evaluate the fatigue implications of metallurgical, manufacturing and service‐induced features were refined by using quantitative fractography to acquire detailed information on the early crack growth behaviour of individual cracks; the crack growth observations were employed in a simple crack growth model developed for use in analysing service crack growth. The use of observed crack growth behaviour reduces the variability which is inherent in EIFS approaches which rely on modelling the whole of fatigue life, and which can dominate EIFS methods. The observations of realistic initial flaws also highlighted some of the significant factors in the fatigue life‐determining early fatigue growth phase, such as surface treatment processes. Although inclusions are often regarded as the single most common type of initiating flaw, processes which include etching can lead to etch pitting of grain boundaries with significant fatigue life implications.