Hydrogen environmentally assisted cracking of aerospace aluminium

: An investigation on the increased susceptibility of novel aircraft alloys to cracking in moist air under static loads

Abstract

Recently, a new generation of 7xxx alloys used for aircraft structural components has been found to be sensitive to intergranular hydrogen environmentally assisted cracking (HEAC) in moist air environments. HEAC can cause subcritical crack growth at stress levels significantly below yield. Thus, some components have been found to be at risk of in-service cracking. The higher sensitivity of these novel alloys has been ascribed to their composition; notably a high Zn/Mg ratio and a low Cu content. There has been substantial research regarding the environmentally assisted cracking behaviour of 7xxx alloys exposed to moist air and other aqueous solutions. However, there is a paucity of data for the cracking behaviour of these novel 7xxx alloys. Additionally, most research has been performed on large pre-cracked fracture mechanics samples. These neglect crack nucleation and microstructurally short cracking, despite these stages dominating the lifetime of components.
The work done in this study can be divided up into five different sections, including three different experiments and the development of two Abaqus models. The first experiment involved the use of static 4-point bending to analyse the propensity to form cracks from smooth samples of AA7075-T651 and AA7449-T7651 exposed to warm moist air. AA7449-T7651 was found to be much more prone to crack formation. For the second experiment, fracture mechanics samples were used to estimate the HEAC crack growth rates of AA7449-T7651 in moist air for range of temperatures. Increasing the temperature was found to increase the crack growth rates and decrease the threshold stress intensity factor for cracking. The third experiment involved in-situ X-ray tomography imaging of small tensile samples of AA7449-T7651 during HEAC. This technique was used to correlate the microstructure with the HEAC crack growth behaviour. Finally, two cohesive zone models (CZM) were developed to be able to simulate the HEAC behaviour for the DCB and in-situ tensile tests. These models were useful to understand how hydrogen was distributed within the samples during the tests.

Date of Award	25 Jan 2022
Original language	English
Awarding Institution	University of Bristol
Sponsors	Engineering and Physical Sciences Research Council
Supervisor	Matthew J Peel (Supervisor) & Nicolas O Larrosa (Supervisor)