3D characterisation of hydrogen environmentally assisted cracking during static loading of AA7449-T7651

Unai De Francisco, Felix Beckmann, Julian Moosmann, Nicolas O Larrosa, Matthew J Peel*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

Abstract

In this investigation, synchrotron X-ray microtomography was used to perform 3D in situ observations of crack initiation and growth during hydrogen environmentally assisted cracking (HEAC) in tensile samples of AA7449-T7651. Two smooth tensile samples with a 1 mm diameter gauge section were held at a fixed displacement (≈30% of yield stress) in warm, moist air (≈76∘C, 73% relative humidity). The samples were then imaged repeatedly using X-ray tomography until they fractured completely. The tomograms showing the nucleation and evolution of intergranular cracks were correlated with electron microscopy fractographs. This enabled the identification of crack initiation sites and the characterisation of the crack growth behaviour relative to the microstructure. The samples were found to fracture within an environmental exposure time of 240 min. Some cracks in both samples nucleated within an exposure time of 80 min (33–40% of the total lifetime). Many cracks were found to nucleate both internally and at the sample surface. However, only superficial cracks contributed to the final fracture surface as they grew faster owing to the direct environmental exposure and the larger crack opening. HEAC occurred prominently via brittle intergranular cracking, and cracks were found to slow down when approaching grain boundary triple junctions. Additionally, crack shielding from nearby cracks and the presence of coarse Al–Cu–Fe particles at the grain boundaries were also found to temporarily reduce the crack growth rates. After prolonged crack growth, the HEAC cracks displayed ductile striations and transgranular fracture, revealing a change in the crack growth mechanism at higher stress intensity factors.
Original languageEnglish
Pages (from-to)93-116
Number of pages24
JournalInternational Journal of Fracture
Volume232
Issue number1
Early online date28 Oct 2021
DOIs
Publication statusPublished - Nov 2021

Bibliographical note

Funding Information:
We would like to acknowledge Deutsches Elektronen-Synchrotron (DESY) and Helmholtz-Zentrum Hereon for providing the synchrotron facilities at PETRA III (Hamburg, Germany), both members of the Helmholtz Association HGF. This support was granted for the beamline experiment proposal I-20190687 EC. This research was supported in part through the Maxwell computational resources operated at DESY, Hamburg, Germany. Additionally, we would like to acknowledge the support of Dr. Haris Paraskevoulakos and Ursula Tietze during the experiment.

Funding Information:
We would like to acknowledge Deutsches Elektronen-Synchrotron (DESY) and Helmholtz-Zentrum Hereon for providing the synchrotron facilities at PETRA III (Hamburg, Germany), both members of the Helmholtz Association HGF. This support was granted for the beamline experiment proposal I-20190687 EC. This research was supported in part through the Maxwell computational resources operated at DESY, Hamburg, Germany. Additionally, we would like to acknowledge the support of Dr. Haris Paraskevoulakos and Ursula Tietze during the experiment.

Publisher Copyright:
© 2021, The Author(s).

Keywords

  • Static loading
  • Hydrogen environmentally assisted cracking
  • AA7449-T7651
  • Synchrotron X-ray tomography

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