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Please find attached hereto an EASA Safety Information Bulletin (SIB) which has already been posted on
our web site and can be viewed at https://ad.easa.europa.eu/ad/2018-04 .
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EASA SIB No.: 2018-04
Safety Information Bulletin
Airworthiness
SIB No.: 2018-04
Issued: 02 February 2018
Subject: Environmentally Assisted Cracking in certain Aluminium Alloys
Ref. Publications:
None
Applicability:
Type Certificate holders, Supplemental Type Certificate holders, equipment manufacturers,
maintenance organisations, production organisations and aluminium alloy producers.
Description:
EASA received reports of brittle cracking of aluminium alloy components. Additional investigation
of some new generation 7xxx series alloys has shown that these have a sensitivity to a
phenomenon known as environmentally assisted cracking (EAC), when subject to certain
conditions in the normal operating environment. The type of EAC encountered appears to be
caused by hydrogen embrittlement along the grain boundaries, leading to crack initiation and
subsequent propagation. These cracks typically start from holes or other areas of stress
concentration and usually propagate in a plane perpendicular to the short transverse (ST)
direction. This phenomenon has been linked to the chemical composition of the alloy, notably a
high zinc/magnesium ratio, combined with low copper content. Brittle fractures have been
reproduced under laboratory environment and cracking has proven to be driven by time exposure
(ageing) and is not fatigue related, although further crack propagation under operative loads
cannot be excluded.
Results of further investigation are in agreement with open scientific literature, and show that an
EAC phenomenon can occur only when the three following conditions are present: (1) susceptible
material alloy, (2) sustained stress in the ST direction and (3) ageing in a typical environment. If
one of those conditions can be eliminated, this form of EAC cracking is unlikely to occur. The
affected materials did pass the “state of the art” qualification requirements for mechanical and
corrosion testing, but the current industry standard of testing for stress corrosion cracking (ASTM
G47) is not capable of adequately detecting the risk of this form of EAC (see Note 1).
Note 1: The subject of this SIB is EAC caused by hydrogen assisted embrittlement resulting in a
decrease of toughness, but in absence of an obvious corrosion reaction. It is different from
classical stress corrosion cracking characterised by anodic dissolution with loss of material
accompanied with findings of active corrosion like pitting or attack of grain boundaries.
This is information only. Recommendations are not mandatory.
TE.CAP.00117-006 © European Aviation Safety Agency. All rights reserved. ISO9001 Certified. Page 1 of 2
Proprietary document. Copies are not controlled. Confirm revision status through the EASA-Internet/Intranet.
An agency of the European Union
EASA SIB No.: 2018-04
Sensitivity to this form of EAC has been confirmed for alloys 7037, 7040 (see Note 2), 7055, 7085,
7099, and 7449. Other alloys with similar compositions might also be affected. The material
temper (i.e. the specified heat treatment and additional processing such as ageing and stress relief
by stretching) and product form can also influence resistance to EAC.
Note 2: Aluminium alloy 7040 has been found sensitive to this form of EAC in T7651 temper only.
Other tempers commonly used with this alloy have not shown similar issues.
Occurrences of this form of EAC cannot be excluded in service and, if not detected, could lead to
crack propagation, possibly resulting in reduced structural integrity. For specific designs that have
already been identified, mandatory inspections and corrective actions have been initiated and
further mandatory actions for other specific designs may follow.
This SIB is issued to raise awareness, in all sectors of the industry, concerning this EAC
phenomenon of these types of aluminium alloys.
Recommendation(s):
EASA recommends all affected organisations to evaluate the extent of the issue, particularly to:
Identify components made of EAC sensitive aluminium alloys.
Evaluate the sensitivity to and criticality of EAC in the component.
Report these evaluation results to EASA.
In addition, EASA requests aluminium alloy producers to establish whether they supply any of the
above mentioned alloys and, if so, to contact EASA and the relevant Design Approval Holder(s).
Contact(s):
For further information contact the EASA Safety Information Section, Certification Directorate.
E-mail:
[email protected].
This is information only. Recommendations are not mandatory.
TE.CAP.00117-006 © European Aviation Safety Agency. All rights reserved. ISO9001 Certified. Page 2 of 2
Proprietary document. Copies are not controlled. Confirm revision status through the EASA-Internet/Intranet.
An agency of the European Union