9Aplus
5th Jul 2012, 16:30
Please note that CRD of NPA-2010-04 "Damage Tolerance and Fatigue Evaluation of Composite Rotorcraft Structures" is now open for consultation on EASA website.See: http://hub.easa.europa.eu/crt/docs/viewcrdpdf/id_88
From document:
2 comment by: Adhesion Associates Pty. Ltd.
Attachment #1
Sir/Madam
I refer to NPA*2010*04 "Damage Tolerance and Fatigue Evaluation of
Composite Rotorcraft Structures". The intent of this amendment is to introduce
a requirement for damage tolerance of primary structural elements on rotary
aircraft, in a similar manner to the FAA NPRM Docket No. FAA*2009*0660;
Notice No. 09*12 dated 06 January 2010. We made a submission to the FAA on
this matter.
Adhesion Associates Pty. Ltd. is a small consultancy in Australia specialising in
composites and adhesive bonding see Adhesion Associates (http://www.adhesionassociates.com). Our
director has over 38 years experience in these fields. He is the primary author
of the FAA document DOT/FAA/AR – TN06/07, Apr 2007, Best Practice in
Adhesive Bonded Structures and Repairs. He has also written an engineering
standard DEF (AUST) 9005 Composites and Adhesive Bonded Repairs, for the
Australian Defence Forces (ADF) and two handbooks on repair design and
repair application technology. He has developed four ADF courses in
composites and adhesive bonded repairs and facilitated the development of
two other ADF courses for technicians.
Adhesion Associates strongly support the incorporation of damage tolerance for
rotary aircraft as proposed by NPA*2010*04. However, we believe that this
amendment alone will NOT provide assurance of continuing airworthiness for
adhesive bonded structures, especially where the bonds involve metals.
Our contention is that adhesive bonds (especially to metals) may under some
circumstances be susceptible to interfacial degradation, which results in a
reduction or even a loss of bond strength. Adhesive bonds rely on chemical
bonds formed at the interface at the time of adhesive cure. Those chemical
bonds provide the strength of the joint, and also control the longer term bond
durability in the operational environment. The mechanism involved in
interfacial degradation is that if the metal oxides are susceptible to hydration,
the chemical bonds formed at the time of manufacture will dissociate to permit
hydration of the oxide layer. For example in aluminium alloys, Al2O3 oxide has
an affinity for the formation of bohemite Al2O3.2H2O. This results in adhesion
failure at the interface between the metal and the adhesive. Hence, bonds
which are initially strong and may pass certification and quality assurance
testing, may degrade in service leading to structural failure once the oxides on
the surface have had time to hydrate.
There is a transition between the full strength cohesion failure to the weak
adhesion failure. During this period the adhesive will fail in a mixed*mode
where there is a combination of cohesion failure and adhesion failure. The
strength of the bond will reduce as hydration progresses.
The deficiency in managing airworthiness by use of damage tolerance is that
damage tolerance acceptance criteria are established by test or analysis on
structures where the interface has not degraded. If these tests demonstrate
adequate strength in the presence of artificial defects, the adhesive
surrounding the defect is pristine and will not fail. If however the same size
bond defect occurs in service, there is a very high probability that the failure is
by time dependent adhesion at the interface, and there is no guarantee that
the interface adjacent to the defect maintains adequate strength. There is
therefore a high risk that the structure may fail with a defect considerably
smaller than that determined by damage tolerance analysis or testing. We are
publishing a paper in June 2010 which explains the sequence of progression
from strong cohesion failures (the type for which damage tolerance is
appropriate) to very weak, interfacial adhesion failures for which damage
tolerance is inappropriate. The real concern is that in the intervening time
between cohesion failure and adhesion failure, the bond displays mixed*mode
failures, where the bond strength may be significantly below that
demonstrated at certification. NDI can detect cohesion and adhesion failures,
but can not detect the weak bonds which signify the onset of mixed*mode
failure.
Similar concerns apply for applying damage tolerance to the effects of micro*
voiding where moisture absorbed by adhesives or nomex core prior to hot*
bonding results in significant weakening of the bond. This results in a loss of
strength form the time of manufacture, and again unless the damage tolerance
testing has been undertaken on bonds with similar reductions in strength,
there is a risk of disbonding or failure. Our report Managing Micro*Voids is
available at http://www.adhesionassociates.com/papers/Managing%20Micro*
Voiding%20of%20Adhesive%20Bonds.pdf. It gives examples drawn from
disbonds in the tail boom structure of AW139 helicopters, where there is
evidence of significant micro*voiding. If the causes of micro*voiding are
addressed at the time of manufacture, then damage tolerance analysis is
appropriate.
Adhesion Associates has recently been involved in a helicopter crash
investigation. The report by the IIC has been submitted to the authorities but
has yet to be released. We are confident that this case clearly demonstrates
that damage tolerance alone will not prevent structural failures in metal
bonded structures
The way to manage these risks is to demonstrate at the time of certification
that the adhesive bonds are resistant to in*service hydration. If there is
certainty that the interface will not hydrate, then the current regulations,
together with the proposed NPA WILL provide assurance of continuing
airworthiness. Therefore, we strongly urge EASA to amend the regulations
(equivalent to FAR 2x.603) to mandate demonstration of the resistance of the
adhesive bond to the operating environment. If that amendment is
incoprporated and hydration of the interface is prevented by appropriate
production processes, then damage tolerance is an effective defence against
in*service disbonding.
We would welcome the opportunity to forward copies of our submission to the
FAA and an advance draft copy of our upcoming paper to relevant personnel in
EASA.
We would be happy to discuss this issue further by email or telephone
Regards
Max
EASA RESPONSE:
Not accepted
The Agency considers that the existing text of CS 29.603 addresses this point
in principle. Nonetheless, the Agency is considering the development of further
regulatory and Guidance Material related to bonded structure, not only in the
field of rotorcraft.
CS 29.603 Materials
The suitability and durability of materials used for parts, the failure of which
could adversely affect safety, must –
(a) Be established on the basis of experience or tests;
(b) Meet approved specifications that ensure their having the strength and
other properties assumed in the design data; and
(c) Take into account the effects of environmental conditions, such as
temperature and humidity, expected in service.
:}
From document:
2 comment by: Adhesion Associates Pty. Ltd.
Attachment #1
Sir/Madam
I refer to NPA*2010*04 "Damage Tolerance and Fatigue Evaluation of
Composite Rotorcraft Structures". The intent of this amendment is to introduce
a requirement for damage tolerance of primary structural elements on rotary
aircraft, in a similar manner to the FAA NPRM Docket No. FAA*2009*0660;
Notice No. 09*12 dated 06 January 2010. We made a submission to the FAA on
this matter.
Adhesion Associates Pty. Ltd. is a small consultancy in Australia specialising in
composites and adhesive bonding see Adhesion Associates (http://www.adhesionassociates.com). Our
director has over 38 years experience in these fields. He is the primary author
of the FAA document DOT/FAA/AR – TN06/07, Apr 2007, Best Practice in
Adhesive Bonded Structures and Repairs. He has also written an engineering
standard DEF (AUST) 9005 Composites and Adhesive Bonded Repairs, for the
Australian Defence Forces (ADF) and two handbooks on repair design and
repair application technology. He has developed four ADF courses in
composites and adhesive bonded repairs and facilitated the development of
two other ADF courses for technicians.
Adhesion Associates strongly support the incorporation of damage tolerance for
rotary aircraft as proposed by NPA*2010*04. However, we believe that this
amendment alone will NOT provide assurance of continuing airworthiness for
adhesive bonded structures, especially where the bonds involve metals.
Our contention is that adhesive bonds (especially to metals) may under some
circumstances be susceptible to interfacial degradation, which results in a
reduction or even a loss of bond strength. Adhesive bonds rely on chemical
bonds formed at the interface at the time of adhesive cure. Those chemical
bonds provide the strength of the joint, and also control the longer term bond
durability in the operational environment. The mechanism involved in
interfacial degradation is that if the metal oxides are susceptible to hydration,
the chemical bonds formed at the time of manufacture will dissociate to permit
hydration of the oxide layer. For example in aluminium alloys, Al2O3 oxide has
an affinity for the formation of bohemite Al2O3.2H2O. This results in adhesion
failure at the interface between the metal and the adhesive. Hence, bonds
which are initially strong and may pass certification and quality assurance
testing, may degrade in service leading to structural failure once the oxides on
the surface have had time to hydrate.
There is a transition between the full strength cohesion failure to the weak
adhesion failure. During this period the adhesive will fail in a mixed*mode
where there is a combination of cohesion failure and adhesion failure. The
strength of the bond will reduce as hydration progresses.
The deficiency in managing airworthiness by use of damage tolerance is that
damage tolerance acceptance criteria are established by test or analysis on
structures where the interface has not degraded. If these tests demonstrate
adequate strength in the presence of artificial defects, the adhesive
surrounding the defect is pristine and will not fail. If however the same size
bond defect occurs in service, there is a very high probability that the failure is
by time dependent adhesion at the interface, and there is no guarantee that
the interface adjacent to the defect maintains adequate strength. There is
therefore a high risk that the structure may fail with a defect considerably
smaller than that determined by damage tolerance analysis or testing. We are
publishing a paper in June 2010 which explains the sequence of progression
from strong cohesion failures (the type for which damage tolerance is
appropriate) to very weak, interfacial adhesion failures for which damage
tolerance is inappropriate. The real concern is that in the intervening time
between cohesion failure and adhesion failure, the bond displays mixed*mode
failures, where the bond strength may be significantly below that
demonstrated at certification. NDI can detect cohesion and adhesion failures,
but can not detect the weak bonds which signify the onset of mixed*mode
failure.
Similar concerns apply for applying damage tolerance to the effects of micro*
voiding where moisture absorbed by adhesives or nomex core prior to hot*
bonding results in significant weakening of the bond. This results in a loss of
strength form the time of manufacture, and again unless the damage tolerance
testing has been undertaken on bonds with similar reductions in strength,
there is a risk of disbonding or failure. Our report Managing Micro*Voids is
available at http://www.adhesionassociates.com/papers/Managing%20Micro*
Voiding%20of%20Adhesive%20Bonds.pdf. It gives examples drawn from
disbonds in the tail boom structure of AW139 helicopters, where there is
evidence of significant micro*voiding. If the causes of micro*voiding are
addressed at the time of manufacture, then damage tolerance analysis is
appropriate.
Adhesion Associates has recently been involved in a helicopter crash
investigation. The report by the IIC has been submitted to the authorities but
has yet to be released. We are confident that this case clearly demonstrates
that damage tolerance alone will not prevent structural failures in metal
bonded structures
The way to manage these risks is to demonstrate at the time of certification
that the adhesive bonds are resistant to in*service hydration. If there is
certainty that the interface will not hydrate, then the current regulations,
together with the proposed NPA WILL provide assurance of continuing
airworthiness. Therefore, we strongly urge EASA to amend the regulations
(equivalent to FAR 2x.603) to mandate demonstration of the resistance of the
adhesive bond to the operating environment. If that amendment is
incoprporated and hydration of the interface is prevented by appropriate
production processes, then damage tolerance is an effective defence against
in*service disbonding.
We would welcome the opportunity to forward copies of our submission to the
FAA and an advance draft copy of our upcoming paper to relevant personnel in
EASA.
We would be happy to discuss this issue further by email or telephone
Regards
Max
EASA RESPONSE:
Not accepted
The Agency considers that the existing text of CS 29.603 addresses this point
in principle. Nonetheless, the Agency is considering the development of further
regulatory and Guidance Material related to bonded structure, not only in the
field of rotorcraft.
CS 29.603 Materials
The suitability and durability of materials used for parts, the failure of which
could adversely affect safety, must –
(a) Be established on the basis of experience or tests;
(b) Meet approved specifications that ensure their having the strength and
other properties assumed in the design data; and
(c) Take into account the effects of environmental conditions, such as
temperature and humidity, expected in service.
:}