Composite Structures

[John R81]

It seems the EASA has noticed that regulations concerning damage and fatigue are all based on metal and may not be relevant to composite structures. I know that composite materials, bonding, delaminations, etc have been regular topics here.

Worth a read, and those with knowledge (unlike me) will no doubt make sure that the rules develop appropriately.

John

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Please note that 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/viewnpa/id_88 (http://hub.easa.europa.eu/crt/docs/viewnpa/id_88)


To place comments please logon at http://hub.easa.europa.eu/crt/ (http://hub.easa.europa.eu/crt/)


For further information please contact Rulemaking Process Support at [email protected] ([email protected])

[blakmax]

Hi John

Sorry. I am not ignoring this message. I am busier than a one-armed bricklayer in Baghdad at present. This action mirrors an FAA NPRM of 06 Jan 10. I have made a submission there. When I get a few minutes I will explain why this proposal will STILL not prevent bond failures. It is of necessity a long explanation and at present I just don't have the time. I'll try to address this in the next few days.

Regards

blakmax

[John R81]

No worries.

And feel free to ignore me - it would make me feel right at home. I am often ignored at my home.

:ok:

[blakmax]

Hi John

Sorry about the delay. Things are still hectic, so I have extracted some information for a conference paper I am currently drafting. Sorry about the length. I have broken it into two messages. My concern is for the continuing airworthines of metal bonds. Here goes.

There are three forms of bond failure:

Cohesion failure usually occurs through the plane of the carrier cloth, which is the weakest plane in an effective bond because of the reduced surface area caused by the presence of the carrier cloth. The surface is rough and often slightly milky in appearance due to shear hackles formed by the failure. Bonds which fail by cohesion exhibit high strength. The causes of cohesion failure include design deficiencies such as inadequate overlap length, thermal stresses or peel stresses, however cohesion failure can also occur from the presence of voids which reduce the available bond overlap length below a critical size.
Adhesion bond failures occur at the interface between the adhesive and the adherend, with residual adhesive remaining at any location on one surface only. The chemical bonds at the interface become weaker than the adhesive strength at the plane of the carrier cloth. The surface of the adhesive is smooth and often replicates surface features from the adherend. Adhesion failures exhibit low strength and may occur with no applied load if degradation of the interface is complete. Causes of adhesion failure include contamination during manufacture, the use of out-of-life adhesive, or inadequate temperature control during production, however such cases should be eliminated by quality assurance tests. The remaining cause is interfacial degradation in service.
Mixed-mode failure exhibits some cohesion failure and some adhesion failure. This is because the interface is partially degraded. In effect mixed-mode failure is just an adhesion failure waiting to occur. The failure exhibits areas of smooth failure as well as areas which are rough. The strength of adhesive bonds exhibiting mixed-mode failure is lower than the cohesion failure strength. The strength of the bond and the proportion of surface smoothness or roughness depend upon the level of degradation of the bond interface.
Continued...

[blakmax]

FAR 2x.573 requires demonstration of damage tolerance for aircraft structures including adhesive bonds. An FAA NPRM issued on 06 January 2010 proposes to extend these requirements to rotary wing aircraft. NPA-2010-04 "Damage Tolerance and Fatigue Evaluation of Composite Rotorcraft Structures"effectively duplicates the process for EASA. These requirements may involve analysis or testing to demonstrate that the structure provides adequate strength in the presence of known and detectable bond defects. Testing generally involves known artificial defects implanted in the bond-line. Production acceptance criteria for tolerable bond defect sizes are established on the basis of static strength tests or analysis on the basis that the bond interface retains full strength. Even service inspection requirements assume that the bond interface retains integrity.
Analysis and testing are meaningless for adhesion or mixed-mode failures because the assumption inherent in these approaches is that the adhesive surrounding the defect maintains an acceptable level of bond strength. In reality, if a disbond occurs in service, it is almost certainly due to degradation of the interface and such degradation is usually not confined to a localised area. The entire interface may be degrading and the bond surrounding the disbond may be weak. It is not possible to predict adhesion disbond growth rates, and defects may grow without any flight loads.
In recent times the FAA has taken steps to address adhesion failure modes. Policy Statement PS-ACE100-2005-10038 and recent amendments to AC 20-107 to “B” status include advice that interfacial failures are unacceptable in the production process and for service defects. These statements are to be commended, but they must eventually be followed up by appropriate NPRM action to amend the FARs to mandate bond durability testing.
Current NDI methods are only generally effective at finding production voids where there is an air-gap. These are the types of defect which cause cohesion failures. The ability of NDI to interrogate interfaces or even to detect weak bonds (kissing disbonds) is extremely limited. Double-sided adhesive tape will pass many NDI inspection methods, especially the tap-test, despite its obvious weakness compared to effective structural bonds. In effect, NDI can only tell that the bond has a defect, it can NOT guarantee the quality of the bond. With reference to NDI for inspection of in-service bonded components, NDI can not detect the onset of insipient disbonding. It is only effective AFTER a disbond has occurred. Hence, because of the ineffectiveness of damage tolerance in assessing the criticality of adhesive bonds with degrading interfaces and the inability of NDI to assess the potential for degradation, there is a risk that a disbond could propagate to a critical size before it is detected.
Therefore, the introduction of damage tolerance will not prevent disbonding from occurring. Worse yet, the assumption that the bond surrounding the defect retains full strength will provide a false sense of security when in fact the bond may be much weaker.

The solution is to mandate bond durability testing using methods which have a demonstrated history of differentiating between good processes and mediocre processes.

[blakmax]

Nice idea cattletruck, but when do you test it? And then what do you do if it passes and you have no more specimens? It also becomes a race between your structure and the specimen to see who fails first, and I am sure Murphy would have a hand in the answer to that question.

There are accelerated tests which can provide a strong indication of the durability of adhesive bonds. Our experience is that when these tests are used, processes which satisfy acceptance criteria and are implemented correctly simply do not fail. As I have said a number of times, our experience was that we turned around a 43% bond failure rate to less than 0.1% and in each of those 0.1% cases we could clearly identify where the technician took a short cut.

My message is that if the processes are CORRECTLY validated, then the current regulations (including the EASA change proposal) are valid and so is NDI, because the interface will not degrade and you will never see mixed-mode or adhesion failures.

It is far better to avoid the problem altogether than try to manage it as the structure weakens around you.

Regards

Blakmax

[John R81]

Perhaps stating the obvious - in which case I am truely sorry - but the point of my original post was to ensure that everyone knew that the consultation was open and that ideas should be posted there. Copy here is fine, but here alone achieves nothing.

[blakmax]

Point taken John.

I have already sent my response to the FAA and receives a polite thanks but no thanks. I will try to find the time in between several impending deadlines to see if I get a different response. I am hoping that it will feel really good when I can stop banging my head against brick walls.

Regards

blakmax

[blakmax]

Response submitted today.

[John R81]

:ok:

Thanks.

I have no expertise whatsoever in this area. Seeing the quality of knowledge floating around on this subject, as an owner / driver I can only benefit by the debate having been informed by those with knowledge and expertise.

Regards

John

[topendtorque]

And feel free to ignore me - it would make me feel right at home. I am often ignored at my home.



Fair Dinkum?

You have made me feel right at home, especially with some recent events of my experience.

Thanks sport.
tet

[blakmax]

Crikey TET,
You remined me of a fellow who used to work with me....No hold on, he used to collect his pay from a common employer.

Anyway, we used to say to him that "Ignorance is Bliss". We could never understand why he wasn't the happiest person in the world.

[Graviman]

blakmax,

Can you recommend any books on composite material properties? I have a background in fatigue calculations for isotropic materials like steel and aluminium, but limited experience of composites. It's something i would like to gain an understanding of. I'm not fazed by stress tensors, so technical texts are ideal.

Thanks in advance,

Mart

[500e]

What with HUMS, And Composites + bonding, no time for flying, So many books so little time.
BM
Note new? helicopter in Qland, Comp blades Could be work closer to home.
Link
Australia Developing the World's First Biofuel Capable Helicopter : TreeHugger (http://www.treehugger.com/files/2010/05/australia-developing-the-world-first-biofuel-capable-helicopter.php)
With this one there appear to actually be a machine, rather than a concept.