Repair of Thrust Reverser Inner Duct Wall Acoustic Panel 2

[Composite Repairman]

Hello!

Could you please help me with composite repair on Thrust Reverser Inner Duct Wall Acoustic Panel.
There is delaminated skin (CFRP) of honeycomb structure.
Skin is consist of heterogeneous structure, quantity of plies varie from 4 in center section and to 10 plies at the edges.
In accordance with SRM I have to remove damaged plies, taper sand repair area, then put there repair plies (overlap 0.260 inch). Red marked the repair area in the picture.
So, how to prepare area of heterogeneous structure and put repair plies (marked with green arrows)?

There is no any information about repair of heterogeneous structure in SRM.

I would be appreciated if you help me.

I uploaded picture one more time,

 

[berkshire]

Composite repair man ,
Your pictures did not come through because of a run time security error. Are you working in a military situation ?
If this is your first time doing a tapered sand out and tapered ply lay up , you should get assistance from a more experienced person.
B.E.

You are judged not by what you know, but by what you can do.

 

[LiftDivergence]

If you are deviating from the SRM, or if there is no instruction in the SRM for your particular case that you can use to see the repair through all the way to the end, then you need to contact engineering for an evaluation.

Either contact the OEM or go to an engineering contractor / DER. The knowledgeable folks on this forum can give you advice, but when the SRM fails you, you need a repair plan / EO / whatever you want to call it and an approval.

It is better to report your situation and have the person who is going to approve it also do the evaluation, than proceed based on what might be stated here.

Keep em' Flying
//Fight Corrosion!

 

[blakmax]

Composite Repair Man

Are you repairing a "delamination" between composite plies, or are you repairing a "disbond" between the laminate and the core? This is important because the approach to damage removal and the consequences of the failure mode to the actual causes of the problem. Repair of a delaminated composite ply is vastly different to the repair for disbonds. Further, the repair for a disbond between the adhesive and face sheet or the adhesive and the core is different than if the adhesive is fractured between the core and the face sheet. See Link

It is of fundamental importance that you undertake a failure investigation based on the evidence contained in the defect rather that just adopting the SRM repair "for this area". A metel based analogy is that a repair in an SRM is usually for a fatigue crack, but if the crack is stress corrosion cracking, the standard SRM repair will not correct the issue because fatigue cracks usually grow perpendicular to the direction that SCC cracks grow. Hence the repair will be ineffective if the cause of the cracking is not identified, even though the SRM says "this is the repair".

I can offer help in adhesive bond failure analysis if you post photos. See the other papers on my web site related to bond failure assessment.

Blakmax

 

[Composite Repairman]

Blakmax,

There is "disbond" between laminate and core.
There was mechanical impact that caused damage laminate and disbond. Core is OK.

LiftDivergence, berkshire

You are right this one repair should be evaluated and supported by engineer, I understand it and I'm waiting for it.
I did many times composite repair with homogeneous structure. A lot of them are covered by SRM.

Now I'm interesting in it: How an engineer does evaluation and define repair in this case?


Thanks

 

[blakmax]

OK CRM

Next, what does the disbond between the laminate and core look like? Has the core fractured? Has the adhesive at the fillet bonds fractured, or has the core simply pulled out of the adhesive with no damage to the core or adhesive?

If it is a core fracture, or if the core has fractured through the fillet bonds, then the repair approach is different to if the core has pulled out of the adhesive with no damage to the core or adhesive. Indeed the implications of the latter failure mode can be significant. I am aware of over twenty rudder failures due to this type of bond degradation on one military aircraft type. Link

If you can work out how to contact me I am happy to discuss this further.

Regards

blakmax

 

[Composite Repairman]

Blakmax,

We had cut out laminate and adhesive with no damage to the core. Adhesive is remained on the laminate, core looks without any damage.
Unfortunately, I don't know how to contact you.

Thanks

 

[SWComposites]

CRM, You need a repair design/disposition from the TR OEM. Period. Full Stop. No other option. The part and loading is way too complicated in the area of damage (as shown in your figure) for anyone on this forum to give you are repair design.

 

[berkshire]

Composite repairman,
This type of repair where the core disbonds, has a nasty habit of extending beyond where you think it should end. Double check the perimeter of your grind out to make sure the honeycomb is firmly bonded.
B.E.

You are judged not by what you know, but by what you can do.

 

[blakmax]

CRM

I totally agree with Berkshire and SW Composites. My expertise is in adhesive bond failure forensics and Berkshire is on the right track.

I strongly urge that you read Link

You are almost certainly dealing with adhesion failure between the core and the adhesive and as Berkshire says, this type of defect often extends much further than you think. You should also look for adhesion failure at the cell nodes because that damage often occurs alongside adhesion fillet bond failures. Both of these defects are discussed in the above paper.

What type of core are you dealing with? Metal? Nomex? If it is metal I would strongly advise you not to attempt to rebond to the "undamaged" core. The mechanism of failure of the bond is that the surface oxides on the metal forma hydrated oxide, and for that to occur the chemical bonds between the adhesive and the metal must dissociate, hence causing adhesion failure. The danger is that the surface of the core is already hydrated so attempting to bond to that core will only result in very weak fillet bonds. Our approach has been to remove the skin around the disbond until you get cohesion failure of the core to adhesive bond. Then remove all of the core and perform core insert repair and apply a doubler over the skin area that was removed.

My guess is that the removal of the degraded material probably would exceed SRM limits, so as SW Composites says, you need to get formal advice. I am not the person to help you there, but if you wish to get advice on adhesive bond failure forensics, try googling my name off the papers then try sending to "enquiry" at my web site.

Regards

Blakmax

 

[berkshire]

Max,
Your link totally nails the problem , I have taken the liberty of downloading that to my archive. Chasing dis-bonds in honeycomb has been the bane of my life on composite aircraft repairs, it always seem that just when you have it, you find a tiny bit more, and a nick that starts out the size of a pea , ends up with a repair the area of a football.
B.E.
I gave you an LPS for that one.


You are judged not by what you know, but by what you can do.

 

[blakmax]

Hi Berkshire

Thanks for the star! In 2012 I became so frustrated with a total lack of knowledge in adhesive bond failure forensics that I created a one day course on damage tolerance and failure analysis. I have delivered this under sponsorship of EASA and FAA on a number of occasions. Of most concern there were errors in assessment of bond failures in official crash investigation reports. In at least one case, the conclusions in relation to the significance of the bond failure were totally in error and should have had a direct influence on the causal findings, but they were considered not significant to the crash. This lack of understanding of the basic fundamentals of bonding mechanisms and importantly the failure mechanisms involved has a direct influence on correctly assessing repair methods, as in the case described by Composite Repair Man. If you understand why the bond failed, there is a higher probability that the repair will be effective.

In particular, for CRM's example I don't know how many SRMs and OEMs would have specified a resin injection repair. If you understand the chemical mechanisms involved in bond formation, you would understand that because the entire surface is non-reactive so simply introducing fresh adhesive will never restore bond strength. It may hide the air gap so that NDT can not find the defect. but the change to bond strength is minimal.

Damage tolerance analysis (DTA) without understanding the mechanisms for in-service degradation is a real concern. Almost all OEMs demonstrate DTA performance in accordance with FAR 2x.573 by using artificial disbonds to demonstrate strength with a given size of defect. If people understood the risks associated with using this approach for service defects, it would make a significant difference to how structural integrity of adhesive bonded joints is managed. see Link and Link

This is an education problem, not an engineering problem. Therefore while SW composites advice for CRM to get formal advice on repairs, the quality of the outcome is totally dependent on the level of (or lack of) knowledge by the repair design authority.

Regards

Blakmax

 

[Composite Repairman]

Thanks a lot for your advises,

I understand all importance of core condition, surface preparation and chemical mechanisms involved in bond formation. But based on my experience, some aviation companies and MRO do not pay enough attention to bond failures in composite repairs. They are guided by SRM, where mostly all composite parts are ''secondary structure'' and they try to use SRM for mostly all composite repairs. They just perform damage assessment and repair to ASAP return aircraft. That's why nobody is not interested in causes of bond failure. What can I say if even SRM is not contain the requirements to find out cause of failure.

Anyway I hope to get quality engineering disposition for my repair from Engineer Department.

Thanks

 

[berkshire]

BlackMax,
Following up on your remarks, Anecdotal story ,a German Salto Sailplane came to the shop showing signs of skin disband on a PVC foam core (Conticell 60) . Factory had this problem before on this same machine, recommended drilling small holes and injecting resin. We ended up removing the outer skins, to find that, nowhere, did the injected resin make an effective bond. we ended up removing the core, replacing it, then the outer skins.
B.E.

You are judged not by what you know, but by what you can do.

 

[WKTaylor]

FYI...On-line versions 'not the most legible' copies.

ADA172236 AFWAL-TR-86-4033 FAILURE ANALYSIS OF COMPOSITE STRUCTURE MATERIALS

ADA250520 WL-TR-91-4032 [DOD/FAA/CT-91/23 Vol I COMPOSITE FAILURE ANALYSIS HANDBOOK - PROGRAM OVERVIEW

ADA250521 WL-TR-91-4032 [DOD/FAA/CT-91/23 Vol II Part 1 COMPOSITE FAILURE ANALYSIS HANDBOOK - VOL II - TECHNICAL HANDBOOK PART 1 - PROCEDURES AND TECHNIQUES

ADA249130 WL-TR-91-4032 [DOD/FAA/CT-91/23 Vol II Part 2 COMPOSITE FAILURE ANALYSIS HANDBOOK - VOL II - TECHNICAL HANDBOOK PART 2 - ATLAS OF FRACTOGRAPHS

ADA249131 WL-TR-91-4032 [DOD/FAA/CT-91/23 Vol II Part 2 COMPOSITE FAILURE ANALYSIS HANDBOOK - VOL IT - TECHNICAL HANDBOOK PART 3 - CASE HISTORIES

[ADA ????] WL-TR-93-4004 [DOD/FAA/CT-96/21 COMPOSITE FAILURE ANALYSIS HANDBOOK - UPDATE 1

Care and Repair of Advanced Composites, 2ED, SAE Product Code R-336, ISBN of 978-0-7680-1062-6

http://books.sae.org/r-336/

DOT/FAA/AR-08/54 Guidelines for the Development of a Critical Composite Maintenance and Repair Issues Awareness Course

DOT/FAA/AR-TN06/57 Best Practice in Adhesive-Bonded Structures and Repairs

ADA141456 AGARD-R-716 COMPOSITE STRUCTURE REPAIR

MIL-HDBK-803 GLASS REINFORCED PLASTICS - PREVENTIVE MAINTENANCE AND REPAIR



Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

 

[LiftDivergence]

WKTaylor,

Just a suggestion, maybe you could start an FAQ with common composite structures resources, similar to the DTE references FAQ in this subforum.

Keep em' Flying
//Fight Corrosion!

 

[blakmax]

Wil

I acknowledge your extensive list of references but I do suggest that some level of additional guidance is necessary. For example I first came across MIL HDBK 337 and MIL HDBK 691 years ago, and immediately recognised the deficiencies in these documents. Many years later I met the engineer responsible for both of these documents and found that there had been no funding for updating these documents since Adam was in short pants. Hence the quality of the advice in references really does depend on the veracity of the content.

As an example DOT/FAA/AR-TN06/57 Best Practice in Adhesive-Bonded Structures and Repairs was written by me in 2004, and if I wrote it with my current knowledge I would update the content significantly.

Kindest regards

Blakmax