AW139 lost tail taxying DOH
So my statement is true then - eddy currents and visual inspections (traditional NDT) don't work but modern techniques (thermography and ultrasound) do.
The advantage of thermography over ultrasound NDT is speed. I don't know exactly about ultrasound, but thermography can also detect weak bonds. No need for any voids. This is all about the heat flow through the bond and some smart data processing.
Arrakis
Join Date: Jul 2008
Location: Australia
Posts: 372
Likes: 0
Received 0 Likes
on
0 Posts
weak bonds?
Hi Arrakis. I have seen some pretty impressive stuff using thermography for larger area disbonds or delaminations, but I was unaware of the ability to find weak bonds. Do you have any published reference material for that? (Interested, not disbelieving.)
I could see the possibility for localised areas of weak bonds, but my understanding is that you need to have areas of good and bad bonds to enable a comparative assessment. In other words, thermography does not provide a numerical output based on a known standard, it relies on variations of temperature within the zone being inspected. If the entire region is weakly bonded, then such a comparative assessment may not be possible.
Regards
blakmax
I could see the possibility for localised areas of weak bonds, but my understanding is that you need to have areas of good and bad bonds to enable a comparative assessment. In other words, thermography does not provide a numerical output based on a known standard, it relies on variations of temperature within the zone being inspected. If the entire region is weakly bonded, then such a comparative assessment may not be possible.
Regards
blakmax
Last edited by blakmax; 6th Sep 2009 at 09:47. Reason: Digital problem. Left finger faster than right. Typo.
Join Date: Jul 2008
Location: Australia
Posts: 372
Likes: 0
Received 0 Likes
on
0 Posts
One more repair option, Old Timer
Sorry Old Timer, I forgot the usual repair method specified by OEMs for disbonds: INJECTION REPAIRS.
Injection repairs involve drilling holes in the skin of sandwich structure and injecting fresh adhesive to "re-bond" the area. Remembering that adhesive bonding involves chemical reactions on clean, chemically active surfaces which are treated to develop hydration resistance, these requirements are physically impossible to achieve by just injecting fresh adhesive. You can not guarantee that the surface is clean even if you flush the holes with solvent. You certainly have done absolutely nothing to activate the surface (solvent cleaning alone does not do that). And for metals, you have not treated the surface to prevent hydration. Let me be blunt: Injection repairs are nothing more than bovine excrement.
The only two things achieved by injection repairs is that the air gap is filled so that (as discussed elsewhere in this thread) NDI no longer finds the gap. The other thing achieved is that the technician can sign off the repair with a warm fuzzy feeling he has fixed the problem. Structurally, he has made not one ounce of difference. This situation is true for any disbond, composite or metal.
If you are looking for a short term fix, my solution (tongue placed firmly in cheek) is to paint the disbond area bright pink, because that is a better structural solution. Firstly, you have not perforated the skin and therefore have prevented moisture ingress which would cause corrosion or further disbonding. Secondly, you have identified the area so that you can cut down future inspection time. Thirdly, if you carefully map the area, you can monitor disbond growth rates. And fourthly, the aditional paint has added to the corrosion protection of ther area. Structurally, the result is the same. You have done nothing to restore the strength of the disbond.
Now I have recently had some comments that my statements are too definitive. Let me stretch the boundary there. If there is one single shread of evidence (apart from removing the NDI trace) from ANY manufacturer, researcher or regulator that injection repairs actually work, please provide the reference.
Does any one want to take a bet of ten pints of Guiness that I get no response?I have a serious number of examples where injection repairs have not rebonded anything. I also have a number of examples where injection repairs have led to in-flight failure of fairing panels approx. 10ft x 8 ft which impacted the fin, HSTAB and rudder. Had this aircraft been in a nose-down pitch with the HSTAB raised, the panel may have jammed the HSTAB with disasterous consequences. All because of an injection repair. In another case, an aircraft lost a rudder due to injection repairs performed at manufacture. Pictures available on request.
So why do we see injection repairs in every repair manual? Because these repairs are "grandfathered" to the next model and the regulators accept that process. In this case "grandfather" has dimentia!
Regards
blakmax
Injection repairs involve drilling holes in the skin of sandwich structure and injecting fresh adhesive to "re-bond" the area. Remembering that adhesive bonding involves chemical reactions on clean, chemically active surfaces which are treated to develop hydration resistance, these requirements are physically impossible to achieve by just injecting fresh adhesive. You can not guarantee that the surface is clean even if you flush the holes with solvent. You certainly have done absolutely nothing to activate the surface (solvent cleaning alone does not do that). And for metals, you have not treated the surface to prevent hydration. Let me be blunt: Injection repairs are nothing more than bovine excrement.
The only two things achieved by injection repairs is that the air gap is filled so that (as discussed elsewhere in this thread) NDI no longer finds the gap. The other thing achieved is that the technician can sign off the repair with a warm fuzzy feeling he has fixed the problem. Structurally, he has made not one ounce of difference. This situation is true for any disbond, composite or metal.
If you are looking for a short term fix, my solution (tongue placed firmly in cheek) is to paint the disbond area bright pink, because that is a better structural solution. Firstly, you have not perforated the skin and therefore have prevented moisture ingress which would cause corrosion or further disbonding. Secondly, you have identified the area so that you can cut down future inspection time. Thirdly, if you carefully map the area, you can monitor disbond growth rates. And fourthly, the aditional paint has added to the corrosion protection of ther area. Structurally, the result is the same. You have done nothing to restore the strength of the disbond.
Now I have recently had some comments that my statements are too definitive. Let me stretch the boundary there. If there is one single shread of evidence (apart from removing the NDI trace) from ANY manufacturer, researcher or regulator that injection repairs actually work, please provide the reference.
Does any one want to take a bet of ten pints of Guiness that I get no response?I have a serious number of examples where injection repairs have not rebonded anything. I also have a number of examples where injection repairs have led to in-flight failure of fairing panels approx. 10ft x 8 ft which impacted the fin, HSTAB and rudder. Had this aircraft been in a nose-down pitch with the HSTAB raised, the panel may have jammed the HSTAB with disasterous consequences. All because of an injection repair. In another case, an aircraft lost a rudder due to injection repairs performed at manufacture. Pictures available on request.
So why do we see injection repairs in every repair manual? Because these repairs are "grandfathered" to the next model and the regulators accept that process. In this case "grandfather" has dimentia!
Regards
blakmax
Join Date: Aug 2009
Location: France
Posts: 12
Likes: 0
Received 0 Likes
on
0 Posts
Thanks blakmax for the info re my question. Maybe the pink paint job would be better, certainly would add to the visual effect overall. As I understand a number of aircraft have now been found suffering from various levels of de-bonding, the next interesting chapter will be the discussion on the approved? Repair scheme from the big A, if and when it reaches this thread. I still think replacement is the safe option.
Join Date: Aug 2009
Location: France
Posts: 12
Likes: 0
Received 0 Likes
on
0 Posts
Sorry blakmax forgot to mention, had a snap sent to me of the approved testing device yesterday, very pretty looking miniature hammer, has a part number and everything. Maybe this move towards high tech is the reason for finding more of those pesky voids that eluded the two pence piece. Retirement has its advantages, a little worrying as off on a holiday soon and flying Airbus, aren’t they mainly composite??? Sorry ignore that, thread drift.
Join Date: Jul 2001
Location: ...in view of the 'Southern Cross' ...
Posts: 1,383
Received 0 Likes
on
0 Posts
Mmmmm ....
Tap Hammers .....
Hey OT .... I gave mine away to an eager apprentice years ago ..... I now find my self back at the Lathe making myself (and a few chosen 139 mates) new tap hammers .... what a way to spend ones leave time????
Wonder if they have to be approved so the "Tap" can be calibrated???
Tap Hammers .....
Hey OT .... I gave mine away to an eager apprentice years ago ..... I now find my self back at the Lathe making myself (and a few chosen 139 mates) new tap hammers .... what a way to spend ones leave time????
Wonder if they have to be approved so the "Tap" can be calibrated???
I hope the 787 designers are taking notes.....
Join Date: Aug 2009
Location: France
Posts: 12
Likes: 0
Received 0 Likes
on
0 Posts
Spin, maybe you have to make a tap test audio recording of a series of toppings and send to A, they then issue a chit. I wonder if the hammer heads are cracked, they could then indicate a false pesky void, hence the sudden increase in the discovery of them. 12 ish in one day seems to be the record, so a mate informs me. Wonder if that’s true? I also understand that sales of pink paint have increased and stocks are being shipped to Italy, so with little hammers and pink paint, problem solved.
OT
OT
Join Date: Sep 2009
Location: Abu Dhabi
Posts: 7
Likes: 0
Received 0 Likes
on
0 Posts
Sorry for my absence over the last week but I've been busy tapping out 139 Tailbooms.
As mentioned before I am the poor technician at the coal face having to deal with composite problems on a daily basis now for nearly 30 years
(yes blakmax I have work with you a long time ago. Say hello to my mate Jimmy S for me.) What I have found over time is that we as technicians, particularly on helicopters, are restricted by repair schemes laid down in the OEM's manuals to the use of cold bonded repairs only.
I know Airbus and Boeing use, in the majority of cases, hot bonded repairs.
Why the difference between fixed wing and rotory wing composite repairs ? This is a question I ask myself everyday, I can only put it down to the fact that most helicopters are operated and maintained in less than favorable locations to carry out hot bonded repairs.
It may also have something to do with the cost associated with having approved hot bonded repair capabilities.
I will go on record as saying I detest cold bonded repairs with rivets as I have seen everyone fail, but I also understand that it would be near impossible in most cases to carry out a hot bonded repair whilst in the field.
What I believe Bell and Agusta should do is place a supplement in their SRM's that allows the customer the option of carrying out a hot bonded repair.
So OK, I'm stuck (no pun intended) as with all other technicians in the field with performing cold bonded repairs ( this is where you come in blakmax) how do we improve our processing of the repair area and parts and to ensure we get the best bond possible? I'm talking surface preparation and core cleaning here...TAKE IT AWAY BLAKMAX
One last thing regarding NDT of composite structures in the field I use the Eurocopter tap hammer which is simply a 2 1/2 inch long cylindrical steel roller bearing. Works a treat and has cost Agusta millions.
As mentioned before I am the poor technician at the coal face having to deal with composite problems on a daily basis now for nearly 30 years
(yes blakmax I have work with you a long time ago. Say hello to my mate Jimmy S for me.) What I have found over time is that we as technicians, particularly on helicopters, are restricted by repair schemes laid down in the OEM's manuals to the use of cold bonded repairs only.
I know Airbus and Boeing use, in the majority of cases, hot bonded repairs.
Why the difference between fixed wing and rotory wing composite repairs ? This is a question I ask myself everyday, I can only put it down to the fact that most helicopters are operated and maintained in less than favorable locations to carry out hot bonded repairs.
It may also have something to do with the cost associated with having approved hot bonded repair capabilities.
I will go on record as saying I detest cold bonded repairs with rivets as I have seen everyone fail, but I also understand that it would be near impossible in most cases to carry out a hot bonded repair whilst in the field.
What I believe Bell and Agusta should do is place a supplement in their SRM's that allows the customer the option of carrying out a hot bonded repair.
So OK, I'm stuck (no pun intended) as with all other technicians in the field with performing cold bonded repairs ( this is where you come in blakmax) how do we improve our processing of the repair area and parts and to ensure we get the best bond possible? I'm talking surface preparation and core cleaning here...TAKE IT AWAY BLAKMAX
One last thing regarding NDT of composite structures in the field I use the Eurocopter tap hammer which is simply a 2 1/2 inch long cylindrical steel roller bearing. Works a treat and has cost Agusta millions.
Join Date: Jul 2008
Location: Australia
Posts: 372
Likes: 0
Received 0 Likes
on
0 Posts
Cold bonded repairs???
G'day skin king. I am still trying to work out who you are so I have sent you a PM. Top right near log-out.
By the quality of your comments about hot bonding vs cold bonding and the integrity of SRM procedures, my persistent nagging over the years has paid off, at least in your understanding of how things could be improved.
You have highlighted one of the issues that stuns me. Why must the SRM only have the dumbest repair? Surely there has to be a grading of capabilities. I fail to understand why SRMs contain only the repair which is easiest to perform while at the same time providing the worst outcome for everyone, including organisations which are capable of higher level repairs. You would presonally know that the only way RAAF at Amberley ever achieved a virtual elimination of repeat repairs was to scrap the OEM processes and adopt scientifically validated procesess which had a verified capability to produce bond durability. This is why I wrote DEF (AUST) 9005. And you would agree, this is not rocket science! I have taught electricians to perform bonded repairs, so what is so difficult that we must continue to use ineffective processes because Blogslovia doesn't have an appropriate level of competency? Can't we have a grading level for authorised repair stations and assign repair approvals on the status of the repair station?
I suggest that many OEMs do not actually comprehend the impact of their own SRM processes on repair integrity and durability. Further, many hide behind a fascade of OEM infallibility which could not sustain knowledgeable scrutiny, as you know from the numerous cases I managed for RAAF. I managed to convince and then assisted a certain large manufacturer in St Louis to re-write their manuals and any organisation who adopted the amended publications gains the benefit of those changes. So it is possible to work with manufacturers to correct deficient bonding practices.
The frustration I have is that the only two ways I can see to force such changes are by 1. Operator pressure to reduce maintenance costs and 2. Legal action after some unfortunate people have died and their dependents seek legal redress. Logic is not effective against a company position of corporate infallibility.
Regards Blakmax
PS respond to the PM.
By the quality of your comments about hot bonding vs cold bonding and the integrity of SRM procedures, my persistent nagging over the years has paid off, at least in your understanding of how things could be improved.
You have highlighted one of the issues that stuns me. Why must the SRM only have the dumbest repair? Surely there has to be a grading of capabilities. I fail to understand why SRMs contain only the repair which is easiest to perform while at the same time providing the worst outcome for everyone, including organisations which are capable of higher level repairs. You would presonally know that the only way RAAF at Amberley ever achieved a virtual elimination of repeat repairs was to scrap the OEM processes and adopt scientifically validated procesess which had a verified capability to produce bond durability. This is why I wrote DEF (AUST) 9005. And you would agree, this is not rocket science! I have taught electricians to perform bonded repairs, so what is so difficult that we must continue to use ineffective processes because Blogslovia doesn't have an appropriate level of competency? Can't we have a grading level for authorised repair stations and assign repair approvals on the status of the repair station?
I suggest that many OEMs do not actually comprehend the impact of their own SRM processes on repair integrity and durability. Further, many hide behind a fascade of OEM infallibility which could not sustain knowledgeable scrutiny, as you know from the numerous cases I managed for RAAF. I managed to convince and then assisted a certain large manufacturer in St Louis to re-write their manuals and any organisation who adopted the amended publications gains the benefit of those changes. So it is possible to work with manufacturers to correct deficient bonding practices.
The frustration I have is that the only two ways I can see to force such changes are by 1. Operator pressure to reduce maintenance costs and 2. Legal action after some unfortunate people have died and their dependents seek legal redress. Logic is not effective against a company position of corporate infallibility.
Regards Blakmax
PS respond to the PM.
Join Date: Jun 2007
Location: foot of a mountain
Posts: 293
Likes: 0
Received 0 Likes
on
0 Posts
Went through my OEM manuals today to look for a repair for a impact mark on Nomex honeycomb with aluminium skin with possible debonding in the surrounding area. Blakmax, you should be happy to here that drilling a hole and injecting something was not an option. In order to comply with the repair I had to get the local aviation approved AMO in who specialises in composite repairs and has stock and experience in working with all the listed(of which there are quite a few) materials to be used during the different stages of the repair.
Sorry, forgot to mention it is not even a structural part or load carrying part at all.
Sorry, forgot to mention it is not even a structural part or load carrying part at all.
Last edited by victor papa; 7th Sep 2009 at 15:45. Reason: Addition
Join Date: Sep 2009
Location: Abu Dhabi
Posts: 7
Likes: 0
Received 0 Likes
on
0 Posts
To anyone at Agusta who is out there and willing to listen,
PLEASE STOP ISSUING REPAIRS FOR LARGE DISBONDS TO AW139 TAILBOOMS !!!!!
They are difficult to accomplish even in a facility with all the resources, use antiquated techniques and simply do not restore the panels to their original strength.
My advice to any operator who chooses to carry out these repairs is to monitor the area around the repair on your daily and after each flight.
Or better still just ask Agusta for a new Tailboom.
I don't want to appear all doom and gloom but in all my years invovled with aircraft I have never come across a more important safety issue in regards to a helicopter structure. It would be nice if Agusta could at least forward an information letter to operators detailing the actions they are taking to fix this immediate problem. Sure they have aluminium core material now in place of Nomex and they say they have improved QA standards, well give us the details...What does this new core material achieve? What are the new QA standards? Are they validating each panel manufactured? I believe we as operators and the pax we carry have a right to know. And if there was an obvious shortfall in QA standards in the manufacture of past assy's we should know that as well. I'm sure if a car maker had a serious safety issue with one of their products it would be recalled regardless of the cost to them.
Time to step up Agusta.
I know, it was more than 2 cents worth
PLEASE STOP ISSUING REPAIRS FOR LARGE DISBONDS TO AW139 TAILBOOMS !!!!!
They are difficult to accomplish even in a facility with all the resources, use antiquated techniques and simply do not restore the panels to their original strength.
My advice to any operator who chooses to carry out these repairs is to monitor the area around the repair on your daily and after each flight.
Or better still just ask Agusta for a new Tailboom.
I don't want to appear all doom and gloom but in all my years invovled with aircraft I have never come across a more important safety issue in regards to a helicopter structure. It would be nice if Agusta could at least forward an information letter to operators detailing the actions they are taking to fix this immediate problem. Sure they have aluminium core material now in place of Nomex and they say they have improved QA standards, well give us the details...What does this new core material achieve? What are the new QA standards? Are they validating each panel manufactured? I believe we as operators and the pax we carry have a right to know. And if there was an obvious shortfall in QA standards in the manufacture of past assy's we should know that as well. I'm sure if a car maker had a serious safety issue with one of their products it would be recalled regardless of the cost to them.
Time to step up Agusta.
I know, it was more than 2 cents worth
Last edited by Skin King; 8th Sep 2009 at 01:51.
Join Date: Jul 2008
Location: Australia
Posts: 372
Likes: 0
Received 0 Likes
on
0 Posts
repairs to tail booms
Skin King, You are saying what a number of others have said to me in PMs. The real (and only valid) solution is to get the manufacturing correct so that disbonds don't occur in the first place.
I am advised by PM from some members that the repairs to which you refer involve large area disbonds and the repairs involve the use of a room temperature curing paste adhesive with lots of blind fasteners added. There are multiple issues with this repair. Firstly, I'd like to know what surface preparation was used on the skin and doubler prior to bonding. (Please don't tell me it is just a solvent wipe!) Secondly, unless they do something to remove the moisture from the core, the adhesive will not bond to the core. Thirdly, the paste adhesive will most probably just run down the cells, leaving barely any adhesive to form fillets to the core. Fourthly, the fasteners will provide a moisture path into the surrounding core making the problem spread wider and faster. Elevated temperature curing adhesives would probably develop stronger bonds, but given that the core to adhesive interface is so weak, the skins would probably pop off the core in the surrounding areas.
A further concern is that these disbonds appear to be driven by a combination of weak interfaces and compression loads. The surrounding areas are probably just as weakly bonded and the only reason they have not failed is that they have not seen the compressive loads. Even if these repairs were successful, there is no guarantee that adjacent areas would fail in a similar manner.
I am told that one approved repair lasted less than 30 minutes. The real solution is to fix the manufacturing processes. I'd be happy to contract to help.
Adhesive bonded repairs can be very effective, but the processes for those repairs must also be just as valid as the processes for manufacture should be.
Does anyone have a contact in Agusta who I can deal with?
Regards
Blakmax
I am advised by PM from some members that the repairs to which you refer involve large area disbonds and the repairs involve the use of a room temperature curing paste adhesive with lots of blind fasteners added. There are multiple issues with this repair. Firstly, I'd like to know what surface preparation was used on the skin and doubler prior to bonding. (Please don't tell me it is just a solvent wipe!) Secondly, unless they do something to remove the moisture from the core, the adhesive will not bond to the core. Thirdly, the paste adhesive will most probably just run down the cells, leaving barely any adhesive to form fillets to the core. Fourthly, the fasteners will provide a moisture path into the surrounding core making the problem spread wider and faster. Elevated temperature curing adhesives would probably develop stronger bonds, but given that the core to adhesive interface is so weak, the skins would probably pop off the core in the surrounding areas.
A further concern is that these disbonds appear to be driven by a combination of weak interfaces and compression loads. The surrounding areas are probably just as weakly bonded and the only reason they have not failed is that they have not seen the compressive loads. Even if these repairs were successful, there is no guarantee that adjacent areas would fail in a similar manner.
I am told that one approved repair lasted less than 30 minutes. The real solution is to fix the manufacturing processes. I'd be happy to contract to help.
Adhesive bonded repairs can be very effective, but the processes for those repairs must also be just as valid as the processes for manufacture should be.
Does anyone have a contact in Agusta who I can deal with?
Regards
Blakmax
Join Date: May 2008
Location: New Zealand
Posts: 7
Likes: 0
Received 0 Likes
on
0 Posts
Hi Blackmax,
with regard to your excellent letter to this thread, you are correct about many misguided constructors out there, in particular reference to sacrificial peel plies and the potential to produce contamination of the adhesive bond.
Also any form of repair by injection of adhesive is flawed as there is no way that the internal structure can be verified as ready for re bonding, anyone can inject and lead themselves to believe that they have corrected a problem but they are only temporarily masking a potentially serious issue. but as this thread is mainly speculative right now it would be silly to try and pre empt a thorough and detailed defect examination. save that the failure of this component is dramatic and the consequences of this event happening in flight would be tragic.
As per your first posting in this thread it would be interesting to see what repair scheme was applied to the boom following the instance of ground contact.
Rumor only, I hear this aircraft type is prone to suffering from exhaust efflux impingement on the boom and coupled with the high ambient temps experienced in the middle east could this be causing disbonding? maybe if it is, a simple mod as per the 212/412 to put a bend in the exhaust duct could help.
Phil
with regard to your excellent letter to this thread, you are correct about many misguided constructors out there, in particular reference to sacrificial peel plies and the potential to produce contamination of the adhesive bond.
Also any form of repair by injection of adhesive is flawed as there is no way that the internal structure can be verified as ready for re bonding, anyone can inject and lead themselves to believe that they have corrected a problem but they are only temporarily masking a potentially serious issue. but as this thread is mainly speculative right now it would be silly to try and pre empt a thorough and detailed defect examination. save that the failure of this component is dramatic and the consequences of this event happening in flight would be tragic.
As per your first posting in this thread it would be interesting to see what repair scheme was applied to the boom following the instance of ground contact.
Rumor only, I hear this aircraft type is prone to suffering from exhaust efflux impingement on the boom and coupled with the high ambient temps experienced in the middle east could this be causing disbonding? maybe if it is, a simple mod as per the 212/412 to put a bend in the exhaust duct could help.
Phil
Join Date: Jul 2001
Location: ...in view of the 'Southern Cross' ...
Posts: 1,383
Received 0 Likes
on
0 Posts
Mmmm ...
Phil read through the whole thread again .... all has been covered with regard
heat & exhaust re-circulation etc etc ...
PS also the exhaust ducts tend to push the hot gas out to the sides on the 139 rather than directly out the back as on the 212/412 ....
Phil read through the whole thread again .... all has been covered with regard
heat & exhaust re-circulation etc etc ...
PS also the exhaust ducts tend to push the hot gas out to the sides on the 139 rather than directly out the back as on the 212/412 ....
Join Date: Nov 2000
Location: Indianapolis, IN, USA
Posts: 151
Likes: 0
Received 0 Likes
on
0 Posts
This episode puts an interesting light on the announcement made a couple of years ago by EC - surely the home of 'plastic' helicopters - that the new EC175 would use an aluminum structure (ostensibly for ease of 'on deck' repair) rather than a composite one.
Join Date: Jul 2008
Location: Australia
Posts: 372
Likes: 0
Received 0 Likes
on
0 Posts
Repair of composite structure
Guys
Several of the last few postings have suggested that repair of composites is too difficult and metallic structures may be easier to maintain. In some ways that is correct, but what we have is more of an education problem than engineering or materials problems. There are some basic rules which become obvious if you understand the basic mechanisms involved in adhesive bonding and importantly the mechanisms involved in bond failure. There are a few production traps, which if addressed correctly should eliminate many of the problems we see in service.
With regard to field level repair, many see adhesive bonding as too hard. The main problem is contamination, including from humidity. We have addressed this by having a portable airconditioner and dehumidifier which we pump into a roughly constructed tent. The positive pressure from blowing air in keeps the contamination out, and the humidity and temperature are controlled. It does not take too long to cofigure such a system once you have spent the effort to get the basic equipment in place. We have used that method for repairs installed in Northern Australia during the wet season.
The real problem is that owners don't want to invest in the equipment and developing the skill levels necessary. They just want to do what they have been doing for years and not to step out of their comfort zones. I don't blame them but they effectively turn their backs on the most effective and efficient method for repairing structures. I have data which shows that the best you can ever achieve for a mechanically fastened repair in metallic structure (assuming that the dmaage is away from edges etc.) is a restoration of about 65% of original strength. For composite structure the efficiency is down to about 45%. With a properly designed and implemented bonded repair, the strength restoration in thin materials is 100%.
With regard to battle damage, most manufacturers use mechanical repairs, but even these have traps such as larger edge distances and fastener separation distances than for metals.
The message is to get the production processes correct in the first place, improve training and then acquire the necessary equipment. Don't dumb down the repair methods.
Regards
blakmax
Several of the last few postings have suggested that repair of composites is too difficult and metallic structures may be easier to maintain. In some ways that is correct, but what we have is more of an education problem than engineering or materials problems. There are some basic rules which become obvious if you understand the basic mechanisms involved in adhesive bonding and importantly the mechanisms involved in bond failure. There are a few production traps, which if addressed correctly should eliminate many of the problems we see in service.
With regard to field level repair, many see adhesive bonding as too hard. The main problem is contamination, including from humidity. We have addressed this by having a portable airconditioner and dehumidifier which we pump into a roughly constructed tent. The positive pressure from blowing air in keeps the contamination out, and the humidity and temperature are controlled. It does not take too long to cofigure such a system once you have spent the effort to get the basic equipment in place. We have used that method for repairs installed in Northern Australia during the wet season.
The real problem is that owners don't want to invest in the equipment and developing the skill levels necessary. They just want to do what they have been doing for years and not to step out of their comfort zones. I don't blame them but they effectively turn their backs on the most effective and efficient method for repairing structures. I have data which shows that the best you can ever achieve for a mechanically fastened repair in metallic structure (assuming that the dmaage is away from edges etc.) is a restoration of about 65% of original strength. For composite structure the efficiency is down to about 45%. With a properly designed and implemented bonded repair, the strength restoration in thin materials is 100%.
With regard to battle damage, most manufacturers use mechanical repairs, but even these have traps such as larger edge distances and fastener separation distances than for metals.
The message is to get the production processes correct in the first place, improve training and then acquire the necessary equipment. Don't dumb down the repair methods.
Regards
blakmax
Last edited by blakmax; 9th Sep 2009 at 23:53. Reason: Ever notice how you read right through typos?
Join Date: Jul 2008
Location: Australia
Posts: 372
Likes: 0
Received 0 Likes
on
0 Posts
Examination of disbond sample photographs
OK, people. I have had some photographs of skin sections cut from a typical disbond in the tail boom sent to me from a reliable source. I am assured they are from an AW139 and from the area which typically disbonds in the tail boom.
In the samples, there are regions just at the edge of the disbond where the core has been in firm contact with the adhesive and has formed what visually appears to be an acceptable bond. To enable core to transfer shear loads, it is essential that the adhesive forms fillets to the core. As the adhesive melts and flows, it wets the surface of the core and by capiliary action flows down the sides for a small distance, thus forming the fillets. These fillets are critical to the performance of bonded sandwich structure, and in the example examined the core had fractured during removal of the sample indicating that these particular cells were well bonded.
However, over the remainder of the surface of the samples I have seen in the photographs there is evidence that the contact between the core and the adhesive has not been sufficient to form the fillets. It appears from these photographs that the capiliary action has only resulted in adhesive contacting just the very end of the core, forming a small bead and not flowing down the sides to form the fillets. The disbonds being investigated when these samples were taken appear to have been due to separation of the beads of adhesive.
To explain the difference, imagine you are to weld one piece of material to another to form a T joint. In the first case, press the sheets together one perpendicular to the other and weld both sides, forming fillets. In the second case, hold the second sheet perpendicular to the other but leave a gap so they are about 5mm apart. Now weld just enough to fill that gap. Clearly the sample which formed the fillets will be much stronger.
There is also extensive micro-voiding in the adhesive layer itself due to the presence of small bubbles. While some micro-voiding is tolerable, the extent in the samples I have seen would not be consistent with what would typically be observed in well bonded structure.
One possible mechanism for disbonding is that the absence of the fillets has led to the loads being transferred by the small bead of adhesive at the end of the cells and in the presence of compression loads which place that bond in out of plane tension, combined with or acting independently of the internal pressure associated with heating, the outcome is that the small amount of adhesive simply can not sustain the loads.
Suffice it to say that the lack of fillet bonds would have meant that this area would not have been as strong as it could have been had the fillets been formed correctly and it would therefore be debatable if the structure could have sustained the loads the design required.
Now, could Agusta have found this with NDI? Probably not. In defence of Agusta, there would have been sifficient contact between the adhesive and the core (even through the small bead at the end of the cells) to enable sound transmission. Hence ultrasonic or tap inspection would have passed this defect. Is it possible to inspect for this defect in current aircraft? Probably not. NDI will only help AFTER the disbond has occurred.
Such a failure to form the fillets results from a number of production issues, and the presence of micro-voiding adds weight to one of the theories I have advanced. There are several other possible production issues that could result in the same defect. The only way to resolve this is to assess the probability of each potential cause occurring within the current production process.
I can not go much further without actually seeing the failed example and assessing AW's production methods and quality processes. Agusta may be able to sort this out themselves, but I am willing to assist if requested. I would much rather help to get the problem fixed than to help some lawyer get rich after some poor b*stard has died.
Sorry if some of the wording is a bit obtuse, but I have tried to take sox6's advice and not to be too prescriptive without actually seeing the articles and assessing the production process.
Regards
blakmax
In the samples, there are regions just at the edge of the disbond where the core has been in firm contact with the adhesive and has formed what visually appears to be an acceptable bond. To enable core to transfer shear loads, it is essential that the adhesive forms fillets to the core. As the adhesive melts and flows, it wets the surface of the core and by capiliary action flows down the sides for a small distance, thus forming the fillets. These fillets are critical to the performance of bonded sandwich structure, and in the example examined the core had fractured during removal of the sample indicating that these particular cells were well bonded.
However, over the remainder of the surface of the samples I have seen in the photographs there is evidence that the contact between the core and the adhesive has not been sufficient to form the fillets. It appears from these photographs that the capiliary action has only resulted in adhesive contacting just the very end of the core, forming a small bead and not flowing down the sides to form the fillets. The disbonds being investigated when these samples were taken appear to have been due to separation of the beads of adhesive.
To explain the difference, imagine you are to weld one piece of material to another to form a T joint. In the first case, press the sheets together one perpendicular to the other and weld both sides, forming fillets. In the second case, hold the second sheet perpendicular to the other but leave a gap so they are about 5mm apart. Now weld just enough to fill that gap. Clearly the sample which formed the fillets will be much stronger.
There is also extensive micro-voiding in the adhesive layer itself due to the presence of small bubbles. While some micro-voiding is tolerable, the extent in the samples I have seen would not be consistent with what would typically be observed in well bonded structure.
One possible mechanism for disbonding is that the absence of the fillets has led to the loads being transferred by the small bead of adhesive at the end of the cells and in the presence of compression loads which place that bond in out of plane tension, combined with or acting independently of the internal pressure associated with heating, the outcome is that the small amount of adhesive simply can not sustain the loads.
Suffice it to say that the lack of fillet bonds would have meant that this area would not have been as strong as it could have been had the fillets been formed correctly and it would therefore be debatable if the structure could have sustained the loads the design required.
Now, could Agusta have found this with NDI? Probably not. In defence of Agusta, there would have been sifficient contact between the adhesive and the core (even through the small bead at the end of the cells) to enable sound transmission. Hence ultrasonic or tap inspection would have passed this defect. Is it possible to inspect for this defect in current aircraft? Probably not. NDI will only help AFTER the disbond has occurred.
Such a failure to form the fillets results from a number of production issues, and the presence of micro-voiding adds weight to one of the theories I have advanced. There are several other possible production issues that could result in the same defect. The only way to resolve this is to assess the probability of each potential cause occurring within the current production process.
I can not go much further without actually seeing the failed example and assessing AW's production methods and quality processes. Agusta may be able to sort this out themselves, but I am willing to assist if requested. I would much rather help to get the problem fixed than to help some lawyer get rich after some poor b*stard has died.
Sorry if some of the wording is a bit obtuse, but I have tried to take sox6's advice and not to be too prescriptive without actually seeing the articles and assessing the production process.
Regards
blakmax