A330/340 fuselage is Al-Li isn't it? don't seem to have much problems..?DFW wrote:In the book Twentieth Century Jet and subsequent documentary(which was mostly a silly piece of marketing), Alan Mullaly went to great lengths to explain why Boeing didn't use Al-Li on certain parts of the B777. His reasoning was that surface cracks, which are tolerable, would none-the-less make customers uneasy.
Are there other disadvantages to Al-Li?
Boeing 787 dreamliner : troubles
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smokejumper
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The A330-340 does not use Al-Li - it uses aircraft grade aluminum. Al-Li has been proposed for use in the current version of the A350. Al-Li is lighter weight than Al and is strong, but can be brittle and can crack. Al-Li has been used in the US Space Shuttle (for lightness) but has exhibited cracks.
just an engineer
I prefer to listen to just an engineer, rather just about anyone else when it comes to safety.
Modern Al-Li is an excellent material for aviation. The so called alloy 8090 was developed some years ago to provide an high fatigue toughnes and crack resistance. It contains (in weight-percents) 2.45 Li, 0.12 Zr, 1.3 Cu, 0.95 Mg.The density of alloy 8090 is 2.55 g/cm3, compared to 2.70 of pure aluminium. One major advantage compared to CFRP is that it protects from ionising radiation twice as good.
It is very simple: The more electrons an atom has, the better will this matter protect from radiation. Lead (Pb) with the atomic number of 82 is widely spread for radiation protection - just think of the lead containing aprons worn by x-ray personnel. CFRP mainly consits of atoms of Carbon (C) and Hydrogen (H), so roughly spoken it would have the same shield effect if it was a little bit over twice as thick as Al-Li alloy 8090. But some CFRPs are almost as heavy as Al-Li with a density of 2.5 g/cm3. (Although I'm sure Boeing will use a variant with a density of around 2.0).
Dear Misako and others,
I got the answer from the teacher:
Chris
our post made me think however. Glare is given a CTE of 16.3 but this must be some sort of average since as you said there must be a big difference between the Al and composite layers. This will eventually result in some thermal residual stresses in the structure itself. And ultimately I could think of delamination problems if the stress becomes too high... Since I don't know the real answer to this I've send an e-mail to a teacher in the hope that he will give me an answer to that subject.
I got the answer from the teacher:
I hope this helpsYou are absolutely right when you state that aluminium has a high CTE (around 23E-06 per degree C), while the glass fibre prepreg layers have a much lower CTE (around 6.1E-06 in fibre direction). So when you bond these layers together in a cure cycle at high temperature and elevated pressures to manufacture Glare, the end result (at room temperature) is that you have created a residual stress field. Since the CTE of the glass fibre prepreg layers is lower than that of the aluminium layers, compressive stresses residual stresses will act in the glass fibre layers and tensile residual stresses in the aluminium layers. So this is similar to what you can expect in bonded repairs with patch materials having a CTE below that of the aluminium skin material of your aircraft.
When you go to cruise altitude temperatures of -50 degrees C, the situation only worsens, because the tensile stresses in the aluminium get higher while the compressive stresses in the glass fibre layers also get higher. However, at each and every operational temperature there can be NO difference in strain (elongation or shortening) between the layers. The strain from one layer to another has to match. That's why you can indeed (as you mention) expect high shear stresses between the different layers to get a balanced whole.
These shear stresses are not so high that the epoxy matrix of the glass fibre prepreg cannot transfer these loads without failing. However, in operation the Glare material is loaded in fatigue both thermally and mechanically and this will eventually lead to crack initiation in the aluminium layers. A crack starts to grow in the aluminium layers which causes stresses to be transferred from the aluminium layer into the fibre layer which is still intact. This in turn increases the shear stresses between the aluminium and fibre layers around the growing crack up to the point of delamination in this area. This delamination continues until a new equilibrium has been reached at which point not more load is transferred through shear from the damaged aluminium layer into the glass fibre layer and the crack in the aluminium layer continues to grow again up to the point that the shear stresses get so high again that more delamination occurs to find a new stress field equilibrium. Thus the mechanism of crack growth and delamination growth in fatigue (operation of the material) is self-balancing and actually causes extremely low crack growth rates compared to aluminium.
To be short: there are shear stresses between the glass fibre prepreg layers and the aluminium layers, which increase when going to cruise altitude temperatures, but since bonded connections are designed to transfer loads in shear and since these shear stresses are low enough not to cause immediate adhesive failure, a self-balancing mechanism between fatigue crack growth and fatigue delamination growth will occur in this material.
Chris
The simple explanation is that sure, the difference in CTE will cause the material to crack, but due to the structure of the laminate, the cracks will grow very slowly, in fact cracks will grow much much slower than in aluminium.
Even shorter: it will break, but very slowly, and the airlines can get away with not inspecting the thing regularly.
Even shorter: it will break, but very slowly, and the airlines can get away with not inspecting the thing regularly.
Dear Chris,
Thank you for your and your teacher's response re CTE problem in GLARE and it is now time for me to surface with my thoughts. I beg to differ from conclusions as, to my mind, they are somewhat misleading or, at best, debatable. If alumunum is placed in high tension via CTE, as is now agreed, and will crack just due to thermal cycling in service as stated in your teacher's response, do we have a decent operational material? If we have shifted and deleteriously impacted the S/N curve of the material re aluminum via imposing a high residual tensile load by bonding to glass fiber, how is that a good thing? Clearly the glass fiber bondline will act as a crack stopper in terms of Griffith factor, but there other tried and tested means to stop such cracks since the Comet investigations of the 50's, however, you have also impacted the modulus significantly downwards, which can lead to other issues. Taking the argument to an illogical extreme, why not just use glass fiber alone and eliminate the problem. of course the modulus goes very low for buckling, but we seem to be heading that way by having the aluminum crack just due to thermal cycling. Also, we would have a lower density in all glass, which is a plus. But if we are worried re low modulus, well, add in some CF and get it up again and have far lower density, no corrosion and lower CTE differential problems. Then we can make it all CF/epoxy and have higher performance with lower density. Frankly, it seems to me that oft times we have materials looking for a usage rather than materials properly designed for a usage.
Materials that crack in service, frankly, are always of concern, even if materials are claimed to fail benignly. If cracks then moisture concerns and so, I think, we have ended up with a complicated hybrid material far more expensive than aluminum, but still with a high density vis a vis CF/epoxy or glass/ CF hybrid. GLARE may help re burn-through and visible denting, I agree,but still has FST issues re epoxy usage and questions of moisture and icing damage, bondline issues and loss of buckling capability also arise. Rubberised adhesives as used have their own problems in my experience and introduce performance issues also. I have strong doubts re efficacy and utility in long term of hybrid materials where it is admitted cracks from thermal cycling. I am no fan of aluminum, but at least it is cheap, well developed, well understood and repairable. Using alumunum in a vestigal and in terms of CTE overloaded manner as in GLARE makes little sense to me. Even if flexible epoxy bondlines don't fail over thirty years of in-service, a huge leap of faith to my mind, the question still remains why build a fuselage from a material which it is now admitted cracks due to CTE issues? It seems the wrong solution to the problem. GLARE has dent resistance, but still has FST issues, a lower in-plane shear strength than all aluminum, a lower bearing strength, probably lower buckling strength, a major CTE cracking issue, moisture issues and the list goes on. CF/epoxy has lots of issues of its own re FST, impact damage and repair, but I think GLARE is no better off. Complexity seldom wins in engineering, Chris.
All thanks again, Chris, for your kind and detailed input, but in summary, the CTE Achilles heel remains for me re GLARE and even more so for ARALL, which has fire and higher CTE issues. Your mullings and excellent insights and thoughts are awaited as ever. Both Airbus and Boeing have major challenges regarding their usage of new and comparatively unproven materials in a commercial aircraft environment, I wish them both luck, but I hope all issues are addressed prior to certification and service.
All best wishes and regards.
Thank you for your and your teacher's response re CTE problem in GLARE and it is now time for me to surface with my thoughts. I beg to differ from conclusions as, to my mind, they are somewhat misleading or, at best, debatable. If alumunum is placed in high tension via CTE, as is now agreed, and will crack just due to thermal cycling in service as stated in your teacher's response, do we have a decent operational material? If we have shifted and deleteriously impacted the S/N curve of the material re aluminum via imposing a high residual tensile load by bonding to glass fiber, how is that a good thing? Clearly the glass fiber bondline will act as a crack stopper in terms of Griffith factor, but there other tried and tested means to stop such cracks since the Comet investigations of the 50's, however, you have also impacted the modulus significantly downwards, which can lead to other issues. Taking the argument to an illogical extreme, why not just use glass fiber alone and eliminate the problem. of course the modulus goes very low for buckling, but we seem to be heading that way by having the aluminum crack just due to thermal cycling. Also, we would have a lower density in all glass, which is a plus. But if we are worried re low modulus, well, add in some CF and get it up again and have far lower density, no corrosion and lower CTE differential problems. Then we can make it all CF/epoxy and have higher performance with lower density. Frankly, it seems to me that oft times we have materials looking for a usage rather than materials properly designed for a usage.
Materials that crack in service, frankly, are always of concern, even if materials are claimed to fail benignly. If cracks then moisture concerns and so, I think, we have ended up with a complicated hybrid material far more expensive than aluminum, but still with a high density vis a vis CF/epoxy or glass/ CF hybrid. GLARE may help re burn-through and visible denting, I agree,but still has FST issues re epoxy usage and questions of moisture and icing damage, bondline issues and loss of buckling capability also arise. Rubberised adhesives as used have their own problems in my experience and introduce performance issues also. I have strong doubts re efficacy and utility in long term of hybrid materials where it is admitted cracks from thermal cycling. I am no fan of aluminum, but at least it is cheap, well developed, well understood and repairable. Using alumunum in a vestigal and in terms of CTE overloaded manner as in GLARE makes little sense to me. Even if flexible epoxy bondlines don't fail over thirty years of in-service, a huge leap of faith to my mind, the question still remains why build a fuselage from a material which it is now admitted cracks due to CTE issues? It seems the wrong solution to the problem. GLARE has dent resistance, but still has FST issues, a lower in-plane shear strength than all aluminum, a lower bearing strength, probably lower buckling strength, a major CTE cracking issue, moisture issues and the list goes on. CF/epoxy has lots of issues of its own re FST, impact damage and repair, but I think GLARE is no better off. Complexity seldom wins in engineering, Chris.
All thanks again, Chris, for your kind and detailed input, but in summary, the CTE Achilles heel remains for me re GLARE and even more so for ARALL, which has fire and higher CTE issues. Your mullings and excellent insights and thoughts are awaited as ever. Both Airbus and Boeing have major challenges regarding their usage of new and comparatively unproven materials in a commercial aircraft environment, I wish them both luck, but I hope all issues are addressed prior to certification and service.
All best wishes and regards.
Had internal note today that GLARE is not working on A-380 as expected, so I am feeling somewhat vindicated at this point re CTE problems of GLARE and other issues as predicted and discussed. Next, I predict will be Boeing 787 center wing box mixed AL and CF issues and fuselage FST issues for Boeing. Please note that I am staying neutral re makers as both have their significant problems. Maybe engineers will do their physics first in future, he added.
There really is so much pontification about the materials of choice for the construction of the Boeing and Airbus products!!
Cant you guys(or gals) accept that with billions of dollars of shareholders money at stake that both manufacturers have considered all your learned discourse, and probably a lot more beside before choosing the paths they have adopted?
It is HIGHLY UNLIKELY that either manufacturer is going to end up compromised by their choice of materials.
If I had a criticism, I suggest that Boeing may have underestimated the downtime likely from ground handling damage, but time will tell.
In the interim, I gave up tertiary education far too long ago to agonize over issues that a lot of very qualified people have put their careers on the line over in Seattle and Toulouse.
LEAVE IT TO THE EXPERTS!
Cheers
Achace
Cant you guys(or gals) accept that with billions of dollars of shareholders money at stake that both manufacturers have considered all your learned discourse, and probably a lot more beside before choosing the paths they have adopted?
It is HIGHLY UNLIKELY that either manufacturer is going to end up compromised by their choice of materials.
If I had a criticism, I suggest that Boeing may have underestimated the downtime likely from ground handling damage, but time will tell.
In the interim, I gave up tertiary education far too long ago to agonize over issues that a lot of very qualified people have put their careers on the line over in Seattle and Toulouse.
LEAVE IT TO THE EXPERTS!
Cheers
Achace
Finally I find the time to respond in this topic again.....
As long as they sort out the difficulties which I'm convinced they'll do, and as long as the passengers won't suffer from any of those problems it's ok.
Kind Regards
Chris
Of course it's never a good thing to have a material which cracks. But Glare will have the bridging effect as explained earlier. So in fact we know the material will crack (quicker than pure Al) but we are in precense of a slow crack growth mechanism. This means that one can schedule and define a specific maintenance program in order to cope with the slow rack growth. On the contrary when you have pure Al, it's the crack initiation which is often taken into account instead of the crack growth since that one is more exponential then slow.misako wrote:If alumunum is placed in high tension via CTE, as is now agreed, and will crack just due to thermal cycling in service as stated in your teacher's response, do we have a decent operational material? If we have shifted and deleteriously impacted the S/N curve of the material re aluminum via imposing a high residual tensile load by bonding to glass fiber, how is that a good thing?Clearly the glass fiber bondline will act as a crack stopper in terms of Griffith factor, but there other tried and tested means to stop such cracks since the Comet investigations of the 50's, however, you have also impacted the modulus significantly downwards, which can lead to other issues.
Well we are still in the very early stages of all those studies and I'm sure much better composite or hybrid materials will developed in the coming years. It will be very interesting though to see Cf and Glare in use in civil aviation. More data will be gathered and it will certainly help research to confirm some theories....Taking the argument to an illogical extreme, why not just use glass fiber alone and eliminate the problem. of course the modulus goes very low for buckling, but we seem to be heading that way by having the aluminum crack just due to thermal cycling. Also, we would have a lower density in all glass, which is a plus. But if we are worried re low modulus, well, add in some CF and get it up again and have far lower density, no corrosion and lower CTE differential problems. Then we can make it all CF/epoxy and have higher performance with lower density. Frankly, it seems to me that oft times we have materials looking for a usage rather than materials properly designed for a usage.
I agree with you. It certainly is not optimal to use a hybrid such as glare. But for me one of the main advantages with respect to CF (for the moment) is "how easy" it is to repair and maintain the structure when it is in service with the airlines. Cf will e much more expensive (high tooling costs and personal training).Materials that crack in service, frankly, are always of concern, even if materials are claimed to fail benignly. If cracks then moisture concerns and so, I think, we have ended up with a complicated hybrid material far more expensive than aluminum, but still with a high density vis a vis CF/epoxy or glass/ CF hybrid. GLARE may help re burn-through and visible denting, I agree,but still has FST issues re epoxy usage and questions of moisture and icing damage, bondline issues and loss of buckling capability also arise. Rubberised adhesives as used have their own problems in my experience and introduce performance issues also. I have strong doubts re efficacy and utility in long term of hybrid materials where it is admitted cracks from thermal cycling. I am no fan of aluminum, but at least it is cheap, well developed, well understood and repairable.
Because studies have shown that eventhough you have small cracks the crack growth is slow, the structural strength doesn't go down (very little), very good fatigue properties, burn thorugh, damage tolerant, easy to repair etc... But indeed the corrosion facor you are raising with the small cracks is an interesting one.the question still remains why build a fuselage from a material which it is now admitted cracks due to CTE issues? It seems the wrong solution to the problem.
The problem here is that both Airbus and Boeing feel the pressure of the other one in my opinion. This means that they need to come out with "revolutionary" planes in order to take the advantage again. This is a very sain competition indeed and will eventually lead to better technology being used. However I have the feeling that both are so much under pressure that they will sometimes use technology which isn't 100% proven yet and hence they will encounter some difficulties.Complexity seldom wins in engineering, Chris.indeed, simplicity is the key word. And indeed Glare is rather complex. But personally I think that it's a nice "invention".
Like Beaucaire I wold be interested to hear what the problems are which they found.Had internal note today that GLARE is not working on A-380 as expected, .
Maybe engineers will do their physics first in future, he added.
As long as they sort out the difficulties which I'm convinced they'll do, and as long as the passengers won't suffer from any of those problems it's ok.
Kind Regards
Chris
Dear Chris and Beaucaire,
Thank you both for yours, they are much appreciated as ever. And Chris, I agree with many of your sentiments, but I worry re design release pressures and very tight schedules re the new technologies in commercial realm. The Military usually have a lots more flexibility and contractors are allowed time to work issues out (I would cite the V-22 here as a prime example having been in development for about twenty years and still having some problems), but in commercial aircraft world there are several aspects such as contractural penalties, tight customer schedules, airworthiness certifications and production ramp-up demands, which puts engineers under tremendous technical and financial pressures and time constraints during development.
And there is always the corporate financial drain aspects due to the heavy investments involved. Usually, companies end up "betting the farm" on major new programs regarding design and development investments and tooling and, for A-380 clearly, their customers are already unhappy and claiming very large compensation payments for delays which only adds to the problems and pushes out breakeven points and lowers ROI. Boeing's ability to meet customers' contractural delivery schedules is unknown to me at this point as later in development than A-380.
All of this pushes commercial towards more proven technolgies usually, but in recent years the marketing competition between Boeing and Airbus coupled with fuel prices, lower fares and customer demands have exacerbated the situation significantly.
Re GLARE specifically, Beaucaire and Chris, I do not yet know all specifics, but am told "No GLARE on A-350XWB as "not suitable" for that aircraft" which makes one pause and wonder re GLARE commercial applications in general at this point.
I like the repair aspects of GLARE, Chris, but I think a lot of technology development is still needed and it might end up that a straight highly improved composite is the best long term answer. I don't pretend to know all answers, but I am thinking current hybrids are not suitable for commercial yet.
With all best regards as ever to you both
Thank you both for yours, they are much appreciated as ever. And Chris, I agree with many of your sentiments, but I worry re design release pressures and very tight schedules re the new technologies in commercial realm. The Military usually have a lots more flexibility and contractors are allowed time to work issues out (I would cite the V-22 here as a prime example having been in development for about twenty years and still having some problems), but in commercial aircraft world there are several aspects such as contractural penalties, tight customer schedules, airworthiness certifications and production ramp-up demands, which puts engineers under tremendous technical and financial pressures and time constraints during development.
And there is always the corporate financial drain aspects due to the heavy investments involved. Usually, companies end up "betting the farm" on major new programs regarding design and development investments and tooling and, for A-380 clearly, their customers are already unhappy and claiming very large compensation payments for delays which only adds to the problems and pushes out breakeven points and lowers ROI. Boeing's ability to meet customers' contractural delivery schedules is unknown to me at this point as later in development than A-380.
All of this pushes commercial towards more proven technolgies usually, but in recent years the marketing competition between Boeing and Airbus coupled with fuel prices, lower fares and customer demands have exacerbated the situation significantly.
Re GLARE specifically, Beaucaire and Chris, I do not yet know all specifics, but am told "No GLARE on A-350XWB as "not suitable" for that aircraft" which makes one pause and wonder re GLARE commercial applications in general at this point.
I like the repair aspects of GLARE, Chris, but I think a lot of technology development is still needed and it might end up that a straight highly improved composite is the best long term answer. I don't pretend to know all answers, but I am thinking current hybrids are not suitable for commercial yet.
With all best regards as ever to you both