Any composites experts out there?

[Oilhead]

I am interested to learn a bit about composite structure and behavior. Is damage easily detectable, and is it cumulative? I fly a composite bird, but really don't know very much about it, except it is supposed to be strong, light and expensive to manufacture.

Thanks!

[Genghis the Engineer]

I won't claim huge expertise, but I've done a few courses on the subject. At least I can start things off, then the real experts can elaborate on the interesting bits.

A composite material is defined as one made up of more than one basic material, they're used in various places - even wood, technically, is a composite (of cellulose and lignin). The most common aerospace composite is FRP or fibre-reinforced-plastic, which covers a fairly wide range of materials. there are others, such as MMC - metal matrix composites, which are starting to be found in some of the more complex parts of engines. Re-inforced concrete is a composite material too, so they're nothing new.

An FRP consists of two parts, a fibrous element, which might be glass, carbon, kevlar (which is really a high strength nylon) or some other more exotic material. Older FRP structures used a random array of fibres which were overly heavy and bulky but had the advantage of giving the same strength in every direction. Mostly now the fibres start as a woven mat, which is much more efficient, but doesn't give the same strength in every direction, which means that the designer has to understand both his material, and the loads it'll be exposed to, very well indeed. This is one of the reasons why the material is so expensive.
The other part is the matrix, which is a plastic surrounding the fibres, and can be either a thermoplastic (one which will melt in heat) or far more likely a thermoset (one which is originally set by heat and ultimately will burn rather than melt). This holds the fibres in position, and the real strength of the composite comes from the junction between the two parts, rather than either individual material.

Composite components can be made in virtually any shape and they can be very light because the designer only needs to provide strength where it's really needed. A metal component by comparison provides strength in every direction, rather than just where it's loaded - in other word there's excess weight there. But, if somebody else in the design team changes the load paths, you are potentially in trouble with a composite component; that's how the EH101 at Westlands lost it's tail rotor, causing the crew to learn that it is possible to parachute from a helicopter !
Failure modes are very complex. Unlike metal fatigue, which is about single crack growth, composites degrade through tiny cracks within the matrix, which reduce the stiffness and ultimately the failure loads. Also any impact starts cracking, leading to what's known as "BVID" or Barely Visible Impact Damage. BVID will cause a step reduction in material strength in the impact area. Frankly nobody understands these well, for which reason all the airworthiness codes require much higher strength from composite components than they do from metal ones - usually 25% to 50% more.

When a composite component does fail, it aint like a metal. It keeps appearing perfectly intact right up to the point of failure, then goes almost instantly - it's not like metal parts which start developing bends. If you remember high school physics terminology, the Hookes law period extends right up to the failure point, there's no plastic deformation.

Finally, there's one area often forgotten about. What happens if a composite burns? The answer is release of huge numbers of very damaging particulates into the surrounding atmosphere, I sat through an RAF fire service briefing on Eurofighter once, if one of those crashes and burns the firemen are going in in NBC suits to protect themselves from the tiny particles (not the toxic gasses, which are there but rather less serious).

Hope this helps a bit, if you're interested in materials in general there's an excellent book called "The new science of strong materials" by JE Gordon that I'd recommend. It's written for the non-engineer, and therefore blessedly free of both maths and molecular structure diagrams, which materials scientists usually seem overly fond of. Gordon does talk a fair bit about aircraft structures, although I'm not sure if he covers the most modern aircraft materials; my copy is about 20 years old.

G

[Charlie O]

My composite experience dates from observing similar failures in the 1950s. Interesting that little has changed. However, I can offer one possible explanation for the failure of the composite tail attachments on AA 587, appparently under "normal" stresses, i.e., wake turbulence: (This combines other reports which may be of interest.)

1) At time the A/C involved in the accident was delivered to American Airlines in 1988, a composite manufacturing fault was found. This consisted of a delamination or separation of the boron fiber bonded material in the attachment blade portion of the fork. Blades and forks connect the tail to the fuselage in six locations. A doubler composed of additional composite material was both bonded and riveted in place.

2) This aircraft suffered an in-flight encounter with wake turbulence in 1994, which was intense enough to shake the A/C to the point of injuring 47 passengers An inspection found no evidence of structural damage.

3)There is substantial evidence which suggests that internal voids between the boron fibers and bonding material may have resulted from the stresses encountered. These voids usually include microscopic cracks which vent into the surrounding air.

4) Any composite structure can have its strength reduced as the result of high altitude to low altitude operations. Any void, no matter how small, in which air might be enter, will eventually lead to material separation. This usually results from microscopic cracks created by prior operational stresses, which can be easily overlooked by inspectors.

5) When an A/C changes from low temperature, high altitude operation (cruise) to a low level, low altitude condition (landing), the low pressure inside the composite material with microscopic cracking will draw in air, which will accumulate water inside due to condensation created by the cold interior. With a return to high altitude, low temperature operation, the water will freeze inside and NOT be expelled on landing even after the water returns to a liquid state.

6) With repeated cycles of low pressure, low temperature operation to areas of high humidity and high pressure, water can accumulate inside a composite material which has voids in which air can enter. Eventually water will accumulate in these voids until its freezing will cause material separations (loss of strength) within a composite assembly. Enough of these microscopic (internal and unseen) separations and operational cycles may lead to complete failure of the composite assembly during "normal" operation.

7) [Another engineer's comment.] The lesson is obviously this: Composite structures (unlike fairings, fillets and other non load-bearing components) get their integrity and strength by being born of a singular process. A mixture of resins during in situ repairs just create weak-points that are always going to be vulnerabilities (as in this case). A glue sitting atop a glue will never make a good bond. When you incorporate a doubler and make that attachment point dissimilar and perhaps even sitting proud by a few mm, the likelihood is that it will be taking a disproportionately higher load (as well as being inherently weaker). That is not good. Thereafter whacking a few rivets through (as a salve to safety and QC) is really just a concession to your realization and admission that it is otherwise a weaker proposition than the other attachment points. The six attachment points have to be both equally strong and share the loads equally, otherwise you have created a path for progressive failure. Structural composites have to be 100% as born from their process -- or ditched and replaced. Repairs, in situ or otherwise, remain a bad idea. One has to wonder how many other "repairs" are out there which will eventually fail under "normal" flight operations.

[GotTheTshirt]

Ghengis,

It is interesting that I was at Dorniers during the eighties when composite was all the rage and they were making the SeaStar out of composites.
They of course had a large laboritory R&D group looking at all the possible types of composites and their usage.
They also had an equally large group trying to find ways of determining continuing structural integrity of these parts using Xray, ultrasonic, HFEC, LFEC and all other know magic. There never was anyone who could put his hand on heart and say - yes this is as good as the day it was made !!! :eek:

[ 16 November 2001: Message edited by: GotTheTshirt ]

[Genghis the Engineer]

Couldn't agree more. I don't think even now that problem's been cracked.

That's why we add the extra safety factors to composite structure. In certification circles it's referred to as "the composite superfactor".

G

[CarbonBrake]

Interesting topic !

I don’t want to start an expert discussion about composite science and technology but I would like to add some comments for some of your statements:

1) Genghis:

Your wrap-up about composites in general is excellent and indeed a composite material which is intrinsically anisotropic does not fail like isotropic metals. However, your statement that composites fail almost instantly compared to metals is simply wrong !
Composites can also fail in a non-catastrophic mode.
Composites also follow the linear elastic behavior until a certain stress and elongation, where tiny microcracks start to form in the matrix material (e.g. an amine cured epoxy resin like the MY720 resin from Ciba/Vantico with a DDS hardener). These microcracks start to form larger cracks within the composite but the component is still far from catastrophic failure. What happens ? The incorporated fibers (e.g. Carbon fibers) act initially as crack-stoppers and further progression of the cracks needs more energy and therefore more stress. Young’s Modulus of the composite is now lower due to the fact that new “inner surfaces” are generated by the cracks. Further loading the composite will lead to the initial failure of individual fiber filaments and yarns with toughening effects like fiber bridging, debonding from the matrix material and finally very important fiber pull-out effects. These effects give the composite a “quasi-plastic” failure behavior leading to non-catastropic failure. Indeed, this depends on the fiber-layout within the composite, i.e. unidirectional layout compared to a 0/90 or 0/45/90/45/0-layout.
Composite science is a very complex area and a lot of work has been done in the past twenty years to understand the micromechanical aspects of damage, both static and/or dynamic.
Reliability of composites, metals and especially ceramics is governed by the laws of crack growth covered in the science and technology of fracture mechanics, again both static and dynamic. Understanding these laws and applying them correctly, combined with the knowledge of environmental ageing is the basis for their use in the entire industry.
There is no doubt, that the knowledge base for composites is smaller compared to metals and may be Airbus has now a problem with the attachments which at the time of their design and manufacturing were proven to be correct. We will learn anyway a lot of new things out of this tragic accident. The knowledge base will increase quite a bit I guess and I am curious to see the outcome of the investigations.
And Genhis, to finally understand why materials are failing, you have to go into the details, e.g. micromechanics, fracture mechanics and environmental ageing.


One comment to your statement about toxic implications, when composites are burning. It is mostly the matrix part of the composite which can be problematic. For military applications high temperature BMI-resins (e.g. Matrimid from Ciba) are used. These can release harmful low molecular components with aromatic rings.
However, the tiny particles you’re talking about are always present when polymeric materials burn. Light your car and look what is happening with your composite parts inside and outside…(well I know, it depends also on the resins in the composites)
There are newer developments on the market like cyanate ester resins which have a high temperature capability and an excellent fire resistance. That’s why they are planned to be used as matrix materials for aircraft indoor panel applications.

2) Charlie O

Are you sure about the boron-fibers ? I know that epoxy/boron-fiber tapes are used for repair purposes. But Boron-fibers as reinforcing fibers for bulk composites in an Airbus-tail ?
The problem with boron-fibers to my knowledge is that the individual filaments are much thicker in diameter (140 microns) than e.g. carbon fibers ( 10 microns) and therefore composite parts with very small design radius are very difficult to make. Also cost-wise, boron-fibers are a drawback.

Your statement “A glue sitting atop a glue will never make a good bond” is wrong !
With our former partner company (Ciba/Vantico) we have proven that interfacial strength of epoxy to epoxy bonds are as good as the intrinsic strength of the epoxy materials itself…. But ONLY if the pretreatment of the bonding surfaces has been done appropriate !
If you ask 20 people what an appropriate clean bonding surface looks like, you get 21 different answers. Once again, we have the problem that most engineers do not understand that a good bonding is only successful with the appropriate pretreatment.

Repairs of composites are very complex and the risk of failure is very big. Composites are not very easy for maintenance staff and reparation cost can be very high. You need highly qualified maintenance staff and you have to pay them a decent salary because those people are rare on the market !

Finally, what about the Comet-accidents ? Using sharp cornered windows in a pressurized aircraft ? Did anyone draw the conclusion that aluminum is a bad material for aircrafts and turned back to balsa and wood ? Once again, initially lack of understanding of the combination of component design and fracture mechanics.

Please gentlemen, be realistic, this accident unfortunately might proove to be another step towards a safer aviation environment. There is no 100% safety out there.

Cheers

CarbonBrake


M.S. & Ph.D. in materials engineering ( polymer and ceramic matrix composites). Now enjoy flying as F/O with a big airline.

[Genghis the Engineer]

CarbonBrake,

Thanks for your comments. I'm intreagued by the quasi-plastic behaviour beyond the Hooke's law region that you refer to. Can you give me any references to that; I'd like to read up on it. I have to say that my own experience is that in a single event, it is fair to consider composites to display Hooke's law behaviour right up to failure.

In your references to Youngs modulus (and UTS) decreasing with damage, yes I agree. However, to quantify that is very hard and I've always regarded that as being the primary mechanism of composite fatigue, rather than the failure mode in a single overload event which was what I was referring to primarily.

G

[widgeon]

Does anybody remember how Rolls Royce nearly went bankrupt by a too early adoptions of composite materials ?.
The RB211 main fan blades were initially designed from Unidirectional graphite fibre . All was fine until they had the first bird strike test and the complete disc had a catastrophic failure . A very expensive lesson was learned . So the use of high strength composites is not really that new . Strength , fatigue and enviromental degradation factors are well established for the most commonly used materials which are based on resin systems developed in the early 70's. What still seems to be a "black art" is non destructive testing , and more importantly in service testing . Ultrasonic inspection still seems to rely to a great degree to expert interpretation of the results. ( we can see something unusual but we are not sure what it is ). Wheras an X ray or Zyglo inspection of a metal part in the most part gives results which are easily interpreted. There is not any new helicopter I know of that does not have composite blades and the safety record of these blades is impeccable , mainly because thay are made from low modulus glass which has a much more gradual failure mode than graphite. The other thing abour helicopter blades is that any change in stiffness will usually be detected by the pilot before it becomes a major problem , i am sure that in a fixed wing this is not the same case.