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.