787 Composites

[blakmax]

Brian Abraham requested that I start a new thread after discussions about the AW139 which lost its tail taxiing in DOH (see Rotorheads forum).

As far as the 787 composite structure is concerned, I am a strong supporter of composites. However, they will fix some problems such as fatigue but create their own peculiar set of materials driven problems. The toughened resin systems used for 787 appear to have addressed the very poor impact resistance in older composite structures. But bolted joints in composites remain a real concern, and 787 uses them every where. The problem is that shear-out strength of composites is exceptionally low compared to metals. Further, the bearing strength of composites is so much lower than the bearing and bending strength of fasteners that the design is bounded purely by the properties of the composite.

My concerns relate to the Boeing proposed repairs. They are mechanically fastened so that Boeing can "dumb-down" the SRM so that Blogslovia Airways can perform repairs. The penalty on parts replacement and overall airworthiness will be paid by more competent operators with reliable bonding shops. Apart from thick composite structures, adhesive bonding still remains the most efficient way to repair damage.

There are differences in rules about edge distance, fastener row separation and fastener pitch which may trap the unwary repair designer who uses the rules applicable to metallic structure.

However, I have every confidence in Boeing's metal bonding processes (at least Boeing Commercial; Boeing's processes on C-17 are alarming!). They are at least aware of the issue of bond durability and the work of Kay Blohowiak (in conjunction with USAF Wright Patterson Lab's Jim Mazza) on the sol-gel process is world leading. Pity C-17 still uses the totally discredited Pasa-jel process.

I am unaware of proposals to repair delaminations and disbonds. If they propose injection repairs, then they need to talk to Walt Disney so that they can suspend reality. Injection repairs for disbonds can never and will never do anything other than fill the air gap so that NDI can not find the defect. There is zero chance of actually restoring any strength. With regard to injection repair for delaminations they should talk to Boeing STL. They at least admit that injection repairs do not restore pre-damage strength and at best slow down delamination propagation. Hence injection repairs can not be considered a permanent repair that restores the certification basis for the aircraft.

I'd like to extend the discussion not just to composites but to other aspects of adhesive bonded repair, especially hot-bonding technology. Is there enough interest out there in pprune land for me to spend my time doing that? Are the Moderators happy with locating such discussions in this thread? Please respond.

Regards

blakmax