NDI of disbonds
Guys, the problem with NDI of bonding problems is not just limited to composites. Adhesive bonds depend on chemical reactions at the interface between materials and in some cases those chemical bonds are susceptible to degradation in service, usually by hydration at the interface. This occurs on composite and metal bond surfaces. NDI can ONLY detect defects which cause an air gap, and hence interupt transmission of sound waves. NDI can NOT detect interfaces which are degrading but have not yet separated. In other words, NDI can tell you if you have a disbond, but it can not tell you that you are about to have a disbond.
I did ask what the repair method was (and go no response) because if it involves injecting fresh adhesive, that will only fill the air gap so that NDI will pass the repair. It will NOT and can never restore bond strength because the surfaces are not chemically active and hence no chemical bonds can be formed.
My next question would be "what was the production method used to prepare the composite bond surface?" because if it relied solely on sacrificial peel plies (removed to produce a "clean" surface) then there is a potential for further problems. The surface must not only be clean, it must be chemically active, and removal of peel plies does not produce a chemically active surface. Worse yet, there are some peel plies which transfer release agent onto the bonding surface, hence causing contamination.
The real problem is that the FARs for certification of aircraft structures require testing for strength and fatigue, neither of which are the cause of bond degradation in service. It is actually possible to fully certify a structure which has a high potential for in service failure through bond degradation. And NDI will not find the disbond until it has occurred. The trick is to find the disbond before it reaches a critical size.
Now, can anyone tell me A: what is the repair method? and B: do they use peel plies during construction?
This is not an attack on this particular manufacturer. There are many well intentioned constructors out there who make the same mistakes.