Brian,
I am well aware of AC 20-107B and I am also well aware of its limitations. Indeed AC 20-107 was updated from A status to B status as a direct result of A08_25_29_Recommendation.pdf and I can claim some credit for that work. Unfortunately the redraft of AC 20 107 only concentrated on adhesion (interfacial) failures. The real crux of the problem is not limited to Cohesion failure (fracture of the adhesive) OR Adhesion failures (pure interfacial failures). There is a third failure mode, which is mixed-mode failure, where there is a combination of adhesion and cohesion failure, adhesion where the interface has degraded, and cohesion where the remaining bond can not sustain the load so the adhesive fractures. It is the transitional zone which is of importance because that is where the interface is degrading but has not reached the extend of degradation that would result in adhesion failure.
Summary:
Cohesion failure is strong,
Adhesion failure is very weak (no strength?) and
mixed-mode is the transition from cohesion to adhesion where the strength is degrading but the level of strength loss depends totally upon the level of degradation. (For metals read Degradation as Hydration).
I say again, that the regulatory framework and AC20-107 if read in isolation beside the FARs still presents the situation where it is possible to certify an adhesive bond which has a definite probability of failure in later service. Static strength testing, fatigue testing and damage tolerance analysis simply do NOT interrogate the resistance of the interface to degradation. It is only when you read the FAA Policy Statement PS-ACE100-2005-10038 that you actually encounter the recommendation for the use of the wedge test to interrogate interfacial resistance.
Spar Web. Thanks for reading the paper. However, the major issue with Grandpa / Grandma's cup is not just contamination. It is almost exclusively driven by selection of a process which has absolutely no chance of survival in a real environment. Changing the adhesive simply changes the colour of the disbond, not the result.
I have since 1997 been advocating a Rule Change (see
However, I ama aware of the difficulty the FAA faces in even suggesting a rule change. I believe that the current preferred method for addressing this issue is to draft another AC which specifically addresses the issue of bond degradation.
I have published a number of papers on this topic at my web site. Google Adhesion Associates and you will find it. Follow through the resume until you find the list of papers.
I say again, if the issue of hydration is addressed and validated correctly, and if the bond is designed such that the adhesive bond is always stronger than the parent structure, then the "grandma's cup" syndrome has no place in a truly scientifically based bonded repair technology.
The real problem is to get over the grandma mind-set. For goodness sake in the latter part of my career I had to deal with a blinkered philistine who insisted that bonded repairs should never be applied over active cracks. The cracks must always be removed, because no aircraft is certified with known cracks. (OK, so Damage Tolerance assumes that the structure is cracked, doesn't it?) When presented with test data that showed that "stop drilled" cracks performed marginally worse than unmodified cracks, and samples where cracks were routed out produced the SHORTEST fatigue life, the blinkers were turned on and "but we don't certify structures with cracks" became the mantra. (I'll elaborate on the mechanisms behind this data if requested.)
So, if I can demonstrate that my bond is resistant to hydration, and I can demonstrate that my bond will always be stronger than the parent material, and I can demonstrate that if my bond is hydration resistant then this is the ONLY case where damage tolerance is valid, and my design precludes conditions where the adhesive stress levels lead to fatigue, then my bond should never fail under any circumstances.
So AC 20-107B and the FARs need to be amended to recognise these conditions, or there needs to be a RELIABLE AC to provide valid guidance in adhesive bonding technology such that Grandma or Grandpa never have to worry about their cup.
I am happy to elaborate on any point.
Regards
Blakmax