Blade condition
I note the discussion about the condition of the blade. The principle objective of the AD to manage paint quality was to prevent undercutting of the adhesive by particle impact. If the paint condition was maintained, then there would be no undercutting of the adhesive, which it had been suggested caused the bond failures experienced prior to the AD.
My personal opinion as an adhesive bond failure forensics specialist is that the basis of the AD is bovine excrement and this photo of the blade proves it. While the paint on the leading edge has been eroded, the paint adjacent to the skin-to-spar bond is not degraded at all. If particle undercutting is the primary cause of these failures, the the paint adjacent to the bond would be absent and it is not. Hence the basis of the AD is to put it mildly CRAP! This blade has disbonded yet the paint adjacent to the bond line is intact, so there is absolutely no evidence of undercutting.
Next, let us examine what we can see of the bond failure. If you look carefully at the image you can see a hatched pattern on the surface of the bond failure. That is the "carrier cloth" a knitted material embedded in the film adhesive when it is manufactured. It is there to provide an ability to handle the adhesive film without the adhesive fragmenting as it is handled.
In terms of failure forensics this carrier cloth is important. There are three failure modes for adhesives. Cohesion failure is the strongest form, and the failure exhibits evidence of the carrier cloth because in the absence of other failure modes that is the plane of weakness because of the stress concentrations associated with the carrier cloth and that this is the plane with the least adhesive material.
The next failure mode is adhesion failure which occurs at the interface between the adhesive and the metal. This is the weakest form of failure and in extreme cases, failure can occur without any load at all. Failure is characterised by the adhesive being on one surface only at any location. Importantly there is no failure through the carrier cloth.
The last form of failure is by mixed-mode, which is a combination of adhesion and cohesion failure, and the strength of the bond depends on the proportion of strong cohesion failure and weak adhesion failure. Importantly, as the bond strength weakens, the locus of failure migrates from the carrier cloth towards the adhesive to metal interface.
Now the above discussions relate to shear failure. If the failure is by peel, then there will be an apparent failure which is a mixture of failure through the carrier cloth but also there will be evidence of failure at the interface but the failure will be within each cell of the carrier cloth. The failure is characterised by evidence of each cell of adhesive being peeled away from the surface, thus presenting the apparent adhesion failure.
Now, what the heck has this to do with this case? I see apparent evidence of the carrier cloth, so the failure of the skin to spar bond is not by adhesion or mixed-mode failure which I have seen in other failures of R44 blades. But if this is true cohesion failure which would expose the carrier cloth, then the bond should have been strong enough to sustain flight loads and (sorry TET) even moderate impact loads such as from contact with flora.
I believe that a closer examination would show peel failure characteristics. So what would cause peel failure? I strongly suspect that there has been weak adhesion or at least mixed-mode failure of the bond to the tip cap retainer fitting aft of the spar and that has caused the skin to peel because of the lack of support for torsional loads in the skin-to spar bond. I would then suspect that the process used to treat the fitting is deficient in providing longer term bond durability.
Now I would suggest that this is a theory and closer examination of the blade would be needed to confirm (or reject) my theory. The key is to examine the skin to tip fitting bond. Is there evidence of adhesion failure?
It is a real pity that the level of understanding of adhesive bond failure forensics appears to be so low within regulators and investigators and appears to be totally absent in manufacturers of bonded structures.
Regards
Blakmax