This defect is a disbond. I just hope that they were not "repaired" using injected liquid adhesive.
I have been involved in an R44 crash investigation which was determined to be most probably due to blade failure due to disbonding.
Let me explain what is happening and why the repair by injection is futile and dangerous. Adhesive bonds depend directly on chemical bonds formed at the interface between the adhesive and the metal and these bonds are formed at the time of manufacture. The bonds form between the adhesive primer material and the metal oxides and hydroxides formed during the preparation process prior to bonding at the factory. Many metal oxides have an affinity to form a hydrated oxide (for example Al2O3 forms AL2O3.2H2O). In this case the steels probably form ferric or ferrous oxides. The moisture comes from the fact that epoxy materials absorb water from the atmosphere which eventually will migrate to the interface as the epoxy saturates. When oxide hydration occurs the chemical bonds between the primer or adhesive and the metal will dissociate to enable the hydration to occur. That results directly in interfacial disbonding and when the disbond is large enough, it may be detected using the tap test.
The disbond near the blade tip (in fact anywhere) is a strong indication that the bond interface is degrading and without any doubt whatsoever the disbond will continue to propagate until bond failure would occur because the adhesive is water saturated and the interface is already hydrating. Despite the AD to maintain paint quality, paint only slows down moisture diffusion- it does not prevent it.
Now to the repair aspect. Adhesion reactions require that the surface to be bonded is not only clean, but must also be chemically reactive. For long-term bond durability (the heart of this current problem) there should also be a step incorporated which will provide resistance to hydration of the oxide layers.
It must be clearly understood that having a clean surface is a necessary but not a sufficient condition to enable adhesion to occur. In the case where disbonds are "repaired" by injecting liquid adhesive into the disbond, it is common to flush the disbond with solvent, dry it and inject fresh adhesive. Such a process can
neverprovide a chemically active surface so the adhesive simply does not bond to the old surface. All that happens is that the air gap caused by the disbond is filled so that it taps out OK. IT IS NOT BONDED.
If there are DERs out there signing off on these repairs then I suggest they read
http://www.adhesionassociates.com/pa...d%20Joints.doc
If Tiny J wants to read this then he may undestand why for this blade design, a disbond so small is really unacceptable.
Regards
Blakmax