This report makes interesting reading from an adhesive bond failure forensics aspect. Probably one of the more important factors is the suggested lack of bond between the core and the rear of the spar. This bond is formed using an expanding foam adhesive that fills part of the gap between the core and the vertical face of the spar. An inadequate bond there has significant influence over the blade behaviour. That foam distributes shear loads from the core into the spar. Normally, the shear loads are not great, mainly because the adhesive shear strength should be orders of magnitude greater than the shear strength of the core itself. However, if that bond is ineffective, then the shear loads can only be transferred through the upper and lower blade skins placing peel stresses on the adhesive bond between the skins and the upper and lower faces of the spar. This may over time lead to fatigue damage in the spar to skin bonds. Given that the observed disbond in this joint on the lower skin occurs at about the middle of the region where the lack of foaming adhesive bond is reported, then this may be a significant contributing factor in this event.
In defence of RHC, it is impossible to inspect the bond between the core and the rear of the spar, either immediately after production or in subsequent service. One consequence of an absence of this bond would be a localised loss of torsional stiffness over the defective bond length, but even that would be difficult to measure.
I am surprised that -5 blades are still in service. I personally would suggest that trying to squeeze the last few hundred hours of life out of these old blades is false economy.
Regards
Blakmax