Hi SLB
Firstly, thanks for confirming that these are the -7 blades.
One of the changes that was made for the -7 blades was to extend the overlap length substantially by extending the skins right up to near the leading edge, so that should give a bit more damage tolerance than the short overlap -5 blades. That is a significant improvement.
The down side is that they have opted for forming the skins by bonding three layers of skin as the blade is formed (four in the regions where the balance weight access slots exist). Now while that should also enhance the fracture toughness of the structure, but also dramatically increases the use of adhesive bonds so the design also increases the chance of bond defects because there is now three times the amount of adhesive being used, and adds a further three times as much interface to be reliably treated.
The fundamental issue with management of bonded structures is that currently the regulations require demonstration of static strength, fatigue resistance and damage tolerance, but there is no regulatory requirement to demonstrate bond durability. In other words, show that the bond is strong enough and fatigue resistance, and that it can tolerate bond defects at the time of manufacture,
but there is no actual requirement to demonstrate that the bond will maintain its strength for the life of the structure. The other significant deficiency is that the way damage tolerance is applied directly implies that the bond surrounding the defect maintains pristine strength, and it can be demonstrated that this fundamental assumption only applies to large voids which occur in production.
For porosity in production or disbonds which occur in service, the assumption that the adjacent bond is pristine is invalid.
See
http://www.adhesionassociates.com/pa...ES%20final.pdf
I can assure you that as a direct result of the investigation of the crash of DQ-IHE the FAA composites people now considers adhesive bond durability to be its most important issue (reported at an FAA meeting on bonded structures in Salt Lake City July 2014). However, whatever the outcomes of their efforts, they can not apply the findings and changes retrospectively, so the current structures will continue to carry risks.
I have personally been attempting to increase the level of understanding within the European and FAA communities by conducting courses on Adhesive Bond Failure Analysis and Prevention, but it is hard to get past the OEM "we know everything" attitude, a lack of understanding of adhesive bond issues for the on-site Designated Engineering Representatives and a lack of understanding for the failure forensics for adhesive bonds within the crash investigation community. Essentially the deficiency in failure forensics competency means that there is no positive mechanism to close the loop which should correct deficient design and production practices.
There are also a number of mantra trotted out by manufacturers when an event occurs. For example I am prepared to wager a good bottle of scotch that the OEM in the current blade case will state that the disbond was caused by corrosion. I can assure you that it is exactly the opposite. Adhesive bonds depend directly on chemical bonds at the interface. For corrosion to occur, the chemical bonds formed at the time of adhesive cure must dissociate for the surface oxides to hydrate as part of the corrosion process.
Therefore, the disbond must occur before the corrosion can commence. The disbond is not caused by the corrosion, the corrosion occurs after the disbond occurs.
Regards
Blakmax