MRB separation history
I have waited to respond to this thread until I saw some suggestions of MRB separation.
I am a specialist in adhesive bond failure forensics, having worked in the past as a scientist for the ADF. Prior to 2007 I was involved in a collaborative investigation with the NZ DTA into a severe cracking problem with one RNZAF UH1H MRB. I sincerely hope that this is not the case with this event.
The primary cause of the RNZAF occurrence was disbonding of the adhesive layer between the blade upper surface and the steel grip pad where the blade attachment fittings clamped onto the blade adjacent to the attachment pin. There were significant concerns in relation to how the adhesive bond design concept was developed. The grip pad is approximately 12mm thick and the end terminates at sharp angle of approximately 45 degrees. There is a common perception that adhesive bonds distribute load over the entire adhesive layer, with a uniform shear stress spread over the joint. Of the order of 76% of bonded structures in the US were found in 2004 to be designed on this basis. This has been known since 1938 to be incorrect. In an adhesive bond the shear stress peaks at the ends and all of the load is transferred just at the ends of the joint with no load whatsoever being transmitted in the centre of the joint. The larger the stiffness imbalance between the adherents, the higher the4 shear stress peak at the end of the joint. In the case of the grip pad bond, there is a very large and rapid increase in thickness and a large difference in elastic modulus (steel vs aluminium) meaning that the shear stresses at the end of the joint are very high.
In the case of the RNZAF blade, post event investigation showed that there was clear evidence of adhesive bond fatigue and environmental disbonding of the adhesive resulting in the entire bond failing as far back as the rotor attachment pin. The consequence of this disbonding was that all of the load that should have been transferred by the adhesive bond was now transferred by increased bearing loads at the attachment pin, and these increased bearing loads initiated fatigue cracking in the MRB. The cracking extended through about 90% of the D spar and extended well into the blade structure itself. It was only because of visual observations of the cracking by the flight crew in their walk around that disaster was avoided..
The RNZAF and ADF managed the problem by visual inspection of the paint layer at the outboard end of the bond. Any cracking in the paint was cause for further investigation. I am unaware of RNZAF findings from these observations but I am aware that a number of suspect blades were removed from service by the ADF and subjected to strip-down investigations that confirmed the presence of disbonds.
Like I have said, I just hope that this is not the cause of the Coffs Harbour incident.
Regards
Blakmax