Arnie
I have discussed this at length in other postings.
There are three types of bond failures: 1. Cohesion failures, where the adhesive material fractures, 2. Adhesion failures, where the bond fails at the interface between the adhesive and the substrate, and 3. Mixed-mode failures where there is a mixture of cohesion and adhesion failures, and mixed mode failures are only adhesion failures which occur before the interface has fully degraded.
Cohesion failures are high-strength failures and are usually related to design deficiencies and result in fracture through the middle of the adhesive layer. Now from my own observations the adhesive RHC use is almost certainly FM 73M and that adhesive has a "carrier cloth" incorporated during production of the film adhesive to aid handling. (Without it the adhesive fragments during handling.) If the failure is due to a design deficiency then the bond will fail by fracture through the plane of the carrier cloth. This type of failure requires a high load to cause failure and one would normally expect that such failures would be identified by certification testing. These are the bond failures you feel would be prevented by coning hinges etc.
In contrast adhesion failures occur at the interface between the adhesive and the adherends. They are low strength failures, and in some cases are ZERO strength failures. These have no relation to loads and are as a direct result of failure of the chemical bonds between the adhesive and the surface of the adherend. THEY CAN OCCUR AFTER NO FLIGHT LOADS WHATSOEVER. Google aair200302820_002 and look at Appendix B items 48 and 49 where disbonds were reported after ZERO flight hours. Ignore the findings of the report because the alleged "fatigue" in Figure 40 is not credible. Fatigue will result in "cohesion" failure and the example in Figure 40 shows "adhesion" failure. The features in Figure 40 are wrongly attributed. Fatigue can only result in cohesion failure which would occur through the plane of the carrier cloth and while the photo identifies features in the plane of the carrier cloth, the actual failure adjacent to the "features' occurs at the interface.
In the extreme, adhesion failures have ZERO shear or peel strength. PM me if you want further elaboration.
Mixed mode failure occurs when a bond which is susceptible to adhesion failure experiences a moderate (within the flight envelope) load before the interface has fully degraded, so there is some adhesion failure and some cohesion failure. Let me be clear. A mixed mode failure is a potential adhesion failure which experienced failure at normal flight loads before it had degraded to zero strength. At production and certification testing, it may and probably will exhibit adequate strength because the interfacial degradation mechanism described in the next para has not had time to manifest itself before testing occurred.
What drives adhesion and mixed-mode failures is the resistance of the interface to degradation and the most common form of degradation is hydration of the oxides on the surface of metals. To form the hydrated oxide, the chemical bonds formed during initial production dissociate so that the metal oxides can hydrate, and the tell-tale sign for this failure is that the bond fails leaving the adhesive on one surface at any given location. In mixed-mode failures, the locus of failure will migrate from the plane of the carrier cloth towards the interface.
Now, how does one control hydration? The surface preparation process used during manufacture is the sole determinant because this will establish resistance (or lack of resistance) to hydration.
The same is true for paint adhesion. The condition of protective paint, the frequency of washing, the buffing of surface finishes and flight loads can not of themselves induce or prevent interfacial or mixed-mode failures unless the surface was already predisposed to adhesion failure. Nor will such stupid specifications of the use of specific wash fluids or stipulations that the blades be washed by (tongue in cheek here) left handed virgins from Gybrobia.
Please read
http://www.adhesionassociates.com/pa...0Explained.pdf and
http://www.adhesionassociates.com/pa...d%20Joints.doc
These papers contain actual photographs of R44 blade failures which exhibit adhesion and mixed-mode failures. Of particular relevance, the latter paper suggests that the current method of managing blade flight safety based on regular inspection MAY be open to review and should be subject to further investigation. The paper suggests that if interfacial degradation occurs in short bond overlap joints (such as RHC R22 and R44 blades) then failure may occur because the bond strength decays below an acceptable level BEFORE a detectable defect can be determined by NDI, especially with methods with low accuracy such as tap testing.
So why do interfacial and mixed mode failures occur? Because there is currently no requirement to demonstrate long-term bond strength. The papers explain why current regulatory requirements are inadequate. To be fair to RHC they probably meet the current requirements.
Will someone please ask the CAAFI why the report on the crash of DQ-IHE has not been released after they have had the report from the IIC for more than two years.
Regards
Blakmax