Originally Posted by
blakmax
The major issue with bonding in blades is with the methodology that establishes the tolerable defect limits for bonded joints. Typically a damage tolerance analysis is undertaken and/or testing is undertaken to demonstrate that the structure can sustain the required loads in the presence of a defect of a certain size, and then NDT is used to demonstrate that all defects are smaller than that size. This valid for PRODUCTION defects, but may not be valid for defects that occur in service.
Production defects are typically voids where volatile products are trapped as the adhesive or resin cures. The defect will not transfer load, but the bond immediately adjacent to the defect will be sound and can transfer load. ANalysis is by assuming a gap in the bond, while testing is almost always based on the inclusion of a non-bonding item such as teflon.
Service disbonds are different. In most cases, service disbonds are caused by degradation of the interface between the adhesive/resin and the structure to which it is bonded. For metallic elements in bonded structures the mechanism is often due to hydration of surface oxides on the metal, for example Al2O3 will degrade to Al2O3.2H2O and in the process the bond will separate at the interface. The degradation is caused by hydration and the rate of degradation depends upon the local moisture level in the interface.
The issue with damage tolerance for service defects is that the degradation of the interface does not abruptly change at the edge of the disbond. There is a region ahead of the disbond where the local strength is dependent on the level of moisture and the degree of hydration of the oxide layer. Hence, the SIZE of the disbond does not accurately represent the extent of local bond strength reduction. Therefore, the use of artificial methods to represent the defect for analysis or testing will result in a possibly unconservative assessment of residual strength for the structure.
The solution to this dilemma is to prevent hydration of the metallic surface and that must be done during surface preparation during blade manufacture. The best test to demonstrate relative hydration resistance is the4 wedge test ASTM D3762, but please make sure you refer to the most recent version of that standard because earlier versions can be misleading as to what constitutes and acceptable bond performance.
Your dissertation comes off as somewhat alarmist and seems to overlook a few very pertinent points that are common in large OEM blade design:
1. Blade designs always include bond test coupon areas, typically within the molded assembly but outside the final machined EOP (as well as prior in the constituent detail parts), that contain all representative adhesive interfaces and laminate structures. Short beam shear, lap shear, block tension, and myriad other destructive tests are performed on these coupons to verify the bonding prep, process, and other specifications were met. It is at this point that defects like porosity and slick bonds (which you mention are undetectable by NDI, which is mostly true) are detected as a requirement prior to acceptance.
2. With respect to moisture susceptibility and "hydration" affecting bond lines, and the inability to reliably detect this, you comments on design allowables omits the fact that responsible manufacturers of composite bonded blades typically use a set of more stringent hot/wet allowables that are derived from destructive testing for precisely this sort of concern.