Originally Posted by
SansAnhedral
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.
Sans…
The failure mode that blakmax describes is not addressed by the hot/wet environmental qualification testing you reference. The hydration mode is an adhesive failure mode at the metal/ adhesive primer interface, initiating at an edge and propagating along a line, creating a disbond. It is driven by exposure to moisture, calendar time, and to a lesser extent, loads.
As you know, the environmental qual tests that establish hot/wet design allowables for structural adhesives strive for a cohesive failure mode, to develop the full strength of the adhesive, with the presumption that the interfaces and adherends will always be stronger than the adhesive. If an adhesive mode failure mode is encountered in such tests, the data would be suspect and the failure surfaces carefully scrutinized. Of course, a failure could also indicate a fundamental problem with the adhesive/primer system, at least in a particular application.
Wedge tests can sometimes reveal the hydration mode, but the exposure time for the test, say up to a month, is orders of magnitude shorter than the years required to create a hydration mode in typical field usage .
To address your concern, the degradation does not occur in a bulk or volumetric sense. It requires an exposed edge that allows moisture ingress, so it is more of a localized and linear initiation. The resulting disbond is detectable by traditional NDI methods, presuming of course it is accessible and is of sufficient size. Propagation is slow, so it can be managed by periodic maintenance.
DS