Absorption of moisture in composites of Navy and Air Force aircraft can cause significant dam-age in two ways:

The first mechanism will greatly weaken the composite structure. In at least two cases, rudders on the F/A-18 have failed in-flight with loss of 80-90% of one of the rudders. Failure analysis of the remaining section indicated extensive honeycomb corrosion near the mid-hinge point due to moisture ingress via the grounding terminal. Fillet bond failure between the core and the graphite/epoxy skin leading to skin delamination was also noted in the lower recovered section [23].

In another example, x-rays detected water in an F/A-18 rudder. The engineering investigation showed aluminum corrosion products in the drained fluid. Ultrasonic C-scans showed a large area of attenuation around the mid-hinge point. Porta Pull plugs from that area showed very low strengths with higher strengths in the surrounding areas. All plugs showed varying degrees of adhesive failure mode.

Moisture in the composite, in the adhesive between the composite and the core, or in the core itself can cause significant damage during repair when the moisture turns to steam and internal pressure can exceed bond strengths. The repair failures are more pronounced with the use of 350F-curing adhesives and have led to change orders authorizing the use of 250F-curing adhesives during Depot level composite repairs [24]. Figure 10 shows an example of such repair-induced damage. During the curing of the repair patch on the left, additional delamination, marked by the black line surrounding the patch, occurred as residual moisture turned into steam.

In-Situ Sensor to Detect Moisture Intrusion and Degradation of Coatings, Composites, and Adhesive Bonds, G.D. Davis, C.M. Dacres, and L.A. Krebs, DACCO SCI Inc.