Accelerated degradation of composite adhesive bonds

Prasanthi Pothakamuri

Recent interest in reducing weight to improve fuel efficiency and incorporating composites in commercial aircraft had renewed interest in the behavior of adhesive bonding and their long term durability. This work considered an adhesive designed for moisture resistance (AF555) and an aerospace composite (Toray T899/39 00-2B). The aims of this study were to investigate the effect of moisture, accelerated degradation and surface preparation on adhesive bonds. The effects of moisture were considered by bonding dry and saturated adherends which were then immersed in water. Degradation was accelerated using constant temperature (-65,140,160 °F) and constant and fluctuating stress (45% to 90% of maximum load). Composite plaques were processed using three types of peel ply to consider surface preparation effects. Some surfaces were given subsequent abrasive treatment to consider benefits of secondary operations. The diffusion coefficients of the adherend and adhesive were found to be 0.00106 mm2/hr and 0.0044 mm2/hr respectively for the saturation levels of 1.25 and 3.0 for the adherend and adhesive respectively. Results of shear and fracture tests showed a decrease in bond strength with increasing moisture content. Examination of the fracture surfaces showed increased adherend failure with moisture content. Adherend shear tests (no adhesive) showed similar moisture sensitivity. The reduced bond strength was, therefore, attributed to the composite rather than the adhesive. The crack growth rate of fractured coupons under fluctuating load was much higher than similar coupons with a constant load of the same magnitude. Additionally, the bond strength of shear coupons under constant load was observed to decrease by 18% over 1000 hours. Those observations suggest that stress can be used to accelerate degradation. Peel ply was observed to have a large effect on the bond strength. Polyester peel ply provided the highest strength followed by nylon and finally by SRB (siloxane coated polyester) peel plies. The peel ply effect was apparently related to debris left on the bond surface. Creep rupture tests showed that degradation was proportional to the bond quality, so that the weaker bonds also degraded faster. Secondary abrasive techniques also had a measurable effect on bond quality. Sanding improved the fracture toughness by 8% while, grit blasted techniques decreased the fracture toughness by 22-50 %. The increase from sanding may be attributed to a cleaner and more uniform bond surface, while the decrease from grit blasting was shown to be due to surface erosion.

Accelerated degradation of composite adhesive bonds

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