Effects of adhesive type on the mechanical properties of adhesive joints between polyurethane top coats and polyurethane-based adhesives after accelerated atmospheric ageing

The performance of a sealed stainless steel bracket system, adhesively bonded with two polyurethane-based adhesives (flexible adhesive, stiff adhesive) onto a marine polyurethane top coat, is investigated. The investigated joint connections on the coatings exhibited a high mechanical stability even after accelerated ageing (salt spray exposure, cyclic temperature variations, high relative humidity). The aged joints retained up to 81% of their tensile strength and up to 92% of their lap-shear strength. The torsional rigidity of the adhesive joint exceeded the required clamping torque of the designed bolt. A cyclical lifetime of >9·10⁶ load cycles was estimated. Effects of cyclic ageing on the creep performance of the adhesive joint were found to be insignificant. Under tensile loads, the joints with stiff adhesive material exhibited a linear-elastic performance without the capability to deform prior to failure. The joints with the flexible adhesive material, in contrast, exhibited a behavior typically for ductile materials featuring a pronounced yield plateau prior to failure. Failure loads were higher, and displacement was notably lower, for the joints with stiff adhesive material. All joint connections met the requirements for a safety factor for the design for marine applications (11.6 for the flexible adhesive; 14.3 for the stiff adhesive). When the flexible adhesive is applied, the cohesive strength of the adhesive material is the limiting design factor. When the stiff adhesive is utilized, the load carrying capacity is higher, and the interface between priming coat and steel substrate is the limiting design area of the joints. The strength utilization of the adhesive materials depended on the particular structure of the entire joint configuration, particularly on the interface between steel substrate and the coating material. A number of assessment factors, namely stress limit factor, coating adhesion factor and safety factor, are introduced and discussed for tensile and shear loads.