Environmental ageing of pretensioned hybrid joints

The load-path-dependent translation of adhesive-level ageing into structural response was quantified for pretensioned hybrid joints and matched bonded and bolted references. Two structural adhesives, a polyurethane (PUR) and an epoxy (EPX), were investigated on organically coated and hot-dip galvanised steel after climatic cycling and salt-spray exposure for up to 6 weeks. The integrated programme comprised 80 exposure-specific condition cells with five nominal replicates, corresponding to 400 matrix entries based on 70 physically distinct joint series and 350 joint specimens; it was complemented by 70 bulk tensile tests and 28 dynamic mechanical analyses. Bonded joints showed the strongest surface- and adhesive-dependent degradation. For EPX on galvanised steel under climatic exposure, the ultimate lap-shear strength decreased from 14.86 to 0.40 MPa and the fracture energy from 38.9 to 1.1 J, whereas the corresponding hybrid peak strength changed only from 22.42 to 22.55 MPa. In the coated PUR salt-spray hybrid series, peak strength changed from 16.04 to 19.91 MPa and energy from 55.3 to 141.0 J. Across the hybrid results, peak-strength retention did not imply unchanged initiation- or dissipation-related response. Bolted joints remained stable in peak response. The results therefore showed that the adhesive-level response measured under a corresponding exposure condition was translated differently according to the available load path, and no single bulk-to-joint relationship described all configurations. The measured joint-level trends were compatible with previous directly instrumented studies, while local mechanisms were not measured in the aged specimens. Preload evolution, specimen-specific bond-line thickness, local load sharing, and interface chemistry were not measured directly; the six-week exposures therefore represent accelerated early-stage ageing rather than service-life qualification.