Environmental Ageing of Pretensioned Hybrid Joints

The ageing response of pretensioned hybrid joints was investigated in comparison with bonded and bolted reference joints for two structural adhesives, namely a two-component polyurethane (PUR) and a two-component epoxy (EPX), using two substrate surface conditions, namely organically coated steel and hot-dip galvanised steel. Climatic cycling and salt-spray exposure were applied for up to 6 weeks. The experimental basis comprised 400 joint tests, supplemented by 70 bulk tensile tests and 28 DMA measurements. Across both adhesive systems and both surface conditions, the hybrid configuration exhibited the most stable ultimate response. Although ageing affected damage-initiation and energy-dissipation measures, the ultimate lap-shear strength generally remained close to the unaged reference level. By contrast, bonded joints showed the strongest surface- and adhesive-dependent degradation, whereas bolted joints remained mechanically stable. The clearest differencewas observed for epoxy joints on galvanised steel under climatic exposure, where the bondedconfiguration decreased from 14.86 to 0.40 MPa in ultimate lap-shear strength and from 38.9 to 1.1 J in fracture energy, whereas the corresponding hybrid joints remained essentially unchanged in peak strength, from 22.42 to 22.55 MPa. For both adhesives, the fracture-surface observations were consistent with the interpretation that hybrid joints retained mixedfailure and avoided the pronounced adhesive- or near-interfacial failure observed in the most severely degraded bonded subsets. Overall, the results suggested that pretensioned hybrid joints provided the most robust ageing tolerance among the investigated configurations. This behaviour was interpreted as evidence that the pretensioned fasteners provided a parallel, mechanically stabilised load path, such that degradation in the adhesive layer or at the adhesive–substrate interphase did not translate directly into a comparable loss of ultimate joint capacity. However, this interpretation was based on joint-level mechanical response and qualitative fracture observations rather than on direct measurements of local load sharing, crack arrest, bond-line thickness, or preload evolution.