Hybrid joints consisting of pre-tensioned bolts and a bonded connection—A comprehensive review part I: Local approach

Hybrid joints combining pre-tensioned bolts and adhesive bonding have been shown to have the potential to increase joint loading capacity for steel structures. In this paper, experimental and numerical research on hybrid joints has been reviewed with a focus on load transfer mechanisms. However, there is still no clear consensus on whether hybrid joints are an extension of a pre-tensioned bolted connection with an increased friction coefficient or adhesively bonded ones. This is due to the complexity of hybrid joints, which require knowledge from two different fields of engineering and can result in incomplete characterisation of adhesives and documentation of manufacturing and testing large-scale samples. The first part of the review paper discusses the experimental evidence gathered on small-scale samples (Napkin and off-axis tests), although not strictly resulting from observations directly made on hybrid joints. The authors that studied the σ-τ–interaction on both tensile and compressive normal stresses provided evidence of a clear distinction to be made between tensile and compressive normal stresses. For tensile normal stresses, there is a clear lower limit below which no shear strength exists, while compressive stresses significantly increase shear strength. However, there is no clear consensus regarding the exact nature of the interaction between compressive normal stresses and shear strength. Some authors assume a linear relationship involving some cohesive strength, while others identified bi-linear or more complex relationships. Adhesives do not exhibit any noteworthy differences in the friction coefficient, and differences related to the failure mode were identified: cohesive failure is likely driven by the intrinsic strength of the adhesive, while adhesive failure can be modelled as a classic friction problem. Relating the aforesaid to the adhesive, or specific properties thereof, proved difficult, as most of the studies do not provide sufficient information. Additional complexity arises if tests are carried out on galvanised or coated surfaces, in which case the performance of the adhesive may be subordinated to that of the specific steel surface. Overall, the review paper provides valuable insights into the load transfer mechanisms of hybrid joints, but further research is necessary to fully understand their behaviour and to optimise their design for use in steel structures.