Experimental investigation of the inelastic tensile behaviour of non-preloadable grade 8.8 bolts

Non-preloadable grade 8.8 bolts are widely adopted in European market of steel constructions. EN 1993:1-8 provides design rules for bolted connections based on simplified elastic perfectly plastic response of bolts, disregarding their ultimate deformation capacity. In case of seismic design of bolted connections, the bolt response is assumed to be unaffected by cyclic loading, even though the quantification of both ductility and low-cycle fatigue is essential to avoid brittle failure of bolted joints. In order to investigate these features, experimental monotonic and both variable and constant amplitude cyclic tests are carried out on non-preloadable grade 8.8 SB (Structural Bolting) assemblies considering three different diameters (i.e. 16, 20 and 24 mm). The results from monotonic tests enable to characterize the force-displacement monotonic response and ductility. The results from variable amplitude cyclic tests allow quantifying the strength degradation induced by cyclic actions, while the constant amplitude low-cycle fatigue tests enabled to investigate the fatigue capacity at different plastic strain and to determine both ε-N (i.e. strain amplitude-number of cycles to failure) and ε/εy-N (i.e. imposed ductility-number of cycles to failure) curves.