Withdrawal behaviour of self-tapping timber screws under cyclic loads

Abstract While the quasistatic behaviour of self-tapping timber screws has been extensively studied, their fatigue behaviour is still not well understood. Consequently, no design rules are available. In this study, 170 quasistatic and 213 cyclic withdrawal tests were conducted to investigate the fatigue behaviour of three screw types at two angles-to-grain (= 90^, 45^) and two effective penetration lengths (l₄₅ = 10d, 15d). Most tests were carried out in CLT. The withdrawal results are compared with the tensile fatigue behaviour of the screws and with findings from previous studies. Considerable scatter was observed in the test data, but could be reduced by accounting for the influence of the specimen density. The fatigue behaviour relative to the short-term withdrawal strength does not differ significantly between the investigated screw types. Withdrawal failure is the dominant failure mode up to N = 10⁵ cycles at high stress levels relative to the short-term strength. At lower stress levels, and thus higher cycle counts, steel tensile failure becomes the critical failure mode. For design purposes, it is recommended to limit the stress level to S = 0. 7 to ensure failure occurs in steel tension, which is more predictable due to lower scatter.