High-Cycle Fatigue Behavior of a Self-tapping Timber Screw Under Axial Tensile Loading

Abstract

The high-cycle fatigue behavior of a self-tapping timber screw is examined for axial tensile loading with R = 0.1. Based on 71 fatigue tests in the finite-life and long-life regime, Wöhler curves are derived for the failure modes thread area failure and head tear off. Comparisons with existing results of similar tests show equal behavior in the long-life regime and slight differences in the finite-life regime which cannot convincingly be attributed exclusively to either varying testing conditions or differences in the examined population. The fatigue verification according to EN 1993-1-9, detail category 50, is found to be highly conservative, whereas detail category 100 is shown to overestimate the observed fatigue behavior in the finitelife regime. Adding to existing observations on the fatigue behavior of a self-tapping screw, a first assessment of the decisiveness of geometrical and procedural differences in the screws can be made.

High-cycle fatigue behaviour of a timber-to-timber connection with self-tapping screws under lateral loading

“…In a sample with 23 specimens, the mean values of outer thread diameter, core diameter, flank inclination angle, head diameter and pitch have been determined as d = 5.96 mm, d c = 4.01 mm, = 39.59 • , d k = 8.15 mm, and p = 3.60 mm. The tensile strength of the material was determined as u,mean = 1236.9 N/mm 2 with CV u = 1.36% in a previous study (Niebuhr and Sieder 2020).…”

“…To allow for a comparison of the given test results in bending and the results in axial tension from an earlier study (Niebuhr and Sieder 2020), the axial bending stresses in the fasteners shall be assessed. The analytical determination of the cross-sectional screw properties is not trivial and has been described extensively by Ringhofer (2017).…”

“…For smaller stress levels the embedment stiffness is higher in relation to the bending stiffness of the fastener, which is assumed to be constant in the linear elastic regime. With increasing embedment stiffness, the location of the maximal bending moment moves closer to the shear plane, ultimately yielding the ideal bearing-type connection assumed for bolted steel connections, where no bending in the fastener is considered (Petersen 2013).…”

“…However, little is known about the behaviour of connections with these fasteners in the high-cycle fatigue regime, which is a decisive limit state in the design of the described structures. Previous studies on this subject are highly limited and consider either small conventional timber and furniture screws that are not suitable for high-performance structural engineering applications (Burmester and Hoffmann 1970;Trübswetter 1973;Bröker and Krause 1991), or consider the fastener itself under axial loading (Ringhofer 2017;Ringhofer et al 2019;Niebuhr and Sieder 2020) or connections with primarily axial loading of the fasteners (Stamatopoulos and Malo 2017).…”

“…A previous study (Niebuhr and Sieder 2020) that examined the same fasteners under axial loading will later be complemented by an ongoing examination of the withdrawal behaviour of the fasteners in spruce wood. That way, all possible failure mechanisms of connections with an inclined orientation of the fasteners have been examined, allowing for a full characterisation of these connections, which can then be compared to connections with lateral loading as examined in this contribution.…”