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

Abstract

The high-cycle fatigue behaviour of a timber-to-timber connection with self-tapping screws is examined with the fasteners under bending due to their alignment lateral to the load direction. The cyclic tests were carried out with a sinusoidal non-reversed load ($$R=0.1$$ R = 0.1 ) with a loading frequency of 5 Hz. The examined connection is designed for the quasistatic failure mechanism with two plastic hinges per shear plane according to Johansen’s theory (European Yield Model), which is mirrored in the observed fatigue failure. Based on 30 cyclic tests on four nominal stress levels $$\left( S=\left\{ 0.47,\;0.41,\;0.31,\;0.20 \right\} \right) $$ S = 0.47 , 0.41 , 0.31 , 0.20 in the finite-life regime the respective Wöhler-curve is obtained, showing high conformity with the test data due to a consideration of the specific density of the individual specimens. It is shown that the examined fasteners show a superior fatigue behaviour under bending compared to axial loading. A simple safe-side approach for the application of Wöhler-curves for axial loading of threaded fasteners to the present case of fastener bending is proposed, extending the field of possible applications for the results of existing and future studies of the behaviour under axial loading.

Estimation of the high-cycle fatigue behaviour of timber connections with inclined screws under tension

“…Lower stress levels led to failure in the fastener itself which has already been examined by Niebuhr and Sieder (2020) for the fastener used here. Beyond that, a distinct influence of the specimen density on the quasistatic capacity and hence on the stress level could be observed (analogous to the findings by Niebuhr and Sieder, 2021), which is why one nominal stress level was found to be sufficient to derive a viable Wöhler-curve for the limited stress-level span 0.75 ≤ S < 1.0. The maximum forces in the cyclic series were determined with the mean ultimate loads from the quasistatic series: F max = S • F ult,mean .…”

“…As described in Section 3.1 some bending in the fastener has to be assumed due to the inclination of the fastener with regard to the force direction. Niebuhr and Sieder (2021) showed that the performance of the examined fastener is superior if the axial fastener stresses are caused by bending rather than an axial force as is known for other threaded fasteners such as steel bolts (cf., e.g., Agatonovic, 1973;Schaumann and Marten, 2009). Hence, a safe side approximation of the behaviour under bending or combined loading is possible with S-N-curves determined with pure axial loading if the actual fastener stresses with consideration of bending are utilised.…”

“…The experiments and reflections in this contribution focus on the screw that has been examined by Niebuhr and Sieder (2020) and Niebuhr and Sieder (2021) (self-tapping, full-threaded carbon steel screw, d n = 6 mm, l = 120 mm). Thus, a holistic description and assessment of the fatigue behaviour of connections with this screw is provided, where individual phenomena and failure mechanisms can optimally be compared (e.g., transition between steel tensile failure and withdrawal, cf.…”

“…However, as that approach focuses on the estimation of the quasistatic ultimate load and relies on plasticity in the ultimate limit state it is not readily suitable to assess the axial forces in the fastener with smaller loads F ext ≪ F ext,ult under linear elastic conditions. Niebuhr and Sieder (2021) describe some reflections on this question for idealised dowel-type connections with α = 90 • .…”

“…( 7) neglects this phenomenon. Niebuhr and Sieder (2021) showed that a decrease of the embedment stiffness and thus an alteration of the location and magnitude of the bending in the fastener has to be assumed. Naturally, this effect also occurs with α ̸ = 90 • with larger impact as α approaches 90 • .…”