Fatigue testing of wood up to one billion load cycles

Abstract

Solid birch wood (Betula pendula) was investigated with a newly developed ultrasonic resonance test set-up enabling accelerated fully reversed fatigue experiments of wood. Ultrasonic fatigue lifetimes were measured at a cycling frequency of about 20 kHz up to 109 load cycles. For comparison, servo-hydraulic fatigue tests at 50 Hz cycling frequency up to 5 × 106 were performed. Numbers of cycles to failure plotted against stress amplitudes determined at both frequencies show comparable slope and standard deviation in the regime of overlapping lifetimes. Birch does not show a fatigue limit. Failures still occurred up to 108 load cycles at stress amplitudes of 30 MPa corresponding to approximately 35% of the ultimate compressive strength. Working principles and control mode of the ultrasonic fatigue test set-up are presented. A modal analysis of excited resonance vibration of the employed load train has been performed by the finite-element-method. Furthermore, the process of determination of applied strain and stress amplitudes was verified by comparative measuring techniques using a contact extensometer.