FATIGUE IN SPRUCE LUMBER UNDER COMPRESSION PARALLEL TO GRAIN

摘要

The low-cycle fatigue (LCF) behaviour of spruce lumber in compression parallel to grain was investigated. Specimens were relatively knotty and grouped according to knot area. Tests employed a sinusoidal waveform, peak load of 0.9 short-term crushing load (F_(ult)), valley load of 0.1F_(ult), and loading frequency of 0.5Hz. The F_(ult) for each fatigue specimen was estimated based on behaviour of a matched control group. Fatigue tests were stopped if there had been no failure within 10 minutes (300 load cycles). Specimens that failed did so mainly due to formation of creases in clear wood regions adjacent to knots. Both cyclic creep deformation (CCD) and cyclic accumulated deformation (CAD) increased as load cycling progressed. Energy dissipation (hysteresis loop area H) was relatively stable except during initial load cycles and near to failure. Beyond the first 5% of the normalised fatigue life, residual stiffness reduced due to load cycling. Residual stiffness was found to be a better damage index than CCD, CAD or H.