Fatigue properties of jointed wood composites: Part I. Statistical analysis, fatigue master curves and constant life diagrams

The primary aim of this work was to assess the fatigue performance of scarf-jointed laminated wood composites used to manufacture wind turbine blades and establish simple fatigue design procedures. Laminates made from poplar (Populus canadensis/serotina), Khaya (Khaya ivorensis) and beech (Fagus sylvatica), incorporating typical scarf joints, were assessed under reversed loading (R=-1). Scarf joints were found to be great equalizers of fatigue performance for wood species with different static strengths. Poplar was investigated at several other R ratios (+ 3, -3, -0.84 and 0.33). The application of 95% survival probability limits derived from pooled data increases the statistical reliability of σ-N curves and gives an improved estimate of a material's minimum performance. The σ-N curves derived for all three wood species at R=-1 were normalized with respect to ultimate compressive strength values and found to be practically coincidental. This allowed the derivation of a master curve for a generic scarf-jointed wood laminate under reversed load conditions. This relationship was verified using data from the literature and found to be a good predictor of fatigue performance. The construction of simple triangulated constant life diagrams based on static tensile and compressive tests and fatigue testing at R=-1 brings about a rapid assessment of the overall fatigue performance of any wood composite. These can then be used in the fatigue design or life prediction of wood composites under cyclic loading.