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 syl vatica), incorporating typical scarf joints, were assessed under reversed l oading (R=-1). Scarf joints were found to be great equalizers of fatigue pe rformance for wood species with different static strengths. Poplar was inve stigated at several other R ratios ( +3, -3, -0.84 and 0.33). The applicati on of 95% survival probability limits derived from pooled data increases th e statistical reliability of sigma-N curves and gives an improved estimate of a material's minimum performance. The sigma-N curves derived for all th ree wood species at R=-1 were normalized with respect to ultimate compressi ve strength values and found to be practically coincidental. This allowed t he derivation of a master curve for a generic scarf-jointed wood laminate u nder reversed load conditions. This relationship was verified using data fr om the literature and found to be a good predictor of fatigue performance. The construction of simple triangulated constant life diagrams based on sta tic 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 c omposites under cyclic loading. (C) 1998 Kluwer Academic Publishers.