Life prediction analysis of thick adhesive bond lines under variable amplitude fatigue loading

Wind turbine rotor blades are exposed to arbitrary wind loadings and consequently their adhesive bond lines along the blade span. Residual stresses, developed due to the manufacturing curing process, are superimposed as steady state components on the wind loadings, shifting the fatigue stress ratios apart from the external imposed load ratios. The prediction of the cohesive failure of the adhesive (transverse to the blade length) is of vital importance since it could propagate in the adjacent laminates, leading potentially into catastrophic failures. A prediction method is proposed regarding the crack initiation and is validated in the adhesive bond line of a generic structural element. The modular method is based on a Goodman diagram and a linear damage accumulation rule. Thus, a generic composite I-beam adhesive joint was designed and manufactured, mimicking the axial-to-shear stress ratio in the bond line between spar caps and shear web of a MW scale wind turbine blade. This was tested under asymmetric three point bending, under static and fatigue variable amplitude loading. The manufacturing residual stresses were calculated analytically based on raw experimental data.