Forensic investigation of failure in a full-scale composite tidal turbine blade

Tidal turbine blades endure uniquely harsh, shear-dominated loads that distinguish them from wind turbines, yet their failure mechanisms remain poorly understood. In this study, we report the first full-scale failure investigation of a tidal blade made of glass fibre reinforced polymer (GFRP), carbon fibre reinforced polymer (CFRP) and cast iron. The blade was subjected to 26 static tests and 17 fatigue tests using a bespoke laboratory setup with three hydraulic actuators and saddle fixtures. Strain gauges, displacement transducers, and digital image correlation (DIC) were coupled to track local deformation and damage. Artificial defects accelerated damage progression, yielding roughly 97,000 fatigue cycles before ultimate failure. Two dominant failure modes emerged: de-bonding between the pressure skin and root connection under quasi-static loading at 216 % of design load, and bondline failure between pressure and suction skins under fatigue at 119 % of design load. A subsequent static test showed residual strength drop to only 68.8 % of the original design load. These findings highlight the critical role of bond interfaces in blade integrity and the contribution of internal ribs in maintaining structural performance. While local stiffness was affected by damage, global stiffness remained largely intact, reinforcing the importance of investigating long-term performance degradation and failure evolution in tidal turbine blades.