Water ageing of offshore wind turbine blades

To optimise the economic and environmental performance of offshore wind turbines, it is necessary to know and predict the material health of the blades in order to extend their service life. When exposed to an aggressive environment, the materials constituting a complex structure such as a blade gradually degrade and become damaged. We focus here on the sandwich material used in the manufacture of the blade shell. During accelerated water ageing by immersion in seawater heated to 40°C, the material health of batches of test specimens is assessed at regular intervals using four-point mechanical bending tests with acoustic emission monitoring of internal damage. Over 88 weeks, the study reveals a three-phase evolution in damage resistance, correlated with acoustic emission signatures. Phase I: during water absorption until saturation, stiffness decreases by approximately 15%, while changes in the acoustic signatures reflect only modifications in acoustic impedance, related to water saturation of the material porosity, with no detected ageing. Phase II: although the mechanical properties remain stable, the acoustic signatures indicate ageing, with a weakening of interfacial strength. Phase III: structural failure is marked by a sudden drop in mechanical properties, and the acoustic signatures reveal a significant weakening of the material resistance to interfacial and volume damage. The main cause is associated with the polyester matrix, which undergoes plasticisation and hydrolysis.