Damage equivalent strain as a metric for tidal turbine blade fatigue scenario comparison

Highlights • Damage equivalent strain as a method of visualising strain and allowing comparison between different operating conditions. • Comparison of damage equivalent strain in a tank test and in a field test of a transverse axis crossflow tidal turbine. First of its kind study. • Ability to show that field turbulence has little or no impact on fatigue when compared to turbine rotations themself for a transverse axis crossflow tidal turbine. A highly significant finding for the operation of a crossflow tidal turbine. Tidal turbine blades are a key component for small, medium, and utility scale tidal turbines given their role as the power capture interface. Common practice is to instrument a turbine blade using strain gauges in order to monitor maximum loads as well as load cycles. The calculation of some fatigue parameters requires the conversion of strain to stress, however, this is not always trivial especially when material properties are highly directional or complex geometries are present. This paper presents a windowed approach using strain as a proxy for stress to enable fatigue scenario comparison. Strain recorded on a transverse axis crossflow turbine blade allows fatigue comparison between a range of scenarios during field testing as well as field-laboratory comparisons. It was found that turbine revolution frequency dominates the fatigue of a transverse axis crossflow turbine eclipsing the fatigue impact of dynamic flow features conducive to a real tidal environment in a way that would not be present for a horizontal axis turbine. This allows blade life to be monitored using a revolution counter. Simplified indicative fatigue comparisons enable rapid identification and mitigation of high damage scenarios beyond the example case presented in this paper facilitating the extension of turbine blade life.