Fatigue crack growth in elastomers for leading edge erosion protection of wind turbine blades

Abstract. Fatigue crack growth has been observed as a prominent damage mode in rain erosion of wind-turbine blades, where it is driven by cyclic pulse loading from liquid-droplet impacts. This study investigates fatigue crack growth in a thermoplastic polyurethane elastomer used for leading-edge protection, linking repeated droplet impacts to controlled cyclic loading in a lab test. The plane-strain tensile double-slit test method is employed to determine the actual tearing energy during fatigue crack growth. A new analysis technique evaluates tearing energy throughout the test by tracking strain energy evolution with crack length. A novel test fixture with circular grip faces was developed to ensure efficient gripping of polymer sheets. It is examined how dwell time (the interval between sinusoidal load pulses) affects fatigue crack growth per cycle, denoted as da/dN.

The material exhibits pronounced visco-elastic behavior, including cyclic stress softening. It may take several hundred cycles to stabilize with repeatable stress–strain loops, requiring a run-in period before crack growth assessment. Tests with shorter dwell times need more cycles to reach stabilization. Two dwell times are applied: 0.1 s and 1.0 s. Longer dwell times allow greater recovery between load pulses, reducing cyclic softening. When da/dN is plotted against peak strain, the cracks grow faster at longer dwell times. However, when plotted against tearing energy, the data collapses onto a single curve, indicating that tearing energy governs fatigue crack growth independently of dwell time. Measured crack growth rates span from 0.6 · 10⁻³ mm to 10 · 10⁻³ mm per cycle, while tearing energies below a threshold of approximately 2100 J/m² result in significantly lower growth values of 3 · 10⁻⁶ mm to 6 · 10⁻⁶ mm per cycle. This testing approach is novel for leading-edge protection materials, and crack growth resistance could become a key parameter in standards, material development, and erosion-safe turbine operation.