Fatigue crack closure assessment by wavelet transform of infrared thermography signals

• Crack closure within a load cycle induces variations in the temperature signal linked to thermoelastic coupling. • An automated method for the measurement of crack closure via the wavelet transform of thermal data is proposed. • The method enhances the detection of cracks compared to conventional first harmonic thermoelastic stress analysis. • The method is validated both via simulation and experiment on DH36 steel. The occurrence of crack closure significantly impacts the fatigue life of materials and structural components. Whether it is induced by the nature of the loading, the fabrication process or the geometry of the structure, its magnitude and effect should be considered to further improve predictive models of fatigue crack propagation. However, the definition of reliable experimental methods for the observation and assessment of fatigue crack closure, and in particular suited to structure testing, remains a challenge. The present study aims to provide a novel approach for the assessment of fatigue crack closure via the continuous wavelet transform of infrared thermography data. The processing of the temperature signal close to the crack in a coherent time–frequency space allows for the identification of crack closing and opening instants associated with high-frequency components. The method is meant to be suited to any testing configuration (conventional compact tension specimen or full-scale structures) with minimum operator-dependent parameters.