Temperature and crack detection of steel rods using an all-optical photoacoustic ultrasound system

•Temperature and crack detection of steel rods using a fiber optical ultrasonic system.•Quantified temporal and spectral changes of ultrasonic signals using spectrograms.•Estimated a rod’s temperature using temporal changes in spectrograms.•Quantified cracks depth using spectral changes in spectrograms. Structural health monitoring (SHM) is crucial for extending service lives and preventing catastrophic failures of deteriorated infrastructures such as bridges, stadiums, and shopping centers from happening. An active all-optical photoacoustic ultrasound system (APUS) had been proposed. This paper investigates potential methods for measuring temperatures and detecting cracks of steel rods using ultrasonic waves generated and received by such APUS. In this study, radial displacements of ultrasonic waves were collected on the surface of steel rods. Short-time Fourier transform (STFT) was applied to obtain their spectrogram. Temperature measurement and crack detection were achieved by using the centroid of the spectrogram at the half-power level. Specifically, time-of-flight (TOF) of the centroid (tc) was used for temperature measurement. Frequency of the centroid (fc) was used for crack detection. It was found that, when temperature increases from 25 °C to 55 °C, tc reduces. Presence of crack was detected by the reduction of fc. Crack quantification was achieved by comparing the fc of damaged steel rods with the one of an intact steel rod. The reduction of fc becomes greater when the crack depth increases from 1 mm to 3 mm. Empirical equations were developed for measuring the temperature of steel rods and crack depths using tc and fc.