Experimentally monitoring the growth of surface-breaking cracks using digital image correlation technique integrated with permanently installed low-cost ultrasonic arrays in fatigue tests

Monitoring crack growth is crucial for ensuring safety, assessing structural health, optimising maintenance strategies and advancing scientific knowledge in the field of structural engineering. It enables cost-effective asset management, proactive maintenance and the growth of innovative solutions for a safer and more sustainable built environment. In this paper, we explore various techniques to experimentally monitor the growth of surface-breaking cracks on a single edge notch Beam (SENB) specimen in four-point bending fatigue tests according to American Society for Testing and Materials (ASTM) standard E647-15. The notch was generated using electrical discharge machining (EDM). In the measurements, a 5 MHz low-cost ultrasonic array with 18 elements was permanently attached to the top surface of a structural steel (SS355) specimen. A clip gauge was installed around the EDM notch mouth to measure the crack mouth opening displacement (CMOD). Additionally, a digital camera was focused on the tip of the EDM notch to measure surface displacements using digital image correlation (DIC). During the four-point bending fatigue tests, experimental data sets were recorded from the ultrasonic array, the clip gauge and the digital camera at each loading cycle. We focused on the development of post-processing techniques for measuring crack length and CMOD using the acquired ultrasonic array datasets and the measured surface displacement and strain distributions from the DIC technique, as well as low-cost ultrasonic array design, fabrication and installation. The proposed post-processing techniques were validated by comparing the results obtained with calculations using the standard procedure in ASTM E1820 based on CMOD values acquired from clip gauge measurements. We demonstrated that both the low-cost ultrasonic array and DIC exhibit reasonable and reliable performance in detecting and monitoring crack growth. The developed post-processing techniques proved effective in accurately measuring key parameters of crack growth, such as crack length and CMOD. These findings validate the feasibility and applicability of using low-cost ultrasonic arrays and the associated post-processing techniques for crack monitoring and characterisation and for supporting structural integrity assessments.