A novel air-coupled ultrasonic technique for assessing the weathering degree of sandstone heritage

In the realm of stone cultural heritage weathering detection, both gel-coupled and non-coupled contact ultrasonic testing are prevalently employed. However, these techniques harbor inherent challenges, such as potential contamination from coupled and the risk of relic damage during contact ultrasonic testing. The advent of air-coupled ultrasonic testing presents a safer alternative for weathering detection of stone cultural heritage, though its stability and applicability in weathering degree detection require further validation. In this research, using sandstone samples from the Yungang Grottoes, we conducted simulated weathering tests under freeze-thaw cycles and acid soaking conditions. An air-coupled ultrasonic test system was set up, and the ultrasonic velocity of sandstone samples was calculated based on the obtained original waveforms. Meanwhile, the samples of sandstone with different weathering degrees were tested by gel-coupled, contact ultrasonic testing and many common non-destructive testing techniques; the value of ultrasonic wave velocity and its change trend with weathering process were analyzed by means of least square regression model and correlation analysis. The results revealed that the wave velocity dispersion of air-coupled ultrasonic in repeated detection is significantly lower than that of the other two traditional ultrasonic detection techniques; the obtained wave velocity is more consistent with the change trend of weathering degree, which has high applicability in the weathering degree detection of stone cultural heritage; the correlation with other non-destructive testing results is stronger. In summary, the air-coupled ultrasonic test technology can provide more accurate, safe, and effective weathering detection data, showcasing strong applicability and broad application prospects in the detection of stone cultural heritage, thus offering important support for their protection.