Abstract: Axial compression tests were conducted on high-strength concrete-filled thin-walled steel tubular short columns with three kinds of wall thickness combined with ultrasonic testing on the middle cross section of the column. The waveforms under different axial loads were analyzed through fast Fourier transform to obtain the frequency spectrum when ultrasonic waves passing through the concrete-filled steel tube. The damage process of concrete in the steel tube can be reflected by the waveform， the maximum amplitude and the frequency spectrum of the waveforms under different loads. The experiment results showed that the damage process of concrete under axial loading takes on a three-stage characteristic， which can be divided into the extension of initial cracks， the compaction due to the hooping effect and the failure of the concrete. The strain changing on the outer wall of the steel tube agrees with the damage process reflected by waveform and spectrum analysis. The ultrasonic testing method can be applied to accurately measure the damage condition of concrete in the steel tube under axial loading.

Key words: ultrasonic wave, thin-walled steel tube, high strength concrete, waveform, frequency spectrum, damage condition