Short time Fourier transform analysis for understanding frequency dependent attenuation in austenitic stainless steel

Short time Fourier transform (STFT) has been used to study the distribution of ultrasonic energy as a function of the frequency of the wave in the backscatter and the back-wall echoes obtained from austenitic stainless steel specimens with different grain sizes in a range of 30–210 μm. A 25 MHz nominal frequency immersion transducer was used in pulse-echo mode for data acquisition. In specimens with larger grain sizes (>100 μm), the frequency content of the first back-wall echo was 4.5–7.0 MHz only whereas the predominant frequency of the backscatter was in a range of 15–25 MHz up to the third/fourth back-wall echo. The amplitude and the frequency content of the back-wall echoes decreased rapidly with the propagation distance, however those of the backscatter decreased very slowly indicating high scattering and low absorption rates in the austenitic stainless steel specimens. The decrease in the center frequency of the first, second and third back-wall echoes has been correlated with the average grain size. The study demonstrates the usefulness of STFT in analyzing the frequency content of the backscatter and back-wall echoes simultaneously and thus understanding the frequency dependent attenuation in high scattering materials for microstructural characterization applications.

► Short time Fourier transform (STFT) is used for microstructural characterization.
► Frequency dependent energy distribution in ultrasonic signal is visualized.
► Peak frequency of back-wall echoes decreases with increasing grain size.
► Frequency of back-scatter is higher than that of echoes for similar time path.
► Echoes submerged in back-scatter could be clearly extracted by STFT.