Detection and monitoring of changes in metallic structures using multimode acoustic signals

A method for detection and monitoring changes in an elongated metallic structure having a wall and an exterior surface, comprising:

• placing at least one acoustic transmitting transducer in vibrational communication with the exterior surface of said metallic structure;
• placing at least one receiving transducer in vibrational communication with the exterior surface of said metallic structure and spaced apart a chosen length from said at least one transmitting transducer;
• generating acoustic frequency chirp signals having a selected signal strength, spectral content, and duration;
• directing the chirp signals to said at least one transmitting transducer;
• producing a baseline signal by:
• receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer;
• averaging a chosen number of received vibrational signals;
• and
• removing any DC component from the averaged received signals;
• producing a monitoring signal by:
• receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer;
• averaging a chosen number of received vibrational signals;
• and
• removing any DC component from the averaged received signals;
• normalizing the monitoring signal to the baseline signal, whereby a maximum value of each of the baseline signal and the monitoring signal is equal to a selected value;
• taking the difference between the monitoring signal and the baseline signal;
• forming a difference signal;
• performing short-time Fourier transform of the difference signal using chosen time and frequency window sizes, and time steps, forming thereby a two-dimensional array in time and frequency;
• calculating the standard deviation of the short-time Fourier transform array along the time-axis for each frequency;
• and
• summing the standard deviations as a function of frequency.

A method for detection and monitoring changes in an elongated metallic structure having a wall and an exterior surface, comprising:

• placing at least one acoustic transmitting transducer in vibrational communication with the exterior surface of said metallic structure;
• placing at least one receiving transducer in vibration communication with the exterior surface of said metallic structure and spaced apart a chosen length from said at least one transmitting transducer;
• generating acoustic frequency chirp signals having a selected signal strength, spectral content, and duration;
• directing the chirp signals to said at least one transmitting transducer;
• producing a baseline signal by:
• receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer;
• averaging a chosen number of received vibrational signals;
• and
• removing any DC component from the averaged received signals;
• producing a monitoring signal by:
• receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer;
• averaging a chosen number of received vibrational signals;
• and
• removing any DC component from the averaged received signals;
• normalizing the monitoring signal to the baseline signal, whereby a maximum value of each of the baseline and the monitoring signals is equal to a selected value;
• performing short-time Fourier Transforms of the baseline and monitoring signals using chosen time and frequency window sizes, and time steps;
• taking the difference between the normalized monitoring signal and the normalized baseline signal, forming thereby a two-dimensional contour map;
• performing a two-dimensional auto-correlation on the two-dimensional contour map wherein the first dimension being time, and wherein the second dimension is frequency, whereby a two-dimensional auto-correlation map;
• identifying auto-correlation crests as crest-crest or trough-trough correlations, and auto-correlation troughs as crest-trough correlations in the two-dimensional auto-correlation map;
• creating a modified two-dimensional auto-correlation map using only negative auto-correlation troughs;
• integrating over the frequency dimension;
• and
• calculating the standard deviation over the time dimension from the modified two-dimensional auto-correlation map, thereby obtaining a number.

A method for detection and monitoring changes in an elongated setalliic structure having a wall and an exterior surface, comprising:

• placing at least one acoustic transmitting transducer in vibrational communication with the exterior surface of said metallic structure;
• placing at least one receiving transducer in vibrational communication with the exterior surface of said metallic structure and spaced apart a chosen length from said at least one transmitting transducer;
• generating acoustic frequency chirp signals having a selected signal strength, spectral content, and duration;
• directing the chirp signals to said at least one transmitting transducer;
• producing a baseline signal by:
• receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer;
• averaging a chosen number of received vibrational signals;
• and
• removing any DC component from the averaged received signals;
• producing a monitoring signal by:
• receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer;
• averaging a chosen number of received vibrational signals;
• and
• removing any DC component from the averaged received signals;
• normalizing the monitoring signal to the baseline signal, whereby a maximum value of each of the baseline signal and the monitoring signal is equal to a selected value;
• performing temperature compensation of the monitoring signal using the baseline signal as a comparison signal, thereby producing a temperature-compensated monitoring signal;
• performing short-time Fourier Transforms of the baseline and temperature-compensated monitoring signals using chosen time and frequency window sizes, and time steps;
• taking the difference between the normalized monitoring signal and the normalized baseline signal, forming thereby a two-dimensional contour map;
• identifying at least one frequency-time mode pair in the contour map indicative of at least one scattering event, where one feature of the at least one frequency-time mode pair has a maximum positive value and the corresponding feature of the at least one frequency-time mode pair has a maximum negative value;
• and
• calculating the amplitude difference between maximum positive value and the maximum negative value.

A method for detection and monitoring changes in an elongated metallic structure having a wall and an exterior surface, comprising:

• placing at least one acoustic transmitting transducer in vibrational communication with the exterior surface of said metallic structure;
• placing at least one receiving transducer in vibrational communication with the exterior surface of said metallic structure and spaced apart a chosen length from said at least one transmitting transducer;
• generating acoustic frequency chirp signals having a selected signal strength, spectral content, and duration;
• directing the chirp signals to said at least one transmitting transducer;
• producing a baseline signal by:
• receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer;
• averaging a chosen number of received vibrational signals;
• and
• removing any DC component from the averaged received signals;
• producing a monitoring signal by:
• receiving the vibrational signals generated in the wall of said metallic structure in response to the chirp signal by the receiving transducer;
• averaging a chosen number of received vibrational signals;
• and
• removing any DC component from the averaged received signals;
• normalizing the monitoring signal to the baseline signal, whereby a maximum value of each of the baseline and the monitoring signals is equal to a selected value;
• performing temperature compensation of the monitoring signal using the baseline signal as a comparison signal, thereby producing a temperatures compensated monitoring signal;
• taking the difference between the temperature-compensated monitoring signal and the baseline signal, forming a difference signal;
• performing short-time Fourier transform of the difference signal using chosen time and frequency window sizes, and time steps, forming thereby a two-dimensional array in time and frequency;
• calculating the standard deviation of the short-time Fourier transform array along the time-axis for each frequency;
• and
• summing the standard deviations as a function of frequency.