Acoustic inspection of structures

• US 6,581,466 B1
• Filed: 03/31/2000
• Issued: 06/24/2003
• Est. Priority Date: 04/02/1999
• Status: Expired due to Term

• Current Assignee: Mississippi State University
• Original Assignee: Mississippi State University
• Inventors: Henderson, Mark E., Costley, R. Daniel, Dion, Gary N.
• Primary Examiner(s): Larkin, Daniel S.
• Assistant Examiner(s): SAINT SURIN, JACQUES M
• Application Number:
  US09/539,553
• Time in Patent Office: 1,180 Days
• Field of Search: 735/84, 735/88, 736/39, 736/49, 738/03, 735/98, 735/99, 736/02, 736/40, 736/43, 736/00, 735/97
• US Class Current: 73/584
• CPC Class Codes: G01N 2291/0232 Glass, ceramics, concrete or stone
• CPC Class Codes: G01N 2291/0258 Structural degradation, e.g. fatigue of composites, ageing of oils
• CPC Class Codes: G01N 29/045 by imparting shocks to the workpiece and detecting the vibrations or the acoustic waves caused by the shocks measuring resonant frequency G01H13/00; measuring strength properties by application of mechanical stress G01N3/00
• CPC Class Codes: G01N 29/225 Supports, positioning or alignment in moving situation
• CPC Class Codes: G01N 29/42 by frequency filtering or by tuning to resonant frequency
• CPC Class Codes: G01N 29/46 by spectral analysis, e.g. Fourier analysis or wavelet analysis spectral signal processing per se G06F17/14

Abstract

A method for inspecting concrete structures includes the steps of exciting the structure to be inspected with a mechanical device such as a chain, sensing resulting vibrations with an acoustical sensor such as a microphone that is not physically coupled to the structure, processing the received signals to exclude signals with frequencies outside of response frequencies of interest corresponding to defects desired to be detected, and examining the received signals. In preferred embodiments, the mean square of the processed signals are compared to a threshold. Areas for which the total energy across the frequency bands of interest exceeds the threshold correspond to defects such as delaminations. The received signals are collected from various locations on the structure. In preferred embodiments, the location of the received signals is also maintained. Also provided is an apparatus for inspecting concrete structures that comprises a mobile platform, such as a wheeled cart or trailer, to which is mounted an excitation device such as a chain, a microphone for receiving acoustic signals from the structure, and a computer for analyzing signals from the microphone.

Claims

1. A method for detecting defects in a structure comprising the steps of:

exciting the structure by direct physical contact with the structure by moving an object across the structure;

receiving acoustical energy generated by the structure while the structure is being excited over a period of time with an acoustical sensor that is not physically coupled to the structure;

processing a signal received from the sensor over the period of time to remove frequencies that do not correspond to defects to be detected;

examining the processed signal to determine the presence of a defect; and

wherein defects in the structure are detected from acoustic waves resulting from vibration of the structure.

2. The method of claim 1, wherein the exciting step is performed by continuously exciting the structure.

3. The method of claim 1, wherein the exciting step is performed by exciting the structure with broadband noise.

4. The method of claim 1, wherein the exciting step is performed using an impulse load.

5. The method of claim 1, wherein the examining step includes the step of calculating the mean square of the signal.

6. The method of claim 5, wherein the mean square of the signal is compared to a threshold to determine the presence of a defect.

7. The method of claim 6, further comprising the step of displaying the mean square of the signal on a display device.

8. The method of claim 7, wherein the exciting, receiving, processing and examining steps are performed for a plurality of time periods, each of the time periods corresponding to different portions of the structure.

9. The method of claim 8, wherein the time periods are of equal length.

10. The method of claim 8, further comprising the step of displaying a graph of the processed energy, the graph having a first axis representing a position along a length of the structure and a second axis representing a position along a width of the structure.

11. The method of claim 10, wherein color is used to represent the mean square in a time period.

12. The method of claim 1, further comprising the step of transforming the acoustical energy from the time domain to the frequency domain.

13. The method of claim 12, wherein the transforming step is performed using a Fourier transform.

14. The method of claim 1, wherein frequencies below approximately one kilohertz are removed in the processing step.

15. The method of claim 1, wherein frequencies above approximately six kilohertz are removed in the processing step.

16. The method of claim 1, wherein the structure is concrete.

17. The method of claim 1, wherein the examining step is performed by a neural network that compares a signal from a known good structure to a signal from a structure being inspected, calculates a distance measurement between the signal from the known good structure and the structure being inspected, and compares the distance measurement to a threshold.

18. The method of claim 17, further comprising the step of calculating linear prediction coefficients from the signal from the structure being inspected, wherein the distance measurement is a measurement of a difference between linear prediction coefficients from the signal from the structure being inspected and linear prediction coefficients from the known good structure.

19. A method for inspecting a concrete structure comprising the steps of:

dragging a chain across the structure;

receiving with a microphone not physically coupled to the structure sounds generated by the structure while the chain is being dragged;

digitizing a signal from the microphone;

processing the digitized signal to remove frequencies outside a frequency range of interest;

calculating the mean square of the processed signal;

comparing the mean square of the processed signal to a threshold to determine whether a defect is present in the structure; and

wherein defects in the structure are detected from acoustic waves resulting from vibration of the structure.

20. An apparatus for detecting the presence of a defect in a structure, the apparatus comprising:

a mobile platform;

an excitation device mounted to the platform such that the excitation device excites the structure when the platform is in close proximity to the structure by moving an object across the structure;

a microphone mounted on the mobile platform such that the microphone is not physically coupled to the structure and is positioned to receive acoustical energy form the structure; and

a processor connected to the microphone, the processor being configured to process a signal received from the microphone over a period of time to remove frequencies that do not correspond to defects to be detected and compare the processed signal to a threshold;

wherein defects in the structure are detected from acoustic waves resulting from vibration of the structure.

21. The apparatus of claim 20, wherein the processor is further configured to transform the signal received from the microphone from the time domain to the frequency domain.

22. The apparatus of claim 21, wherein a Fourier transform is used to transform the signal received from the microphone.

23. The apparatus of claim 22, wherein the processor is further configured to calculate the mean square of the signal received over the period of time.

24. The apparatus of claim 20, further comprising a sound dampening box having an open end, the box being mounted on the mobile platform, the open end facing the structure, the microphone being mounted in the box.

25. The apparatus of claim 24, further comprising a wheel attached to the platform and an odometer attached to the wheel and connected to the processor for correlating the time period to a location on the structure.

26. The apparatus of claim 25, further comprising a display device connected to the processor, the processor being further configured to display the processed signal.

27. An apparatus for inspecting a concrete structure, the apparatus comprising:

a cart;

a sensor attached to the cart and connected to a processor;

a sound dampening box mounted to the cart, the sound dampening box having an open end, the sound dampening box being positioned such that the open end faces a surface of structure to be inspected when the cart is moved across the surface;

a plurality of chains attached to the cart, the chains being positioned to drag on the surface when the cart is moved across the surface;

a microphone mounted in the box and positioned such that the microphone is not in contact with the surface when the cart is moved across the surface;

an amplifier connected to the microphone;

an analog to digital converter connected to the amplifier;

a processor connected to the analog to digital converter, the processor being configured to perform the steps of processing the signal from the converter to remove frequencies outside of a frequency range of interest, calculating a mean square of the processed signal, comparing the mean square of the signal to a threshold to determine whether a defect is present, and correlating signal to a location on the surface using the sensor; and

a display connected to the processor for displaying a location of a defect;

wherein defects in the structure are detected from acoustic waves resulting from vibration of the structure.

28. The apparatus of claim 27, wherein the sensor is a global positioning sensor.

29. The apparatus of claim 27, wherein the sensor is an odometer.