Method and system for monitoring structural damage

• US 20070034009A1
• Filed: 08/09/2005
• Published: 02/15/2007
• Est. Priority Date: 08/09/2005
• Status: Active Grant

• Current Assignee: The Boeing Co.
• Original Assignee: The Boeing Co.
• Inventors: Pado, Lawrence
• Granted Patent: US 7,426,447 B2
• Time in Patent Office: Days
• US Class Current: 73/579
• CPC Class Codes: G01N 2291/0258 Structural degradation, e.g. fatigue of composites, ageing of oils
• CPC Class Codes: G01N 2291/02827 Elastic parameters, strength or force
• CPC Class Codes: G01N 2291/2694 Wings or other aircraft parts
• CPC Class Codes: G01N 29/12 by measuring frequency or resonance of acoustic waves measuring frequency or resonant frequency of mechanical vibrations or acoustic waves in general G01H1/06, G01H3/04, G01H13/00; acoustic resonators G10K11/04; vibration or shock testing of structures G01M7/00
• CPC Class Codes: G01N 29/348 with frequency characteristics, e.g. single frequency signals, chirp signals measuring frequency of mechanical vibrations or acoustic waves in general G01H1/06, G01H3/04; measuring frequency or analysing frequency spectra G01R23/00
• CPC Class Codes: G01N 29/42 by frequency filtering or by tuning to resonant frequency

Abstract

A method for monitoring damage to a structure having an actuator and a sensor. The method includes exciting the actuator across a predetermined frequency range to excite the structure, measuring a vibrational characteristic of the structure across the predetermined frequency range in response to the excitation of the actuator using the sensor, calculating a transfer function for the actuator and the sensor using the measured vibrational characteristic, determining a change in the vibrational characteristic across the predetermined frequency range using the transfer function, and analyzing the determined change in the vibrational characteristic across the predetermined frequency range to facilitate determining whether the structure is damaged.

Claims

1. A method for monitoring damage to a structure having an actuator and a sensor, said method comprising:

exciting the actuator across a predetermined frequency range to excite the structure;

measuring a vibrational characteristic of the structure across the predetermined frequency range in response to the excitation of the actuator using the sensor;

calculating a transfer function for the sensor using the measured vibrational characteristic;

determining a change in the vibrational characteristic across the predetermined frequency range using the transfer function; and

analyzing the determined change in the vibrational characteristic across the predetermined frequency range to facilitate determining whether the structure is damaged.

2. A method in accordance with claim 1 wherein the step of analyzing the determined change includes determining whether the change is nonlinear, thereby indicating the structure is damaged.

3. A method in accordance with claim 1 wherein the structure includes a plurality of actuators and a plurality of sensors, and wherein said step of measuring the vibrational characteristic comprises measuring a vibrational characteristic at each sensor, said step of calculating the transfer function comprises calculating a transfer function for each actuator/sensor pair using the measured vibrational characteristics, said step of determining the change comprises determining a change in each calculated transfer function, and said step of analyzing the determined change comprises determining whether the change in each transfer function is nonlinear, thereby indicating the structure is damaged.

4. A method in accordance with claim 1 wherein said method is substantially unaffected by changes in structure temperature.

5. A method in accordance with claim 2 wherein the step of analyzing the determined change includes determining an amount of nonlinearity in the change and comparing the amount of nonlinearity to a predetermined threshold.

6. A method in accordance with claim 5 wherein the step of determining the amount of nonlinearity in the change is performed using:

Shift(Freq)=(Coef₁×

Freq²)+(Coef₂×

Freq)+Coef₃.

7. A method in accordance with claim 6 further comprising determining at least one of said Coef₁, said Coef₂, and said Coef₃ by:

choosing an initial value for at least one of Coef₁, Coef₂, and Coef₃;

shifting a sample transfer function based on at least one of the vibrational characteristic with respect to a baseline vibrational characteristic by adding shift hertz at each frequency interval across the predetermined frequency range;

interpolating a frequency interval of the shifted sample transfer function back into a frequency interval of the unshifted sample transfer function;

calculating a difference between the shifted sample transfer function and a baseline transfer function by summing a magnitude difference at each frequency interval across the predetermined frequency range; and

optimizing at least one of said Coef₁, said Coef₂, and said Coef₃ to facilitate reducing the difference between the shifted sample transfer function and the baseline transfer function.

8. A method in accordance with claim 6 wherein said comparing the determined the amount of nonlinearity comprises comparing Coef₁ to a threshold Coef₁.

9. A method in accordance with claim 8 further comprising determining whether the structure is damaged by assessing whether the Coef₁ is greater than the threshold Coef₁.

10. A method in accordance with claim 1 wherein the structure includes a plurality of actuators and a plurality of sensors, each of said sensors being paired with one of said actuators, wherein measuring the vibrational characteristic comprises measuring a vibrational characteristic at each sensor, and wherein said method further comprises:

ranking each of the actuator-sensor pairs based on an indication of damage for each pair based on the determined change in the vibrational characteristic of each pair; and

defining an area of the structure most likely to contain damage based on the pair rankings.

11. A method in accordance with claim 10 wherein defining an area of the structure comprises:

defining a first area of the structure extending perpendicular to a line drawn between the highest ranked pairs; and

defining a second area of the structure located at least partially within the first area based on the pair rankings.

12. A system for monitoring damage to a structure comprising:

an actuator operatively connected to the structure for exciting the structure;

a sensor operatively connected to the structure for measuring a vibrational characteristic of the structure in response to excitation of the structure; and

a processor operatively connected to the actuator and the sensor programmed to;

excite the actuator across a predetermined frequency range to excite the structure;

receive from the sensor a measured vibrational characteristic of the structure across the predetermined frequency range in response to the excitation of the actuator;

calculate a transfer function for the sensor using the measured vibrational characteristic;

determine a change in the vibrational characteristic across the predetermined frequency range using the transfer function; and

analyze the determined change in the vibrational characteristic across the predetermined frequency range to facilitate determining whether the structure is damaged.

13. A system in accordance with claim 12 the processor is programmed to determine whether the change is nonlinear, thereby indicating the structure is damaged.

14. A system in accordance with claim 12 wherein the processor is configured to determine an amount of nonlinearity in the change and compare the amount of nonlinearity to a predetermined threshold.

15. A system in accordance with claim 14 wherein the amount of nonlinearity in the change is determined using:

Shift(Freq)=(Coef₁×

Freq²)+(Coef₂×

Freq)+Coef₃.

16. A system in accordance with claim 15 wherein the processor is configured to determine at least one of said Coef₁ said Coef₂, and said Coef₃ by:

choosing an initial value for at least one of Coef₁ Coef₂, and Coef₃;

shifting a sample transfer function based on at least one of the vibrational characteristic with respect to a baseline vibrational characteristic by adding shift hertz at each frequency interval across the predetermined frequency range;

interpolating a frequency interval of the shifted sample transfer function back into a frequency interval of the unshifted sample transfer function;

calculating a difference between the shifted sample transfer function and a baseline transfer function by summing a magnitude difference at each frequency interval across the predetermined frequency range; and

optimizing at least one of said Coef₁, said Coef₂, and said Coef₃ to facilitate reducing the difference between the shifted sample transfer function and the baseline transfer function.

17. A system in accordance with claim 15 wherein the processor is configured to compare the determined the amount of nonlinearity comprises comparing Coef₁ to a threshold Coef₁.

18. A system in accordance with claim 17 wherein the processor is configured to determine whether the structure is damaged by assessing whether the Coef₁ is greater than the threshold Coef₁.

19. A system in accordance with claim 12 further comprising a plurality of actuators and a plurality of sensors, each of said sensors being paired with one of said actuators, wherein said processor is configured to:

rank each of the actuator-sensor pairs based on an indication of damage for each pair based on the determined change in the vibrational characteristic of each pair; and

define an area of the structure most likely to contain damage based on the pair rankings.

20. A system in accordance with claim 19 wherein the processor is configured to:

define a first area of the structure extending perpendicular to a line drawn between the highest ranked pairs; and

define a second area of the structure located at least partially within the first area based on the pair rankings.