Measuring crack growth by acoustic emission

• US 6,062,083 A
• Filed: 01/29/1998
• Issued: 05/16/2000
• Est. Priority Date: 10/31/1995
• Status: Expired due to Fees

• Current Assignee: Score Atlanta, Inc.
• Original Assignee: Dunegan Engineering Consultants, Inc.
• Inventors: Dunegan, Harold L.
• Primary Examiner(s): Williams, Hezron
• Assistant Examiner(s): MILLER, ROSE MARY
• Application Number:
  US09/016,050
• Time in Patent Office: 838 Days
• Field of Search: 73/587, 73/602, 73/1.82, 73/649, 73/652, 73/654, 73/763, 73/774, 73/778, 73/514.01, 73/514.16, 73/514.29, 73/514.34, 73/526, 73/598, 73/60 0, 310/328, 310/329, 310/336, 310/369
• US Class Current: 73/587
• CPC Class Codes: G01H 1/00 Measuring characteristics of vibrations in solids by using direct conduction to the detector G01H9/00, G01H11/00 take precedence
• CPC Class Codes: G01H 1/12 of longitudinal or not specified vibrations
• CPC Class Codes: G01N 2291/014 Resonance or resonant frequency
• CPC Class Codes: G01N 2291/02854 Length, thickness
• CPC Class Codes: G01N 2291/0289 Internal structure, e.g. defects, grain size, texture
• CPC Class Codes: G01N 2291/0422 Shear waves, transverse waves, horizontally polarised waves
• CPC Class Codes: G01N 2291/0423 Surface waves, e.g. Rayleigh waves, Love waves
• CPC Class Codes: G01N 2291/0427 Flexural waves, plate waves, e.g. Lamb waves, tuning fork, cantilever
• CPC Class Codes: G01N 2291/0428 Mode conversion
• CPC Class Codes: G01N 2291/2632 flat
• CPC Class Codes: G01N 29/14 using acoustic emission techniques echo of particles G01N29/046; measuring mechanical vibrations or acoustic waves in solids in general G01H1/00
• CPC Class Codes: G01N 29/2437 Piezoelectric probes
• 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 29/36 Detecting the response signal , e.g. electronic circuits specially adapted therefor
• CPC Class Codes: G01N 29/40 by amplitude filtering, e.g. by applying a threshold or by gain control
• CPC Class Codes: G01N 29/42 by frequency filtering or by tuning to resonant frequency
• CPC Class Codes: G10K 11/004 Mounting transducers, e.g. provided with mechanical moving or orienting device mountings specially adapted to a particular sound-producing device, see the preceding groups G10K1/00 - G10K9/00, e.g. G10K1/26, G10K1/28, G10K9/22; arrangements of sonic watch equipment on submarines B63G8/39; buoys B63B22/00

Abstract

A method and an apparatus for detecting and measuring cracks in plate-like structures using acoustic emission technique are disclosed. A false aperture transducer is designed to provide a criterion for filtering out extraneous noise in the acoustic emission signal based on modal analysis by computing the ratio of the high-frequency peak amplitude to low-frequency peak amplitude of the signal. A calibration curve correlating crack depth to the amplitude ratio can be obtained by simulating crack growth in a fracture specimen coupled to a test structure or field structure, and measuring acoustic emission signal in the structure by the false aperture transducer. The calibration curve correlates simulated crack depth percentage with computed peak amplitude ratio of the measured signal. Using the calibration curve and acoustic emission signal sensed by a false aperture transducer in a field structure, a crack in the structure can be detected and its depth measured by computing the peak amplitude ratio of the signal and identifying the crack depth that correlates with the ratio from the calibration curve.

Claims

1. A method of detecting crack growth in a structure, comprising the steps of:

sensing acoustic emission signal in a structure, said signal including an in-plane high-frequency component and an out-of-plane low-frequency component;

determining a high-frequency peak amplitude of said high-frequency component and a low-frequency peak amplitude of said low-frequency component; and

computing the ratio of said high-frequency peak amplitude to said low-frequency peak amplitude in order to detect crack growth.

2. The method of claim 1, wherein said step of determining a high-frequency peak amplitude and a low-frequency peak amplitude includes:

splitting said signal into said high-frequency component and low-frequency components;

amplifying said high-frequency component;

filtering said high-frequency component with a high-pass filter;

detecting a high-frequency peak amplitude;

amplifying said low-frequency component;

filtering said low-frequency component with a bandpass filter; and

detecting a low-frequency peak amplitude.

3. The method of claim 2, wherein said high-pass filter has a frequency range above 100 kHz and said bandpass filter has a frequency range of 20 to 60 kHz.

4. The method of claim 1, wherein said sensing step is performed with a transducer that is calibrated with a sensitivity ratio (R₁).

5. The method of claim 4, wherein said computing step includes:

identifying crack growth having a sensitivity ratio greater than R₁.

6. The method of claim 5, wherein said identifying step includes producing an audio tone.

7. The method of claim 4, wherein R₁ equals 1.

8. The method of claim 7, wherein said comparing step includes:

generating a high-frequency audio tone from said high-frequency peak amplitude;

generating a low-frequency audio tone from said low-frequency peak amplitude;

comparing said high-frequency audio tone and said low-frequency audio tone; and

identifying crack growth with said high-frequency audio tone producing a higher amplitude said low-frequency audio tone.

9. The method of claim 1, wherein the sensing step includes mounting a transducer onto said structure at a first location.

10. The method of claim 9, further comprising the step of computing a distance of said crack growth source from said transducer, said distance forming a first circle with a radius D1 from said transducer.

11. The method of claim 10, wherein said step of computing a distance includes:

determining a high-frequency threshold detection time of said high-frequency signal;

determining a low-frequency threshold detection time of said low-frequency signal;

starting to count said cycles of a 1-MHz clock at said high-frequency threshold detection time;

stopping to count cycles of said clock at said low-frequency threshold detection time to obtain a cycle count;

computing a time delay from said cycle count; and

computing said distance of said crack growth source from said transducer.

12. The method of claim 10, wherein said step of computing a distance includes:

determining a high-frequency threshold detection time, of said high-frequency signal;

determining a low-frequency threshold detection time of said low-frequency signal;

computing a time difference, Δ

T, between said high-frequency threshold detection time and said low-frequency threshold detection time;

computing said distance D1 by the following;

space="preserve" listing-type="equation">D1=Δ

T(1/V.sub.s -1/V.sub.e).sup.-1

wherein

V_s =shear velocity in said structure and

V_e =extensional velocity in said structure.

13. The method of claim 10, further comprising the step of locating said crack growth source, said step including:

mounting said transducer onto a second location of said structure;

computing a distance of said crack growth source from said transducer at said second location, said distance forming a second circle with a radius D2 from said transducer, said second circle intersecting with said first circle along a line of intersection;

mounting said transducer onto a third location of said structure;

computing a distance of said crack growth source from said transducer at said third location, said distance forming a third circle with a radius D3 from said transducer, said third circle intersecting with said line of intersection at a point of intersection, said crack growth source being located at said point of intersection.

14. The method of claim 10, further comprising the step of estimating a crack depth of said crack growth source from a ratio of said high-frequency peak amplitude to said low-frequency peak amplitude using a calibration curve which correlates crack depth percentage with said ratio.

15. The method of claim 14, further comprising the step of spatial-filtering said acoustic emission signal with said radius D1.

16. The method of claim 1, further comprising the step of estimating a crack depth of said crack-like source from a ratio of said high-frequency peak amplitude to said low-frequency peak amplitude using a calibration curve which correlates crack depth percentage with said ratio.

17. The method of claim 16, further comprising the step of time-filtering said acoustic emission signal.