Apparatus and method for detecting fatigue cracks using infrared thermography

• US 5,111,048 A
• Filed: 09/27/1990
• Issued: 05/05/1992
• Est. Priority Date: 09/27/1990
• Status: Expired due to Term

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Sparks, Joseph M., Kania, Janet S., Devitt, John W., Bantel, Thomas E.
• Primary Examiner(s): Fields, Carolyn E.
• Assistant Examiner(s): BEYER, JAMES
• Application Number:
  US07/588,909
• Time in Patent Office: 586 Days
• Field of Search: 374/5, 374/4, 250/341, 250/342, 250/330
• US Class Current: 250/342
• CPC Class Codes: G01N 25/72 Investigating presence of flaws

Abstract

A method and apparatus for detecting a defect in a workpiece are disclosed. The workpiece is positioned with a surface of the workpiece to be inspected in an optical path of an infrared radiation detector. A selected portion of the workpiece is heated by scanning with electromagnetic radiation for a selected duration to cause an increase in radiance from any defect present in the selected portion. Any defects, present in the workpiece selected portion, which may cause a failure of the workpiece, may be detected and distinguished from minor surface anomalies by analyzing a transient response of the irradiance received by the infrared radiation detector.

Claims

1. A method for detecting a defect in a workpiece, comprising the steps of:

(a) positioning a surface of the workpiece to be inspected in an optical path of an infrared radiation detector;

(b) generating a graph of irradiance versus distance along a selected portion of the workpiece to establish an original irradiance intensity level;

(c) heating the selected portion of the workpiece by electromagnetic radiation for a selected duration to cause an increase in radiance from any defect present at the workpiece surface in the selected portion, the radiation detector operating in a different spectral band than the electromagnetic radiation wherein any defect has a substantially higher emissivity relative to the workpiece surface surrounding the defect to cause an enhanced signal-to-noise ratio so as to allow distinction of a flaw which may cause a failure of the workpiece from another minor surface anomaly;

(d) generating a series of graphs of irradiance versus distance along the selected portion as the irradiance decays from a peak intensity level back to the original intensity level after the heat is removed, the series of graphs representing a transient response of the irradiance from the selected portion; and

(e) detecting defects in the selected portion of the workpiece by comparing the graphs to one another to analyze the transient response of the radiance received from the selected portion by the infrared radiation detector and to distinguish any flaw which may cause a failure of the workpiece from another minor surface anomaly.

2. The method of claim 1, wherein step (d) comprises the steps of:

(d1) generating a series of images of the heated selected portion from the irradiance detected by the infrared radiation detector; and

(d2) generating the series of graphs of irradiance versus distance along the selected portion from the images.

3. The method of claim 1, wherein step (c) comprises the step of focusing a laser beam on the selected portion.

4. The method of claim 3, wherein the laser beam is generated by a YAG laser operating at a wavelength of about 1.06 microns and the laser beam is substantially normally incident upon the selected portion.

5. The method of claim 1, wherein the radiation detector is an infrared imaging radiometer operating at a wavelength band between about 3 microns and about 12 microns.

6. The method of claim 1, further comprising the step of reducing an emissivity of the workpiece surface by cleaning the workpiece surface to remove any coatings.

7. The method of claim 1, further comprising the step of preheating the workpiece surface to a selected temperature before step (b).

8. The method of claim 7, wherein step (c) comprises the step of scanning the workpiece surface with a laser beam.

9. The method of claim 1, wherein step (e) further comprises the step of distinguishing a fatigue crack which may cause a failure of the workpiece from another minor surface anomaly by analyzing an intensity level of the irradiance during step (c) and a decay rate of the irradiance intensity level after removing the heat.

10. The method of claim 1, further comprising the step of applying a stress to the workpiece to cause any subsurface defects proximate to the surface under inspection to be detectable at the workpiece surface.

11. The method of claim 10, wherein the stress applied has an intensity level below a characteristic damage threshold stress intensity factor of a material from which the workpiece is made.

12. The method of claim 1, wherein the optical path is substantially parallel to the surface normal of the workpiece selected portion.

13. A method for detecting a fatigue crack in a component, comprising the steps of:

(a) positioning a surface of the component to be inspected in an optical path of an infrared imaging radiometer;

(b) generating a graph of irradiance versus distance along a selected portion of the surface to establish an original irradiance intensity level;

(c) selectively heating the selected surface area of the component by scanning with a laser beam to cause an increase in radiance from the selected surface area, the infrared imaging radiometer operating in a different spectral band than the laser beam;

(d) generating a series of video images of the heated surface area from the irradiance received by the infrared radiometer during and immediately after scanning with the laser beam;

(e) generating a series of graphs of irradiance versus distance along the selected surface area from the video images, the graphs corresponding to a transient response of the irradiance during and immediately after scanning with the laser beam; and

(f) distinguishing between a minor surface anomaly and a crack which may cause a failure of the component by analyzing an intensity level of the irradiance and a decay rate from a peak intensity level to the original irradiance intensity level after laser scanning, the decay rate for a crack which may cause a failure of the component being more than about 2 times as long as the decay time for a minor surface anomaly.

14. The method of claim 13, wherein the optical path is substantially parallel to the surface normal of the selected surface area.

15. The method of claim 13, wherein the laser beam is generated by a YAG laser operating at a wavelength of about 1.06 microns and the laser beam is substantially normally incident upon the selected surface area.

16. The method of claim 13, further comprising the step of preheating the workpiece surface to a selected temperature before step (b).

17. The method of claim 16, wherein the selected temperature is between about 50°

C. and about 150°

C.

18. The method of claim 13, further comprising the step of applying a stress to the component to cause any subsurface defects which are proximate to the surface to be detectable at the component surface.

19. An apparatus for detecting a crack in a component, comprising:

means for generating a graph of irradiance versus distance along a selected surface area of the component to establish an original irradiance intensity level;

means for heating the selected surface area of the component to cause an increase in radiance from any crack present at the component surface, wherein any crack which may cause a failure of the component has a substantially higher emissivity relative to the component surface surrounding the crack to cause an enhanced signal-to-noise ratio so as to allow distinction of any crack which may cause a failure from other minor surface anomalies;

means for receiving the radiance from the selected surface area; and

means for generating a series of graphs corresponding to the radiance received from the selected area immediately after heating as the irradiance decays from a peak intensity level back to said original intensity level, said series of graphs providing a transient response of the radiance received to distinguish between a minor surface anomaly and a crack which may cause a failure of the component.

20. The apparatus of claim 19, wherein said heating means comprises:

a laser; and

optical means for scanning the laser across the selected surface area.

21. The apparatus of claim 20, wherein said laser and said optical means are positioned to provide a laser beam which is substantially normally incident upon the selected surface area.

22. The apparatus of claim 19, wherein said radiance receiving means comprises an infrared imaging radiometer positioned in an optical path substantially parallel to the surface normal of the selected surface area.

23. The apparatus of claim 19, further comprising:

means for generating at least one image of the irradiance from the selected surface area.

24. The apparatus of claim 23, further comprising:

means for digitizing the at least one image to facilitate storage and analysis of the at least one image to distinguish between a minor surface anomaly and a crack which may cause a failure of the component.

25. The apparatus of claim 19, further comprising

means for applying a stress to the component to cause any subsurface defects which are proximate to the surface to be detectable at the selected surface area, said stress having an intensity level below a characteristic damage threshold stress intensity factor of the component.