WO1999010733A1

WO1999010733A1 - Method for nondestructive evaluation of a weld

Info

Publication number: WO1999010733A1
Application number: PCT/US1998/017700
Authority: WO
Inventors: Steven M. Shepard
Original assignee: Thermal Wave Imaging, Inc.
Priority date: 1997-08-26
Filing date: 1998-08-26
Publication date: 1999-03-04

Abstract

A method for nondestructive evaluation of spot welds, wherein the sample weld (12) is thermally excited (10) and then allowed to cool. As the thermal energy level on the weld area's outer surface changes, a series of positionally-fixed infrared images of the weld are acquired. The acquired images compose the infrared signature of the spot weld. The infrared signature is digitized and analyzed to determine the size and acceptability of the spot weld.

Description

METHOD FOR NONDESTRUCTIVE EVALUATION OF A WELD

BACKGROUND OF THE INVENTION

This invention relates generally to a method for nondestructive evaluation of welds, and more particularly relates to a method of determining certain characteristics, such as strength, size, and acceptability, of spot welds.

DESCRIPTION OF THE PRIOR ART

Welding, in particular, spot welding, is used to join two or more pieces of metal by forming a metallurgical bond therebetween. Spot welds are often used in the automotive industry, for example, to join the frame and body components of the automobile. Spot welding employs the combination of heat and pressure in a localized area between two abutting metal pieces. This combination of heat and pressure forms a metallurgical bond (known as a "nugget") between the two metal workpieces in the localized area. The presence and size of the "nugget" is one widely used indicator of weld quality. In most industries, the strength of a particular weld is evaluated using destructive techniques such as a chisel or "pull" test that effectively destroys the bond between the metals. Such destructive evaluation methods are very expensive and wasteful. Moreover, the results generated by such crude testing techniques are extremely subjective. Alternatively, several manufacturing industries overcompensate for the unpredictability of certain welds by forming redundant welds throughout a workpiece. For example, in the automotive industry, the typical automobile will contain 30-40% redundant welds. This method of overcompensation is also very costly and inefficient.

Methods are known using infrared (IR) imaging techniques for nondestructive evaluation (NDE) of laminar composite materials. For example, known methods in the art of infrared NDE involve thermally exciting a painted object with a heat source, such as a hot air gun, for a finite period of time, thereby imparting thermal energy to the object. After the target object has been thermally excited, the object is allowed to cool, and thermal energy earlier absorbed by the object then begins to dissipate. As the object cools, an infrared camera acquires a series of successive positionally fixed IR images of the external surface of the object. The images are acquired at fixed time intervals, starting relative to the beginning of the thermal excitation period, and continuing during the period while the object cools, until a pre-determined time period has elapsed. Analysis of the infrared signature emitted from the painted object can identify portions where the paint adherence is poor and portions where the paint adherence is good.

SUMMARY OF THE INVENTION

The present invention is a procedure for evaluation of spot welds that are typically used to join pieces of sheet metal.

The method of the present invention is based on the thermal behavior of the weld in response to an instantaneous heat pulse. It exploits the fact that in a properly formed weld, the nugget has different heat transfer properties than the metal in the surrounding heat affected zone. Since there is a metallurgical bond between the work pieces at the nugget, thermal energy deposited on the upper layer of the joint is transferred to the lower layer through the nugget. Immediately after energy is deposited on the upper layer, there is a brief period of time during which conduction of heat through the weld is the dominant cooling mechanism for the top surface. However, at later times after heating, lateral heat flow in the upper layer becomes the dominant cooling mechanism, regardless of the condition of the weld.

The present invention utilizes a brief pulse of light from either a laser or optical flashlamp to heat the weld area. The heat deposited at the upper surface diffuses laterally into the upper layer as well as through the nugget into the lower layer. The rate of heat conduction into the lower layer depends on the presence and size of the nugget at the interface between the work pieces. Areas adjacent to the nugget may either be in mechanical contact or bonded to each other, but in either case, heat conduction is less effective in these areas than directly through the nugget due to lack of metallurgical bonding. As a result, immediately after the surface is heated, the temperature of the upper surface directly above the nugget will be temporarily cooler than adjacent surface areas since both lateral heat flow and conduction through the nugget are available as cooling mechanisms. However, for smaller nuggets, the effect is less pronounced.

These and other features and objects of this invention will become apparent to one skilled in the art from the following detailed description and the accompanying drawings illustrating features of this invention by way of example.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic depiction of heat flow which takes place between two panels during the welding process.

Figure 2 is a schematic diagram of the apparatus of the present invention.

Figure 3 is a graph depicting the infrared signature emitted from a specimen as it decays over time.

DESCRIPTION OF THE PREFERRED AND ALTERNATIVE EMBODIMENTS

In the preferred embodiment of the present invention, a laser pulse 10 is used to heat the surface above the nugget 12 and create an infrared signature for the weld area 14. The infrared signature indicates the thermal characteristics of the weld area 14. The laser 10 is particularly useful since it can deliver a considerable amount of energy in a very short pulse (e.g. 30 Watts in a few microseconds can be accomplished easily with off-the-shelf commercial hardware). Moreover, lasers do not generate heat at the source like incandescent sources or flashlamps, which operate by sending an electric current arc through a gas to create plasma. The infrared signature is monitored during the heat pulse and subsequent cooling using an infrared camera 16. The data from the infrared camera 16 is either digitized internally or transmitted to a computer 18 where it is digitized using a highspeed frame grabber. Given the excellent thermal conductivity of metals such as aluminum or steel, it is desirable to use a high-speed focal plane array integrated circuit (e.g. > 200 Hz) for most spot weld situations.

For a brief interval of time after the heating pulse, the presence of the nugget 12 and its approximate size can be detected by analyzing the data from the infrared camera 16 with a computer 18 (this is necessary because the nugget 12 is only visible for a period on the order of a few milliseconds, and cannot be directly viewed by the unassisted eye). The phenomenon is illustrated in the amplitude vs. time plot for an ideal spot weld shown in Fig. 3. The surface directly above the nugget 12 location cools quickly to a near equilibrium state, while the temperature of the surface 20 surrounding the nugget 12 location cools much more gradually. The net result is that there is a brief time interval (e.g. between 20 and 50 msec in Fig. 3) during which, for a good nugget 12, the surface above the nugget 12 has almost completely cooled, i.e. the slope of the temperature versus time curve of each pixel during this interval is quite small, while the surrounding area 20 is still cooling, resulting in a negative slope. During this time interval the nugget 12 can be seen in the image of the slope (rate of change) of each pixel in the digital image of the weld with respect to time, over an interval of several consecutive video frames. The nugget 12 appears as a "hot" spot in the slope image. It is also possible to detect the presence of the nugget 12 by viewing the amplitude of each pixel, although this method is more susceptible to variations due to ambient temperature variations and sample surface preparation. In order to accurately determine the nugget size, it is necessary to capture the image at the earliest possible time after flash heating, since lateral heat flow from the rest of the weld area 14 toward the nugget 12 area will cause the image of the nugget 12 to become less distinct.

Alternatively, this method can also be applied using "through transmission," i.e. placing the laser 10 on one side of the weld 14 and the camera on the opposite side. In this case, the amplitude vs. time profile has the opposite polarity of the single side case. Specifically, the area above the nugget 12 is heated rapidly, while surrounding areas 20 are heated more gradually. The net result is that there is an interval of time where the slope of the good weld is very small while the slope of the surrounding area 20, or of a bad weld, is a large number. As a result, a good weld appears as a "cold" spot in the slope image.

It is also possible to use sources other than a laser 10 to heat the sample. For example, a photographic flashlamp can be used to successfully image the nugget 12. However, since flashlamps generate heat as well as light, they also generate stray infrared radiation that contributes unwanted artifacts to the image.

For bare metals, results are enhanced if the sample is first coated with a thin layer of paint, water or other moist material (for example dampened paper), to improve infrared emissivity and optical absorption. However, this may be avoided if a sufficiently intense laser 10 is used, and the camera 16 is placed at an oblique angle.

The preceding description is exemplary rather than limiting in nature. A preferred embodiment of this invention has been disclosed to enable one skilled in the art to practice this invention. Variations and modifications are possible without departing from the spirit and purview of this invention, the scope of which is limited only by the appended claims.

Claims

CLAIMSI claim:

1. A method for nondestructive evaluation of a weld, comprising the steps of: heating a predetermined surface of a weld area thereby creating an infrared signature of said weld area; monitoring said infrared signature; and analyzing said infrared signature to determine certain characteristics of said weld area.

2. The method of claim 1, wherein said heating step utilizes a laser.

3. The method of claim 1 , wherein said weld area includes a spot weld having an upper surface and a side surface.

4. The method of claim 3, wherein said spot weld includes a nugget.

5. The method of claim 4, wherein said predetermined surface is said upper surface of said spot weld located above said nugget.

6. The method of claim 4, wherein said predetermined surface is said side surface of said spot weld located adjacent to said nugget.

7. The method of claim 1, wherein said monitoring step utilizes an infrared camera.

8. The method of claim 7, wherein said analyzing step further includes the step of digitizing said infrared signature.

9. The method of claim 8, wherein said digitizing step is performed internally within said infrared camera.

10. The method of claim 8, wherein said digitizing step is performed externally by a computer.

11. The method of claim 1 , wherein said monitoring step is performed during an initial heat pulse and subsequent cooling period.

12. The method of claim 1 , further comprising the step of coating said predetermined surface to improve infrared emissivity and optical absorption.

13. The method of claim 12, wherein said coating step utilizes a thin layer of paint.

14. The method of claim 12, wherein said coating step utilizes a thin moist substance.

15. The method of claim 1, wherein said certain characteristics include size and acceptability of said weld area.

16. A method for nondestructive evaluation of a weld, comprising the steps of: heating a predetermined upper surface of a weld area thereby creating an infrared signature of said weld area; monitoring said infrared signature with an infrared camera; and analyzing said infrared signature to determine certain characteristics of said weld.

17. The method of claim 16, wherein said certain characteristics include size and acceptability of said weld area.

18. The method of claim 16, wherein said weld area includes a spot weld.

19. The method of claim 18, wherein said spot weld includes a nugget.

20. The method of claim 19, wherein said predetermined upper surface is located above said nugget.

21. The method of claim 16, wherein said analyzing step further includes the step of digitizing said infrared signature.

22. The method of claim 21, wherein said digitizing step is performed internally within said infrared camera.

23. The method of claim 21 , wherein said digitizing step is performed externally by a computer.

24. The method of claim 16, wherein said monitoring step is performed during an initial heat pulse and subsequent cooling period.

25. The method of claim 16, wherein said heating step utilizes a laser.

26. A method for nondestructive evaluation of a weld, comprising the steps of: heating a predetermined side surface of a weld area thereby creating infrared signature of said weld area; monitoring said infrared signature with an infrared camera; and analyzing said infrared signature to determine certain characteristics of said weld.

27. The method of claim 26, wherein said certain characteristics include size and acceptability of said weld area.

28. The method of claim 26, wherein said weld is a spot weld.

29. The method of claim 28, wherein said spot weld includes a nugget.

30. The method of claim 29, wherein said predetermined side surface is located adjacent to said nugget.

31. The method of claim 26, wherein said analyzing step further includes the step of digitizing said infrared signature.

32. The method of claim 31, wherein said digitizing step is performed internally within said infrared camera.

33. The method of claim 31 , wherein said digitizing step is performed externally by a computer.

34. The method of claim 26, wherein said monitoring step is performed during an initial heat pulse and subsequent cooling period.

35. The method of claim 26, wherein heating step utilizes a laser.