Synthetic reference thermal imaging method

Details Synthetic reference thermal imaging method Synthetic reference thermal imaging method

2000-05-10

2002-04-09

Gutierrez, Diego (Department: 2859)

Thermal measuring and testing

Distance or angle

Thickness, erosion, or deposition

C374S005000, C250S341600, C250S330000

Reexamination Certificate

active

06367969

ABSTRACT:

TECHNICAL FIELD
The present invention relates to thermographic nondestructive testing techniques for determining the thickness of an object. More particularly, the present invention relates to a infrared transient thermography method that utilizes a synthetic thermal reference in determining the wall thickness of metal turbine rotor blades or the like.
BACKGROUND
Over the years, various nondestructive ultrasonic measurement techniques have been utilized to determine cross-sectional thickness of cast metal and other solid objects. Conventionally, the object is probed with ultrasonic waves which penetrate the surface and are reflected internally at the opposite side or surface of the object. Based upon the time required to receive a reflected wave, the distance to the opposite (back) side can be determined—giving the thickness of the object at that point. Unfortunately, conducting ultrasonic measurements of this sort to examine the cross-sectional thickness for most of an object would usually necessitate a cumbersome and time-consuming mechanical scanning of the entire surface with a transducer. In addition, to facilitate intimate sonic contact between the transducer and the object surface, a stream of liquid couplant must be applied to the surface or, alternatively, total immersion of the object in the couplant must be accommodated. Such accommodations, however, are most often not very practical or even feasible for numerous structural and material reasons. For example, ultrasonic systems capable of scanning and analyzing geometrically complex parts are typically very expensive and complicated. In addition, a mechanical scanning of the transducer over the surface of a large object can literally take hours.
Moreover, when conducting ultrasonic measurements on certain metal objects, the internal crystal orientation and structure of the metal can cause undesirable noise and directional effects that contribute to inaccuracies in the acquired data. This inherent limitation of ultrasonic measurements proves to be a serious drawback when testing components constructed of crystalline or “directional” metals such as often used in contemporary turbine airfoils.
In contrast, infrared (IR) transient thermography is a somewhat more versatile nondestructive testing technique that relies upon temporal measurements of heat transference through an object to provide information concerning the structure and integrity of the object. Since heat flow through an object is substantially unaffected by the micro-structure and the single-crystal orientations of the material of the object, an infrared transient thermography analysis is essentially free of the limitations this creates for ultrasonic measurements. In contrast to most ultrasonic techniques, a transient thermographic analysis approach is not significantly hampered by the size, contour or shape of the object being tested and, moreover, can be accomplished ten to one-hundred times faster than most conventional ultrasonic methods if testing objects of large surface area.
One known contemporary application of transient thermography, which provides the ability to determine the size and “relative” location (depth) of flaws within solid non-metal composites, is revealed in U.S. Pat. No. 5,711,603 to Ringermacher et al., entitled “Nondestructive Testing: Transient Depth Thermography”; and is incorporated herein by reference. Basically, this technique involves heating the surface of an object of interest and recording the temperature changes over time of very small regions or “resolution elements” on the surface of the object. These surface temperature changes are related to characteristic dynamics of heat flow through the object, which is affected by the presence of flaws. Accordingly, the size and a value indicative of a “relative” depth of a flaw (i.e., relative to other flaws within the object) can be determined based upon a careful analysis of the temperature changes occurring at each resolution element over the surface of the object. Although not explicitly disclosed in the above referenced Ringermacher patent, the “actual” depth of a flaw (i.e., the depth of a flaw from the surface of the object) can not be determined unless a “standards block”, having voids at known depths, or an “infinite” (thermally thick) reference region on the object is included as part of the thermographic data acquisition and analysis for comparison against the relative depth values.
To obtain accurate thermal measurements using transient thermography, the surface of an object must be heated to a particular temperature in a sufficiently short period of time so as to preclude any significant heating of the remainder of the object. Depending on the thickness and material characteristics of the object under test, a quartz lamp or a high intensity flash-lamp is conventionally used to generate a heat pulse of the proper magnitude and duration. However, the specific mechanism used to heat the object surface could be any means capable of quickly heating the surface to a temperature sufficient to permit thermographic monitoring—such as, for example, pulsed laser light. Once the surface of the object is heated, a graphic record of thermal changes over the surface is acquired and analyzed.
Conventionally, an infrared (IR) video camera has been used to record and store successive thermal images (frames) of an object surface after heating it. Each video image is composed of a fixed number of pixels. In this context, a pixel is a small picture element in an image array or frame which corresponds to a rectangular area, called a “resolution element”, on the surface of the object being imaged. Since, the temperature at each resolution element is directly related to the intensity of the corresponding pixel, temperature changes at each resolution element on the object surface can be analyzed in terms of changes in pixel contrast. The stored IR video images are used to determine the contrast of each pixel in an image frame by subtracting the mean pixel intensity for a particular image frame, representing a known point in time, from the individual pixel intensity at that same point in time.
The contrast data for each pixel is then analyzed in the time domain (i.e., over many image frames) to identify the time of occurrence of an “inflection point” of the contrast curve data, which is mathematically related to a relative depth of a flaw within the object. Basically, as applied to an exemplary “plate-like” object of consistent material and thickness L, a heat flux pulse impinging on an object takes a certain “characteristic time”, T
c
, to penetrate through the object to the opposite side (back wall) and return to the front surface being imaged. This characteristic time, T
c
, is related to the thickness of the object, given the thermal diffusivity of the material, by the following equation:
T
c
=4
L
2
/&pgr;
2
&agr;  Equ.(1)
where L is the thickness (cm) of the object and &agr; is the thermal diffusivity (cm
2
/sec) of the material.
From empirical observations it is known that after a heat pulse impinges on a plate-like object, the surface temperature observed from the same side of the object (i.e., the front) rises in a fashion that is also dependent on the thickness and the thermal diffusivity of the material. Moreover, from a graph of the time vs. temperature (T-t) history of the surface, one can determine the characteristic time, T
c
, in terms of a unique point on the T-t curve, called the “inflection point.” This inflection point, t
infl
, is indicated by the point of maximum slope on the T-t curve (i.e., peak-slope time) and is related to the characteristic time, T
c
, by the following equation:
t
infl
=0.9055
T
c
  Equ.(2)
This relationship between the inflection point and the characteristic time, as expressed by Equ. (2) above, is precise to approximately 1% for one-dimensional (1-D), as well as two-dimensional (2-D), heat flow analysis. Once an inflection point, t
infl
, is determined from the T-t response, a relative thickness, L, of the object

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