Abstract:
A method for detecting surface discontinuities in a test specimen. The method includes applying a one or more substances including a detection medium to the test specimen wherein the detection medium enters at least one surface discontinuity in the test specimen. The specimen surface is monitored for discontinuity signatures produced by the detection medium. The monitoring includes monitoring the detection medium to detect a temperature differential indicative of a surface discontinuity in the test specimen wherein the discontinuity signatures include a warm signature emitted by the detection medium that has entered the surface discontinuity.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to non-destructive inspection methods and, more particularly, to inspection methods that use an endothermic or exothermic reaction at a discontinuity to detect the discontinuity using thermography. 
     BACKGROUND OF THE INVENTION 
     Maintaining the structural integrity of certain structures is very important in many fields because of safety concerns, downtime, cost, etc. Loss of structural integrity is typically caused by material defects, such as cracks, disbonds, corrosion, voids, etc. that may exist in or on the structure. For example, it is important in the power generation industry that reliable techniques are available to examine the structural integrity of turbine engine, generator and other plant equipment to ensure the components and systems do not suffer failure during operation. In particular, the structural integrity of turbine blades and rotors requires monitoring through inspections to facilitate the long term service life of the turbine engine. A common method for detection of a crack or defect is visual examination by skilled personnel. However, it is known that cracks or defects that may affect the integrity of structural components may not be readily visible without the use of special techniques to aid the examiner. Therefore, various techniques have been developed in the art for non-invasive and non-destructive analysis of different structural components and materials in various industries. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the invention, a method is provided for detecting surface discontinuities in a test specimen, the method comprising: applying a liquid detection medium to the test specimen wherein the liquid detection medium enters at least one surface discontinuity in the test specimen through capillary action; and monitoring the surface of the test specimen for discontinuity signatures produced by the liquid detection medium including monitoring the liquid detection medium to detect a temperature differential indicative of a surface discontinuity in the test specimen; wherein the discontinuity signatures comprise a warm signature emitted by the liquid detection medium that has entered the surface discontinuity. 
     The temperature signature may be determined by detecting the warm signature relative to a cooler signature measured on an area of the test specimen surrounding the surface discontinuity. 
     The warm signature may comprise the result of an endothermic reaction following application of the liquid detection medium to the test specimen. 
     The endothermic reaction may comprise evaporation of the liquid detection medium from the area of the test specimen surrounding the surface discontinuity at a faster rate than evaporation of the liquid detection medium that has entered the surface discontinuity. 
     The liquid detection medium may comprise a volatile liquid, and may comprise at least one of alcohol, acetone and ethylene. 
     The warm temperature signature may comprise the result of an exothermic reaction following application of the liquid detection medium to the test specimen, and the exothermic reaction may be produced by applying a reacting medium to the liquid detection medium. 
     The liquid detection medium may be substantially removed from an area surrounding the surface discontinuity prior to application of the reaction medium, and the reaction medium may be applied to an area including both the surface discontinuity and the area surrounding the surface discontinuity. 
     One of the liquid detection medium and the reaction medium may be chemically alkaline, and the other of the liquid detection medium and the reaction medium may be chemically acidic. 
     The temperature signatures may be produced on the test specimen without application of an excitation energy to the test specimen. 
     The monitoring may comprise thermally monitoring the liquid detection medium by acquiring infrared images of the test specimen and liquid detection medium. 
     The discontinuity may be a crack formed in the surface of the test specimen. 
     In accordance with another aspect of the invention, a method is provided for detecting surface discontinuities in a test specimen without application of an external excitation energy to the specimen, the method comprising: applying a detection medium to the test specimen wherein the detection medium enters at least one surface discontinuity in the test specimen; and monitoring the surface of the test specimen for discontinuity signatures produced by the detection medium including monitoring the detection medium to detect a temperature differential indicative of a surface discontinuity in the test specimen; wherein the discontinuity signatures comprise a warm signature emitted by the detection medium that has entered the surface discontinuity, and the warm signature is detected relative to a cooler signature measured on an area of the test specimen surrounding the surface discontinuity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that the present invention will be better understood from the following description in conjunction with the accompanying Drawing Figures, in which like reference numerals identify like elements, and wherein: 
         FIG. 1  is a schematic plan view illustrating an enlarged section of a test specimen coated with a detection medium in accordance with a step of a first embodiment of the invention; 
         FIG. 1A  is an enlarged cross sectional view taken along line  1 A- 1 A in  FIG. 1 , and illustrating a monitoring step for detecting a surface discontinuity in accordance with the first embodiment of the invention; 
         FIG. 2  is a schematic plan view illustrating an enlarged section of a test specimen coated with a detection medium in accordance with a step of a second embodiment of the invention; 
         FIG. 2A  is an enlarged cross sectional view taken along line  2 A- 2 A in  FIG. 2 ; 
         FIG. 3  is a schematic plan view illustrating the test specimen of  FIG. 2  in which a residual amount of the first detection medium surrounding a surface discontinuity has been removed in accordance with a step of the second embodiment of the invention; 
         FIG. 3A  is an enlarged cross sectional view taken along line  3 A- 3 A in  FIG. 3 ; 
         FIG. 4  is a schematic plan view illustrating the test specimen of  FIG. 2  coated with a reacting medium in accordance with a step of the second embodiment of the invention; and 
         FIG. 4A  is an enlarged cross sectional view taken along line  4 A- 4 A in  FIG. 4 , and illustrating a monitoring step for detecting a surface discontinuity in accordance with the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description of the preferred embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, specific preferred embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention. 
     According to one aspect, the invention provides methods for detecting surface discontinuities in a component or test specimen, such as a component for use in turbo-machinery (e.g. gas or steam turbines). Surface discontinuities detected by the methods of the invention may in particular comprise, for example, linear cracks, porosity, etc, or similar discontinuities that may characterize a defect formed or located on the surface of a component or test specimen. The methods provide an active thermography technique in which one or more chemicals may be applied to the surface of the specimen, the chemical or chemicals produce a reaction to provide an indication to a thermal imaging device of a surface discontinuity location without application of an external excitation energy. 
       FIG. 1  illustrates a component or test specimen  10  in accordance with a first embodiment of the invention. A surface discontinuity is indicated by reference numeral  12  and extends inwardly from a surface  14  of the specimen  10 , see  FIG. 1A . In an initial step of the method of the present embodiment, a liquid detection medium  16  is applied to at least a portion of the surface  14  to be monitored. The liquid detection medium  16  is applied to substantially coat the surface  14  as well as to transport into the discontinuity  12 , such as through capillary action drawing the liquid detection medium  16  into a subsurface cavity  18  defined by the discontinuity  12 . That is, the adhesion of the liquid detection medium  16  to the inwardly extending surfaces of the cavity  18  interacts with the surface tension of the liquid to cause the liquid detection medium  16  to move into the cavity  18 . The liquid detection medium  16  may be applied by any known technique to substantially wet the specimen surface  14  with the detection medium  16 , with sufficient liquid being applied to cause the liquid detection medium  16  to transport under capillary action into the discontinuity  12  to fill the cavity  18 . 
     The liquid detection medium  16  in accordance with the first embodiment comprises a chemical substance that readily produces an endothermic reaction on the surface  14 . In particular, the liquid detection medium  16  preferably comprises a volatile liquid, i.e., a liquid that readily vaporizes or evaporates at approximately room temperature (approximately 22° C.). For example, the liquid detection medium  16  may comprise, without limitation, alcohol, acetone or ethylene. The liquid detection medium  16  is preferably selected with reference to the material of the specimen  10 , such that the liquid detection medium  16  does not cause deterioration of the specimen  10  through contact with the surface  14  or within the cavity  18 . 
     As seen in  FIG. 1A , a system  20  for implementing the present invention includes a thermal imaging device, depicted herein as comprising an infrared camera  22  directed at the surface  14  and connected to a display screen  24  capable of displaying thermal images acquired by the infrared camera  22 , whereby real-time images may be displayed to an observer or inspector to determine the location and characteristics of the discontinuity. Alternatively, in place of the display screen  24 , a digital processor may be provided connected to the camera  22 , e.g., a digital camera, for implementing the discontinuity detection method in an automated system, such as for implementing a computer-aided non-destructive examination process. A computer-aided examination process for the present method may comprise computer software implementation of known auto-defect recognition techniques. 
     In a step of monitoring the surface  14  to detect the surface discontinuity  12 , the camera  22  acquires thermal images of the surface  14  following application of the liquid detection medium  16 . The thermal images comprise discontinuity signatures where an endothermic reaction of the liquid detection medium  16  on the surface  14 , due to vaporization or evaporation (indicated by arrows  26 ), produces a temperature differential relative to an endothermic reaction of the liquid detection medium  16  present within the discontinuity  12 . Specifically, the liquid detection medium  16  within the discontinuity  12  has a greater volume and will evaporate more slowly than the liquid detection medium on the surface  14 . Hence, the discontinuity signature acquired by the camera  22  corresponds to a lower frequency infrared emission  28  from the discontinuity  12  and will appear as a warm signature relative to the area of the surface  14  surrounding the discontinuity  12 , where the liquid detection medium  16  has a lower volume and will evaporate more quickly to create a cooler temperature signature. 
     The system  20  is substantially sensitive to small temperature changes. In particular, the camera  22  is capable of detecting changes at least as small as approximately 0.5° C., and preferably comprises a detection capability of approximately 10 millikelvin. Accordingly, although the discontinuity  12  being detected may be small, e.g., a crack, with a correspondingly small volume for the cavity  18  to receive the liquid detection medium  16 , the additional volume of the cavity  18  is sufficient to provide a detectable warm signature relative to the temperature signature of the surrounding surface  14 . Since the discontinuity signature provided by the temperature differential, and associated temperature signatures, of the discontinuity  12  and the surrounding surface  14  are produced by an endothermic reaction of the liquid detection medium  16 , no additional energy input to the specimen, such as by ultrasonic stimulation of the specimen  10  or heat input to the specimen  10 , is required to provide measurable results for locating the discontinuity  12  using the present system  20 . 
       FIG. 2  illustrates a component or test specimen  110  in accordance with a second embodiment of the invention. A surface discontinuity is indicated by reference numeral  112  and extends inwardly from a surface  114  of the specimen  110 , see  FIG. 2A . As in the previous embodiment, in an initial step of the method of the present embodiment, a detection medium  116  is applied to substantially coat at least a portion of the surface  114  to be monitored as well as to transport into the discontinuity  112 . For example, the detection medium  116  may be a liquid detection medium and may transport into the discontinuity  112  through capillary action drawing the detection medium  116  into a subsurface cavity  118  defined by the discontinuity  112 . The detection medium  116  may be applied by any known technique to substantially cover at least a portion of the specimen surface  114  to be monitored with the detection medium  116 , with sufficient liquid being applied to cause the detection medium  116  to transport under capillary action into the discontinuity  112  to fill the cavity  118 . 
     Following application of the detection medium  116  to the surface  114 , and entry of the liquid detection medium into the cavity  118  of the discontinuity  112 , the detection medium  116  is wiped or substantially removed from the surface  114 , such that only the detection medium  116  within the cavity  118  remains, as is illustrated in  FIGS. 3 and 3A . 
     Referring to  FIG. 4 , a reacting medium  130  is applied to the same portion of the specimen surface  114  that previously received the application of the detection medium  116 . The reacting medium  130  may be applied by any known technique to cover a substantial portion of the surface surrounding the discontinuity  112 , as well as to enter the cavity  118 . 
     The detection medium  116  and the reacting medium  130  comprise different chemical substances that mix and react with each other to produce an exothermic reaction. For example, one of the detection medium  116  and the reacting medium  130  may comprise a substance that is chemically alkaline, and the other of the detection medium  116  and the reacting medium  130  may comprise a substance that is chemically acidic. Further, one of the detection medium  116  and the reacting medium  130  may be a liquid, and the other of the detection medium  116  and the reacting medium  130  may comprise either a liquid or a solid, e.g., a powder. The detection medium  116  and the reacting medium  130  are preferably selected with reference to the material of the specimen  110 , such that the detection medium  116  and reacting medium  130  do not cause deterioration of the specimen  110  through contact with the surface  114  or within the cavity  118 . 
     As seen in  FIG. 4A , a system  120  for implementing the present invention includes a thermal imaging device. As with the previous embodiment, the thermal imaging device may comprise an infrared camera  122  directed at the surface  114  and connected to a display screen  124  capable of displaying thermal images acquired by the infrared camera  122 , whereby real-time images may be displayed to an observer or inspector to determine the location and characteristics of the surface discontinuity. Alternatively, in place of the display screen  124 , a digital processor may be provided connected to the camera  122 , e.g., a digital camera, for implementing the surface discontinuity detection method in an automated system, such as for implementing a computer-aided non-destructive examination process. 
     In a step of monitoring the surface  114  to detect the discontinuity  112 , the camera  122  acquires thermal images of the surface  114  following application of the reacting medium  130 . In particular, upon application of the reacting medium  130  to the surface  114 , at least a portion of the reacting medium  130  will enter the cavity  118  and mix or interact with the detection medium  116 , as indicated at  132 . The interaction of the detection medium  116  and the reacting medium  130  comprises an exothermic reaction at the location of the discontinuity  112 , and defines a temperature differential relative to the portion of the surface  114  surrounding the discontinuity  112 . The thermal images acquired by the camera  122  comprise discontinuity signatures of the exothermic reaction at the discontinuity  112  relative to the surface  114  surrounding the discontinuity  112 . In particular, the discontinuity signature acquired by the camera  122  corresponds to a lower frequency infrared emission  128  from the discontinuity  112  and will appear as a warm signature relative to the area of the surface  114  surrounding the discontinuity  112 , where the detection medium  116  has been substantially removed and no reaction occurs upon application of the reacting medium  130 , to provide a cooler temperature signature surrounding the discontinuity  112 . 
     Since the discontinuity signature provided by the temperature differential, and associated temperature signatures, of the discontinuity  112  and the surrounding surface  114  are produced by an exothermic reaction of the detection medium  116  reacting with the reacting medium  130 , no additional energy input to the specimen, such as, for example, ultrasonic stimulation of the specimen  110  or heat input to the specimen  110 , is required to provide measurable results for locating the discontinuity  112  using the present system  120 . 
     From the above description, it should be apparent that the present invention provides a method of detecting a discontinuity on a specimen through a readily implemented chemical surface treatment, providing a sensitive thermal imaging indication of the discontinuity. 
     While particular embodiments of the present invention have been illustrated and described, it would be obvious to those skilled in the art that various other changes and modifications can be made without departing from the spirit and scope of the invention. It is therefore intended to cover in the appended claims all such changes and modifications that are within the scope of this invention.