Abstract:
Methods for examining through holes of a component according to prior art generally use hot gases for the thermographic detection of blockages. The inventive method for examining the structure of through holes of a component considerably simplifies said techniques, using a medium which has at least one absorption edge in the region of the wavelength of the camera and thus appears opaque in the camera image.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is the US National Stage of International Application No. PCT/EP2003/010172, filed Sep. 12, 2003 and claims the benefit thereof. The International Application claims the benefits of European Patent application No. 02024601.3 EP filed Nov. 4, 2002, both of the applications are incorporated by reference herein in their entirety. 
     FIELD OF INVENTION 
     The invention relates to a method of checking the structure of through-holes of a component according to the precharacterizing clause of claim  1 . 
     BACKGROUND OF INVENTION 
     Through-holes of components, for example laser-drilled holes, in particular cooling-air holes of gas turbine blades, often have complex geometries that differ from a cylindrical form. The diameter of the hole that is effective for flow, the location of the hole in the wall of the component, the position and the location and the offset of the diffusors (outflow region widened in cross section) of these holes vary on account of tolerances of the casting, laser or erosion process for example, or on account of the respective production conditions. 
     The effectiveness of the cooling-air bores on the airfoil profile of the turbine blade results from the complex interrelationship between these stated variables. Up to the present time, they cannot be determined or measured in an automated manner or without great technical expenditure. 
     With the conventional methods, the continuity of bores is checked by the detection of the heated component surface, i.e. if the hole is blocked, no heating of the material at the bore hole will occur. The disadvantage of this method is that a small opening (partial closure of the bore) also allows air to pass through and heat up the material. In a thermographic image it is scarcely possible to distinguish between partially closed bores and open bores. 
     Both DE 35 33 186 A1 and DE 197 20 461 A1 show thermographic methods in which a heated gas is forced through the cooling-air bores. The supply of warm air entails considerable expenditure on apparatus. The conventional thermographic method records the temperature distribution on the component surface which is heated by the warm air. However, conclusions concerning the form of the bore cannot be drawn from the information which can be obtained. 
     SUMMARY OF INVENTION 
     It is therefore the object of the invention to overcome this problem. 
     The object is achieved by a method of checking the structure of through-holes of a component according to claim  1 . 
     Further advantageous refinements of the method are listed in the subclaims. 
     A visual representation of the flow of gas flares emerging from the through-holes with the aid of the camera images produced and image analysis provides a wealth of information on the formation and location of the through-holes and permits both process control and design verification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a device  1  with which the method according to the invention can be carried out, and 
         FIG. 2  shows a camera image taken with the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The device  1  according to  FIG. 1  comprises, inter alia, a computer  3  with a screen, to which a camera  13 , for example an infrared camera  13 , possibly a source of illumination  28  and further control elements  19  are connected. 
     Also connected to the computer  3  for example is a supply of medium  7 , which controls the flow of a medium (gas, fluid) into the interior of the component  10 . 
     This medium then emerges again from through-holes  25  in the surface  22  ( FIG. 2 ) of the component  10 , for example at the diffusor that is present. 
     The component  10 , or at least a through-hole  25 , is irradiated by the source of irradiation  28 . The source of irradiation  28  has a specific wavelength range. The source of irradiation  28  may also be ambient light. The rays of the source of irradiation  28  impinge on the surface  22  of the component  10 , where they are reflected and absorbed. The reflected rays are recorded by the camera  13 . 
     The medium has in the region of the wavelength(s) used by the source of irradiation  28  at least an absorption line, edge or strip. 
     Since the medium absorbs the rays of the source of irradiation in the region of the through-hole  25 , the rays of the source of irradiation  28  which impinge in the region of the through-hole  25  are consequently at least attenuated, and do not reach the camera  13 , or only in an attenuated form. 
     The wavelength or wavelength range of the source of irradiation ( 28 ) can consequently be detected by the camera ( 13 ). 
     The surface  22  of the component  10  is for example recorded by an infrared camera  13 . 
     In order to measure the entire surface  22 , the component  10  is for example arranged on an adjusting unit  16 , which is movable, for example rotatable. Similarly, the component  10  may be fixedly arranged and the infrared camera  13  is moved in relation to the surface  22  of the component  10 . The component  10  and the adjusting unit  16  may also be movable in all three spatial directions. 
     According to the invention, the medium, for example a gas, absorbs in the range of the wavelength(s) of the source of irradiation  28  that are used. 
     The medium is, for example, carbon dioxide (CO2), which has an absorption band in the range of the wavelength of 3–5 (μm). The source of irradiation  28  possibly has at least a wavelength in the range of 3–5 (μm). The camera can detect at least this one wavelength of the source of irradiation  28 . 
     Consequently, it is possible to distinguish this gas from the surroundings in the camera image as an opaque matter. The gas CO2 is particularly well suited, since it has similar fluid-dynamic properties to air, which is used for example as the cooling medium. 
     The evaluation of the camera image at individual through-holes  25  leads to the concentration distribution or propagation direction of the medium flowing out from the through-hole  25 . Given sequential expulsion of CO2 clouds and integration of the concentration values, the determination of the amount, and consequently of the through-flow capacity, of the respective bore hole is possible from the concentration distribution. With this information, the production parameters, for example of the laser and erosion processes, can be optimally adapted for individual through-holes  22 . 
     The observation by means of stereo perspective or the variation of the observation angle of the camera  13  and component  10  makes it possible to determine the three-dimensional propagation of a flare of medium via the diffusor and the adjacent outer profile region. This results in possibilities, for example numerical models, for verifying the flow distribution at the through-hole  22  and diffusor. 
     After that, the analysis of a gas flare allows further statements to be made concerning the bore diameter and effects of the geometry of the hole on the outflow behavior—and also the angle of emergence, etc. 
     The flowing medium may have the same temperature as the component  10 ; therefore, by contrast with the previously known thermographic methods, it does not have to be heated up. 
     Similarly, however, it is possible to heat up the medium, if an absorption band in the wavelength range of the camera  13  that is used is achieved by the heating up. 
     Warm gas or fluid may also flow through the through-hole  25 , in order to investigate the outflow behavior of warm gases. For example, gas with a temperature that is greater than room temperature also flows through a cooling-air bore of a turbine blade during operation. 
     Control elements  19  coordinate for example the movement of the camera  13 , the source of irradiation and the component  10  and also the medium flow  7  in relation to one another. 
       FIG. 2  shows a camera image taken with the method according to the invention. 
     On the surface  22  of the component with the through-hole  25 , the medium flowing out appears for example as black against the considerably lighter-appearing surface. 
     The outflow region of the gas after emergence from the through-hole  25  also appears for example as black against the surroundings.