Abstract:
The present invention relates to a testing device for detecting and determining material inhomogeneities in electrically conductive samples ( 10 ), comprising a support ( 30 ) for the samples ( 10 ) to be tested, a temperature regulating device ( 30, 50, 60 ) for configuring a temperature profile in the sample ( 10 ), a drive connected to the support ( 30 ) for changing the position of the sample ( 10 ) and at least one measuring sensor ( 20 ) for contactless measurement of the magnetic field outside the sample ( 10 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a testing device for detecting and localizing material inhomogeneities in electrically conductive subjects or samples. 
     According to the state of the art, with the testing of electromagnetic inclusions the subject is premagnetized and subsequently scanned with a magnetic field measuring apparatus as published by J. Tavrin and by J. Hinken at the “ 7 . Europäischen Konferenz für zerstörungsfreies Testen” (7th European Conference for non-destructive testing) in Copenhagen 1998 and in the document of the Institute Dr. Forster 04/95. By way of the scanning in at least two planes one may infer the depth position of the inclusions. With the testing for non-ferromagnetic inclusions or inhomogeneities the subject is brought into an external magnetic field, wherein this may also be the naturally present earth&#39;s field. On account of the succeptibility fluctuations in the subject the magnetic field outside the subject is location-dependent. With measurement with a magnetometer one may draw conclusions on the non-ferromagnetic inhomogeneities, as is known from the publication by J. P. Wikswo in IEEE Trans. Appl. Supercond., Volume 3, No. 1 of March 1993. Both measuring methods do not use a directed temperature change of the subject. 
     Thermoelectric effects were up to now only used for the sorting of similar materials, not for the detection and localization of inhomogeneities, as is known from a publication by McMaster in “Non-destructive Testing Handbook”, Second Edition, Volume 4, Electromagnetic Testing of the American Society for Non-destructive Testing of 1996 and from a publication by A. S. Karolik and A. A. Lukhvich in Sov. J. Nondestruct. Test., Volume 26, No. 10 of October 1990. Furthermore for this an electrical and mechanical contacting of the component is necessary. 
     The apparatus for magnetic field measurement described according to the state of the art, i.e. based on the remanence and the succeptibility have the disadvantage that the measuring signals are not strong enough to ascertain and to quantify also small inhomogeneities lying far below the surface. Measuring apparatus with thermoelectric effects have not yet been used for the detection and localization of inhomogeneities. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the invention, with magnetic field-supported, non-destructive testing of electrically conductive subjects to intensify the magnetic field signals and thus to increase the measuring resolution. This applies to inhomogeneities close to the surface as well as to those which lie deep below the surface. 
     On account of the temperature profile set in a sample by way of the temperature setting means, the magnetic field signals of the material of the probe, in particular the segregations, are increased in a manner such that material inhomogeneities may be detected and localized when the magnetic field outside the sample is measured during a position change. By way of this, material inhomogeneities on the surface and also deep below the surface of the sample may be detected in a non-destructive and exact manner. 
     The testing device according to the invention measures and tests in a non-destructive manner, wherein the device sets the temperature or the temperature gradient in the measured object in a targeted manner and measures the magnetic field outside the measured object. Characteristic magnetic field signatures arise on account of various physical effects. To these there belong temperature dependency of the succeptibility, thermoelectric effects and thermomagnetic effects. 
     Measuring signals which are based on succeptibility differences become stronger when this difference is greater. Now the succeptibility of many materials becomes larger with a reducing temperature. It is often roughly proportional to the inverse value of the absolute temperature. A cooling of the subject therefore increases the succeptibility of the base material and the inclusion and thus also the difference of both, as is known from the publication by W. Schultz “Dielektrische und magnetische Eigenschaften der Werkstoffe” (Dielectric and magnetic properties of the materials), Vieweg, Braunschweig of 1970. With this the cooling is contrast-intensifying. This method based on the succeptibility difference permits the detection also of inhomogeneities lying deep below the surface. 
     Of the thermoeletric effects in this context amongst others the Seebeck effect and the first Benedicks effect are used, which are known from the publication by Joachim Schubert: “Physikalische Effect” (Physical effects), Physik publishing house, Weinheim 1984. 
     If two contact locations lie between two different materials at different temperatures, between them there arises an electrical voltage. This is the thermoelectric voltage, the effect is the Seebeck effect. In the component to be tested these contact locations are formed by the border layer between the base material and the inclusion. If a temperature gradient lies over the inclusion there is created the condition for the existance of thermoelectric voltages and thermoelectric currents. These currents in turn also outside the tested object produce a magnetic field which may be detected with a magnetic field measuring apparatus. The mentioned temperature gradient may be created by cooling or heating. The polarity of the produced magnetic field together with the polarity of the temperature gradient give indications as to the material class of the inclusions. The inclusions to be detected with this must be electrically conductive. 
     Fractures or insulating inclusions in otherwise homogeneous material may be detected by way of the first Benedicks effect. According to the Benedicks effect in a homogeneous conductor there arises a thermoeletric voltage when there is present a high temperature slope. This thermoeletric voltage in turn results in thermoelectric currents whose distribution is disturbed by fractures and insulating inclusions. Corresponding changes in the magnetic field which are produced outside the tested object by way of these currents may be detected. 
     According to the invention the thermoeletric effects are observed without creating an electrical and mechanical contact. This has the advantages that the errors by way of unreproducable contacts are avoided, that the components may be scanned with more degrees of freedom and that with this there are left no traces of scratches. This measuring method based on thermoelectric effects permits inhomogeneities lying deep below the surface to be detected. 
     Further advantageous embodiments of the invention are the subject-matter of the dependent claims. 
     With the use of the thermoelectric effects the temperature slope in subsequent measurements may be differently set in a targeted manner. The measuring signals resulting therefrom give further information on the examined inhomogeneity, as e.g. an improved localization and shape detection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details, features and advantages of the invention result from the subsequent desription of one advantageous embodiment form of the invention by way of the drawings. 
     There are shown in 
     FIG. 1 a schematic representation of a testing device according to the invention; 
     FIG. 2 a graphic representation of a measuring signal of a sample, which has been determined by the testing device of FIG. 1; and 
     FIG. 3 a graphic representation of a measuring signal of a sample which has been determined by the testing device of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 there is shown an advantageous embodiment example of a testing device for detecting and localizing material inhomogeneities in a measured object or a sample  10 , which in particular is pre-magnetized. The sample  10  is a circular disk which is carried by a short tube piece  21  which serves as a distancer and a cold bridge. The lower side of the tube  21  is cooled with cooling fluid, in particular liquid nitrogen. In the sample  10  itself thus there arises a temperature gradient, i.e. a temperature slope with which at the top there is present a higher temperature and at the bottom a lower temperature. The sample  10  is rotated and at its upper side the magnetic field is scanned with a magnetic field measuring apparatus or a gradiometer  20 . As a magnetic field measuring apparatus there is used a Squid gradiometer  20  of the second order (HMT), as shown in FIG. 1, which measures the normal component of the magnetic field on the surface of the subject or of the sample  10 . This magnetic field measuring apparatus  20  consists of three individual Squid sensors  22  which are manufactured of high-temperature superconductors. For operation they are filled with liquid nitrogen. The three sensors  22  and their electronic channels are mechanically and electronically matched such that the background fields are extremely supressed. Only signals from the neighboring sample  10  are indicated, and specifically with a particularly high sensitivity. This measuring system thus does not require any magnetic shielding around the sample  10  and the sensors  22 , as is otherwise often necessary with Squid measuring systems. 
     There are various cooling methods, as shown in FIG. 1, which are based on the use of a cooling fluid. With the use of a first method  50  the probe  10  is cooled over a large surface on the lower side, and there sets in a certain temperature slope in the sample  10 . According to a second method  60  a tube piece  21  is cooled whose diameter may be suitably selected and varied. With the variation of the temperature slope, inhomogeneities present may be localized. The sample  10  may be measured from both sides by turning round. With this, mostly a polarity change and an amplitude change arc expected. The gradiometer  20  or the cryostat with gradiometer, in particular with “epoxy dewar” or epoxy-pole has a height of approx. 800 mm, wherein the diameter of the lower part is approx. 90 mm. The gradiometer  20  may be varied in its height above the sample  10  in order in subsequent measurements to determine the depth of an inhomogeneity. 
     The three Squid sensors  22  are normally as described above, connected to a gradiometer  20  of the second order. In FIG. 1 three Squid sensors  22  are connected to an electronic device  40 , wherein the electronic device  40  indicates a measuring result in ((d 2 Bz)//dz 2 ))(t) as is indicated by the arrow leading away from the electronic device  40 . This connecting may be simply changed so that the lower two and also the upper two Squid sensors  22  may in each case be connected to gradiometers of the first order. In this manner it is possible with these two magnetic field measuring apparatus to simultaneously measure at different distances to the sample  10  and furthermore to carry out a depth detection of inhomogeneities present. 
     The FIGS. 2 and 3 show graphic representations of measuring signals which were recorded with a testing device of FIG. 1, wherein the disk consisted of a nickel base alloy Waspaloy with a disk diameter of approx. 180 mm and a disk thickness of approx. 40 mm. In the FIGS. 2 and 3 the x-axis indicates the rotational angle of the sample  10  between 0° and 360°, wherein the y-axis indicates the magnetic field strength in (d 2 Bz)//dz 2 ). On the surface, by way of segregation sets at an angle φ which represents the minimum of the graphs, a hard-α segregation was recognized and localized. 
     FIG. 2 shows a distinct measuring signal at the location of the segregation, created by currents which according to the Seebeck effect flow in the sample. In FIG. 2 the temperature which means the measuring signal is very distinctive. 
     Under conditions which are otherwise the same, FIG. 3 shows the measurement with a weekly set temperature gradient with a correspondingly less strong measuring signal with the minimum of the graphs at φ=190°.