Instrument for testing materials
BACKGROUND OF THE INVENTION 
     This invention provides an instrument for testing paramagnetic and diamagnetic metallic materials, as well as ferromagnetic materials above their Curie points, for material defects and dimensional accuracy. It is particularly useful for testing tubes and slabs above 1000° C. The instrument essentially comprises an electromagnetic transducer (EMT) having two magnetic pole shoes, an eddy current exciting winding, a receiving winding, and an electronic signal processing unit for processing signals from the receiving winding. 
     In the pursuit of quality control and optimum production efficiency, it is advantageous to test the quality of a product at an early stage of its manufacture so that, in the event that a defect is detected, correction can be made and product will not be wasted. For example, in the manufacture of seamless tubes, especially steel tubes, it is important to test tolerance accuracy while the tube is hot in order to detect erroneous settings of production line equipment while there is still sufficient time to control the roll stands further down the line. 
     From literature on this subject, it is apparent that many different techniques are known for measuring hot test pieces. These known techniques can be classified into contacting measuring techniques and non-contacting measuring techniques. 
     One such technique uses an ultrasonic piezoelectric oscillator with a high-pressure flow water coupling. At high pressure a stretch of water is generated between the test piece and the ultrasonic oscillator. The test results achieved with such an arrangement are highly dependent upon the surface quality of the test piece. The test procedure is relatively slow and requires a relatively large quantity of water. Furthermore, the coupling water causes local cooling of the test piece which can cause strains and cracks in the test piece. 
     Another technique uses piezoelectric oscillators with pressure couplings. Such arrangements permit the direct coupling of the oscillator at high pressure within the range of a roll body. However, such arrangements limit the positioning of the measuring apparatus since measurements cannot be made at any desired point of the train of rollers. Also, it is impossible to take a measurement for tolerance accuracy. 
     It is also known to excite ultrasonic pulses by using a high capacity pulse laser to locally heat the test piece. However, the use of this instrument for industrial testing is limited. 
     In addition to the ultrasonic arrangements, it is also known to test materials in production using radiological methods. These methods require working with X-rays, beta rays or gamma rays and therefore present problems of protection from the radiation. Also, radiological testing requires a relatively long measuring time to obtain the accuracy of measurement normally required. 
     Another known technique is the electrodynamic excitation of ultrasonics. This technique is based on the principle that a force is exerted on a current-carrying body in a magnetic field, acting vertically on the plane that is charged by the magnetic field vector and the current vector. A solenoid generates the magnetic field via a pole shoe configuration while an eddy current pulse is excited in the test piece using a transformer coil. The intensity of the power pulse generated in such a manner and thus the force of the ultrasonic pulse is a function of the magnitude of the eddy current and on the strength of the magnetic field. 
     In order to be able to generate a sufficiently strong magnetic field and a large eddy current to permit ultrasonic evaluation of the test piece, the EMT head must be positioned very close to the test piece. Depending upon the design of the EMT head itself, different types of ultrasonic waves can be generated. Know methods and instruments are described in the German specification No. 26 55 804 and German application No. 26 21 684 and No. 28 43 804. However, these known instruments have an operational disadvantage. Dependng on its specific design, an EMT head is suitable only for generating transverse waves, guided waves or Lamb-waves, which at high temperatures in the test piece are strongly damped since the shearing module is highly decreasing and thus they cannot be exploited technically. In addition thereto, all of the abovementioned arrangements have the disadvantages that they are restricted to the use of ferritic materials or to such geometries of test pieces that fit between the pole shoes of the EMT. This means that the thickness of the test piece is limited to some millimeters. 
     The longitudinal wave transducer required for measurements taken on hot test pieces or such made of paramagnetic or diamagnetic metals are described, e.g., in the British Journal of Non-Destructive Testing, 20, 1978, No. 5, September, pages 242 to 247, in Material Evaluation, 34, 1976, No. 4, April pages 81 to 90; and in Ultrasonics, 16, 1978, No. 7, July, pages 151 to 155. However, these publications describe only transducers that are not suitable for use at high temperatures and within small distances of the test piece, as they permit only a maximum test piece temperature of 1000° C. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the present invention to provide an instrument for testing of paramagnetic and diamagnetic metallic materials, as well as of ferromagnetic materials above their Curie points for material defects and dimensional accuracy, and especially for testing tubes and slabs above 1000° C. The test instrument essentially comprises an EMT having two magnetic pole shoes, an eddy current exciting winding, an eddy current receiving winding, and an electronic signal processing unit for processing signals from the eddy current receiving winding. The EMT according to the present invention overcomes the major operational problems associated with prior art devices. Specifically, it generates longitudinal waves, it can be moved within 1 mm of the test piece, and is insensitive to high temperatures. 
     A primary feature of the EMT is that the pole shoes are designed so as to produce a magnetic field parallel to the test piece. As an additional feature, the eddy current exciting and receiving windings are heat-insulated on their respective sides facing the test piece. These windings, on their respective sides facing the test piece, are embedded in a heat-insulating ceramic layer, and on their respective sides facing away from the test piece are embedded in a heat-conducting ceramic layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be further described with reference to the following Figures, wherein: 
     FIG. 1 is a schematic block diagram of the test instrument according to the present invention; 
     FIG. 2 is a more detailed schematic diagram of adapter member 4, shown in block diagram form in FIG. 1; 
     FIG. 3 is a cross-sectional view of transformer 1 shown schematically in FIG. 1; 
     FIG. 4 is an expanded view of a portion of the transformer shown in FIG. 3; 
     FIG. 5 is a further expanded view of a portion of the transformer shown in FIG. 3 and illustrating a magnetic field generated by the transformer; and 
     FIG. 6 is a sectional view of FIG. 5. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the Figures, the testing and measuring instrument according to the present invention comprises an EMT 1 having a magnetic field exciting winding 22, magnetic pole shoes 8 and 11 (shown in FIG. 3), an eddy current exciting winding 5 and an eddy current receiving winding 3; and an electronic signal processing unit 23. 
     Eddy current exciting winding 5 and receiving winding 3 are made heat-resistant thereby permitting their placement within a mm of a hot test piece 2. To provide this heat resistance, there is provided a heat-insulating protective ceramic layer 6 (shown in FIG. 3) of, e.g., a thickness of approximately 0.5 mm, preferably of aluminum oxide, on the sides of windings 3 and 5 facing the test piece. Even in the extreme case of a test piece temperature of 1300° C., the temperature on the EMT will not exceed approximately 350° C. The insulated wire, e.g., copper wire, used for the windings 3 and 5, can withstand a continuous heat load up to approximately 200° C. A heat-conducting ceramic layer 7 is applied to that side of each winding facing the EMT (facing away from hot test piece 2), to further cool the windings so that the continuous heat loading capacity will not be exceeded. Ceramic layer 7 provides heat conduction and preferably seals the gap to pole shoes 8 and 11 that is required by the expansiveness of heat-insulating ceramic layer 6. In the preferred embodiment, layer 7 is formed by a heat-conducting ceramic bonding agent. The surface of windings 3 and 5 facing the transformer is preferably additionally cooled by means of a cooling liquid flowing through a cooling liquid passage 24 (see FIG. 3). 
     In addition to the heat-insulation of the windings 3 and 5, the design of pole shoes 8 and 11 is very important. In order to be able to excite longitudinal waves in the test piece, one must generate a rather strong magnetic field 25 (shown in FIG. 5) parallel to the surface of the test piece. Within the range of the eddy currents only horizontal components are desired. Vertical components excite transverse waves which in hot test pieces do not propagate and cause disturbing echoes when cold paramagnetic and diamagnetic materials are used. Therefore, in order to obtain a field as shown in FIG. 5, it is especially preferably to make one pole shoe 8 (pole shoe 8) of conical design. The angle of conical surface 35 is determined by the width required for eddy current exciting winding 5 and eddy current receiving winding 3. In order to obtain a symmetric magnetic field independent of the distance between transformer 1 and test piece 2, it is desirable that the surface 9 of pole shoe 8 be positioned parallel to the test piece, and be approximately of the same size as the corresponding surface 10 of the counter-pole shoe 11. It is also desirable to incline the pole shoe 11 against the plane of the pole shoe surfaces 9 and 10 at an angle α of at least 20°. 
     The inclination of the edge surface 12 of pole shoe 11 should preferably correspond to approximately the angle of the conical surface 35 of pole shoe 8. In order to prevent a substantial loss of field strength, the edge formed by the surface 13 (see FIG. 4) of pole shoe 11 and it edge surface 12 is broken so as to define an edge surface 34. Using this design, it is possible to avoid an especially powerfully developed magnetic field at this place that would otherwise be formed due to the point effect (by reducing the distance). 
     Special importance must be attached to the construction and inclination of the pole shoes 8 and 11 for another reason. In order to avoid the eddy currents induced by he windings 3 and 5 in the pole shoes 8 and 11, that could lead to ultrasonic pulses in the pole shoes that could return again and again to their places of origin by multiple reflection or appear as set noise in the receiver, it is preferable to choose the inclination angle of pole shoe 11 in such a manner that these interfering impulses will flow outwardly along pole shoe 11. Pole shoe 8 is preferably formed from sheets insulated from each other. This also results in an expansion of the tolerance range for the distance between transformer 1 and test piece 2. 
     To protect pole shoes 8 and 11 from heat so that the pole shoe material will keep its functional properties below its Curie point, the pole shoes are cooled. Pole shoe 11 preferably has a cooler 14 (see FIG. 3) of non-ferritic material, through which a cooling agent flows. Pole shoe 8, on the other hand, is provided with a cooling channel 26 (see FIG. 3), passing a cooling agent also along heat-conducting ceramic layer 7 and magnetic field exciting winding 22, and in that manner providing for the cooling of these members. The cooling agent is pumped in a closed or open loop system by a pump 27 (see FIG. 1) through EMT 1. 
     Referring now specifically to FIG. 1, there is shown in schematic and block diagram form the electrical construction of the instrument according to the invention. To generate the ultrasonic pulses, a transmitter 28 via an adapter means 29 supplies a signal to eddy current exciting winding 5. Echoes from test piece 2 are received via receiving winding 3. Through an active adapter means 4 the signals are coupled to a display amplifier 30 and then to a display 31. The total operation is activated by a control 32. The magnetic field is controlled via a direct current source 33. 
     Referring now specifically to FIG. 2, active adapter means 4 of electronic signal processing unit 23 preferably comprises an electronically controllable damping means 16, adapter means 17, a transmit-receive switch 18, a band-pass filter 19, a pre-amplifier 20 and a line driver 21. Adapter means 17 comprises one or more matching capacitors. 
     The signals from receiving winding 3 are scanned by adapter means 17, transferred via the pre-amplifier 20 and thereafter via a line driver 21 to a line, which can be of any length in this configuration. To protect pre-amplifier 20 from overmodulating, there is provided a transmit-receive switch 18, essentially comprising an analog switch and diverse protective diodes. 
     The electronically controllable damping means 16 located in the input circuit forms a resonant circuit winding 3 and adapter means 17. Transmission can be damped to such an extent that there will be no inadmissibly long dead time during which echoes cannot be received. If for tolerance reasons the distance between EMT 1 and test piece 2 is changed, this may require a variation of the damping of the oscillatory circuit resulting in a longer transmitting dead time and a broadening of the echo pulses. A broadening of the echo pulses, however, would decrease resolution capacity. On the other hand, active adapter means 4 according to the invention provides an adequate damping of the transmit and echo signals. This is realized by electronically controllable damping means 16 and band-pass filter 19. 
     The following non-limitative examples represent typical embodiments of the instrument according to the invention: 
     EXAMPLE 1 
     
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Test Piece:        Tube with a wall                                       
                   thickness of 40 mm                                     
Material:          Ferritic steel                                         
Temperature:       1200° C.                                        
Measurement:       Wall thickness                                         
Transmitter:       Length of pulse 1 s at                                 
                   300 V on 12 ohms                                       
Transformer distance:                                                     
                   1 mm (tolerance ± 0.5 mm)                           
Transformer windings:                                                     
                   Transmitter 20, 0.1 mm                                 
                   copper wire                                            
                   Receiver 50, 0.05 mm                                   
                   copper wire                                            
Accuracy of measurement:                                                  
                   ± 0.1 mm                                            
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     EXAMPLE 2 
     
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Test piece:        Aluminum band with a                                   
                   thickness of 20 mm                                     
Temperature:       Ambient temperature                                    
Measurement:       Thickness of band                                      
Transmitter:       Pulse length 1 s at                                    
                   300 V on 12 ohms                                       
Transformer distance:                                                     
                   0.85 mm (tolerance ± 0.3                            
                   mm)                                                    
Transformer windings:                                                     
                   Transmitter 29, 0.1 mm                                 
                   copper enamelled wire                                  
                   Receiver 50, 0.05 mm                                   
                   copper enamelled wire                                  
Accuracy of measurement:                                                  
                   ± 0.05 mm                                           
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     The invention is not restricted to air as a working energy, and is also operating under a heavy radial load, e.g., in hot cells of nuclear plants. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims which scope it to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures.