GB2277157A

GB2277157A - Method and test probe for the non-destructive testing of the surfaces of electrically conductive materials.

Info

Publication number: GB2277157A
Application number: GB9406230A
Authority: GB
Inventors: Klaus Dobler; Hansjoerg Hachtel; Stanislaw Bodzak
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1993-04-02
Filing date: 1994-03-29
Publication date: 1994-10-19
Anticipated expiration: 2014-03-29
Also published as: GB2277157B; DE4310893C2; GB9406230D0; DE4310893A1; GB9718522D0; FR2703465B1; FR2703465A1

Description

A 2277157 Method and test Drobe for the non-destructive testina of the

surfaces of electricallv conductive materials

Prior art

The invention is based on a method and a test probe of the type described in the independent Claims 1 and 3. A number of publications have already proposed carrying out nondestructive tests of materials with the aid of the eddy current method. An introduction to the method and possible test coil designs can be found, for example, on pages 129 to 141 of the book by Heptner and Stroppe, entitled "WerkstoffprUfung" ("Materials testing"). According to this, among other things the eddy current method is suitable for testing the mechanical hardness of a material and the thickness of particularly thin layers of metal. Examples of suitable coil designs are an exploring coil which is placed on the workpiece surface or an internal coil which is used for testing very thick-walled pipes or holes, for example.

From pages 213 to 218 of issue No. 4 1986 of the publication 11HTMI1, the proposal is known to use an eddy current method of measurement to determine incremental permeability. Information about the depth of the heat-influenced layer can be obtained via the coercive field strength derived from this. The measurement set-up essentially comprises a coil and a pick-up arranged separate from the coil. Although a range of good test information can be obtained with this measurement arrangement, particularly as regards hardness and thickness of the surface layer, the measurement set-up and the procedure are comparatively complicated.

2 A further use of the eddy current method of measurement for the nondestructive measurement of the connecting layer thickness of samples nitrided in a salt bath is known from pages 89 to 94 of vol. 2 of the 11Zeitschrift fUr wirtschaftliche Fertigung11 ("Economic Production Journal,'). In this case the layer thickness is determined with the aid of the impedance measured for the material under test, with fixed frequency of the test coil a.c. voltage. A coil is used as the measuring probe. This method does not provide any information about the hardness of the layer tested and it has only limited suitability for frequencies under 3 MHz because permeability variations in the base material then exert a stronger influence.

The object of the invention is to propose a method which quickly supplies sufficiently reliable information particularly about the hardness and thickness of the surface of an electrically conductive material by means of a sensor of simple construction. This object is achieved by a method and a test probe with the features of Claims 1 and 3. The method according to the invention supplies information about the hardness and/or thickness of a tested surface layer in a simple way by measuring the coil inductance. In this case use is made of the essentially known relationship between the depth of penetration of the alternating magnetic field generated by the coil, the coil inductance and the hardness and/or thickness of the material tested.

To implement the method, test measurements are carried out in each case for two different frequencies of the coil alternating current. The first frequency is selected to be so high, e.g. 3 MHz, that the resulting depth of penetration of the alternating magnetic field generated by the coil into the material under test is small compared with a desired overall thickness of the surface layer tested. From the inductance of the coil that arises, information about the hardness, structure or surface texture can be derived depending on the

41 3 R. 26053 pre-treatment of the material under test. In particular.it is possible to determine whether the tested surface has a desired minimum thickness, whether a hardened layer is present at all and, if necessary, whether these have cavities, cracks, burrs or defects of form.

The second frequency of the coil alternating current is selected to be so low, e.g. 10 KHz, that the depth of penetration of the alternating magnetic field into the material under test approximately corresponds to a desired thickness of the tested surface layer. Information about the overall quality of the surface over the thickness range tested can be derived from the coil inductance that arises in this way.

A two-stage method of this kind provides information about the hardness of a thin layer which forms the actual surface, by means of the measurement which takes place at high frequency, as well as information about the quality of the layer under the surface by means of the measurement which takes place at lower frequency. This overcomes the problem posed by the fact that on the one hand, a high-frequency measurement only can lead to an incorrect assessment particularly of the thickness of a surface layer and on the other hand that a low-frequency measurement only provides no information about the conditions on the actual surface.

The method is particularly suitable for testing the surface quality of nitrided metallic materials.

Claims

The method according to the invention is advantageously supplemented by

combining with a method with the distinguishing features of Claim 3. By means of an additional measurement at a third frequency which is particularly suitable for this, the material under test is examined for material defects. In this case the measurement frequency is so selected that a measurement signal is only generated if a

4 material defect is present. A material test of this kind can also be easily carried out by unskilled persons.

If effects deriving from changes in the alloy elements or alloy contents of the tested surface are to be kept to a minimum it is appropriate to provide a reference measurement by means of a reference coil. The differential signal of the test coil and the reference coil is used as the measurement signal, for example, the reference coil detecting the inductance of a non-heat-treated base material.

In the case of materials with surfaces treated so as to obtain special properties, such as nitrided metals, in order further to improve the accuracy it can be of advantage to carry out a reference measurement with a reference material, a measurement on the test and on the reference material being undertaken both before and after the treatment of the material under test.

The test probe used to implement the method is suitably a coil arranged at the tip of a coil holder. The pick-up winding is matched to the shape and/or task of the device under test. It is meaningful for the size of the coil used to be selected according to whether information about the average hardness of a fairly large surface of the device under test will be adequate or whether a detailed grid of the test surface is required.

In some applications, such as internal measurements in holes, it can be of advantage to provide the pick-up coil with covers and to carry out the measurement of the surface under test via the uncovered part of the test coil only. A profile of the condition along the internal circumference of the hole can be obtained by rotating a pick-up coil constructed in this way.

A ferrite core arranged in the centre of the coil advantageously increases the measurement effect of the test coil.

Instead of the coil inductance, its a.c. resistance can also be measured. The advantages and implementation of the method remain unchanged.

Embodiments of the proposed invention will be described in greater detail below with the aid of the drawings.

Drawings Fig. 1 shows a section through a test probe placed on a surface under test, Fig. 2 a section through a special design of a test probe with an associated test body, Fig. 3 a detail of this probe and Fig. 4 the theoretical construction of evaluation electronics.

Description Fig. 1 shows the theoretical construction of a test probe required in order to implement the proposed method. It essentially comprises a coil holder 11 at whose tip a sensor coil 10 is arranged. Its ends are connected to an alternating current generator arrangement which is not shown via supply leads 13. The leads 13 can be fed in a hole or holes inside the coil holder 11. A ferrite core 12 can be provided in the centre of the sensor coil 10. The surface of the test probe is matched to the surface 16 of the material under test 15. In the example in Fig. 1 the material under test has a flat surface 16, and so the surface of the test probe is also flat.

Fig. 2 shows a possible embodiment of the test probe for a nozzle body with a valve at its tip. In this case the surface under test 16 is the conical valve face at the end of a hole introduced into the material under test. The tip of the test 6 probe with the sensor coil 10 is correspondingly also conical. In embodiments of the test probe intended for testing internal surfaces, the sensor coil supply leads 13 are suitably always led inside the coil holder 11 in holes, as shown in Figs. 2 and/or 1, or in grooves arranged on the circumference of the coil holder.

The use of the test probe will now be described through the example of the testing of a nitrided surface. For the test operation the test probe is placed closely and firmly onto the surface under test 16. If the surface under test 16 is an internal surface as in Fig. 2, the test probe is pushed into the test body until it rests as closely as possible on the surface under test, the valve face in Fig. 2. A highfrequency alternating current to generate an alternating magnetic field in the coil 10 is applied to the coil 10 in the known way in respect of the inductive and/or eddy current method. The alternating magnetic field penetrates into the material under test 15 depending on the frequency of the alternating current and on the electrical and magnetic properties of the material tested. In its turn the material under test influences the alternating magnetic field according to the depth of penetration and its electrical and magnetic properties. A characteristic alternating magnetic field which is measurable is generated. The coil inductance and/or the alternating current resistance value of the pick-up winding is the measured variable.

To implement the method the frequency of the coil alternating current is initially so set that the depth of penetration of the alternating magnetic field generated is small, for example in the range from 3-10 pm, compared with the overall thickness of the treated surface layer 17. The precise frequency value is either calculated in the known way or determined experimentally in reference measurements. It is assumed that the material under test actually possesses the desired material composition. Small variations in the material 1 7 composition do not usually affect the measurement result. substantially and can be tolerated.

The measurement signal obtained for the coil inductance corresponds with good accuracy to the average hardness of the surface layer 17 of the tested material as far as the depth of penetration determined by the frequency. The measurement at high frequency therefore provides information as to whether the test material has a desired minimum hardness, at least immediately at the surface 16.

In the second step of the method the frequency of the coil alternating current - which is called the carrier frequency below - is so set that the depth of penetration of the alternating magnetic field into the material under test 15 called the device under test below - lies in the range of a minimum thickness desired for the surface layer 17 in question; typically this thickness is a few 100 pm. This measurement at lower frequency provides information about the average hardness of the entire tested layer 17. Again, information about the abrasion resistance of the tested layer 17 can be derived from it, for example. In Fig. 1 the different depths of penetration according to the carrier frequency in question are shown by dotted lines in the device under test 15, T1 denotes the depth of penetration at high carrier frequency, T2 at low carrier frequency. Practical values for the ratio T2:T1 can be substantially greater than shown in Fig. 1 and can, for example, readily assume the value T2:T1 = 60.

The combination of the two measurement steps enables materials to be evaluated in respect of their serviceability much more reliably than is possible with only one measurement at one frequency. For example, devices under test whose surfaces certainly have a desired property, particularly a desired hardness, but which are not sufficiently thick to withstand expected friction losses can be rejected.

8 Both method steps are, for example, carried out one immediately after the other by means of a single test probe, the carrier frequency of the test probe being switchable between the high and the low frequency. It is also possible to use two test probes each with fixed carrier frequencies, the device under test being exposed first to the one and then to the other test probe. The measurement results obtained from the two method steps are suitably evaluated by comparing them in a simple manner with specified reference values which correspond to a faultless item. If one of the measurement results differs from the reference values the device under test is declared to be defective.

A further meaningful supplement to the method, which can also be used as an independent test method, consists of testing the device under test for material defects. This can be simply carried out by skilful selection of a further carrier frequency which is different from the two used to implement the method according to Claim 1. This makes use of the fact that when an alternating magnetic field acts on a material under test, on the one hand a ferroelectric effect which intensifies the alternating field it is based on the alignment of the elementary magnets in the test material - and on the other hand an eddy current effect which weakens the alternating field in a known way occurs in the material. Both effects are frequency-dependent, the ferromagnetic effect decreasing and the eddy current effect increasing as the alternating field frequency increases. For a test for material defects the frequency of the alternating field is selected precisely so that the measurement signal of a faultless material corresponding to the material under test precisely corresponds to the measurement signal with respect to air. -If the test probe is placed on the material under test at this frequency, there will be no measurement signal provided that the test material is faultless. If a measurement signal is produced there is a material defect. Defects of form in the material under test are suppressed in i 9 this test method and so it is suitable, for example, for. devices under test with a considerable manufacturing tolerance or wide tolerances of form.

For the more accurate testing of internal surfaces, particularly holes, as shown in Fig. 3 it can also be appropriate partially to cover the sensor coil 10 with an electrically conductive material 18 so that the alternating magnetic field only reaches the test surface through the uncovered area 14. By rotating the test probe, curves are obtained which reproduce the qualitative configuration of the surface finish, particularly as regards its hardness, over the circumference of the internal surface of holes. Internal contour defects such as out-ofroundness, grooves or cracks can be identified in this way in addition to deviations from specified minimum hardness values.

In materials with treated surfaces, such as nitrided metals, the composition of the treated, particularly nitrided base material (Fe alloys) affects the measurement result. This is why it can be appropriate to carry out measurements related to a reference variable. The effects that are attributable to changes in the contents of the alloy elements can be reduced by reference measurements. The reference measurement is suitably carried out by means of an additional reference coil which detects the inductance of an untreated base material. If a very high degree of test accuracy is required it can also be meaningful to carry out measurements for the device under test with the test coil and the reference coil both before and after treatment.

Fig. 4 shows a simplified wiring diagram of an electrical arrangement suitable for implementing the proposed method. Essential components of the arrangement are the alternating current generator 51 for generating the coil alternating current and a measurement device comprising a resistor 52, a capacitor 53 and the sensor coil 10. The test device 15 with the treated sur face layer 17 is allocated to the sensor coil. The voltage signal applied to the sensor coil 10 is rectified via the diode 54, smoothed in the network 56 comprising the peak detector 57 and low pass 58 and amplified in the amplifier network 55 to the output voltage UM. The arrangement shown in Fig. 4 is by way of an example of a multiplicity of other arrangements which can easily be constructed by a skilled person. In particular it is possible to detect the change in the coil alternating current resistance instead of a change in inductance. The basic procedure and the interpretation of the measurement results can be applied unchanged in this case also.

The proposed method is not, of course, in principle restricted to a use for testing the surfaces of metallic materials. It can also be used to examine other materials or layers either directly or following suitable modification.

11 Claims 1. Method for the non-destructive testing of the surfaces of electrically conductive materials by means of a test coil which generates an alternating magnetic field to which the material under test is exposed, characterized by the following steps: establishment of an initial coil alternating current frequency, at which the depth of penetration of the alternating magnetic field into the material under test (15) is small compared with a desired thickness of the treated surface layer (17), measurement of the change in inductance which the establishing alternating magnetic field in the coil (10) creates, repetition of these steps with a second coil alternating current frequency at which the depth of penetration of the alternating magnetic field into the material under test (15) approximately corresponds to a desired thickness of the treated surface layer (17), - derivation of information as to whether the tested surface (16, 17) has a desired material property from the measurement results.

2. Method according to Claim 1, characterized in that the information about the material property concerns the hardness and/or the thickness of the tested surface (17, 18).

3. Method for the non-destructive testing of the surfaces of electrically conductive materials by means of a test coil 12 which generates an alternating magnetic field to which the material under test is exposed, characterized by the following steps: establishment of a coil alternating current frequency, at which the change in inductance in the coil (10) caused by a faultless material (15) precisely corresponds to the change in inductance in the coil (10) occurring in air, measurement of the change in inductance which the establishing alternating magnetic field in the coil (10) creates, derivation of information about the material quality of the surface (16, 17) of the material under test (15) from the measurement result.

4. Method according to Claim 1, characterized by the further steps: establishment of a third coil alternating current frequency, at which the change in inductance in the coil (10) caused by a faultless material (15) precisely corresponds to the change in inductance in the coil (10) occurring in air, - measurement of the change in inductance which the establishing alternating magnetic field in the coil (10) creates, derivation of information about the material quality of the surface (16, 17) of the material under test (15) from the measurement result obtained with the third coil alternating current frequency.

5. Test probe for the non-destructive testing of the surfaces of electrically conductive materials with a coil holder (11) and a sensor coil (10) arranged at its tip, characterized in that the tip of the test probe with the sensor coil (10) coincides with the surface under test (16) as regards its shape.

6. Test probe according to Claim 5, characterized by a conical tip.

13 7. Test probe according to Claim 5, characterized in that the sensor coil (10) is partially covered with electrically conductive material (18) on the side with which it is placed on the material under test (15).

8. Either of the methods of non-destruction testing substantially as herein described with reference to the accompanying drawings. 9. A test probe substantially as herein described with reference to the accompanying drawings.