GB2070248A

GB2070248A - Measurement of the thickness of a surface layer of a body subjected to surface structure modification treatment

Info

Publication number: GB2070248A
Application number: GB8103366A
Authority: GB
Original assignee: Centro Ricerche Fiat SCpA
Current assignee: Centro Ricerche Fiat SCpA
Priority date: 1980-02-26
Filing date: 1981-02-04
Publication date: 1981-09-03
Also published as: DE3105002A1; IT1129066B; IT8067288A0; FR2476831A1

Abstract

A device (20) is described for the non-destructive measurement of the thickness of a surface layer of a body (10) subjected to surface structure modification treatment. It comprises a transmitting transducer (11) and a receiving transducer (12) to be disposed facing each other on diametrically opposite sides of said body (10); an oscillator (16) which supplies said transmitting transducer (11); and a phase difference measurement circuit (17) having a first input connected to said transmitting transducer (11) and a second input connected to said receiving transducer (12), and an output at which a signal is present which depends on the phase difference between the signals present at said first and second inputs. <IMAGE>

Description

SPECIFICATION Device for the non-destructive measurement of the thickness of a surface layer or a body subjected to surface structure modification treatment The present invention relates to a device for the non-destructive measurement of the thickness of a surface layer of a body subjected to surface structure modification treatment.

In particular, the present invention relates to a device for the non-destructive measurement of the hardening depth.

Devices are known at the present time for making non-destructive measurements of the hardening depth, these being based on the parasite current principle, and comprising generally an induction coil, an oscillator which supplies said coil, and means for measuring the voltage and current supplied to the coil.

When in use, the induction coil is disposed so that it faces the body to be examined, in such a manner that the magnetic field generated by it passes through the surface layer of the body. Consequently, parasite currents are set up in this latter, and generate a magnetic field in the opposite direction to that induced by the coil, and thus modify the intensity of the current which the coil takes from the oscillator, according to the thickness of the layer subjected to hardening. This is because the material of a hardened layer has a resistivity which is greater than that of a similar nonhardened material, and thus hardening treatment leads to a reduction in parasite currents and in the magnetic field generated thereby.

The presence of a hardened layer therefore leads to a variation in the current absorbed by the coil, or rather a variation in the impedance across the coil, and the extent of this variation depends on the thickness of the layer. Although devices of the aforesaid type are widely used, they are unsuitable for measuring hardening depths of the order of 10-15 mm. This is because the penetration of the magnetic field into the body to be examined is influenced by the skin effect, which limits the frequency range of the signals supplied by the oscillator to values of between 3 and 20 Hz, which correspond to maximum penetrations of the order of 2-3 mm. Greater hardening depths cannot be measured by these devices, because the measurement errors can even reach 100% of the effective thickness of the layer to be measured.

The object of the present invention is to provide a device which is particularly able to make a non-destructive measurement of the hardening depth, and which is free from the drawbacks of the aforesaid devices of known type.

The present invention provides a device 20 for the non-destructive measurement of the thickness of a surface layer of a body 10 subjected to surface structure modification treatment, characterised by comprising: a transmitting transducer 11 and a receiving transducer 1 2 to be disposed facing each other on diametrically opposite sides of said body 10; an oscillator 1 6 which supplies said transmitting transducer 11; and a phase difference measurement circuit 1 7 having a first input connected to said transmitting transducer 11 and a second input connected to said receiving transducer 12, and an output at which a signal is present which depends on the phase difference between the signals present at said first and second inputs.

The present invention will be more apparent from the description of a preferred embodiment given hereinafter by way of a nonlimiting example with reference to the accompanying drawings in which: Figures 1 and 2 are a theoretical illustration of the arrangement adopted for a measurement of hardening depth; Figure 3 is a diagrammatic perspective view of a device according to the present invention; and Figure 4 is a graph showing the variation in a quantity measured by the device of Fig. 3.

With particular reference to Fig. 1, the reference numeral 10 indicates a homogeneous test piece of thickness L, on which two electromechanical transducers, namely a transmitting transducer 11 and receiving transducer 12, for example of piezoelectric type, are disposed on diametrically opposite sides. A sinusoidal signal having a frequency of the order of a few units of MHz, for example 5 MHz, is fed to the transducer 11 in a manner not shown in Fig. 1.

In Fig. 2, the reference numeral 10 indicates the same test piece as in Fig. 1 after it has undergone hardening treatment, the consequence of which is the creation of a surface layer 1 4 of depth a, and a change dL in the thickness of the test piece 1 0.

Fig. 3 shows the test piece 10, of cylindrical structure, immersed in a liquid such as water or oil contained in a vessel 1 5. Support slides are provided for the test piece 10 in a manner not shown, and are also able to transmit to the test piece an axial movement and/or a rotation about its axis. The transducers 11 and 1 2 are supported by opposite walls of the vessel 1 5 in such a manner that the test piece 10 is positioned along the path through which the ultrasonic waves generated by the transducer 11 pass in reaching the transducer 12, as shown in Fig. 2.In Fig. 3, the transducer 11 is connected to the output of a sinusoidal oscillator 16, which generates said 5 MHz signal and which also supplies a first input of a phase difference measurement circuit 1 7. This latter has a second input to which the electric signal generated by the receiving transducer 1 2 is fed, and an output at which an electric signal is present which is proportional to the phase difference between the signals present at the two said inputs, and which is connected to an indicator 18.

The transducers 11 and 12, the oscillator 16, the circuit 1 7 and the indicator 1 8 constitute overall a device 20 for measuring the hardening depth of the test piece 10 in the manner specified hereinafter.

With reference to Fig. 4, the reference numeral 21 indicates a test piece comprising a hardened layer 22 of depth a which extends longitudinally for a length d. A graph is shown below the test piece 21 which indicates the variation in the phase difference signal f generated by the circuit 1 7 as the axial position of the test piece 21 relative to the transducers 11 and 1 2 varies.

The operation of the device 10 will firstly be explained from the theoretical aspect with particular reference to Figs. 1 and 2, and using the symbols f, c, c - IS cl to indicate the frequency of the signal emitted by the transducer 11, the speed of sound within the test piece 10, and the speed of sound within the hardened layer 14 of the test piece 10 respectively.

Under these conditions, the phase difference +, between the signal received by the transducer 1 2 and that emitted by the transducer 11 in Fig. 1 is given by:

If the test piece 10 is hardened (see Fig. 2), the phase difference between the signal emitted and the signal received becomes:

Because of the effect of the surface hardening treatment of the test piece 10, a change in the phase displacement (80 occurs between the emitted and received signals, given by::

74 It is plausible that the expansion (d L) in the test piece 10 as a result of the hardening treatment is proportional to the depth (a) of the surface layer 14, so that SL/a can be considered a constant which is characteristic of the material, as is 15/c. In the specific considered case of hardening treatment, dL/a is of the order of 10-3, whereas Id cl/c is of the order of 10-2; consequently 1/2 SL/a is negligible relative to ld cl/c; consequently equation (3) can be simply written:

where K depends exclusively on the characteristics of the material fQr equal values of the frequency f.When in use, the constant K can be determined once and for all for a certain type of material by measuring +1 and +2 for the same test piece and determining the effective value of a by means of a destructive test, so that as 8 f and a are known, K can be obtained from (4). With K known, equation (4) gives the value of a when the phase difference values +, and +2 are known for any test piece.

With particular reference to Fig. 3, the measurement of the hardening depth of the test piece 10 is carried out in two separate stages, during the first of which +, is measured for a non-hardened reference test piece (Fig. 1), and during the second of which 02 is measured for a hardened test piece (Fig. 2).

The values of +, and +2 are calculated by the circuit 1 7 which compares the electrical signals fed to the transmitting transducer 11 and emitted by the receiving transducer 1 2 respectively.

The distribution of the hardened zone often assumes the pattern shown in Fig. 4. In this case, the axial movement of the test piece gives both +, and 02 in successive periods of time, so that the value of d 9, i.e. 2 = +1, is effectively the required value for substitution into equation (4) in order to calculate the hardening depth a.

Finally, the indicator 1 8 could be conveniently graduated as a function of phase measurements for a specific test piece, so that it indicates the value of the hardening depth a directly.

From an examination of the characteristics of the device 20 according to the present invention, it can be seen that it enables the aforesaid drawbacks of known devices to be overcome.

In this respect, the proportionality relationship (see equation 4) which relates the phase difference (5(p) to the hardening depth (a) is valid whatever the value a of the hardening depth. In all cases, the device 20 is able to provide an indication of the hardening depth in a simple and precise manner, and can therefore be advantageously used even by non-specialised personnel.

Finally, it is apparent that modifications can be made to the device 20 according to the present invention without leaving the scope of the inventive idea.

For example, although the device 20 has been described with particular reference to hardening depth measurement, it could be used for measuring the thickness of a surface layer of a body subjected to any surface structure modification treatment, for example to treatments involving the surface diffusion or absorption of foreign substances. In an even more general case, the data provided by the device 20 could constitute a useful source of information for the discovery of internal defects in the body under examination, provided such defects produce a change in the speed of sound relative to the normal value of the speed of sound in the test piece.

Claims

1. A device (20) for the non-destructive measurement of the thickness of a surface layer of a body (10) subjected to surface structure modification treatment, characterised by comprising: a transmitting transducer (11) and a receiving transducer (1 2) to be disposed facing each other on diametrically opposite sides of said body (10); an oscillator (1 6) which supplies said transmitting transducer (11); and a phase difference measurement circuit (17) having a first input connected to said transmitting transducer (11) and a second input connected to said receiving transducer (12), and an output at which a signal is present which depends on the phase difference between the signals present at said first and second inputs.

2. A device as claimed in claim 1, characterised by comprising a vessel (15) containing a liquid in which said transmitting transducer (11) and receiving transducer ( 1 2) are immersed, and in which said body (10) to be examined is immersed.

3. A device as claimed in claim 1 or 2, characterised by comprising means for supporting said body (10) and for adjusting the position of said body (10) relative to said transducers (11, 12).

4. A device as claimed in any one of the preceding claims, characterised in that said oscillator (16) generates a sinusoidal signal.

5. A device as claimed in any one of the preceding claims, characterised in that said oscillator (16) emits a signal at a frequency of the order of units of MHz.

6. A device as claimed in any one of the preceding claims, characterised in that said transducers (11, 12) are of piezoelectric type.

7. A device for measuring the thickness of the surface layer of a body (10) subjected to surface structure modification treatment, as described with reference to the accompanying drawings.