Process and device for determining the mass fraction of one constituent of a layer, and effecting the quality control of a composite metal sheet

A process for controlling the weldability of a sheet (1) of the composite metal sheet type having a layer (7) which is interposed between two metal facing sheets (3,5) and comprises a polymer (8) having a filler of conductive particles (9). This process comprises measuring by means of a signal of given frequency f applied to two electrodes (11,13) disposed in facing relation to each other, each electrode being in contact with a corresponding metal facing sheet (3,5), the voltage/current phaseshift .phi. due to the resistive and capacitive properties of the composite sheet (1). The response of the composite sheet (1) to this signal corresponds to that of an equivalent parallel RC circuit. Thereafter, the mass fraction of the conductive particles (9) is calculated from the value of .phi.. By comparing the determined mass fraction with prefixed thresholds, the weldability and the vibration-damping capacity of the composite sheet (1) are evaluated.

The invention relates to the quality control of sheets of the composite 
metal sheet type and more particularly to the continuous control of the 
weldability of such sheets. 
Composite metal sheets have vibration-damping properties which are used in 
particular for reducing the propagation of noise. They are used for 
example for manufacturing vehicle body elements in the motor industry or 
covering panels of domestic appliances of electrical household equipment. 
Known composite metal sheets usually comprise two metal facing or cladding 
sheets separated by a film of synthetic material, such as for example a 
polymer which constitutes not only an acoustic but also an electric 
insulation. 
For assembling composite metal sheets of large size it is current practice 
to interconnect the two sheets by resistance welding. 
Now, as the polymer layer is insulating, the very widely used technique of 
resistance welding cannot be applied to conventional composite metal 
sheets. 
To overcome this drawback, when manufacturing composite metal sheets, the 
layer of synthetic material is provided with a filler of a conductive 
material usually formed by balls of nickel so that the latter are in 
contact with the two metal facing sheets to ensure the electrical 
continuity of the sheet. To interconnect two sheets manufactured in this 
way, two of their end portions are superimposed and gripped between two 
electrodes through which a high electric current is passed. The passage of 
the electric current through the nickel balls distributed in the film of 
synthetic material results in a high heating that locally melts the metal 
facing sheets of the composite metal sheets which become welded under the 
effect of a pressure exerted by the electrodes. 
It will therefore be understood that, in order to ensure the weldability by 
resistance of such composite metal sheets, the filler of conductive 
particles must be distributed in a sufficient amount in the synthetic 
material; an excessive amount of filler should however be avoided in order 
to conserve the acoustic insulating properties of the composite metal 
sheet. Generally, a filler of conductive particles of about a 20% mass 
fraction of the layer interposed between the metal facing sheets is used 
for ensuring the resistance weldability of composite metal sheets, at 
least a mass fraction of 5% being necessary. 
To guarantee the resistance weldability of composite metal sheets, 
measuring means must be used for controlling the quantity of the filler of 
conductive particles in the layer interposed between the two metal facing 
sheets. 
At the present time, two processes are known for evaluating the resistance 
weldability of composite metal sheets. 
A first process comprises taking a sample of a composite metal sheet and 
effecting a few spot welds, namely about thirty spot welds, on this 
sample. The great drawback of this control or inspection process resides 
in the fact that it is destructive since the sample is rendered unusable 
by the control spot welds for a subsequent application. A thorough control 
of the manufacture of composite metal sheets requires a large number of 
samples which considerably increases the manufacturing costs. 
Further, this process cannot be applied to the continuous control of a 
strip of composite metal sheet, for example a sheet leaving an 
installation manufacturing such sheets. 
A second process is based on X-ray photographs which reveal the presence of 
conductive particles contained in the composite metal sheets. The main 
drawbacks of this process are the high cost of carrying out the process 
and the danger to the personnel owing to the powerful X-ray radiation in 
an industrial context. Moreover, this process rapidly reaches its limits 
of application when the thickness of the facing sheets of the composite 
metal sheet becomes excessive. 
An object of the invention is to overcome these drawbacks by providing a 
process for controlling the weldability of a composite metal sheet which 
is non destructive, reliable, inexpensive and presents no danger to the 
personnel. 
The invention therefore provides a process for determining the mass 
fraction of at least one of two constituents, one of which is formed by 
the conductive particles and the other is formed by an electrically 
insulating material, of a layer interposed between two metal facing sheets 
which together form with said interposed layer a composite metal sheet, 
said conductive constituents establishing an electric contact between the 
two metal facing sheets, characterized in that it comprises: 
measuring, by means of a signal of given frequency f applied to two 
electrodes disposed in facing relation to each other, each electrode being 
in contact respectively with a corresponding metal facing sheet, the 
voltage/current phase shift,.phi., due to the resistive and capacitive 
properties of the composite metal sheet, 
likening the portion of the composite metal sheet between the electrodes to 
an equivalent parallel RC circuit, and 
calculating from the value of tan .phi. which is the expression of the 
ratio between the capacitive component and the resistive component of the 
impedance of the equivalent RC circuit, the mass fraction Y.sub.R,Y.sub.C 
of at least one of said constituents, the mass fractions of these 
constituents being given by the following respective relations: 
##EQU1## 
Where .lambda. is a constant determined by the frequency f of the signal 
applied to the two electrodes, by the electrical properties of each 
constituent of the layer interposed between the metal facing sheets and by 
the densities of said constituents. 
The invention also provides a process for the quality control of a 
composite metal sheet comprising two metal facing sheets between which 
there is interposed a layer comprising two constituents, namely an 
electrically insulating material and conductive particles establishing an 
electric contact between the two metal facing sheets, characterized in 
that it comprises: 
determining the mass fraction of one of the two constituents of the layer 
interposed between the two metal facing sheets and applying the process 
defined hereinbefore, 
fixing at least one threshold of the mass fraction of the constituent whose 
mass fraction has been determined, said threshold corresponding to a 
required property of the composite metal sheet, and 
comparing the value of the measured mass fraction with the fixed threshold 
for determining whether the composite metal sheet satisfies or does not 
satisfy the required property. 
The invention further provides a device for effecting the quality control 
of a composite metal sheet which employs the process defined hereinbefore, 
characterized in that it comprises two electrodes disposed in facing 
relation to each other, each electrode being in contact respectively with 
a corresponding metal facing sheet of the composite metal sheet and being 
connected to the input of a unit for measuring the voltage/current phase 
shift,.phi., the output of the measuring unit being connected to the input 
of a utilization unit which calculates the mass fraction of said 
constituents of said layer and compares the mass fraction obtained with at 
least one predetermined threshold corresponding to a property of said 
composite metal sheet for determining whether the composite metal sheet 
satisfies or does not satisfy the required property.

FIG. 1 shows a composite metal or plymetal sheet 1 comprising two metal 
facing or cladding sheets 3 and 5. A layer 7 comprising an electrically 
and acoustically insulating material 8, for example a polymer, and a 
filler of conductive particles 9, for example balls of nickel, interposed 
between said facing sheets 3 and 5. The nickel balls 9 generally have a 
diameter of 50 .mu.m and are uniformly distributed in the layer 7. The 
spacing e between the metal facing sheets 3 and 5 is such that the nickel 
balls 9 are in contact with both of the two metal facing sheets 3 and 5 to 
ensure an electrical continuity between the facing sheets. 
An electrode 11, 13 is respectively applied to each metal facing sheet 3,5. 
These two electrodes are disposed in facing relation to each other and the 
composite metal sheet is interposed therebetween. 
To characterize the electrical properties of such a composite metal sheet 1 
in response to a signal of frequency f applied to the electrodes 11 and 
13, the passage of a current from one facing sheet to the other is 
analysed along two current lines 15 and 17, one line 15 only passing 
through the polymer 8 of the layer 7, the other line 17 passing through a 
nickel ball 9. 
Along the current line 15, the polymer 8 constitutes an electrical 
insulation between the two metal facing sheets 3 and 5. In this region, 
the composite metal sheet may be likened to an elementary capacitance dC. 
Along the current line 17, the current passes through a nickel ball 9 which 
establishes an electric current between the two metal facing sheets 3 and 
5, the current being limited by the electric resistance of the ball which 
depends on the particular resistivity .sigma..sub.R of the nickel. In this 
region, the composite metal sheet may be likened to an elementary 
resistance dR. 
Consequently, the layer 7 formed by the polymer 8 including a filler of 
conductive particles 9 and interposed between the facing sheets 3 and 5 of 
the composite metal sheet may be likened to a succession of elementary 
resistances and capacitances, respectively dR and dC, arranged in parallel 
as shown in the lower part of FIG. 1. 
The arrows at the ends of the current lines 15 and 17 indicate which one of 
the elementary electrical properties of the composite metal sheet is 
associated with each current line 15,17. 
The elementary impedance of an association in parallel of dR and dC 
carrying an alternating current of frequency f is given by the following 
relation: 
##EQU2## 
The elementary resistance dR and the elementary capacitance dC are 
expressed as a function of the elementary areas dA.sub.R and dA.sub.C of 
passage of the current lines respectively in the resistive and capacitive 
parts of the sheet. The total area A.sub.R +A.sub.C of passage of the 
current lines corresponds to the area of the confronting metal facing 
sheets through which the whole of the current passes. In a first 
approximation, it is considered that this total area A.sub.R +A.sub.C 
corresponds to the area of contact between the metal facing sheets 3,5 and 
the electrodes 11,13. 
The elementary resistance dR for the conductive particles 9 composed of a 
material of resistivity .sigma..sub.R is determined by: 
##EQU3## 
In considering the dielectric permittivity .epsilon. of the polymer 8 
between the facing sheets 3 and 5, there is obtained the elementary 
capacitance dC: 
##EQU4## 
By combining the equations (1), (2) and (3) there is obtained the total 
impedance Z.sub.T given by the arrangement in parallel of the elementary 
impedances dZ: 
##EQU5## 
The resistive and capacitive properties of the layer 7 are therefore given 
by the connection in parallel of a resistance corresponding to a total 
resistive effect proportional to the amount of conductive particles and a 
resistance corresponding to a total capacitive effect proportional to the 
amount of polymer. The proportion of each of these effects is equal, apart 
from a constant factor, to the ratio of the sections of passage of current 
A.sub.R and A.sub.C through the conductive particles and through the 
polymer respectively. In considering the equation (4) and in calculating 
in the known manner tan .phi., where .phi. is the voltage/current phase 
shift caused by the total impedance Z.sub.T, there is obtained, taking 
into account the fact that the thickness e of the layer 7 is constant, the 
ratio of the volumes V.sub.C and V.sub.R occupied respectively by the 
polymer 8 and the conductive particles 9 in this layer 7 by the following 
relation: 
##EQU6## 
It is this ratio which permits deducing the mass fraction of one of the two 
constituents of the layer 7 interposed between the two metal facing sheets 
from the measure of a magnitude related to the resistive and capacitive 
properties of the composite metal sheet. 
Knowing the densities .rho..sub.C and .rho..sub.R of the polymer and the 
conductive particles respectively, there is obtained the mass fraction of 
the conductive particles Y.sub.R distributed in the polymer from the 
relation: 
##EQU7## 
The mass fraction of the polymer Y.sub.C is obtained from the relation: 
##EQU8## 
In combining each equation (6a) and (6b) with the equation (5), there are 
then obtained the mass fractions Y.sub.R and Y.sub.C expressed with known 
parameters from the following relations: 
##EQU9## 
where .lambda. is a constant for a given frequency f. .lambda. is given by 
the relation: 
##EQU10## 
The mass fractions of the polymer and the conductive particles always 
satisfy the following relation: 
EQU Y.sub.C +Y.sub.R =1 (9) 
Consequently, the determination of one of the two mass fractions Y.sub.C or 
Y.sub.R permits deducing the other by subtraction. 
To determine the mass fraction of the conductive particles by applying the 
relation (7a), the phase shift .phi. may be measured by two known methods. 
A first method comprises measuring the impedance Z.sub.R between the two 
electrodes 11 and 13 in contact respectively with one of the two metal 
facing sheets 3 and 5 by means of a signal of given frequency f. There are 
then extracted from the value of the impedance Z.sub.T the equivalent 
resistance R and the reactance X. Then tan .phi. is determined by 
calculating the ratio between the reactance X and the equivalent 
resistance R. 
A second method comprises measuring by means of a signal of given frequency 
f the delay dT between an alternating voltage applied to the electrodes 
11, 13 and the alternating current produced by this voltage. Knowing the 
frequency f and said delay dT, the phase shift .phi. and then tan .phi. 
are calculated. 
An example of a device for determining the mass fraction and effecting the 
quality control of a composite metal sheet is shown in FIG. 2. 
This arrangement comprises two electrodes 31 and 32 in the form of rollers 
between which a section of a travelling strip of composite metal sheet 1 
is gripped, the widths of the rollers 31 and 33 and the strip being 
substantially equal. Of course it is not absolutely necessary that the 
width of the rollers 31 and 33 be equal to that of the strip. The two 
rollers 31 and 33 are rotatively mounted in a frame (not shown) so that 
the gripping force exerted on the composite metal sheet by the rollers 
31,33 shown by the arrows F, may be adjusted, the axes of rotation of the 
rollers 31 and 33 being substantially perpendicular to the direction of 
travel of the strip. The strip of the composite metal sheet 1 travels 
between the rollers 31 and 33 and the determination of the mass fraction 
of one of the two constituents of the layer 7 according to the process of 
the invention is effected during this travel. The rollers 31 and 33 are 
made of a material having a low electric resistance, for example copper. 
Each of the two electrodes formed by the two rollers 31 and 33 is 
electrically connected to an input of a measuring unit 35, for example an 
impedometer known per se. The impedometer 35 delivers an output signal 
corresponding to the module of the voltage/current phase shift .phi.. As 
the composite metal sheet only has resistive and capacitive properties, 
the phase shift signal is determined. The output of the impedometer 35 is 
connected to the input of a utilization unit 37, for example a PC type 
computer. This PC 37 is programmed to calculate the mass fractions of the 
two constituents of the layer 7 by applying the relations (7a and (7b) or 
the relation (9) in combination with either of the relations (7a) and (7b) 
and to record them. The PC 37 is advantageously fed with a computer 
program which enables it to carry out the thresholding and comparison 
operations, described hereinafter, for determining whether the composite 
metal sheet 1 has or does not have one of the required properties such as 
weldability or vibration damping. Further, the PC 37 controls a device for 
marking the sheet (not shown) in the event that the latter does not meet 
the required conditions of quality. 
To control the quality of a composite metal sheet as concerns its 
resistance weldability and its vibration damping properties, there is 
first of all determined, by means of the aforementioned device, the mass 
fraction for example of the conductive particles 9 uniformly distributed 
in the polymer 8 of the layer 7. Thereafter, the value obtained is 
compared with a predetermined minimum threshold Y.sub.R.sup.min, for 
example Y.sub.R.sup.min =5%, and with a predetermined maximum threshold 
Y.sub.R.sup.MAX, for example Y.sub.R.sup.MAX =20%. Below Y.sub.R.sup.min, 
the welding of the sheet is no longer possible and above Y.sub.R.sup.MAX, 
the vibration damping capacities are greatly diminished. If the given 
value of the mass fraction is not within the range defined by these 
minimum and maximum thresholds, it is considered that the composite metal 
sheet is of poor quality. In the opposite case, the composite metal sheet 
meets the required conditions of quality. 
It will be understood that, for the purpose of quality control, it is also 
possible to determine the mass fraction Y.sub.C of the insulating material 
8 by fixing the corresponding minimum threshold Y.sub.C.sup.min and the 
maximum threshold Y.sub.C.sup.MAX. Below Y.sub.C.sup.min, the vibration 
damping capacities are greatly diminished and above Y.sub.C.sup.MAX the 
welding of the sheet is no longer possible. 
According to another embodiment of a device for determining the mass 
fraction and effecting the quality control of a composite metal sheet (not 
shown), the arrangement comprises two electrodes in the form of wheels 
which are moved transversely over the entire width of the travelling strip 
of composite metal sheet so as to achieve a scanning of the strip during 
its travel. These two rotary wheels then replace the two rollers and are 
mounted to be rotatable and movable in translation in a frame similar to 
that described in respect of the embodiment shown in FIG. 2.