Apparatus for non-destructive testing of spot welds using an electromagnetic field

A method of and apparatus for nondestructive testing of spot welds by producing in a test weld high and low frequency electromagnetic fields to induce eddy currents in the weld zone. The difference between the phase values of the resultant electromagnetic field intensities is indicative of the quality of the weld. The apparatus includes an eddy current transducer, high and low-frequency measuring channels, an indicator, and a controlled commutator for alternate connection of the eddy current transducer to the low-frequency measuring channel or to the high-frequency measuring channel. A memory unit is provided for storing the output signal of the high-frequency measuring channel, and a comparator is provided for comparing the signal stored in the memory unit with the output signal of the low-frequency measuring channel. A control unit alternately generates signals for simultaneously connecting the eddy current transducer to the high-frequency measuring channel and for storing the output signal of the high-frequency measuring channel in the memory unit, and signals for simultaneously connecting the eddy current transducer to the low-frequency measuring channel, and for comparing the signal of the low-frequency measuring channel with the signal stored in the memory unit.

FIELD OF THE INVENTION 
The present invention relates to the field of weld testing, and more 
particularly to methods of and apparatus for nondestructive testing of 
spot welds. 
The present invention can be most advantageously used in machine, 
shipbuilding, aircraft, and automotive industries for flaw detection and 
nondestructive evaluation of properties of various metal structures. 
BACKGROUND OF THE INVENTION 
Spot welding is of wide application for joining part. For example, a car 
has over 22,000 spot welds. However, the use of spot welding in critical 
parts is impossible without a reliable method of and apparatus for 
evaluating the quality of the nugget of a spot weld. 
Known in the art is a method of nondestructive testing of spot welds, 
residing in that the test weld is examined by X-rays which are 
non-uniformly absorbed by various regions of the weld nugget because of 
inhomogeneity of its chemical composition. X-rays penetrate through the 
test weld and fall onto an X-ray film, whereon, after development, 
liquation rings are formed, the diameter of the weld nugget being 
evaluated from the dimensions of said rings. 
The apparatus realizing this method comprises an X-ray source and a 
detector, e.g. X-ray film on which X-rays having passed through the test 
weld are recorded. 
The abovementioned method of and apparatus for nondestructive testing of 
spot welds suffer from the disadvantage residing in that it is impossible 
to detect such a dangerous and popular welding effect as the lack of 
fusion for materials which do not exibit sharply defined inhomogeneity of 
chemical composition in the weld nugget section. 
Besides, said method and apparatus have a low efficiency and are of high 
cost. 
It is possible to improve the efficiency of the method and to reduce the 
cost of the equipment by the use of an eddy current method of spot weld 
testing based on the fact that the nugget of the test spot weld and a 
welded material outside the nugget zone possess different conductivities. 
The apparatus based on the eddy current method of testing should provide a 
means for tuning out from the interfering effect of variations of the 
lift-off formed between a superimposed eddy-current transducer and a test 
weld due to an indentation left by electrodes in the spot weld zone, 
tuning out from the effect of local inhomogeneity of a test weld in its 
chemical composition, tuning out from the action of temperature of the 
test weld, and tuning out the action caused by variation in the structure 
of the test weld material as a result of their mechanical treatment. 
There is well known and widely used a method of nondestructive testing of 
spot welds, residing in that a primary electromagnetic field is produced 
in the test weld zone, which field induces in said zone eddy currents 
generating a secondary field, whereupon a phase value of the resultant 
electromagnetic field intensity is determined, and the presence and 
quality of the weld nugget are evaluated in accordance with said values. 
The apparatus realizing this method of nondestructive testing of spot welds 
comprises a sinusoidal oscillator, a reference channel connected to one of 
the outputs of the oscillator and representing a phase shifter having its 
output connected to one of the inputs of a phase meter, and a measuring 
channel connected to the other output of the oscillator and representing 
an unbalanced bridge circuit with a superimposed eddy-current transducer 
included into one of its arms and with its output connected to the other 
input of the phase meter. 
The output voltage of the sinusoidal oscillator is supplied to the phase 
shifter which serves to set the phase of the reference voltage and to the 
bridge circuit with one of its arms including the eddy current transducer 
placed upon a test weld. The transducer complex resistance and hence the 
output signal of the bridge circuit wherein the transducer is included 
vary according to the weld quality. The bridge circuit is adjusted so that 
the phase of the output voltage is independent of the lift-off between the 
superimposed transducer and the article to be tested and is determined 
only by variations in the electric conductivity of the tested zone, which 
in turn is dependent upon the weld quality. From the output of the phase 
shifter the signal is supplied to one of the inputs of the phase meter, 
and from the output of the bridge circuit it is fed to the second input of 
the phase meter. The transducer being placed upon the reference weld, the 
phase of the reference voltage is changed by means of the phase shifter so 
that the phase shift between the reference and measured voltages, in case 
of a quality weld, should be equal to zero. Thereupon, the transducer is 
placed upon the test weld, and the weld quality is evaluated according to 
indications of the phase meter. 
The abovementioned method of and apparatus for nondestructive testing of 
spot welds make it possible to tune out from the interfering effect of 
variations in the lift-off between the superimposed eddy-current 
transducer and the test weld only in those cases when the phase of the 
output voltage of the unbalanced bridge circuit is approximately linearly 
dependent upon the conductivity of the test weld material and upon the 
size of the lift-off. As soon as this linear dependence is distorted, it 
becomes practically impossible to tune out completely from the effect of 
variations in the lift-off size, as a result of which the phase of the 
output voltage of the unbalanced bridge circuit will also depend upon the 
lift-off value. 
This method of and apparatus for nondestructive testing of spot welds do 
not permit to find out unambiguously whether the conductivity of the test 
weld material has changed, as compared to that of the reference weld 
material, due to a quality welding or under the influence of undesirable 
factors, such as: 
local inhomogeneity of the test weld material in its chemical composition, 
variations in the ambient temperature, and 
variations in the structure of the test weld material as a result of its 
mechanical treatment. 
What is more, this method of nondestructive testing of spot welds and the 
apparatus implementing the same make it possible to evaluate the weld 
nugget zone only dependently upon the change in its one parameter, namely 
upon the change in electrical conductivity of the weld nugget zone 
material. This results in that the reliability of the test is declined. 
Besides, the apparatus must be preadjusted by placing the eddy-current 
transducer upon the reference weld. However, for a number of materials, 
such as aluminum-magnesium alloys, a reference weld cannot be revealed by 
nondestructive methods. 
Known in the art is a method of nondestructive testing of spot welds (Cf. 
U.S. Pat. No. 3,526,829), residing in that two pulsed electromagnetic 
fields are produced, which are locally applied to the test weld and to the 
reference weld. Thereupon, the depth of penetration of two pulsed 
electromagnetic fields into the welds is determined by dynamic impedance 
measurements of the effect of the induced eddy currents on the applied 
electromagnetic fields. 
The apparatus realizing this method comprises eddy current transducers 
placed upon the reference and test welds, an impedance comparator having 
its inputs connected to the eddy current transducers, and a controlled 
switch having its output connected to a stored energy source. The output 
of the comparator is connected to a threshold circuit having its output 
connected to an information display unit wherein an information signal 
varying in accordance with the difference between the measured impedance 
values in the reference and in the test welds is displayed in an 
acceptance-rejection form. 
By means of this method and the apparatus it is possible to test spot welds 
of small thickness. 
Simultaneous measurements of electromagnetic parameters of the test weld 
and reference weld and comparison of the results of such measurements make 
it possible to eliminate to some extent the interfering effect of ambient 
temperature, assuming that the temperatures of the test weld and of the 
reference weld are equal. 
However, this method of and apparatus for nondestructive testing of spot 
welds are rather difficult to be applied to the welding of materials 
having substantial variations in physical-and-chemical characteristics 
within the same brand of materials. 
Also known in the prior art is a method of nondestructive testing of spot 
welds (Cf. USSR Inventor's Certificate No. 336,587), residing in that a 
superimposed eddy current transducer produces in a test weld nugget zone a 
primary low-frequency electromagnetic field inducing in said zone eddy 
currents which produce a secondary low-frequency electromagnetic field. 
Thereupon, the phase value of the resultant low-frequency electromagnetic 
field intensity is determined, according to which the quality of the weld 
nugget zone is evaluated. 
The apparatus realizing this method comprises a low-frequency measuring 
channel including a low-frequency generator, a T-shaped unbalanced LCR 
bridge circuit connected to the output of the generator, a main eddy 
current transducer included in the T-shaped bridge as an L element, an 
electronic indicator of the bridge output signal, a phase shifter, 
frequency multipliers, and a phase detector which are connected across the 
output of the generator and the main superimposed eddy current transducer. 
Fixed in the center of the main eddy current transducer is an additional 
eddy current transducer which forms in combination with a capacitor a 
measuring circuit for measuring the lift-off depth. The measuring circuit 
of the additional eddy current transducer is connected to a high-frequency 
generator and to the indicator of the lift-off depth. 
Sinusoidal voltage of the low-frequency generator is applied to the input 
of the T-shaped bridge circuit and to the phase shifter. The bridge 
circuit is balanced when the main superimposed eddy current transducer is 
placed on the reference weld. Thereupon, the main eddy current transducer 
whose parameters vary according to the weld quality is placed upon the 
test weld, and an error signal dependent upon the weld quality appears at 
the bridge output. 
The amplitude of the error signal is measured by the electronic indicator. 
To measure the phase, the signal is taken directly from the main 
superimposed eddy current transducer and applied through the frequency 
multiplier to the phase meter. A reference voltage is supplied from the 
low-frequency generator to the phase meter through the phase shifter and 
frequency multiplier. To determine the lift-off depth, voltage is supplied 
from the high-frequency generator to the measuring circuit. The signal 
corresponding to the lift-off depth is supplied from the measuring circuit 
to the lift-off depth indicator. 
This method and apparatus make it possible to perform tuning out from the 
interfering effect of variations in the lift-off between the superimposed 
eddy current transducer and the test weld. 
However, the aforementioned method and apparatus provide low reliability of 
testing since they fail to reveal unambiguously whether the conductivity 
of the test weld material has changed as compared to that of the reference 
weld material due to a quality weld or under the influence of undesirable 
factors, such as: 
local inhomogeneity in chemical composition of the test weld material, 
variations in the ambient temperature, and 
variations in the structure of the test weld material, caused by its 
mechanical treatment. 
Besides, the use of two superimposed eddy current transducers arranged 
coaxially considerably increases the overall dimensions of the apparatus. 
Also known is a method of nondestructive testing of spot welds (see "Heads 
of Reports of the Second Higher School Conference on the Problems of 
Nondestruction Quality Testing", Riga, RPI, 1975, pp. 140-143). According 
to this method, as a first step calibrating, is performed i.e. dependence 
of the phase value of intensity of each of the applied frequencies on the 
metered parameters of the spot weld, namely on the depth of fusion and on 
the nugget diameter, is determined at standart points thereupon a curve is 
plotted. 
Then a primary low-frequency electromagnetic field is produced in the test 
weld nugget zone, which field induces in said zone eddy currents 
generating a secondary low-frequency electromagnetic field which, 
interacting with the primary low-frequency electromagnetic field, forms a 
resultant low-frequency electromagnetic field, afterwards the phase value 
of the resultant low-frequency electromagnetic field intensity is 
determined. Next, a primary high-frequency electromagnetic field is 
produced in the test weld nugget zone, which field induces in said zone 
eddy currents generating a secondary high-frequency electromagnetic field 
which, interacting with the primary high-frequency electromagnetic field, 
forms a resultant high-frequency electromagnetic field, afterwards the 
phase value of the resultant high-frequency electromagnetic field 
intensity is determined. 
The spot weld quality is evaluated using the above curve on which points 
corresponding to the resultant phase values of intensity of the resultant 
low-frequency and high-frequency electromagnetic fields are located. 
the apparatus realizing this method comprises a superimposed eddy current 
transducer, a low-frequency measuring channel including a low-frequency 
generator, an unbalanced bridge circuit, and a phase detector, all 
elements being connected in series, a high-frequency measuring channel 
including a high-frequency generator, an unbalanced bridge circuit, and a 
phase detector, all connected in series, and an indicator. 
The above mentioned method and apparatus permit the testing reliability to 
be increased to some extent, since the spot weld nugget quality is 
evaluated according to two parameters, namely depth of fusion and diameter 
of the spot weld nugget zone. 
However, said method and apparatus fail to provide a sufficient reliability 
since the quality of the spot weld nugget is evaluated based on absolute 
phase values of intensity of the resultant electromagnetic fields. 
What is more, said method and apparatus do not permit to find out 
unambiguously whether the conductivity of the test weld material has 
changed as compared to that of the reference weld material due to a 
quality weld or under the influence of undesirable factors such as local 
inhomogeneity in chemical composition of the test weld material variations 
in the ambient temperature, and variations in the test weld material 
structure caused by its mechanical treatment. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of an 
apparatus for nondestructive testing of spot welds, enabling the 
reliability of testing to be improved. 
Another object of the present invention is to provide a method of and 
apparatus for nondestructive testing of spot welds, making it possible to 
perform testing regardless of variations in chemical composition of the 
material, in the structure of the material caused by its mechanical 
treatment, and in the environmental conditions, such as an ambient 
temperature. 
Still another object of the present invention is to provide a method of and 
apparatus for nondestructive testing of spot welds, enabling to perform 
testing without the need to pre-adjust it on a reference weld. 
Yet another object of the present invention is to provide a method of and 
an apparatus for nondestructive testing of spot welds, enabling the spot 
welding defects to be classified i.e. to distinguish spot welds having a 
sticking-type defects from spot welds having defects of through 
penetration type. 
With these and other objects in view, there is provided a method of 
nondestructive testing of spot welds, comprising the steps of producing in 
a test weld nugget zone a primary low-frequency electromagnetic field 
inducing eddy currents in said zone, generating a secondary low-frequency 
electromagnetic field which, interacting with the primary low-frequency 
electromagnetic field, forms a resultant low-frequency electromagnetic 
field, of determining in said zone the phase value of the resultant 
low-frequency electromagnetic field intensity, of producing in the test 
weld nugget zone a primary high-frequency electromagnetic field inducing 
eddy currents in said zone, generating a secondary high-frequency 
electromagnetic field which interacting with the primary high-frequency 
electromagnetic field forms a resultant high-frequency electromagnetic 
field, of determining in said zone the phase value of the resultant 
high-frequency electromagnetic field intensity, and of evaluating the spot 
weld nugget quality according to the phase values of intensity of the 
resultant high-frequency and low-frequency electromagnetic field, wherein, 
accordng to the invention, a difference between the phase values of the 
resultant high-frequency electromagnetic field intensity and that of the 
resultant low-frequency electromagnetic field intensity is determined, and 
the quality of the weld nugget is evaluated directly from that difference. 
Determination of the difference between the phase values of the resultant 
electromagnetic field intensities provides reliable testing of spot welds 
even in the presence of interfering factors. 
Under the influence of such interfering factors as inconstancy of the 
material chemical composition of joined parts, variations in the structure 
of the material in the course of its mechnical treatment, and variations 
in the ambient temperature, there occur considerable changes in the 
material conductivity, commensurable with or even greater than those 
caused by welding. 
Since these changes in conductivity, caused by said interfering factors, 
are practically the same both in the surface layer and in the weld nugget, 
the difference between the values of conductivities or the difference 
between the phase values of field intensities respectively proportional to 
conductivities in these zones, is independent of said interfering factors 
and is determined only by variations in the structure of the material 
caused by welding. 
It is advisable to produce a primary high-frequency electromagnetic field 
in a zone disposed in the immediate vicinity to the weld nugget, which 
field induces eddy currents in said zone, generating a secondary 
high-frequency electromagnetic field forming, while interacting with the 
primary high-frequency electromagnetic field a resultant high-frequency 
electromagnetic field, to determine the difference between the phase value 
of the intensity of the resultant high-frequency electromagnetic field 
produced in the zone disposed in the immediate vicinity to the weld nugget 
and that of the resultant high-frequency electromagnetic field produced in 
the weld nugget zone, and to evaluate the quality of the spot weld nugget 
directly from that difference. 
Generation of the primary high-frequency electromagnetic field in the zone 
disposed in the immediate vicinity to the weld nugget makes it possible to 
induce eddy currents in said zone and to measure the phase value of the 
resultant high-frequency electromagnetic field intensity in a surface 
layer not subjected to thermal treatment during the welding process. 
In case of the sticking-type defect present in the spot weld, variations in 
the material structure in the surface layer of the weld joint cause but a 
little change in its conductivity which only slightly differs from the 
conductivity of the material subjected to no thermal treatment. 
The presence of the through penetration defect is characterized by a 
substantial variations in the material structure in the surface layer and, 
consequently, by substantial variations in the material conductivity. 
With these and other objects in view, there is also provided an apparatus 
for nondestructive testing of spot welds, comprising a superimposed eddy 
current transducer, a low-frequency measuring channel including a 
low-frequency signal generator, an unbalanced bridge circuit and a phase 
detector, connected in series, a high-frequency measuring channel 
including a high-frequency signal generator, an unbalanced bridge circuit 
and a phase detector, connected in series, and an indicator, which 
apparatus, according to the invention, further comprises a controlled 
commutator for alternate connection of the superimposed eddy current 
transducer to the unbalanced bridge circuits of the low-frequency and 
high-frequency measuring channels, a memory unit adapted for storage of 
the output signal of the phase detector of the high-frequency measuring 
channel and connected to the output of the phase detector of the 
high-frequency measuring channel, a comparator for comparing the signal 
stored in the memory unit with the signal of the phase detector of the 
low-frequency measuring channel, and a control unit having its outputs 
connected to the control inputs of the controlled commutator, of the 
memory unit and of the comparator, and which alternately generates signals 
for simultaneously connecting of the superimposed eddy current transducer 
to the unbalanced bridge circuit of the high-frequency measuring channel 
and for storing the output signal of the phase detector of the 
high-frequency measuring channel in the memory unit, and signals for 
connecting the superimposed eddy current transducer to the unbalanced 
bridge circuit of the low-frequency measuring channel and for comparing 
the signal of the phase detector of the low-frequency measuring channel 
with the signal stored in the memory unit. 
The control unit, controlled commutator, memory unit, and comparator make 
it possible to determine the difference between the phase values of the 
intensities of the resultant high-frequency electromagnetic field and the 
resultant low-frequency electromagnetic field said difference being 
dependent upon the difference between conductivities of the material in 
the surface layer of the test weld and in the test weld nugget zone, 
conditioned only by the process of welding. 
It is advisable that the apparatus be further provided with controlled 
commutator adapted to switch a second input of the comparator from the 
phase detector of the low-frequency measuring channel and to the phase 
detector of the high-frequency measuring channel, the control unit being 
provided with a fourth output connected with a control input of the 
additional controlled commutator and generating signals to connect 
simultaneously the superimposed eddy current transducer to the unbalanced 
bridge circuit of the high-frequency measuring channel, the phase detector 
of the high-frequency measuring channel to the second input of the 
comparator, and to compare the signal from the phase detector of the 
high-frequency measuring channel with the signal stored in the memory 
unit. 
Such an embodiment of the apparatus makes it possible to compare two 
signals which are proportional to the conductivity of the test weld 
material in the surface layer in the weld nugget zone and in the surface 
layer in the zone disposed in immediate vicinity to the weld nugget. 
Other objects, features and advantages of the invention will become more 
apparent upon consideration of the following detailed description of its 
embodiments taken in conjunction with the accompanying drawings, in which:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The method of nondestructive testing of spot welds is realized as follows. 
A superimposed eddy current transducer 1 (FIG. 1) is placed over a test 
weld 2. To suppress the effect of variations in the lift-off between the 
superimposed eddy current transducer 1 and the test weld 2, the transducer 
1 is included into an unbalanced bridge circuit which is adjusted by a 
conventional method so that the phase of the output voltage of the 
unbalanced bridge circuit is independent of the lift-off value between the 
superimposed eddy current transducer 1 and the test weld 2, and is 
determined only by variations in the conductivity in a test zone, the 
variations in conductivity depending upon variations in the structure of a 
material, i.e. upon the quality of the weld nugget and weld defects 
present therein. 
By means of the superimposed eddy current transducer 1 powered by a 
sinusoidal high-frequency voltage, a primary electromagnetic field is 
produced, which induces eddy currents in the test weld 2. 
The frequency of the sinusoidal voltage is chosen so that the 
high-frequency electromagnetic field produced by the superimposed eddy 
current transducer 1 would penetrate only into the surface layer of the 
test weld 2 without reaching a depth where a nugget 3 of test weld 2 may 
be located. 
Eddy currents produce a secondary high-frequency electromagnetic field 
which, interacting with the primary high-frequency electromagnetic field, 
forms a resultant high-frequency electromagnetic field whose phase of 
intensity depends upon the conductivity of the material, i.e. upon the 
material structure of the surface layer of the test weld 2. 
The resultant high-frequency electromagnetic field acts upon the 
superimposed transducer 1 varying its parameters according to the weld 
quality. 
The phase of the resultant high-frequency electromagnetic field intensity 
is measured and translated into a voltage proportional thereto. 
Thereupon, the frequency of the sinusoidal voltage powering the 
superimposed eddy current transducer 1 is varied, this frequency being 
chosen to be a lower value. 
The frequency of the sinusoidal voltage is chosen so that a primary 
low-frequency electromagnetic field produced by the superimposed eddy 
current transducer 1 would penetrate to a depth where the nugget 3 of the 
test weld 2 may be located. 
The primary low-frequency electromagnetic field induces eddy currents in 
the test weld 2. 
Eddy currents produce a secondary low-frequency electromagnetic field 
which, interacting with the primary low-frequency electromagnetic field, 
forms a resultant low-frequency electromagnetic field whose phase of 
intensity depends upon the conductivity, i.e. upon the structure of the 
surface layer as well as of the nugget 3 of the test weld 2. 
The resultant low-frequency electromagnetic field acts upon the 
superimposed eddy current transducer 1 varying its parameters dependently 
upon the weld quality. 
The phase of the resultant low-frequency electromagnetic field is measured 
and is translated into the voltage proportional thereto. 
Thereupon, the voltage obtained due to the action of the primary 
high-frequency electromagnetic field on the test weld 2 is compared with 
the voltage obtained due to the action of the primary low-frequency 
electromagnetic field on the test weld. 
When the weld nugget is of high quality, the structures of the material in 
the surface layer and in the zone of the weld nugget 3 are different, and 
the material in these zone has different conductivities. If the difference 
between the voltage corresponding to the surface layer and the voltage 
corresponding to the zone of the weld nugget 3 exceeds a preset value 
U.sub.o which corresponds to the reference weld quality, the weld 2 is of 
high quality, and if this difference is below a preset value, the test 
weld is of poor quality. 
Shown in the graph of FIG. 1 are a curve 4 corresponding to a voltage 
U.sub.1 proportional to the phase of the resultant high-frequency 
electromagnetic field intensity, and a curve 5 corresponding to a voltage 
U.sub.2 proportional to the phase of the resultant low-frequency 
electromagnetic field intensity, the abscissa being the time, t, over 
which testing is performed, while on the ordinate the voltage, U, is 
plotted. 
It is evident from the graph that, since the voltage difference 
.DELTA.U=U.sub.1 -U.sub.2 is greater than U.sub.o, the test weld 2 is of 
high quality. 
FIG. 2 shows a kind of a spot weld wherein a test spot weld has no nugget. 
The absence of the weld nugget determines little difference between the 
structures of the surface layer and a zone 7 wherein the nugget of a spot 
weld of high quality should be located. 
Accordingly, conductivities of these zones differ insignificantly. 
The voltage obtained due to the action of the primary high-frequency 
electromagnetic field on the test weld 6 is only slightly different from 
that obtained due to the action of the primary low-frequency 
electromagnetic field on the test weld 6. 
Referring to a curve 8, it can be seen that, since the structure of the 
surface layer in the test weld 6 is similar to that in the test weld 2, 
the voltage corresponding to this zone in the test weld 6 is equal to 
U.sub.1. 
Referring now to a curve 9, it can be seen that the voltage U.sub.3 
corresponding to the zone 7 is only slighty different from the voltage 
U.sub.1. The difference .DELTA.U.sub.1 =U.sub.1 -U.sub.3 is smaller than 
U.sub.o, i.e. the test weld 6 is of poor quality. 
FIG. 3 shows another kind of the spot weld, wherein a nugget 10 of a test 
spot weld 11 reaches the surface of the test weld 11. 
Since the nugget 10 extends substantially across the whole width of the 
test weld 11, there is a little difference in structures of the surface 
layer and the nugget 10. 
Accordingly, conductivities of these zones differ insignificantly. 
The voltage obtained due to the action of the primary high-frequency 
electromagnetic field on the test weld 11 is only slightly different from 
that obtained due to the action of the primary low-frequency 
electromagnetic field on the test weld 11. 
A curve 12 corresponds to the voltage U.sub.4 proportional to the phase of 
the resultant high-frequency electromagnetic field intensity, and a curve 
13 corresponds to the voltage U.sub.2 proportional to the phase of the 
resultant low-frequency electromagnetic field intensity. 
Since the structure of the zone wherein the nugget 10 of the test weld 11 
is located is similar to that wherein the nugget 3 of the test weld 2 is 
located, the voltage corresponding to the zone of the nugget 10 is equal 
to U.sub.2. 
It can be seen from the curve 12 that the voltage U.sub.4 corresponding to 
the surface layer of the test weld 11 is only slightly different from the 
voltage U.sub.2. 
The difference .DELTA.U.sub.2 =U.sub.4 -U.sub.2 is smaller than U.sub.o, 
i.e. the test weld 11 is of poor quality. 
Given below are the examples of the embodiments of the proposed method of 
and of the apparatus for nondestructive testing of spot welds under 
various conditions. The compositions of the steels used in the examples 
are as follows: 
______________________________________ 
C, % Mn, % Si, % P, % S, % 
______________________________________ 
Steel 15 KP 
0.12-0.20 
0.35-0.65 0.17-0.37 
0.045 0.040 
Steel 08 KP 
0.05-0.11 
0.25-0.50 0.30 0.040 0.40 
______________________________________ 
EXAMPLE 1 
______________________________________ 
Brand of the test weld material 
15 KP steel with 
DC magnetizing 
Thickness of the test weld 
3 + 3 mm 
material 
Frequency of the high-frequency 
120 kHz 
sinusoidal voltage 
Frequency of the low-frequency 
3 kHz 
sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the 
0.26 V 
action of the primary high- 
frequency electromagnetic 
field on the test weld, U.sub.1 
Voltage obtained due to the 
0,10 V 
action of the primary low- 
frequency electromagnetic 
field on the test weld, U.sub.2 
Voltage difference, 0.16 V 
.DELTA.U = U.sub.1 - U.sub.2 
______________________________________ 
Since .DELTA.U is greater than U.sub.o, the test weld is of high quality 
and corresponds to the spot weld shown in FIG. 1. 
EXAMPLE 2 
______________________________________ 
Brand of the test 15 KP steel with 
weld material DC magnetizing 
Thickness of the test 3 + 3 mm 
weld material 
Frequency of the high- 
120 kHz 
frequency sinusoidal 
voltage 
Frequency of the low-fre- 
3 kHz 
quency sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the 
0.26 V 
action of the primary high- 
frequency electromagnetic 
field on the test weld, 
U.sub.1 
Voltage obtained due to the 
0.22 V 
action of the primary low- 
frequency electromagnetic 
field on the test weld, U.sub.3 
Voltage difference .DELTA.U.sub.1 between 
0.04 V 
the voltage U.sub.1 and the volt- 
age U.sub.3 
______________________________________ 
Since .DELTA.U.sub.1 is smaller than U.sub.o, the test weld is of poor 
quality and corresponds to the spot weld shown in FIG. 2. 
EXAMPLE 3 
______________________________________ 
Brand of the test weld material 
15 KP steel with DC - magnetizing 
Thickness of the test weld ma- 
3 + 3 mm 
terial 
Frequency of the high-frequency 
120 kHz 
sinusoidal voltage 
Frequency of the low-frequency 
3 kHz 
sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the 
0.15 V 
action of the primary high-fre- 
quency electromagnetic field 
on the test weld, U.sub.4 
Voltage obtained due to the 
0.10 V 
action of the primary low- 
frequency electromagnetic 
field on the test weld, U.sub.2 
Voltage difference .DELTA.U.sub.2 between the 
0.05 V 
voltage U.sub.4 and the voltage U.sub.2 
______________________________________ 
Since .DELTA.U.sub.2 is smaller than U.sub.o, the test weld is of poor 
quality and corresponds to the spot weld shown in FIG. 3. 
EXAMPLE 4 
______________________________________ 
Brand of the test weld material 
08 KP steel with DC 
magnetizing 
Thickness of the test weld 
3 + 3 mm 
material 
Frequency of the high frequency 
120 kHz 
sinusoidal voltage 
Frequency of the low-frequency 
3 kHz 
sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the 
0.6 V 
action of the primary high- 
frequency electromagnetic 
field on the test weld, U.sub.1 
Voltage obtained due to the 
0.36 V 
action of the primary low- 
frequency electromagnetic 
field on the test weld, U.sub.2 
Voltage difference .DELTA.U between 
0.24 V 
the voltage U.sub.1 and the voltage 
U.sub.2 
______________________________________ 
Since .DELTA.U is greater than U.sub.o, the test weld is of high quality 
and corresponds to the spot weld shown in FIG. 1. 
EXAMPLE 5 
______________________________________ 
Brand of the test weld material 
08 KP steel with 
DC magnetizing 
Thickness of the test weld ma- 
3 + 3.5 mm 
terial 
Frequency of the high-frequency 
120 kHz 
sinusoidal voltage 
Frequency of the low-frequency 
3 kHz 
sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the 
0.68 V 
action of the primary high- 
frequency electromagnetic 
field on the test weld, U.sub.1 
Voltage obtained due to the ac- 
0.63 V 
tion of the primary low-fre- 
quency electromagnetic field 
on the test weld, U.sub.3 
Voltage difference .DELTA.U.sub.1 between 
0.05 V 
the voltage U.sub.1 and the voltage 
U.sub.3 
______________________________________ 
Since .DELTA.U.sub.1 is smaller than U.sub.o, the test weld is of poor 
quality and corresponds to the spot weld shown in FIG. 2. 
EXAMPLE 6 
______________________________________ 
Brand of the test weld material 
08 KP steel with 
DC magnetizing 
Thickness of the test weld 
3 + 3.5 mm 
material 
Frequency of the high-fre- 
120 kHz 
quency sinusoidal voltage 
Frequency of the low-frequency 
3 kHz 
sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the 
0.40 V 
action of tbe primary high- 
frequency electromagnetic 
field on the test weld, U.sub.4 
Voltage obtained due to the 
0.36 V 
action of the primary low-fre- 
quency electromagnetic field on 
the test weld, U.sub.2 
Voltage difference .DELTA.U.sub.2 between the 
0.04 V 
voltage U.sub.4 and the voltage U.sub.2 
______________________________________ 
Since .DELTA.U.sub.2 is smaller than U.sub.o, the test weld is of poor 
quality and corresponds to the spot weld shown in FIG. 3. 
In order to distinguish a type of defect of the low-quality spot welds, 
i.e. to reveal sticking-type spot welds and through penetration-type spot 
welds, the superimposed eddy current transducer 1 (FIG. 4) is installed on 
the low-quality spot weld 11 in the immediate vicinity to the nugget 10. 
The superimposed eddy current transducer 1 powered by a sinusoidal 
high-frequency voltage generates a primary high-frequency electromagnetic 
field inducing eddy currents in the test weld 11. 
The frequency of the sinusoidal voltage is chosen so as to provide 
penetration of the primary high-frequency electromagnetic field produced 
by the superimposed eddy current transducer 11 but only into the surface 
layer of the test weld 11 without reaching a depth at which the nugget of 
the high-quality spot weld should be located. 
Eddy currents produce a secondary high-frequency electromagnetic field 
which, interacting with the primary high-frequency electromagnetic field, 
forms a resultant high-frequency electromagnetic field whose phase of 
intensity is determined by the conductivity of the material, viz. by the 
structure of the surface layer material in the zone disposed in the 
immediate vicinity to the weld nugget. 
The resultant high-frequency electromagnetic field acts upon the 
superimposed eddy current transducer 1 varying its parameters according to 
the conductivity of the material in this zone. 
Next, the phase of the resultant high-frequency electromagnetic field 
intensity is measured and transformed into a voltage proportional thereto. 
Thereupon, the intensity produced by the primary high-frequency 
electromagnetic field acting on the test weld 11 in the zone located in 
the immediate vicinity to the weld nugget is quantitatively compared with 
the intensity due to the action of the primary high-frequency 
electromagnetic field on the test weld 11 in the nugget zone, whereby the 
type of the spot weld defect is discerned. 
When the weld nugget is of poor quality, the structure of the material of 
the surface layer disposed in the immediately vicinity to the weld nugget 
zone differs from the structure of the material of the surface layer 
disposed directly in the weld nugget zone. This results in that the 
materials in said zones have different conductivities. If the difference 
between the voltage corresponding to the surface layer disposed in the 
immediate vicinity to the weld nugget zone and the voltage corresponding 
to the surface layer disposed directly in the weld nugget zone is above 
the preset value U.sub.o, the test spot weld suffers from a 
through-penetration the defect, whereas if said difference is lower than 
the preset value U.sub.o, the test spot weld suffers from a sticking-type 
defect. 
FIG. 4 shows a kind of the spot weld 11 with the nugget 10 extending to the 
surface of the spot weld 11. The extension of the weld nugget 10 
substantially throughout the whole thickness of the test spot weld 11 
results in a marked difference in the structures of the surface layer 
disposed in the immediate vicinity to the weld nugget zone and of the 
surface layer disposed directly in the weld nugget zone. 
Accordingly, conductivities of said zoned differ significantly. 
The graph of FIG. 4 shows voltage U plotted versus time t. A curve 14 
corresponds to a voltage U.sub.5 proportional to the phase value of the 
intensity of the resultant high-frequency electromagnetic field produced 
in the surface layer disposed in the immediate vicinity to the weld nugget 
zone, while a curve 15 corresponds to a voltage U.sub.4 proportional to 
the phase value of the intensity of the resultant high-frequency 
electromagnetic field applied to the surface layer disposed directly in 
the weld nugget zone. 
It is clear from the graph that since the voltage difference .DELTA.U.sub.3 
=U.sub.5 -U.sub.4 is greater than U.sub.o, the test weld 11 has a defect 
of a through-penetration type. 
FIG. 5 shows the spot weld 6 free from the weld nugget. 
The absence of the weld nugget determines a little difference between the 
structures of the surface layer disposed in the immediate vicinity to the 
the weld nugget zone and of the surface layer disposed directly in the 
weld nugget zone. 
Accordingly, conductivities of these zones differ insignificantly. 
Shown on the grapth of FIG. 5 are a curve 16 corresponding to voltage 
U.sub.5 proportional to the phase of the intensity of the resultant 
high-frequency electromagnetic field produced in the surface layer 
disposed in the immediate vicinity to the weld nugget zone, and a curve 17 
corresponding to voltage U.sub.1 proportional to the phase of the 
intensity of the resultant high-frequency electromagnetic field produced 
in the surface layer disposed directly in the weld nugget zone. 
It is evident from the graph that since the voltage difference 
.DELTA.U.sub.4 =U.sub.5 -U.sub.1 is smaller than U.sub.o ', the test weld 
6 suffers from a sticking-type defect. 
Given below are the examples of the embodiments of the proposed method of 
nondestructive testing of spot welds. 
EXAMPLE 7 
______________________________________ 
Brand of the test weld material 
15 KP steel with DC 
magnetizing 
Thickness of the test weld 
3 + 3 mm 
material 
Frequency of the high-frequency 
120 kHz 
sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the 
0.26 V 
action of the primary high- 
frequency electromagnetic 
field on the weld nugget zone, 
U.sub.1 
Voltage obtained due to the 
0.31 V 
action of the primary high- 
frequency electromagnetic 
field on a portion disposed in 
the immediate vicinity to the 
weld nugget zone, U.sub.5 
Voltage difference .DELTA.U.sub.4 = U.sub.5 - U.sub.1 
0.05 V 
______________________________________ 
Since .DELTA.U.sub.4 is smaller than U.sub.o, the low-quality spot weld has 
a sticking-type defect. 
EXAMPLE 8 
______________________________________ 
Brand of the test weld material 
15 KP steel with 
DC magnetizing 
Thickness of the test weld 
3 + 3 mm 
material 
Frequency of the high-fre- 
120 kHz 
quency sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the 
0.15 V 
action of the primary high-fre- 
quency electromagnetic field 
on the test spot weld in the 
weld nugget zone, U.sub.3 
Voltage obtained due to the action 
0.31 V 
of the primary high-frequency 
electromagnetic field on a por- 
tion of the test spot weld, dis- 
posed in the immediate vicinity 
to the weld nugget zone, U.sub.5 
Voltage difference .DELTA.U.sub.3 = U.sub.5 - U.sub.4 
0.16 V 
______________________________________ 
Since .DELTA.U.sub.3 is greater than U.sub.o, the test spot weld of low 
quality suffers from a sticking-type defect. 
EXAMPLE 9 
______________________________________ 
Brand of the test weld material 
08 KP steel with 
DC magnetizing 
Thickness of the test weld 
3 + 3.5 mm 
material 
Frequency of the high-frequen- 
120 kHz 
cy sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the action 
0.68 V 
of the primary high-frequency 
electromagnetic field on the 
test spot weld in the weld 
nugget zone, U.sub.1 
Voltage obtained due to the 
0.74 V 
action of the primary high- 
frequency electromagnetic 
field on a portion of the test 
spot weld, disposed in the immediate 
vicinity to the weld nugget zone U.sub.5 
Voltage difference, .DELTA.U.sub.4 = U.sub.5 - U.sub.1 
0.06 V 
______________________________________ 
Since .DELTA.U.sub.4 is smaller than U.sub.o, the test spot weld of poor 
quality suffers from a sticking-type defect. 
EXAMPLE 10 
______________________________________ 
Brand of the test weld material 
08 KP steel with 
DC magnetizing 
Thickness of the test weld 
3 + 3.5 mm 
material 
Frequency of the high-frequen- 
120 kHz 
cy sinusoidal voltage 
Preset voltage, U.sub.0 
0.10 V 
Voltage obtained due to the 
0.40 V 
action of the primary high- 
frequency electromagnetic 
field on the test spot weld in the 
weld nugget zone, U.sub.4 
Voltage obtained due to the ac- 
0.74 V 
tion of the primary high-fre- 
quency electromagnetic field 
on a portion of the test spot 
weld, disposed in the immedi- 
ate vicinity to the nugget zone, 
U.sub.5 
Voltage difference, .DELTA.U.sub.3 = U.sub.5 - U.sub.4 
0.34 V 
______________________________________ 
Since .DELTA.U.sub.3 is greater than U.sub.o, the test spot weld of poor 
quality suffers from a through-penetration type defect. 
The apparatus realizing the method described hereinabove comprises a 
high-frequency measuring channel 18 (FIG. 6) including a high-frequency 
signal generator 19, an unbalanced bridge circuit 20 and a phase detector 
21, connected in series, and a low-frequency measuring channel 22 
including a low-frequency signal generator 23, an unbalanced bridge 
circuit 24 and a phase detector 25, connected in series. The output of the 
phase detector 21 is connected to the input of a memory unit 26 having its 
output connected to a first signal input of a comparator 27 whose output 
is connected to an indicator 28. 
The output of the phase detector 25 is connected to a second signal input 
of the comparator 27. 
A control input of the comparator 27 is connected to one of the outputs of 
a control unit 29. A second output of the control unit 29 is connected to 
a control input of the memory unit 26. A third output of the control unit 
29 is connected to a control input of a controlled commutator 30. 
The superimposed eddy current transducer 1 is connected to an input of the 
controlled commutator 30 whose outputs are respectively connected to the 
unbalanced bridge circuit 20 and to the unbalanced bridge circuit 24. 
In one embodiment shown in FIG. 7, the output of the phase detector 21 is 
connected to one of the inputs of a controlled commutator 31, while the 
output of the phase detector 25 is connected to another input of the 
controlled commutator 31. A control input of the controlled commutator 31 
is connected with a fourth output of the control unit 29. The output of 
the controlled commutator 31 is connected to a second signal input of the 
comparator 27. 
The proposed apparatus realizing the method of nondestructive testing of 
spot welds operates as follows. 
The superimposed eddy current transducer 1 (FIG. 1) is placed over the test 
spot weld 2. 
The control unit 29 (FIG. 6) is energized to supply a control signal to the 
controlled commutator 30 which switches the coil of the superimposed eddy 
current transducer 1 into the unbalanced bridge circuit 20 included into 
the high-frequency measuring channel 18. The sinusoidal high-frequency 
voltage from the high-frequency signal generator 19 is applied to the 
unbalanced bridge circuit 20. 
The unbalanced bridge circuit 20 is adjusted so that the phase of the 
output voltage is independent from the size of the lift-off formed between 
the superimposed eddy current transducer 1 and the test spot weld, and is 
determined only by variations in the conductivity of the test region, 
which in turn is dependent upon the weld quality. 
The current flowing through the coil of the superimposed eddy current 
transducer 1 produces a primary high-frequency electromagnetic field which 
induces in the surface layer of the test weld 2 (FIG. 1) eddy currents 
producing a secondary high-frequency electromagnetic field. 
The resultant high-frequency electromagnetic field acts on the coil of the 
superimposed eddy current transducer 1, thus varying its impedance 
according to the value of the conductivity of the material in the zone of 
eddy currents. 
From the output of the unbalanced bridge circuit 20 (FIG. 6) the voltage 
whose phase of value is dependent upon the conductivity of the material in 
the zone of action of the primary high-frequency electromagnetic field 
produced by the superimposed eddy current transducer 1, is applied to the 
input of the phase detector 21. 
Simultaneously with the supply of the control signal from the control unit 
29 to the controlled commutator 30, a signal is supplied from the control 
unit 29 to the control input of the memory unit 26. From the output of the 
phase detector 21 the voltage proportional to the phase value of the 
output voltage of the unbalanced bridge circuit 20 viz. to the 
conductivity of the material in the surface layer of the test weld 2 (FIG. 
1), is fed to the input of the memory unit 26 (FIG. 6) wherein the voltage 
is stored in an analog form. 
After the voltage is stored in the memory unit 26, the supply of the signal 
to its control input from the control unit 29 is cut off. Simultaneously 
with the cut-off of signal supply to the control input of the memory unit 
26 from the control unit 29, the signal from the latter is supplied to the 
control input of the comparator 27 and to the controlled commutator 30. 
After the signal is fed to the controlled commutator 30, the coil of the 
superimposed eddy current transducer 1 is disconnected from the unbalanced 
bridge circuit 20 and switched into the unbalanced bridge circuit 24 
included into the low-frequency measuring channel 22. 
The low-frequency sinusoidal voltage is applied from the low-frequency 
signal generator 23 to the unbalanced bridge circuit 24. 
The unbalanced bridge circuit 24 is adjusted so that the phase of the 
output voltage is independent of the size of the lift-off formed between 
the superimposed eddy current transducer 1 and the test spot weld, and is 
determined only by variations in the conductivity in the test region, 
which, in turn, is dependent upon the weld quality. 
Current flowing through the coil of the superimposed eddy current 
transducer 1 produces a primary low-frequency electromagnetic field which 
induces eddy currents in the zone of the weld nugget 3 (FIG. 1) of the 
test weld 2. Eddy currents produce, in turn, a secondary low-frequency 
electromagnetic field. 
The resultant low-frequency electromagnetic field acts on the coil of the 
superimposed eddy current transducer 1, thus varying its impedance 
according to the conductivity of the material in the zone of eddy 
currents. 
From the output of the unbalanced bridge circuit 24 (FIG. 6) the voltage 
whose phase depends upon the conductivity of the material in the zone of 
action of the primary low-frequency electromagnetic field produced by the 
superimposed eddy current transducer 1, is applied to the input of the 
phase detector 25. 
From the output of the phase detector 25 the voltage proportional to the 
phase of the output voltage of the unbalanced bridge circuit 24 viz. to 
the conductivity of the material in the weld nugget zone (FIG. 1) is fed 
to the second signal input of the comparator 27 (FIG. 6), the voltage from 
the memory unit 26 being applied to the first signal input thereof. 
The comparator 27 compares the signals supplied from the phase detector 25 
and from the memory unit 26, whereupon the difference of this signals is 
applied to the indicator 28. 
The spot weld is of high quality if the difference between the signals on 
the display of the indicator 28 exceeds a preset value. 
If the difference between the signals on the display of the indicator 28 is 
below a preset voltage value, the spot weld is of poor quality. 
Then the supply of signals from the control unit 29 to the controlled 
commutator 30 and to the control input of the comparator 27 is cut off. 
The superimposed eddy current transducer 1 is disconnected from the 
unbalanced bridge circuit 24, the supply of the signal difference from the 
output of the comparator 27 to the indicator 28 is cut off, and the cycle 
of testing residing only in distinguishing high-quality spot welds from 
low-quality spot welds comes to its end. 
In case it is necessary to recognize a type of defect of the low-quality 
spot welds, the superimposed eddy current transducer 1 (FIG. 7) is 
installed on the test article in the immediate vicinity to the weld. The 
control unit 29 is energized thus supplying a signal to the control input 
of the controlled commutator 30 which switches the coil of the 
superimposed eddy current transducer 1 into the unbalanced bridge circuit 
20. At a time, signals from the control unit 29 are applied to the control 
inputs of the controlled commutator 31 and of the comparator 27. In doing 
so, the controlled commutator 31 connects the output of the phase detector 
21 with the second signal input of the comparator 27. 
The current flowing through the coil of the superimposed eddy current 
transducer 1 produces a primary high-frequency electromagnetic field 
inducing eddy currents in the surface layer of the test weld in the zone 
disposed in the immediate vicinity to the weld nugget zone. The resultant 
high-frequency electromagnetic field acts on the coil of the superimposed 
eddy current transducer 1, thus varying its impedance according to the 
conductivity of the material in said zone. 
From the output of the unbalanced bridge circuit 20, the voltage is fed to 
the input of the phase detector 21 initiating at its output the voltage 
proportional to the phase of the output voltage of the unbalanced bridge 
circuit 20. 
From the phase detector 21, the output voltage is fed to the second signal 
input of the comparator 27 through the controlled commutator 31. 
The comparator 27 compares the signals supplied from the memory unit 26 and 
from the phase detector 21, the signal being recorded in the memory unit 
26 when the coil of the superimposed eddy current transducer 1 installed 
in the weld nugget zone was included into the unbalanced bridge circuit 
20. 
Then the signal difference is applied to the indicator 28. 
If the difference between the signals on the display of the indicator 28 is 
above a preset voltage value, the spot weld suffers from a through 
penetration type defect. If the signal difference is below a preset 
voltage value, the spot weld suffers from a sticking-type defect. 
Next, the control unit 29 cuts off supply of the signal to the controlled 
commutator 31 and to the input of the comparator 27. The supply of the 
signal difference from the output of the comparator 27 is cut off, and the 
cycle of testing comes to its end. 
Thus, the present invention makes it possible to carry out continuous 
nondestructive testing of spot welds and to reveal defective spot welds. 
Besides, the invention makes it possible to perform testing without 
previous adjustment on a reference weld and to improve the reliability of 
testing due to elimination of such interfering factors as: 
variations in the chemical composition of the test weld material; 
variations in the structure of the test weld material; and 
variations in the environmental conditions such as an ambient temperature. 
What is more, the present invention makes it possible to reveal a type of 
defect of the weld nugget, namely to distinguish spot welds with a 
sticking-type defect and spot welds with a through penetration type 
defect. 
Metallic structures in which the spot welds suffer from a through 
penetration type defect can be used in operation under restricted cycles 
of dynamic load, while metallic structures in which the spot welds suffer 
from a sticking-type defect must be subjected to repeated welding for 
rectifying this defect, thus reducing a number of rejected metallic 
structures. 
From the spicific embodiments of the present invention considered 
hereinabove, it is readily apparent to those skilled in the art that all 
the objects and advantages of the invention can be accomplished within the 
scope of the appended claims. It is also apparent that insignificant 
changes in the steps of the method and in the construction of the 
apparatus realizing this method can be made without departing from the 
spirit of the invention. All these insignificant changes are considered to 
be within the spirit and scope of the invention as defined in the claims 
below.