V. A. GLUSHCHENKOV et al.: A NON-DESTRUCTIVE METHOD FOR EVALUATING THE ADHESION ...
3–8
A NON-DESTRUCTIVE METHOD FOR EVALUATING THE
ADHESION BETWEEN THE LAYERS OF
METAL-POLYMER-METAL COMPOSITE MATERIALS
NEPORU[NE METODE ZA OVREDNOTENJE ADHEZIJE MED
PLASTMI KOMPOZITNIH MATERIALOV V SESTAVI
KOVINA-POLIMER-KOVINA
Vladimir A. Glushchenkov
1
, Heinz Palkowski
2,1,3
, Rinat Y. Yusupov
1
,
Denis A. Matveev
1
, Mikhail V. Khardin
1,*
1
Samara National Research University, Metal Forming Department, Samara, Russia
2
Clausthal University of Technology, Institute of Metallurgy, Clausthal-Zellerfeld, Germany
3
Clausthal Center of Materials Technology, Clausthal-Zellerfeld, Germany
Prejem rokopisa – received: 2023-02-07; sprejem za objavo – accepted for publication: 2023-07-31
doi:10.17222/mit.2023.727
Development of a non-destructive measuring technique for evaluating the bonding quality of metal-polymer-metal composite
sheet materials is introduced. Sufficient adhesion is necessary to ensure further shaping steps or bearing loads when used with-
out failure due to delamination. Much attention is paid to determining the quality of gluing. According to the patent-literature
review, no non-destructive measuring or quality control of layers’ bonding quality over the entire surface has been found. Thus,
a new technical solution is proposed for defining the quality of bonding between the layers by means of electrodynamic repul-
sion forces arising from the pulse current passing over the metal layers of a composite in the counter direction. The feasibility of
the method is tested in laboratory conditions. For its implementation, the necessary values of the test parameters are obtained,
i.e., the values of the current transmitted through metal layers. In addition, the possibility of detecting local defects of different
shapes and sizes with the proposed method is estimated.
Keywords: sandwich composite sheet, adhesion strength, magnetic pulse, non-destructive measurement
V ~lanku avtorji predstavljajo razvoj tehnik neporu{nega testiranja in ovrednotenja kvalitete vezave med plastmi kompozitne
"sendvi~" plo~evine vrste kovina-polimer-kovina. Dobra vezava (adhezija) med plastmi te vrste kompozita je potrebna zato, da
plo~evina prenese nadaljnje stopnje preoblikovanja ali obremenitve, ne da bi pri tem pri{lo do delaminacije oziroma
razslojevanja. Veliko pozornosti je namenjeno dolo~evanju kvalitete lepljenja. Med pregledom razpolo`ljive patentne literature
za te vrste metod, metod neporu{nega merjenja ali kontrolnih metod kvalitete vezave med posameznimi plastmi po celotni
povr{ini so avtorji ugotovili, da le-te v pisni obliki ali predpisih ne obstajajo. Avtorji v ~lanku predstavljajo novo tehni~no
re{itev za dolo~itev kvalitete vezave med plastmi polimera in kovine na osnovi nastajajo~ih elektrodinami~nih odbojnih sil pri
prehodu preko kovinskih plasti kompozita v nasprotni smeri. Izvedljivost metode so avtorji testirali v laboratorijskih pogojih. Za
izvedbo te metode so ugotovili potrebne testne parametre, kot so na primer vrednosti jakosti toka, ki se prena{a s kovinskih
plasti. Dodatno so ocenili tudi kak{ne so mo`nosti, da s predlagano metodo dolo~ijo lokacijo prisotnih napak, njihovo obliko in
velikost.
Klju~ne besede: kompozitne "sendvi~" plo~evine, adhezijska trdnost, magnetni pulzi, neporu{ne metode testiranja
1 INTRODUCTION
Among the composite materials used in mechanical
engineering, a special one is a multilayer sheet material
consisting of covering metal layers and a polymer core,
thus having a "metal/non-metal/metal" combination.
1,2
In
the automotive industry, steel-polymer-steel sheets or
combinations of aluminum and polymers are already in
use.
3,4
A new 3-layered material of aluminum cover sheets
and a PP/PE polymeric core, abbreviated as Al-P-Al, for
aerospace and automotive applications is developed.
Similar lightweight materials can also be used for medi-
cal purposes, e.g., for prosthetic applications.
5
As shown in Figure 1, for the industrial use the man-
ufacturing of such materials includes gluing of the metal
and the polymer. To improve the quality of bonding, an
epoxy is used in such cases as the promoter. However, in
the case of biomedical use, epoxies have to be avoided.
Here, other solutions have been developed.
5–7
For testing the bonding quality, destructive test meth-
ods are used in general, such as shear tests or pull-off
tests. They require the cutting of the samples for testing
(witnesses), which helps us define, for example, the
strength of gluing layers in case of their separation. For
such pull-off tests, a glue with a bonding strength higher
than that between the layers of a sandwich has to be used
between the open surfaces of a metal and bracket. For
metals, metal bonding glues with a pull-off strength of
more than 25 MPa are available, making sure that failure
by rupture will occur between the layers of a sandwich
Materiali in tehnologije / Materials and technology 58 (2024) 1, 3–8 3
UDK 544.23: 621.792.4 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(1)3(2024)
*Corresponding author's e-mail:
Khardin.mv@ssau.ru (Mikhail V. Khardin)

with their lower bonding strength. For its testing, a sand-
wich is glued to the brackets of a testing machine and
this set-up is loaded with tension until failure of the
sandwich or the brackets. At the same time, mechanical
adhesion is estimated, i.e., the fastening strength of the
glued surfaces.
8
Up to now, no non-destructive method
for determining the quality of the adhesive strength has
been known.
A number of non-destructive methods for detecting
local defects (glueline/separation defect) with acoustic,
radioscopic, optical and other physical methods are
known and available on the market.
9–11
However, they do
not allow estimating the adhesive strength quantitatively
in the local area of a defect.
12
The absence of a reliable quantitative non-destructive
method of controlling the quality of glued layers in a
sheet composite material makes its use more difficult in
the industry manufacturing high-quality parts. On the
other hand, highly controlled quality of bonding opens
up the possibility of a wider use of sheet stamping opera-
tions in manufacturing parts from a multi-layer compos-
ite material.
13
Consequently, the development of a
non-destructive quality control method for the analysis
of the bonding quality between the glued metal and poly-
mer is of specific interest.
2 EXPERIMENTAL PART
2.1 Material and methods
Currents induce magnetic fields. When a pulse cur-
rent is fed in the oncoming direction along two closely
located conductors, a pulsed magnetic field is formed
around each of them. The interaction of these fields leads
to the creation of electrodynamic repulsion forces.
14
Using this physical phenomenon, a non-destructive
testing method for determining the quality of bonding
between layers of Al-P-Al composite materials is pro-
posed as sketched in Figure 2.
Metal sheets (1 and 3) with a polymer foil (2) be-
tween them are connected to the output of the magnetic
pulse installation (MIU).
15
Their electrical closure is car-
ried out using the bracket (5) and clamping slats (4 and
6).
When the battery is discharged by MIU capacitors to
the metal sheets, the pulsed current I flows in the counter
direction. In this case, electrodynamic forces arise, af-
fecting the separation. If the value of force F, generated
by the field, is greater than the predetermined adhesive
strength between the layers, delamination will occur; if it
is lower (with a given stock ratio), the integrity of the
material is not violated and the material is recognized as
suitable for further processing.
2.2 Samples for experimental verification of the pro-
posed technical solution
For our experiments, a layered composite (Al-P-Al)
was used with a total thickness of 0.9 mm, consisting of
two aluminum sheets (AlMn) – each 0.3 mm thick – sep-
arated by a 0.3 mm thick polymer foil. The mechanical
properties of the AlMn alloy in the annealed state are
listed in Table 1;
16
so are the ones for the polymer, a
thermoplastic polyolefin (polypropylene, polyethylene
and talc, PP/PE).
17
Table 1: Mechanical properties of the AlMn alloy and polymer, PP/PE
Material
state
Young’s
modulus,
GPa
Yield
strength,
MPa
Ultimate
tensile
strength,
MPa
Elongation
to rupture,
%
AlMn annealed 69 127.4 49 20
PP-PE 11 – 33 ± 11 500
2.3 Breakdown voltage of the polymer layer
In the process of a working cycle, when connecting
to the MIU, a high-voltage potential is generated be-
tween the plates, which can lead to an electrical break-
down of the polymer. That is why it is necessary to de-
termine its breakdown voltage. The samples are
connected to the high-voltage output of the AID-70M
testing machine allowing the testing of electrical insulat-
ing materials (dielectrics) up to 70 kV. To prevent sur-
V. A. GLUSHCHENKOV et al.: A NON-DESTRUCTIVE METHOD FOR EVALUATING THE ADHESION ...
4 Materiali in tehnologije / Materials and technology 58 (2024) 1, 3–8
Figure 1: Basic technological operations used for manufacturing multilayer composite materials Al-P-Al
Figure 2: Scheme of the non-destructive testing set-up for magnetic
pulse installation (MIU) to determine the bonding quality in Al-P-Al
sandwich composites

face breakdowns (over the ends), the polymer foil pro-
trudes the metal sheets by about 15 mm at all sides for
the tests.
For the experimental investigation, samples sketched
in Figure 3 were used. As a result, the breakdown volt-
age of the polymer is 17 kV. This makes it possible to
carry out experiments to assess the quality of gluing in a
wide range of MIU modes, with different values of the
current transmitted through the layers, and determine the
geometric sample dimensions to verify the proposed
technical solution for the gluing quality control.
2.4 Determination of the adhesion strength between
the composite layers
At this stage of processing MPM materials, their ad-
hesive strength is appraised using experimental results of
the pull-off or shear tests, therewith defining a tolerable
process window and keeping the processing parameters
within this field. Up to now, there has been no indicator
for evaluating the adhesive strength to be used in-line
and as a non-destructive test. The method introduced be-
low could be a solution to this issue.
The evaluation of the adhesion strength is carried out
on Al-P-Al sheet samples of 10 × 10 mm
2
. Extension
cords are attached to the metal parts by spot laser weld-
ing to fix them in the taps of a Tinius Olsen H5KT test
machine. Figure 4 shows the typical force-elongation di-
agram.
Up to point A a pure elastic deformation of the test
sample is given, while seperation between the Al-P lay-
ers occurs within section A–B, see Figure 4.
The experiments performed allow determining the
adhesion strength indicator to be ~(40–50) N. This indi-
cator has to be compared with the experimental values of
the electrodynamic forces obtained with the proposed
test method.
2.5 Determination of electrodynamic efforts
The magnitude of the electrodynamic forces between
two infinitely thin conductors of the same length is de-
fined with equation (1)
18
FI I k k =⋅⋅⋅ ⋅
 0
12
4
F
(1)
where the magnetic air permeability
μ
0
equals 4π×10
–7
H/m,
I
1
, I
2
– currents flowing in the parallel conductors,
k – geometric factor or circuit coefficient of the electro-
dynamic forces,
k
F
– coefficient taking into account the shape and size of
conductors.
The geometric factor depends on the length of the
conductors, i.e., their range distances:
k
l
a
a
l
a
l
=⋅+
⎛ ⎝ ⎜ ⎞ ⎠ ⎟ −
⎡ ⎣ ⎢ ⎢ ⎤ ⎦ ⎥ ⎥ 2
1
2
(2)
l – length of the conductors,
a – axial distance between them.
Given that the distance between the conductors is sig-
nificantly lower than their length, we get:
k
l
a
=
2
k
F
takes into account the shape of the cross-section of
the conductors, that is, the form of the power lines of the
magnetic field. This calculation is carried out with the
help of Dwight’s diagrams.
19
The discharge current of the capacitor battery is vari-
able. The value of I
max
is tens of kA. For the calculation,
the following current equation is used:
V. A. GLUSHCHENKOV et al.: A NON-DESTRUCTIVE METHOD FOR EVALUATING THE ADHESION ...
Materiali in tehnologije / Materials and technology 58 (2024) 1, 3–8 5
Figure 3: Sample drawing for experimental studies of the adhesion
strength
Figure 4: Indicator chart of testing for the separation of Al-P-Al sand-
wich

I
I
D
=
max
2
Given the same current value for both conductors (in
our case, for both sheets of the composite) we obtain an
expression to calculate the maximum electrodynamic
force:
FI
l
a
k
max
=⋅⋅⋅
−
10
72
DF
(3)
For the sample dimensions used in the experiments
(a = 0.6 mm, b = 0.3 mm, H=10mm,L = 100 mm) it is
not possible to find the proper k
F
coefficient in Dwight’s
diagram, as with (a–b)/(h+b) = 0.03 and h/l = 30; a curve
with such values is missing there.
To find the fitting values of the electrodynamic forces
affecting the separation of the composite layers, the
method of their determination through the kinematics of
the strip moving under the action of these forces is pro-
posed. On two non-glued plates, the current also skips in
the counter direction. The emerging electrodynamic
force causes acceleration on one of the sheets (F = m·a).
High-speed recording of this process allows us to photo-
graph the kinematics of the central free-moving strip in
the initial time stage and calculate the value of F.
Figure 5 present the principal sketch of the measur-
ing arrangement created for this purpose and the devices
used.
20
The principle of the operation is as follows: launch-
ing of the camera (6) and charging of the battery to the
desired level of the magnetic-pulse installation capaci-
tors take place simultaneously (1). Both the light source
and the synchronization unit (3) are activated (7) due to
the signals from the computer (4) and the camera (6).
The synchronization unit with a defined delay gives off a
triggering pulse to the discharge of the magnetic-pulse
installation and the oscilloscope sweep (2).
The delay time (delay) of the discharge current is less
than 1 μs. At the same time, the signal of the discharge
current from sensor 9 is recorded with the oscilloscope.
A diagram of the operation of the measuring set-up is
shown in Figure 6.
The first graph shows the longest cycle of the camera
sweep drive. Below is the charging cycle of the capaci-
tors that provide the subsequent pulsed discharge. Next,
the backlight turns on to ensure filming. When the lamp
is in the operating mode during time t
1
, synchronization
occurs; after interval t
2
the magnetic-pulse installation is
turned on and the capacitor bank is discharged. Simulta-
neously with the beginning of the discharge, the camera
is turned on, carrying out frame-by-frame shooting.
The frame frequency (FPS) of shooting in the experi-
ments is 29740 frames/s. Consequently, the time be-
tween the frames is 33.6 μs. Figure 7 shows four frag-
ments of the sample movement. Starting with the third
6 Materiali in tehnologije / Materials and technology 58 (2024) 1, 3–8
V. A. GLUSHCHENKOV et al.: A NON-DESTRUCTIVE METHOD FOR EVALUATING THE ADHESION ...
Figure 7: Fragments of recording the layer repulsion process
Table 2: Dependence of the sample movement on time
Sample movement, mm Time t,s ·10
–6
00
0 33.6
0.4·10
–3
67.2
1.2·10
–3
100.8
Figure 5: Sketch of the measuring arrangement for recording the
movement of the strip under the influence of electrodynamic load
1 – magnetic-pulse installation, 2 – oscilloscope, 3 – synchronization
unit, 4 – computer, 5 – display, 6 – high-speed film camera, 7 – pulse
backlight, 8 – sample, 9 – pulse current sensor
Figure 6: Diagram of the operation of the measuring set-up

frame (67.2 μs), the sample is observed, as indicated by
the arrows.
Numerical results of processing the records are given
in Table 2.
The obtained values allow calculating the accelera-
tion (a = 531462.49 m/s
2
) and, knowing the mass of the
moving section (m = 0.25 × 10
–3
kg), the electrodynamic
force (F = 132.87 N).
In addition, the data obtained allow us to get the
value of the k
F
and apply it in further calculations:
k
F
Ik
F
D
=
⋅⋅⋅
=
⋅⋅ ⋅
=
−−
max
.
.
210
13287
2 10 295602 125
0006
72 7
Thus, all the necessary values included in Equation
(1) are obtained to determine the electrodynamic forces
arising from the non-destructive quality control of the
composite layers.
3 RESULTS
The purpose of the experimental studies is to find the
dependence of the force of the metal strip – adhesive
strength – on the value of the current transmitted by the
metal layers, i.e., to find the minimum current value, at
which a high-quality sandwich sample remains undam-
aged while delamination is observed in a sample with a
weak bond.
After each discharge, a visual and instrumental con-
trol of the samples is carried out for the impairment of
the adhesive strength of the layers: possible deformation
of the metallic layers is recorded using a simple dial
gauge.
In the absence of any defects, indicated by deforma-
tions, a sample is connected again to the MIU and a
higher current is created. The current is measured using a
RGA belt (a current sensor) and its value is presented on
the oscilloscope screen. Results of testing defect-free
specimens are listed in Table 3.
Table 3: Experimental results: height of sample delamination h at cur-
rent flow I
d
Sample no. Current I
d
,k À
Height of delami-
nation h,mm
1
12.83 0
13.59 0
14.16 0
16.57 0
17.50 0.19
2
15.86 0
17.11 0.17
3
15.80 0
17.19 0.15
Thus, with a current value of (17.11–17.50) kA, a
delamination between metal an polymer is observed,
having a bulge of around (0.15–0.19) mm. This current
value corresponds to an electrodynamic force of
(44–46) N, which corresponds to the experimentally ob-
tained value of the force of (40–50) N (see Figure 4).
The results obtained allow a construction based on
the dependence of the electrodynamic force on the value
of the pulse current passed through the metal layers of
the three-layered Al-P-Al composite (Figure 8).
If the adhesion strength (F
set
) is specified for a com-
posite material in technical conditions, then, considering
the safety coefficient – for example, 1.2 (F
set
/1.2) – we
find the adequate value of the current being tested (cur-
rent I) with the help of the constructed graph (Figure 8),
and through it, we determine the condenser discharge en-
ergy of the magnetic-pulse installation, which must be
used for a non-destructive test. If, with this current value
(discharge energy of the capacitor battery), no delami-
nation is observed, the logical conclusion is that the
bonding quality corresponds to the required one.
V. A. GLUSHCHENKOV et al.: A NON-DESTRUCTIVE METHOD FOR EVALUATING THE ADHESION ...
Materiali in tehnologije / Materials and technology 58 (2024) 1, 3–8 7
Figure 8: Dependence of the electrodynamic force on the current flow
through the metal layers
Figure 9: Samples with local defects

4 DISCUSSION
4.1 Possibility to use this method to detect local defects
This non-destructive method is suitable for determin-
ing the quality of bonding in general. The question to be
answered now is how sensitive it is, e.g., when detecting
local defects in a bonding structure. Therefore, three-lay-
ered Al-P-Al samples with the same dimensions as be-
fore were used, but with pre-established macroscopic lo-
cal defects of different dimensions in the core material.
Circles with diameters of 3, 6, 8 mm, located in the cen-
ter of the composite as single circles or several ones set
in a row were cut into the polymer, see Figure 9.
An increase in the diameter of the defects leads to a
decrease in the current, at which delamination of a sam-
ple occurs. In all the cases, the place of the gap defect is
clearly seen when the current is passing through.
The analysis of defective samples carried out during
the test has shown that delamination in an Al-P-Al sand-
wich with a circular defect of 3 mm and height of
0.1–0.3 mm is observed at a current, I
d
, of (16 ÷ 17) kA.
The variation in the values for one testing set is ex-
plained by the differences in the bonding quality of each
sample (standard deviation) and placing the defects (geo-
metrical deviations). However, according to the data ob-
tained we can see some dependence of the current mag-
nitude required for delamination on the size of the
defect: the smaller the diameter of the defect, therewith
the ratio of the defect, the higher is the force (current) re-
quired for delamination of the layers.
The observed local deformation of the strip above the
defect proves the possibility of using the proposed
method of control for detecting local defects as well.
5 CONCLUSIONS
A non-destructive method for testing the quality of
bonding between the layers in Al-P-Al composite sheet
materials, i.e., metal/dielectric intermediate layer/metal,
is introduced. The first experimental tests show its basic
applicability. Defects due to delamination can be de-
tected and correlated with electrodynamic forces and
currents. Moreover, implemented defects of a macro-
scopic size lead to a characteristic change in the currents
needed for delamination, depending on the size of a de-
fect. This method also has a potential for detecting
smaller defects in such composites. Up to now it has not
been possible to quantitatively assess the value of bond-
ing, but we can determine whether the bonding is suffi-
cient for an estimated load, based on the failure/non-fail-
ure of the part under defined current loads.
Further tests have been performed to develop an in-
dustrially fitting measuring device, a tool able to provide
quantitative measures and values for on-line detection of
the bonding quality in three- or even multilayer sandwich
structures of the Al-P-Al type, consisting, in general, of
metal/polymer/metal, or similar structures.
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8 Materiali in tehnologije / Materials and technology 58 (2024) 1, 3–8