EMBEDDED PVDF SENSORS FOR IMPACT AND AE DETECTION IN
COMPOSITE STRUCTURES
C. Caneva, I.M. De Rosa and F. Sarasini
Department of Chemical and Materials Engineering, University of Rome “La Sapienza”
Via Eudossiana 18, 00184 Rome (Italy)
claudio.caneva@ingchim.ing.uniroma1.it, igor.derosa@uniroma1.it, fabrizio.sarasini@uniroma1.it
ABSTRACT
This work focuses on the assessment of the damage due to low-velocity impact on composite laminates. Damage of polymeric
composite structures through impact events is perhaps one of the most important aspects of mechanical behavior which limits
the wide applications of these materials and it is critical to the Structural Health Monitoring system (SHM). This kind of damage
can occur during assembly or in service. An embedded piezoelectric (PVDF) thin film sensors system for Acoustic Emission
(AE) was realized to investigate, in real time, post impact damage in aramid woven fabric reinforced epoxy [1].
Aramid fiber/epoxy composite specimens with embedded PVDFs previously impacted at different energies, namely 5J, 10J
and 15J, were tested using three-point bending tests. The mechanical behavior of the specimens was investigated in order to
assess that these sensors had a negligible effect on the mechanical properties of the impacted laminates. It appeared from the
mechanical tests, that the flexural strength decreased passing from non-impacted specimens to those impacted with the
highest energy and that the embedment of PVDFs in the laminates did not affect the structural integrity of the impacted
composites, since the mechanical response of specimens with and without PVDF sensors was almost identical. Moreover, to
verify that the PVDF sensors are reliable during the impact loading without experiencing any kind of damage, impacts at
energies as high as 20J were performed on the area over the sensor. The lack of damage was assessed by using the HsuNielsen source.
The degree of impact damage, represented by the decrease in mechanical properties has been correlated with the AE activity
by means of a parametric analysis of the acoustic emission signals detected during post impact mechanical tests. This allowed
us to better understand the microscopic fracture processes leading to the final failure of the composites. It was also verified
that there is no great difference in the AE detected by PVDF sensors embedded and by those surface mounted.
Introduction
In composite structures low-energy impact can produce damage which is barely visible and that exists in the form of extensive
subsurface matrix cracks, backside fiber failure and delaminations and this can significantly degrade a structure’s
performance. Because the existence of barely visible impact damage (BVID) can be a significant safety threat, the capability to
have a better understanding of both the impact response of composite laminates and their structural performance is of great
utility.
The impact damage mechanism in a laminate is a very complex process. It is a combination of matrix cracking, surface
buckling, delamination, fiber shear-out and fiber fracture which usually interact with each other. Very often, damage in
composites begins on the non-impacted surface or in the form of delamination. Impact damage is generally not considered as
a threat in metal structures because, owing to their ductile nature, large amounts of energy may be absorbed. On the contrary,
composites can fail in a wide variety of modes and contain barely visible damage which severely reduces their structural
integrity [2-8]. Most composites are brittle and so can only absorb energy in elastic deformation and through damage
mechanisms and not via plastic deformation.
The impacts performed in this work can be classified as low-velocity impacts. Generally, impacts are categorized into either
low or high velocity, but there is not a clear distinction between categories. In a high-velocity impact the structure has not time
to respond, leading to a very localized damage. In low-velocity impact, the contact duration is long enough for the entire
structure to respond to the impact and in consequence more energy is absorbed elastically. Based on these considerations,
Cantwell and Morton [8] classified low velocity as up to 10ms-1, by considering the test techniques which are normally used in
simulating the impact event (instrumented drop weight impact test, Charpy, Izod and so on).
The heterogeneous and anisotropic nature of fiber reinforced polymer laminates give rise to four main modes of failure [7]:
•
•
•
•
Matrix mode – cracking occurs parallel to the fibers due to tension, compression or shear;
Delamination mode – produced by interlaminar stresses;
Fiber mode – in tension fiber breakage and in compression fiber buckling;
Penetration – the impactor completely perforates the impacted surface.
In this work, no attempt has been made to definitively characterize the different damage mechanism and their evolution during
the impact of aramid reinforced composite laminates. The impact damage was employed so as to introduce damage within
composite materials and verify whether or not the AE signals detected by embedded PVDF sensors could be able to
discriminate between amount of induced damage and the post impact mechanical behavior.
Materials and specimens
SP106 with Slow Hardener (mix ratio 5:1 by volume), a multi-purpose Epoxy System from SP Systems, was used as matrix
material. The aramid fabric reinforcement used was an RA175H4 from SP Systems, which is a 4-Harness Satin weave
containing Kevlar49 fibers and with a Fiber Areal Weight (FAW) of 170 g/m2.
The specimens were manufactured using the Hand Lay Up technique. The use of this method was determined by the
possibility of controlling in a more efficient way the parameters of the whole process. Furthermore, this process made the
embedment of PVDF sensors easier. Ten layers of fabric were used to make the test panels so that the final panel thickness
was about 3 mm and the fiber volume fraction was about 0.40. The fabrication procedure is briefly described below. The
woven fabric was first cut into pieces of 250 mm long and 250 mm wide. The right number of layers were aligned and stacked
together, and put inside a metallic mold whose interior dimensions match those of the preform (250 x 250 mm). The mould had
been previously covered with a release agent (Polyvinyl alcohol or PVA). The preform was then impregnated with the matrix
mixture. During the layer stacking process, PVDF sensors were embedded in the middle of the stacking sequence of the
composite [1, 9, 10]. The mould was finally closed using another aluminum mould and subjected to the cure cycle. The cure
cycle was 19h at room temperature and under low pressure followed by 24h at 75°C. Upon completion of the cure cycle, the
composite laminate panel was removed from the mould and ready for cutting into test specimens.
The volume fractions Vf of the composite samples were calculated from the preform basic weight (weight per unit area) and
they ranged between 0.37 and 0.40. From the plates were cut the three-point bend specimens having a length of 250 mm, a
width of 30 mm and a thickness of 3 mm. The specimens were divided in four groups with six specimens in each group. One
group was not impacted (served as a reference material, labeled as KFR for specimens without PVDFs embedded and KFE
for specimens with two PVDFs embedded) whilst the others were impacted at 5, 10 and 15J energies (labeled as Kr_5J, 10J,
15J for those without PVDFs embedded, and Ke_5J, 10J, 15J for those with two PVDFs embedded). Another one series of
specimens was manufactured with only one PVDF sensor embedded in the centre. These specimens were impacted at
different energies (5, 10, 15 and 20J) on the area over the sensor.
Figure 1. Schematic representation of a DT series element (left) and photograph showing a typical DT1-052K sensor (right)
The PVDF sensors used in this work, DT1-052K, were manufactured by Measurement Specialties Inc. The DT series of piezo
film sensors are rectangular elements of piezo film with silver ink screen printed electrodes and are supplied with a thin
urethane coating over the active sensor area; the lead attachment legs are free of the insulating urethane coating [11]. Figure
1 shows a schematic representation of the DT series element with a detailed description of the sensors used. Table 1 lists
some typical properties of the piezo film [11]. The PVDF sensors for the types KFR and Kr were bonded to the specimen
surface using a double-side tape.
Table 1. Typical properties of piezo film
Symbol
d31
d33
g31
g33
k31
kt
Y
p
ε
ε/ε0
ρm
T
Parameter
Piezo Strain Constant
Piezo Strain Constant
Piezo Stress Constant
Piezo Stress Constant
Electromechanical
Coupling Factor
Electromechanical
Coupling Factor
Young’s Modulus
Pyroelectric coefficient
Permittivity
Relative Permittivity
Mass Density
Temperature Range
PVDF
23
-33
216
-330
Units
10-12 (m/m)/(V/m)
10-12 (m/m)/(V/m)
10-3 (V/m)/(N/m2)
10-3 (V/m)/(N/m2)
12%
14%
2-4
30
106-113
12-13
1.78
-40 to 80...100
109 N/m2
10-6 C/m2 K
10-12 F/m
3
3
10 kg/m
°C
Experimental
The specimens were impacted and then subjected to post-impact three-point bend test. The impact point was located at the
centre of the specimens. The impact energy was changed varying the mass of the hemispherical drop-weight impactor thus
having a constant velocity of 2.5m/s. Impact tests were performed on an instrumented impact tower (ATI-MIO-16E-2) fitted
with an anti-rebound device. Before being impacted, the specimens were clamped to ensure the right transmission of the
impact force to the specimens.
The flexural tests were carried out in accordance with ASTM D-790 (3-point loading). These tests were performed in a
universal testing machine (Instron 5584), at a constant cross-head speed of 2.5mm/min and the span-to-thickness ratio in
these three-point bending tests was 20:1. The PVDF sensors embedded were located within the span length. Ten specimens
were tested for each composite system. The specimens were tested so as to have the face opposite to the impact point in
compression during the flexural tests. These tests were monitored by acoustic emission until final fracture occurred using an
AMSY-5 AE system by Vallen Systeme GmbH. The AE acquisition settings used throughout this experimental work are as
follows: threshold = 40dB, Rearm Time = 0.4ms, Duration Discrimination Time = 0.2ms and total gain = 34 dB. Figure 2 shows
an example of the specimens tested.
Figure 2. Photograph showing specimens KFR and KFE
To check whether or not the sensors were able to support impacts, calibration tests (through Hsu-Nielsen source or pencil lead
break method) were performed on specimens having the same dimensions of those tested in flexure but with only one sensor
embedded in the middle and impacted just above it. Figure 3 shows the experimental set-up used.
PVDF
Impact Point
Pencil lead
break point
Figure 3. Experimental set-up used in the post-impact calibration
Three specimens were tested at each impact energy. The impact energies were 5, 10, 15 and 20J. The distance between
each pencil lead break point was 2cm.
Results and discussion
Figures 4-5 show post impact photographs of both front and rear target faces of aramid reinforced laminates subjected to 5, 10
and 15J impacts. As can be seen, the only visible damage observed on the impacted surface for all specimens was a very
slight indentation of the surface coupled with some whitening effect (Figure 4), while no visible damage was observed on the
rear face except for the specimens impacted at the highest energies which showed some matrix cracks (Figure 5). This
behavior is due to the thickness of the laminates inasmuch as thick laminates are generally less susceptible to impact damage
as well as to the high impact toughness of aramid fibers. In fact, for resistance to low-velocity impact, the ability to store energy
in the fibers is the fundamental parameter.
Figure 4. Impacted surface of a Ke_10J (left) and Ke_15J (right)
Figure 5. Rear surface of a Ke_15J
Figure 6 shows three-point bending strength of specimens without and with PVDFs embedded impacted at different energies.
Figure 6. Bending strength vs. impact energy for specimens without and with PVDFs embedded
It can be noticed that there is a decrease in the bending strength passing from the non-impacted material to that impacted with
the highest energy. It is to be emphasized that the presence of PVDF sensors embedded does not affect the mechanical
response of the laminates [1], even in the case of impact damage, since the trend for the two kind of aramid reinforced
specimens is almost identical. From these graphs is furthermore clear the role that plays impact damage in limiting mechanical
strength and life of composite materials, even if it does not produce large amount of visible damage. While passing from nonimpacted to impacted materials, aramid reinforced laminates experience less decrease in bending strength than E-glass ones,
for instance, because of their inherent higher impact toughness, which enables the composite to undergo major deformation
thus better absorbing the impact energy.
Figure 7. AE signal attenuation curves for PVDF sensors embedded within aramid reinforced composite laminates impacted at
different energies
Before discussing the results of the AE monitoring of post-impact flexural tests, it is worth showing the results of calibration
performed on aramid reinforced laminates impacted at different energies. The results of these calibration tests are
summarized in Figure 7. The overall trend resembles the one detected for specimens non-impacted, that is to say a decrease
of the amplitude values detected as a function of the increasing distance from the sensor embedded. From the results it can be
inferred that the damage due to impact does not affect the signal response and sensitivity of PVDF sensors. The values
detected are similar to the corresponding ones of the non-impacted specimens and there is not a significant difference among
the values corresponding to the different impact energies. The scatter of data found for the aramid reinforced laminates is still
so reduced to point out a particular damage mechanism of the sensors.
Figure 8. Load and AE response (left), Load and amplitude distribution for KFR specimens (right)
Figure 9. Load and AE response (left), Load and amplitude distribution for KFE specimens (right)
Aramid specimens impacted at different energies exhibited a particular behavior. In fact, all of the specimens tested showed
the same behavior, very similar to that of the specimens undamaged; that is to say, significant AE activity started at the onset
of the “yielding” stage and there were no remarkable shift towards lower loads, as can be seen in Figures 8-10.
700
600
110
110
Load
Amplitude
Load
Amplitude
600
100
500
90
100
500
90
300
70
80
300
70
Amplitude (dB)
80
Load (N)
400
Amplitude (dB)
Load (N)
400
200
200
60
100
50
0
0
50
100
150
200
Time (s)
250
300
350
40
400
60
100
50
0
0
50
100
150
200
250
300
40
350
Time (s)
Figure 10. Load and amplitude distribution for specimens Ke_5J (left) and Ke_15J (right)
As passing from non-impacted to impacted specimens at the highest energy, it is worth noting that there is a progressive
decrease in the overall emissivity, in terms of cumulative number of hits detected. The high impact toughness of aramid fibers
accounts for this particular behavior. In fact, during impact, the induced damage in aramid reinforced laminates is absorbed by
the tough fibers thus enabling the composite to undergo the full deformation, the opposite of what happens, for example, in
glass reinforced laminates where the inherently brittle behavior of the epoxy matrix is not balanced by the fibers, which are
brittle as well. This causes the occurrence of matrix cracks and fiber ruptures which are responsible of the premature presence
of signals as increasing the impact energy. On the contrary, in aramid laminates, this tendency is less pronounced. Since the
signals detected in the first stages are usually characterized by low amplitudes, the lower sensibility of PVDF in aramid
laminates together with the extremely localized damage formation could collaborate in order to make these signals hardly
detectable. Even the amplitude distributions for the impacted specimens were very similar to each other, as can be seen in
Figure 11 (left). In addition to a major number of signals for specimens undamaged, the trend of the curves is almost identical.
110
300
5J
10J
15J
0J
200
Hits
5J
10J
15J
100
AE Signal Amplitude (dB)
250
150
100
90
80
70
60
50
50
0
40
50
60
70
AE Signal Amplitude (dB)
80
90
100
40
0
500
1000
1500
2000
2500
3000
Duration ( µ s)
Figure 11. Peak amplitude distributions for aramid laminates impacted at 0, 5, 10 and 15J (left), Amplitude vs. Duration for
aramid laminates impacted at 5, 10 and 15J (right)
The AE results confirm the better damage tolerance of aramid fibers. Differences among the AE responses of aramid
reinforced specimens were found in plotting the amplitude against duration, as shown in Figure 11 (right). As passing from 5 to
15J, the signals tend to be more concentrated in the region corresponding to matrix cracking (low amplitudes and short
durations, visible also on the rear surface) while there is substantially the same number of signals belonging to delaminations
(intermediate amplitudes and medium/long durations) and fiber ruptures for the specimens impacted at 10 and 15J.
Specimens impacted at 5J seem to have a behavior which is intermediate, with less tendency to delamination damage. These
results seem to confirm the decrease in bending strength with increasing impact energy. The lower emissivity has been taken
into account considering the cumulative amplitude normalized to that of non-impacted specimen. The results are shown in
Figure 12. The sharpest decrease occurs at 5J: this points out that increasing the energy does not increase markedly the level
of damage obtained for a 5J impact. This is due to the presence of aramid fibers. Another significant decrease occurs at 15J,
where the combination of different damage mechanisms in terms of matrix cracking and delaminations becomes strengthlimiting.
Figure 12. Normalized cumulative amplitude vs. impact energy
Conclusions
Aramid reinforced specimens were impacted at different energies, namely 5J, 10J and 15J, and their residual mechanical
strength was analyzed by post-impact three-point flexural tests. These tests were monitored by AE in order to obtain valuable
information on the damage induced through variations in the AE responses detected by embedded PVDF sensors. The results
of the AE analysis demonstrated that damage affects remarkably the AE activity patterns and parameters and that these
variations are detected by PVDF sensors.
As a consequence, this system enables the detection of whether the damage is present in the structure, damage location,
quantification of damage severity and evaluation of remaining structural integrity and risk assessment.
In conclusion, results from this study indicate that the embedment of PVDFs in aramid woven fabric/epoxy laminates has
negligible influence on their mechanical behavior and that the use of Acoustic Emission detected by embedded PVDFs has
great potential for the investigation of the development of damage mechanisms and, as a consequence, for the establishment
of an effective SHM system based on it which is thought to be reliable, low cost, effective and in situ. This could result in
avoiding catastrophic failures and in predicting residual life thus leading to ultra-reliable and extremely safe structures. In
addition, these structures would be maintained only when actually needed, thus substantially reducing maintenance and
related life cycle costs.
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