http://www.revmaterialeplastice.ro MATERIALE PLASTICE ♦55♦No. 3 ♦2018 364Assessment of Delamination in Tensylon® UHMWPE Composites by Laser-induced… [629407]

http://www.revmaterialeplastice.ro MATERIALE PLASTICE ♦55♦No. 3 ♦2018 364Assessment of Delamination in Tensylon® UHMWPE
Composites by Laser-induced Shock
LUMINITA CRISTINA ALIL1, MICHEL ARRIGONI2*, LORENA DELEANU3, MARCEL ISTRATE4
1Military Technical Academy Ferdinand I, Doctoral School Engineering of Systems for Defense and Security, 39-49 George
Cosbuc Av., 050141, Bucharest, Romania
2ENSTA Bretagne, IRDL FRE CNRS n°3744, 2 rue Francois Verny, 29806 Brest, France
3Dunarea de Jos University of Galai (UGAL), 47, Strada Domneasca, Galati, Romania
4STIMPEX SA, 46-48 Nicolae Teclu, 032368, Bucharest, Romania
Ultra-High Molecular Weight Polyethylene (UHMWPE) composites are the result of recent developments in
material research for ballistic protection due to their ability to absorb the kinetic energy of the bullet by
various mechanisms of dissipation, among which an important one is delamination. In order to study this
mechanism independently, the laser induced shock wave testing procedure has been used on thin Tensylon®
laminate samples. Laser-induced shock represents a modern approach that can be used for assessing the
interlaminar bond strength between two plies of a composite material, in dynamic conditions, at high strain
rates representative for a ballistic impact. Through this technique, a delamination failure stress threshold
can be determined. In the present work, the laser induced shock technique was applied on the commercial
UHMWPE material called Tensylon®. The delamination threshold of this material was determined by using
the Novikov approach, and, compared to the literature, the results match the values determined by other
means of measurement.
Keywords : delamination stress threshold, laser-induced shock wave, ballistic protection, polymer composites,
Tensylon®
* email: [anonimizat] industry of ballistic protection is focused on
developing lightweight solutions for their strategic use. First,
light weight armors increase the autonomy of vehicles in
battlefields by reducing the fuel consumption determinedby the weight penalty of metal shielding materials. At the
same time, they offer to enhance the level of personal
protection and increase the mobility of the users. Ultra-High Molecular Weight Polyethylene (UHMWPE) – based
composite assemblies have been identified as some of
the most relevant solutions for light weight armors. Thehard (hot-pressed) ballistic laminates available under the
tradename of Tensylon
® (manufactured by DuPont®, USA)
are UHMWPE-based materials that could fulfillrequirements related to ballistic protection.
Delamination (i. e. interlaminar fracture toughness) is a
phenomenon specific to laminates. Figure 1 illustrates thedelamination associated phenomena on a 50 cm x 50 cm
Tensylon® hard panel, consisting of 196 layers (initially
approximately 23 mm thick), impacted by five 7.62*39mm Full Metal Jacketed, Lead Core (FMJ-LC) bullets from
a distance of 10 m, at ~700 m/s. The plate has been
sandwiched, before being hit, between two thick steelplates (2.5 mm). The back and forth travel of the shock
wave, enhanced and prolonged by the presence of the steel
sheets, determined a massive delamination throughout thevolume of the panel.
Delamination is, among processes as fiber melting,
bulging (which could produce curling or buckling of thefailed filaments/layers) and stretching (which leads to back
face deformation), one of the physical mechanisms of
UHMWPE-based and other composite materials fordissipating the kinetic energy during shock and impact
events. However, in modelling approaches, delamination
is often neglected, as it is considered to have smallinfluence on other characteristics of interest, such as fiber
strength or ply stiffness [1-2].Nevertheless, delamination is considered to be a key
mechanism in the material response, as a sign of softening[3], which subsequently is reported to actually enhance
the ballistic resistance of laminates [4-6]. Therefore, it is
useful to know more about the dissipative role played bythe delamination process in the prospect of enhancing the
performance of composite-based light weight armor.
The laser induced shock wave technique, at the base of
the laser adhesion test (LASAT) [7] is a modern approach
especially used in studying the adhesive bonding of light-
weight, multilayered composites and adhesively bondedassemblies used in the aeronautic and naval structures.
Its ability in establishing the performance of multilayered
materials under dynamic loadings, at a local scale, ishighlighted in numerous articles and thesis [7-9].
The work presented in this paper is an experimental
approach conducted on Tensylon® laminate, a material
Fig. 1. (a) Tensylon® plate impacted by five 7.62*39 mm FMJ-LC
bullets; (b) section through the impact zone in the plate, via
waterjet-cutting; (c) detailed view of the impact zone, obtained
with a Keyence VHX-5000 digital microscope, showing the
UHMWPE mechanisms of stopping the bullet penetration

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Fig. 2. Microscopic observations of polished Tensylon® laminate,
using (a) 5 µm grit silicon carbide paper and (b) 0.6 µm alumina
polishing suspension on felt. One ply is approximately 57 µm thick
Fig. 3. Tensylon® laminate samples dimensions
Fig. 4. a. Schematic view of the laser-induced pressure (direct/
confined) in time; b. Laser-matter interaction and generation of the
shock wave within the target; b. Confined laser-matter interaction
Fig. 5. Maximal pressure
induced by laser shock
wave versus power density
in a target [15-16]which has not been extensively studied at mechanical
level, to the authors’ knowledge, in spite of its
acknowledged ballistic performances. This paper presentssome of the results obtained by testing 1.25 and 2.45 mm
thick Tensylon® laminate samples subjected to the laser
induced shock wave technique.
The article is organized as follows. After the introduction,
section two provides a brief description of the
manufacturing process of the Tensylon® compositematerials involved in this study. Section three contains the
layout of the experimental procedure of laser induced
shock wave technique and the description of the usedequipment. Section four presents test results in terms of
time resolved free surface velocity (FSV) diagrams. Section
five expands the approach for assessing the failure stressthreshold of the studied UHMWPE materials. The last
section is dedicated to general conclusions and future
prospects.
Experimental part
Description of the materials
Tensylon® is a representative material for the highly
oriented polyethylene tapes, obtained through an industrial
procedure, called solid state extrusion. Unlike Dyneema®and other types of fibrous UHMWPE worldwidely used for
a variety of purposes, the oriented filaments are difficult to
be characterized individually and the matrix constituentcannot be easily replaced in order to study its influence on
the overall composite performance, as it represents a
different phase of the same polyethylene material [10].However, the authors are mainly interested in the ballistic
proficiency of these materials, apart from any other use
and, from this point of view, Tensylon® is an affordablesolution that exhibits similar performance to the most
ballistic-efficient versions of fiber-based UHMWPE
composites [11].
Currently, there is only one type of Tensylon
® tape
commercially available under the name Tensylon® HSBD
30A, which is manufactured in the shape of a 1.60 m wide
double-layered bi-directional criss-cross tape, having a total
length of approximately 300 m [12]. Tensylon® laminatescan be obtained by pressing several cut-to-size layers, at
15.2 MPa and 120
oC maximum temperature. Its internal
structure is similar to other UHMWPE-based products, suchas Dyneema
® HB26 or HB50, which are [0°, 90°]nsequences, i.e. a criss-cross pattern. Each layer consists inunidirectionally oriented filaments and about 20% adhesivematrix (percentage characteristic to all oriented UHMWPE
materials, either fiber or filament-based) [13]. Figure 2
depicts microscopical cross sections, obtained by waterjet cuttting from Tensylon® laminate samples, revealing
its layered structure composed of a succession of plies of
about 57 µm in thickness. In order to obtain a more detailedmicroscopic observation, polishing with very fine particles
sized down to 40 nm is required [14].The Tensylon® laminate samples (having the internal
structure depicted in fig. 2) have been obtained by water
jet cutting of small pieces (10 mm x 20 mm) from 25 cmx 30 cm rigid plates, specially hot-pressed for these tests,
having a thicknesses of 1.15±0.05 mm (obtained by
pressing together 20 layers) and 2.45±0.05 mm (obtainedby pressing together 40 layers) (fig. 3).
Description of the setup
Testing procedure
During the irradiation, a thin layer (submicronic) of
material is sublimated in plasma (fig. 4) in a very short
time (< ns, similar to a detonation). The plasma expansion
pushes into the sample, resulting in a shock wave (figure4-b). When a layer of water or another transparent material
is added at the place of irradiation (fig. 4-c), it plays a
confinement role as it retains the plasma expansion. Itresults in an increase by a factor of 3 of the generated
pressure and by a factor of 3 to 5 of the duration of the
pressure pulse (fig. 4-a).
The general condition of achieving a laser induced shock
wave is that the power density ( φ) should be of the order of
GW/cm
2, but it must remain below 4 GW/cm² in air and 8
GW/cm² in water, in order to avoid breakdown at ambient
conditions in front of the sample. This condition sets the
maximal laser spot size at impact, in air and under water.The density of power is calculated with relation (1):
(1)
wheremaxE is the maximum energy provided by the laser
source, τ is the duration of the laser-induced pressure and
Sis the laser spot size on the sample. This relation between
the power density and the induced peak pressure in water
confined regime was determined empirically by Berthe
and Sollier [15-16], based on the graph depicted in figure5.

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Fig. 6. Testing configuration for
the laser induced-shock
impacts, performed on
Tensylon® UHMWPE laminatesEquipment and setup
In these experiments, the shock was created by the
laser-matter interaction in water confined geometry [15-16]. This configuration requires focusing the laser beam
on one face of the sample, in such way that its propagation
is normal to the sample thickness (out-of-plane direction)(fig. 6).the form of a sinusoid response with a modulated period
that is a function of the Doppler shift, i.e. the velocity of the
sensed material.
HetV signals have been processed with
Cafeine®
software [18] that performs a step by step Short TimeFourrier Transform (STFT) with a temporal window of 800samples (20 ns width) and 1 sample of overlapping (0.025
ns). It gives a time resolved velocity under the form of
spectrograms, with a ∆V . ∆T≥ 1E
-7 accuracy, if velocity is
stable, and ±10 m/s accuracy, if velocity is unstable. The
data extraction lead to the time resolved FSVs that were
unfortunately sensitive to the selected region of interest ofthe spectrogram and the STFT. In order to avoid averaging
that may be detrimental to the accuracy of data extraction,
additional care has been taken by choosing the region ofinterest using a polygonal selection of the most intense
parts of the spectrogram.
In order to tune the density of power (i.e. the pressure
peak), distance
l2 (fig. 6) has been changed several times,
aiming to obtain a higher energy density by changing the
laser spot size, which has been checked using aphotosensitive paper. It was also possible to modify the
laser energy to get two different densities of power for a
similar laser spot size.
The velocity record was triggered on the laser source
firing event. The laser beam took, therefore, about 3 ns to
reach the target. In order to estimate the time taken by thelaser-matter interaction and the acquisition chain delay
time response (electronic devices + coaxial cables),
strikes haves been performed on pure aluminum sample.From HetV measurements, the longitudinal sound velocity
on aluminum could then be calculated by knowing the
aluminum thickness and the time between waves backand forth in the sample. It is estimated to be 6400 m/s ±
50 m/s. By measuring the first transit time, a correction
delay is fixed in order to get the correct longitudinal velocity
for low amplitude shock waves in order to reduce the
damping effect on the transit time. This process has beenrepeated 9 times, yielding to an average delay time of 110
ns with 3 ns of standard deviation. The speed of sound in
honey at ambient temperature of 20°C is about 2030 m/s[24]. The estimated time taken by the shock to transit the
16 µm Al layer and 10 µm honey is, then, of the order of 8
ns. This time delay is the time to be added to the zero timeof the trigger to obtain the instant of the shock generation
on the sample (the authors consider the laser matter
interaction that creates the shock is less than 1 ns).
Method for interpreting the results
The mechanism of the laser-induced delamination in
multilayered composites is illustrated in figure 7. It
represents the basis for the interpretation of the results
obtained in the present work.
In such laminates, the laser-induced shock wave travels
through the whole composite thickness, along with a
release wave that follows after the loading ends, reaching,one after the other, the back face. After the shock wave is
reflected as release waves at the free surface, these
release waves will cross the release waves of theunloading, coming from the front face. This situation marks
the place of a dynamic tensile strength known for
provoking spall fracture [19]. In case of non-spalling (travelbetween t
1 and t3 in space-time diagram and dashed line
in velocity-time diagram on fig. 7), it means the induced
stresses were lower than the failure stress and, so, theshock wave travels back and forth in the whole target
thickness several times.The laser source used was a Quanta-Ray
® Pro-350-10
that could provide a maximum measured energy of 2 J at
a wavelength of 532 nm and 3.9 J at 1064 nm, with a
Gaussian like pulse of 9 ns of duration at half maximum.The incident energy per shot was measured with a
Gentec® Maestro® Joulemeter containing an attenuator
protection. The laser pulse duration was measured withan Alphalas® fast response photodiode and a 40 GS/s
Agilent oscilloscope. Both pulse duration and energy have
been regularly checked along the experimental sessionand the measured parameter resulted in a less than 2 %
variability.
As the laser-matter interaction also depends on the
irradiated material, it has been chosen to add a sacrificial
layer of a 16 µm thick of aluminum foil on the shocked
face, considering that the pressure dependence on thedensity of power is known for aluminum [15-17]. This
aluminum layer was stuck on the Tensylon® sample with
acacia honey and strongly pressed by hand. The totalthickness of the sample is then measured and, by
deduction, the honey thickness is estimated to be 10 ±5
µm. Acacia honey is a material of interest, well known byacousticians since it results in a good acoustic coupling
between materials, by avoiding the presence of bubbles.
In laser induced shock experiments, one of the most
accessible physical parameters is the free surface velocity,
obtained by the use of contactless methods like laser
Doppler interferometry, which is fast enough for observingsuch rapid phenomenon. In order to measure the time
resolved particle velocity on the free surface of the
UHMWPE samples, Heterodyne Velocimetry (HetV inEnglish), also called Photonic Doppler Velocimetry (PDV
in U.S.A.) equipment manufactured by IDIL® [18], has
been installed in such way that it measured the free surfacevelocity on the opposite face of the laser irradiation,
coaxially with the irradiation spot (fig. 6). This requires
aligning the laser probe of the HetV with the incident laserbeam. The used HetV equipment has an operating
wavelength of 1550 nm (invisible) and a diameter of the
laser beam of 150 µm. Basically, the HetV system will
measure the light phase shift (Doppler shift) between a
reference beam and the collected light reflected from the
free surface of the sample. The collected signal is under

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Fig. 7. (a) Schematic 1D Time/position diagram – the shock
(compression) is represented in solid full line, the release
(tension) in dashed lines; (b) Associated back face velocity versus
time schematic graph; dashed line – no spallation, full line-
spallation
Fig. 8. Schematic representation of velocity signals of the back
(free) surface. The red (upper line) and blue represent,
respectively, a signal slightly above and below damage threshold.
The blue signal has been normalized to the red signal maximum
Fig. 9. Post-mortem appearance of the
samples (a) strike face; (b) and (c) back face
deflections (illustrated for samples T-9 and
T-10, respectively)Delamination or spalling (fig. 7, marked by instants t1,
t2, t2’ on the space-time diagram and by full line in velocity-time diagram) means that the shock surpassed the strength
of the material. Voids are created inside the material,
adding an inner free surface that will affect wavepropagation.
It is understood that what is called
spall or delamination
strength is loading- and sample-dependent [19]. In the
proposed approach, it means the ability to resist the tensile
load consecutive to the laser induced shock wave in the
sample of interest, taking into account its material andthickness aspects. Spallation process is actually initiated
by a tensile state provoked by the occurrence of release
waves that propagate in opposite directions: one coming
from the shock reflection at the free surface, the other
from the unloading at the impacted face. Ideally, accordingto [20], the transition from non-spalling to spalling behavior
of a bonded assembly occurs when the free surface
velocity – time diagram changes (schematically) aspresented on figure 8. It illustrates the velocity signals
obtained in a homogenous medium for pressures slightly
above and below the damage threshold. The peak free-surface velocity,
u0, and the free-surface velocity just before
the arrival of the spall pulse, um, are determined directly
from the free-surface velocity profile. In order to evaluatethe dynamic tensile strength
σspall, the following linear
approximation ([19-20]) is extended to our composite
materials, as considered in previous studies [9]:
(2)
where ∆u = u0 – um is the so-called velocity pullback (the
velocity gap measured between the top of the first velocity
peak and the take-off point). In some cases, as in the case
of CFRPs studied in [9], an extra small acceleration afterthe first velocity peak may appear and this will be ignored,
as it is induced by a wave reflection in the outer epoxy
layer.Results and discussions
A series of Tensylon® samples, numbered from T-1 to
T-10, have been impacted by the laser beam, at variousdensities of power. The samples did not only suffer inside
delamination phenomena, but were also affected on the
outside. As a consequence, after each strike, the authorscould immediately observe one major consequence on
the stricken face: the aluminum foil has been destroyed,
approximately marking the diameter of the size of the laserbeam due to the laser-matter interaction – figure 9(a).
Another feature exhibited by the samples whose spall
strength was initiated by the laser power is the appearanceof a small bulge on the free surface, similar to the deflection
exhibited by a thicker protection panel in the case of
ballistic impact. This represents a sign of internal changes(such as delamination, spallation, elongation or a
combination of them), (fig. 9(b) and 9(c)).
Strikes on samples T-7 and T-8 were too week to allow
for the HetV system recording with a suitable signal to
noise ratio. Therefore, the authors were not able to extractvelocity signals from the corresponding spectrograms. For
all the other strikes, the experimental data is gathered in
table 1, in the appendix.
It should be noted, as a remark, that the extractions of
time resolved velocities have been done with difficulty
because of the semi-transparency of Tensylon® UHMWPEto the 1550 nm wavelength of the HetV laser, illustrated in
figure 10, on the example of sample T-9 spectrogram.
Indeed, on figure 10, at least three rising fronts, delayed
of 33 ns, can be observed. The first rising front indicated in
the figure is very subtle; the second rising front reaches a
velocity signal at half amplitude of the highest observablepeak, corresponding to the third rising front. Considering
the longitudinal velocity in the sample, the authors suspect
a pre-existing crack or porosity at 76±5 µm from the free
surface. This explanation matches with the fact that the
material velocity inside the sample is half the one of the
free surface. Since the highest peak of velocity is obtainedfor the third rising front, the authors kept this last as being
the shock breakout at the free surface.
By further analyzing the spectrogram in figure 10, it could
be obtained the speed of sound for the Tensylon® laminate
material. A sharp rising edge can be observed at around
0.615 ± 0.005 µs. It indicates the breakout of the laserinduced shock wave at the free surface and corresponds

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Fig. 10. Spectrogram of the free surface
velocity, sample T-9 (thickness 1.15 ±0.01 mm,
density of power 1.80 GW/cm²)
Table 1
RESULTS OF LASER
SHOCK TESTING ON
UHMWPE TENSYLON®
SAMPLES*
to an average shock velocity of 2300 ± 50 m/s. Extracted
velocity data gives a peak velocity of 267 m/s at 0.615 µs.
A valley is noted at 0.654 µs, with a velocity dropping downto 210 m/s. The material velocity behind the shock front
within the material is half of the free surface velocity, so,
about 135 m/s. Next the relation (3) between shockvelocity
Us and material velocity Up is used:
(3)
with s = 3.73, the coefficient given by Lässig [23] for
Dyneema®. By assuming the same s value for Tensylon®,
it leads to C0=1797 ± 50 m/s, close to the value of 1898
m/s, given by Lässig for 980 kg/m3 Dyneema® [22]. By
considering formula (4) as an estimation of the strain rate
during delamination:
(4)the strain rate is estimated at about 2.21e6 s-1, a maximum
during the process of delamination.
In figure 11, the authors have superposed the recorded
velocities for several samples (using spectrograms in figures
12 to 18 from, in the appendix), at different power densities,
in order to compare their shapes. Analyzing thespectrograms, one may notice different amplitudes of the
free surface velocities, for a range of power densities (0.64-
1.80 GW/cm
2). As expected, the transit time of the shock
wave is smaller in thinner samples. Highest velocities are
obtained on samples T-9 and T-10 (1.15 mm thick),
respectively, for 1.80 and 0.71 GW/cm². The arrival time isnearly the same in both cases (around 0.6 µs), which
means a transit velocity of about 2300 m/s, available as
well for samples T-1, T-2 and T-3. For sample T-4, thedensity of power is significantly lower than for previous
ones. In the case of samples T-5 and T-6, their
spectrograms are obtained at power densities of 1.18 and0.64 GW/cm², respectively. Both samples T-5 and T-6 are

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Fig. 11. Free surface velocities extracted from spectrograms
recorded with the HetV system, according to experimental
conditions specified on table 1, appendix
Table 2
NOVIKOV APPROACH IN TENSYLON®
SAMPLES*
Fig. 15. Spectrogram of Sample T-4Fig. 14. Spectrogram of Sample T-3Fig. 12. Spectrogram of Sample T-1
Fig. 13. Spectrogram of Sample T-2
2.45 mm thick, thus, the shock propagation is longer and
the break out at the free surface occurs in about 1.7 µs.
Shots on samples T-4 and T-6, however, did not yield to avisible bulging at the free surface. Likewise, their free
surface velocity signals do not exhibit a clear fall after the
shock break out at free surface, as it is the case for other
strikes. Thus, the authors consider that these shots did not
spall the sample (there is no delamination), without moreconvincing diagnostics.
Spectrograms of T-9 and T-10 samples have some
oscillations appearing around the peak, which is an evidentsign of delamination for the reasons explained in figure 8.
In addition, samples T-9 and T-10, as well as samples T-1,
T-2, and T-3, show a bulged free surface and a clear velocityfall after the shock break out on spectrograms. The authors
considered these samples as spall (delaminated).
Spall strength assessment for Tensylon®
Considering the shapes of the FSV signals, the Novikov
[20] approach was eligible for spectrograms of shots onsamples T-1, T-2, T-3, T-5, T-9 and T-10. The Novikov
equation (2) indicates the maximum tensile stress at
rupture. In this formula, ∆u (also referred to as
pullback
velocity) is the first velocity drop or the difference between
the maximum amplitude and that of rebound, as illustrated
in figures 7 and indicated in figure 10. The sound velocityconsidered in the calculations was the one determined
earlier, C
0=1797 m/s. The resulted stress and strain rate at
delamination were gathered on table 2, in the appendix.
There is a certain uncertainty about the spall strength
determination in Tensylon® composites by the Novikov
approach. This strength is ranging from 32 MPa to 58 MPa.These discrepancies can be explained by the fact that the
interfacial debonding depends on the local crack initiation
and crack propagation energies. It is known that compositematerials have some defects, such as porosity and
preexisting cracks that may affect the amount of energy
required for initiating and propagating cracks. Thus, theobtained values appear reasonable, as it is known fromthe literature that UHMWPE matrix is very weak and shear
stresses could be lower than 10 MPa for Dyneema
®
laminates under quasi static loadings, according to
O’Masta [21], and around 50 MPa under high strain rate
loadings in tensile model, according to Lassig et al. [6].
The fact that the delamination stress at high strain rate ishigher than that in quasi static situation is also reported by
Ecault [8] and Gay [9] for composite materials and in [19]
for metallic ductile materials.

http://www.revmaterialeplastice.ro MATERIALE PLASTICE ♦55♦No. 3 ♦2018 370Fig. 18. Spectrogram of shot no 6 /Sample T-10Fig.17. Spectrogram of Sample T-6Fig. 16. Spectrogram of Sample T-5
Conclusions and future prospects
Laser induced shock definitely represents a technique
to be taken into account in order to study the delamination
mechanism of UHMWPE composite materials used in
ballistic protection since it provides high strain rates,
representative for a ballistic impact situation. This techniqueis also appreciated for its local aspect of strength
assessment.
These tests represent the first attempt in evaluating the
delamination phenomena of UHMWPE materials by laser
induced shock wave, i.e. under very high strain rates (>1e6
s
-1).
The spall strength estimated is, of course, dependent on
the strain rate, but the order of magnitude is close to that
obtained on other types of UHMWPE composites, such asDyneema®.
Overall, the results prove that the studied UHMWPE
composites (Tensylon® brand) are materials with a certainamount of internal defects, roughness and wavy effects
that vary locally throughout the thickness, a fact that,
subsequently, significantly affects their performance atboth quasi-static and dynamic tests.
Further investigation using tools such as micro-
tomography is advisory. For future experiments, more carewill be taken concerning the transparency of the material
to the 1550 nm wavelength of the HetV system. A thin
metal coating obtained by vapor deposition could helpremedy this aspect.
Numerical simulation could also help understanding the
shock propagation and the spall process, particularly forthe stress estimation by an inverse approach. An effort will
be done on this direction in the future.
ACKNOWLEDGMENTS: The authors acknowledge the ERASMUS+
program of the European Commission, which facilitated an internship
for Luminia-Cristina Alil at ENSTA Bretagne, during her PhD studies,between september 2015 and march 2016. The authors are also grateful
for the help received from Thomas Bonnemains (IUT Brest), who
prepared the cut-to-size samples used in these tests, via waterjet
cutting.
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