1Numerical and experimental investigation of impact on bilayer aluminum-
rubber composite plate
Amin Khodadadi1, Gholamhossein Liaghat1, 2*, Davoud Shahgholian-Ghahfarokhi1, 
Mahmoud Chizari3, Bin Wang4
1Department of Mechanical Engineering, Tarbiat Modares University, Tehran, Iran
2Department of Mechanical Engineering, Kingston University, London, United Kingdom
3School of Engineering and Technology, University of Hertfordshire, Hatfield, United Kingdom
4Departement Mechanical and Aerospace Engineering, University of Brunel, London, United Kingdom
* Corresponding author, Ghlia530@modares.ac.ir and G.Liaghat@kingston.ac.uk
Abstract
This paper aims to investigate the performance of an aluminum–rubber composite plate under impact 
loading. The impact resistance of the plate has been evaluated using both experimental and numerical 
methods. The experimental tests were carried out using gas gun at velocities of 75, 101, 144 and 168 m/s. 
The energy absorption of composite plates has been closely examined for all samples. The effect of rubber 
layer positioning either on front face or on back face of the aluminum plate was also evaluated. It was 
found that the composite plate with rubber on front face provides higher performance to absorb the 
energy. In parallel to the experiment, a finite element model was created using the finite element software 
LS-DYNA to simulate the response of the aluminum–rubber composite plate under a high energy rate 
loading condition. The data obtained from finite element modeling shown a close agreement with the 
experimental results in terms of failure mechanism and energy absorption. In addition, a parametric 
study was carried out incorporating different impact velocities, rubber formulation, rubber layer 
thickness, interface bonding strength between rubber and aluminum layers and ballistic performance of 
aluminum-rubber sandwich panel. It was concluded that by increasing the rubber layer’s thickness the 
energy absorption of the composite plate will be increased, especially when rubber layer placed in front 
face of the aluminum plate. Although at high interface bonding of rubber and aluminum layer, the 
composite with rubber layer in front face has better performance, but low bonding of interface lead to 
higher energy absorption in back face configuration. 
Keywords: Impact loading; Numerical simulation; LS-DYNA; Energy absorption; Bilayer aluminum-
rubber composite.
21. Introduction
The protection capability of armor plates made of strong aluminum alloys has been a topic of 
interest for many years due to their low density, reasonable formability and high impact strength. 
Many publications deal with the ballistic performance of aluminum plates under impact of 
projectile through experimental and numerical analysis [1-5].
In recent years, researchers have made significant efforts to improve the performance of 
metallic structural protections against impact threats [6-8]. One major development has been the 
use of elastomeric coatings on hard substrates to decrease the damages of blast load and 
penetration of projectiles. Elastomers can be used to dissipate kinetic energy associated with 
impacts and shocks. Due to ability of absorption of considerable amount of energy before failure 
elastomers have been considered as a protective coating for structural and composite system 
under dynamic loading induced by blast, ballistic and other impact events. Several elastomers 
have been shown promising results in these applications. Amini et al. [9, 10] investigated the 
response of monolithic steel plates and steel-polyurea bilayer plates subjected to impulsive and 
direct pressure pulse. The research was carried out experimentally and numerically, focusing on 
the deformation and failure modes of the plates. Their results suggested that the polyurea layer 
can have a significant effect on the response of the steel plate onto dynamic impulsive loads. 
They have considered the failure mitigation and energy absorption of the plate, if the layer 
attached on the back face of the plate. Roland et al. [11, 12] reported the ability of polyurea 
coatings to increase the impact resistance of high hardness steel plates, where they observed the 
effect of different layer configurations on the residual velocity. They showed that when polyurea 
applied to the strike face of steel plates provides a significant enhancement in the ballistic 
resistance of these plates. They have concluded that the most possible reason for this 
3improvement against impact resistance of the polyurea-coated steel is a phase transition of the 
polyurea from the rubbery phase to the glassy phase. This hypothesis was supported by Grujicic 
[13] using a computational model to evaluate the energy absorption when a deformation-induced 
glass transition occurs.
Natural rubber (NR) is an appropriate material which can be used as a layer on a rigid 
substrate. Rubber materials have been widely used in shock absorbers, impact resistance panels 
and other engineering applications [14, 15]. High level of damping property [16], high level of 
flexibility [17], and excellent puncture and tear resistance [18] are the specific properties of NR. 
These features make NR a good candidate to be used as reinforcement in a composite structure 
[19-22]. To convert a raw NR into a material with desired properties, some ingredients such as 
fillers, activators, sulfur or other equivalent curatives and accelerators should be added to the 
raw NR. Variation of compound ingredients alters the mechanical properties of rubber [23]. 
These additives modify the rubber by forming cross-links between polymer chains. One of the 
most important ingredients is filler including carbon black and calcium carbonate [24-26]. These 
fillers are added to rubber formulation to improve the mechanical properties of NR.
As highlighted above, although there are some researchers who investigated the impact 
response of metallic plates coated by polyuria elastomer but impact response of bilayer 
aluminum-rubber composite is not investigated yet. Therefore, there is a knowledge gap in this 
area to understand the effect of the rubber panel on the energy absorption of an aluminum plate. 
In order to explore the ballistic performance of these composites, experimental studies were 
conducted using hemispherical projectiles with impact velocities of 75, 101, 144 m/s and 168 
m/s. During the experiments, the focus was on the significance of positioning the rubber layer 
onto the front face or onto the back face of the aluminum plate. Intention was to look for 
4probable configuration of bilayer aluminum-rubber composite which provides more energy 
absorption. In parallel, the failure mode of layer was closely monitored and the observation was 
mimicked to create a computer simulation. The numerical model was then used to carry a series 
of parametric studies to investigate the parameters which affect the impact resistance of bilayer 
aluminum-rubber composite.
2. Experimental procedure
2.1. Materials and specimen preparation
Aluminum alloy has been used as a candidate material in many engineering applications due to 
its low density and high ductility and its reasonable strength. In this study, Aluminum 2024-T3 
was used to carry the experimental tests. The stress-strain curve of Aluminum 2024-T3, which 
was obtained from tensile test, was shown in Fig 1 and its mechanical properties were reported 
in Table 1.
Fig. 1 Tensile stress–strain curve of aluminum 2024-T3
5Table 1 Mechanical properties of aluminum 2024-T3
Property Value
Density, ρ (kg/m3) 2700
Elastic modulus, E (MPa) 72200
Yield stress,  (MPa)
y 350
Poisson ratio, ν 0.32
εf 0.18
Natural rubber (SMR 20) with Mooney viscosity of 65 was used to carry the experiments in 
this study. The SMR 20 material was supplied by the Rubber Research Institute of Malaysia. 
Compound ingredients named fillers such as carbon black and calcium carbonate were added to 
the rubber formulation to improve its mechanical properties. In rubber compound, ZnO, stearic 
acid, accelerators and sulfur constitute the vulcanization system which is used for crosslinking 
of the matrix phase. To evaluate the behavior of rubber with different components at high strain 
rates, two types of rubber with different formulation were used. The NR compounds formulation 
for two types of compounds with high hardness (HH) and low hardness (LH) is presented in 
Table 2. Compounding were performed on an open two-roll mixing mill (Polymix 200 L, 
Germany) and were cured under hydraulic pressure according to the rheometer results which is 
presented in Fig.2 for both LH and HH rubber.
Table 2 Formulation of the rubber compounds
Loading (Phr)
Ingredients
Formulation 1 Formulation 2
NR 100 100
Carbon Black (N330) 60 40
Zink oxide 5 5
Calcium carbonate 30 30
Spindle oil 15 30
Sulfur 2 1.5
Volcacit 0.7 0.7
6Fig. 2 Rheometer curves
To prepare the specimens, a layer of aluminum with thickness of 0.5 mm and rubber layer 
with 2 mm were bonded together by BYLAMET S2 adhesive. Before bonding, the aluminum 
plate was carefully cleaned using acetone. To obtain a strong bonding between aluminum and 
rubber plates, a pressure was applied on the specimen. Fig. 3 shows the bilayer aluminum-rubber 
specimen and its dimensions.
Fig. 3 (a) Aluminum-rubber specimens (b) Dimension of specimens
2.2. Impact tests
Impact tests were performed using a gas gun as shown in Fig. 4. The gas gun was made of a 
pressure vessel with 120 bar capacity, a high speed firing valve and a hollow steel barrel with 6 
7m long. The inside diameter of barrel was 10 mm. The exact impact velocity of each projectile 
was measured with a chronograph (model M-1, Chrony Canada) before and after impacting the 
target. Fixture for holding the specimens was located in the target chamber. The projectile used 
for ballistic tests was made of steel and were hardened by heat treatment to minimize projectiles’ 
deformation. The physical properties of projectile were presented in table 3.
Fig. 4 (a) Gas gun (b) Target chamber (c) Fixture (d) Projectiles
Table 3 Physical properties of projectile
Length (mm) Diameter (mm) Weight (g) Hardness (RC)
16.75 10 9.32 55-56
2.3. SHPB experiment
High strain rate tests on the rubber sample were conducted using Split Hopkinson Pressure Bar 
(SHPB) to obtain the samples stress–strain properties at different strain rates. The conventional 
steel SHPB helps to test metallic materials, but it cannot precisely determine the dynamic responses 
8of soft materials like rubber [27]. The tests were performed using nylon bars instead of metal bar 
owing to this limitation. The mechanical impedance of nylon bars is much closer to that of the 
rubber specimens. Thus, the transmitted wave is sufficiently large for measurement. The Split 
Hopkinson Pressure Bar (SHPB) system, striker bar and rubber specimen are presented in Fig. 5. 
The ratio of optimal length-to-diameter (L/D) in the specimens for the SHPB test is 0.5, which was 
used to minimize inertia and friction effects [28]. To ensure homogeneous deformation and stress 
equilibrium during the experiment, the length of soft material specimens must be sufficiently short. 
In this study, the length of the specimen was designed to be 5 mm.
Fig. 5 Split Hopkinson Pressure Bar test (a) SHPB machine (b) Striker bar (c) rubber Specimen.
3. Numerical analysis
3.1. Geometry modelling
The commercial finite element software, LS-DYNA V9.71, was used to simulate the response 
of aluminum-rubber composite under impact loading. LS-DYNA is a non-linear dynamic 
modeling software that benefits explicit formulation. The numerical model consists of a 
9projectile with initial velocity and a bilayer aluminum-rubber composite plate. Fig. 6 shows the 
finite element model of projectile and target for rubber layer located on the impact face (front 
face; FF) and second the rubber layer located on the face opposite to the impact side (back face; 
BF). aluminum-rubber composite was modeled with the dimensions of 100×100 mm. Clamped 
boundary condition was assigned to the model to restrict all degrees of freedom at edges as 
shown in Fig. 6(c). To include out-plane stress components, the composite model was meshed 
with 8-node reduced integration solid element. Mesh sensitivity was checked by varying the 
number of the elements along the bilayer aluminum-rubber composite thickness to obtain the 
residual velocity of projectile with adequate accuracy. The mesh of bilayer plate included a total 
number of 400000 elements. 160000 elements in aluminum layer with the mesh size of 
0.5×0.5×0.125 mm and 240000 elements with the mesh size of 0.5×0.5×0.33 mm for rubber 
layer.  Also the projectile’s mesh had 12900 elements.
Fig. 6 Finite element model (a) rubber layer located on the impact face (b) rubber layer located on the 
opposite of the impact face (c) Boundary conditions
10
3.2. Material modeling
3.2.1. Aluminum plate
Material model 3 in LS-DYNA software (MAT_PLASTIC_KINEMATIC) was chosen to 
describe elastic–plastic behavior of the aluminum plate. This material model uses an isotropic 
constitutive based on isotropic and kinematic hardening [29]. Also, strain rate effects are 
estimated by Cowper–Symonds constitutive relationship.
1/
[1 ]( )( / )
P
Py dyn y st Pc E      (1)
In Eq. (1), P and C are empirical constants, and for aluminum alloys are 4 and 6500 1/s, 
respectively [30, 31]. Also, Ep is calculated as shown in Eq. (2).
Ep = E Et / (E - Et) (2)
Where Et is tangent modulus of bilinear stress–strain curve.
The plastic-kinematic material model which was used with Cowper–Symonds constitutive 
relationship, is an appropriate to model the failure and fracture of aluminum plate. Fracture 
strain was considered as criteria to model failure. As the projectile hits the aluminum plate its 
nose pushes the front surface of it which caused generation of compressive stress at the target 
surface. Further movement of the projectile created the tensile stretching of the material due to 
which thinning of the material was observed particularly close to contact region between the 
target and the projectile. Finally, by exceeding the element strain from fracture strain, the 
element is deleted from simulation.
3.2.2. Rubber layer
LS-DYNA offers several material models of rubber-like materials. In this research Mooney-
Rivlin model has been chosen which the strain energy function is given by:
11
1 210 01
( - 3) ( - 3)W C CI I (3)
Where I1 and I2 are the principal invariants of the left Cauchy–Green deformation tensor, 
defined by: 
22 2
1 31 2trCI     
2 22 2 2 22 2
2
1
[ ] 3 31 2 1 22
( ) trtrC CI         
3 1 2 3
det CI    
(4)
Where λ1, λ2 and λ3 are the principal stretches. The Mooney–Rivlin model does not take the 
strain rate effect into consideration. However, with certain adjustment, the Mooney–Rivlin 
model can be used in the simulations. Fig. 7 shows the stress–strain curves at different strain 
rates obtained by SHPB tests for two LH and HH natural rubber. The strain rate that the material 
undergoes during the penetration process was estimated by impact simulation on pure rubber 
panel. The study used the strain rate about 4000 s-1 for the rubber and fitted it with Mooney-
Rivlin material model, using least squares approach. The calibrated coefficients of C10 and C01 
are 5.6 and 0.5, respectively for HH rubber and 2.9 and 0.4 for LH rubber. The maximum 
principal strain is used as the failure criterion of the rubber. The pure rubber panel was modeled 
by LS-DYNA and impact response of panel was simulated and verified by the authors’ pervious 
experimental work [32]. Although, results of quasi-static test show the HH and LH rubber strain 
to break are about 2.2 and 3.5 under quasi-static test but from a series of simulations, it was 
estimated that the fracture strain for the rubber material is 1.20 and 1.70 under high strain rate. 
Split Hopkinson pressure bar results show that in high strain rates, rubber material behaves 
stiffer. It can be seen that by increasing the strain rate, the strain-stress curve transfers to higher 
values, which means higher stiffness. On the other hand, it was shown by Roland [11], when an 
12
elastomer was loaded in high strain rates, its behavior transfers from elasticity to brittle behavior. 
This means its elongation capacity decreases and rubber fails at lower strain values.
Fig. 7 Stress-Strain curves of rubber at different strain rates (a) high hardness rubber (b) low hardness 
rubber
3.3. Contact modeling
To allow the removing of elements due to the impact of projectile, 
CONTACT_ERODING_SURFACE_TO_SURFACE was added between the projectile and 
target layers. Also results of impact tests on bilayer aluminum-rubber composite indicated that 
debonding is an important failure mode. This fact led to the necessity to implement debonding 
in the modelling of composite. Debonding in LS-DYNA was modeled through the 
TIEBREAK_SURFACE_TO_SURFACE contact algorithm. Tiebreak is active for nodes which 
are initially in contact. This contact algorithm incorporates failure criteria that, when achieved, 
release the tied interface between the contacting faces and the constraint is transformed to 
surface-to-surface contact that allows sliding between the faces while preventing the penetration 
of nodes between the parts in contacts. Debonding occurs when the following equation is 
satisfied:
13
2 2
1( ) ( )
NFLS SFLS
   (5)
Where  and  are the normal and shear stresses at the interface, and SFLS and NFLS are  
the interface normal and shearing strengths. Because there was no any bonding strength 
experimental data at the time, a trial and error method was employed to reproduce the 
experimentally observed debonding by adjusting the value of the interface bonding strength. 
In a final simulation, the SFLS and NFLS were taken to be 80 MPa and 50 MPa, respectively. 
Debonding of composite obtained experimentally and numerically for two BF and FF 
configurations are shown in Figs. 8 and 9. The comparison of the experimental results and the 
numerical predictions, shows that the estimated data for bonding strength values are accurate 
enough to predict the debonding of composite layers under impact loading.
Fig. 8 Comparison of (a) experimental and (b) numerical results with the rubber on the back face at the 
impact velocity of 144 m/s
Fig. 9 Comparison of (a) experimental and (b) numerical results with the rubber on the front face at the 
impact velocity of 144 m/s
14
4. Results and discussion
The main objective of this study is to investigate the behavior and impact resistance of 
aluminum-rubber composite plate. The effect of rubber layer on energy absorption and failure 
mechanism of the composite plate has been studied. To carry on the study a series of 
experimental tests and numerical analysis were performed on single layer aluminum plate and 
bilayer aluminum-rubber samples. 
4.1. Impact on aluminum plate
To evaluate the impact resistance of aluminum plate, impact tests were conducted using gas gun 
at five different gas reservoir pressure of 6, 9, 12, 15 and 18 bar. These pressure launched the 
projectile with the velocities of 75, 96, 109, 122 and 129 m/s respectively. These values were 
chosen to be greater than ballistic limit. The hemispherical projectile impacted the 0.5 mm 
aluminum plate and residual velocity, global deformation and the fracture mechanisms were 
evaluated. The scare of the impact on the plate at penetration zone, had 4 petals. By increasing 
the impact velocity, the larger petals were shown at penetration zone. 
A numerical simulation was performed to investigate the aluminum plate behavior under 
impact loading. This was done in parallel with the experiment to validate the energy absorption 
of the target plate at velocities which experiments were performed. The failure mode also was 
compared with the experiments. Fig. 10 shows the perforated aluminum plate at incidence 
velocity of 109 m/s. Four petals formed at this velocity have been compared experimentally and 
numerically in Fig. 10a-b. The compared results show, the predicted numerical model is in close 
agreement with the experiment. Plate was deformed under impact of projectile and when the 
stress is beyond the tensile strength of material, cracks appear in vicinity of the impact zone. 
15
Cracks will be propagated until the projectile perforates the plate. The perforation process is 
shown in Fig. 11.
Fig. 10 Comparison of (a) experimental and (b) numerical results at initial velocity of 109 m/s.
The projectile’s residual velocity versus impact initial velocity of the aluminum plate is shown 
in Fig. 12. The residual velocities from the simulations show good agreement with the 
experimental test results. To determine the ballistic limit velocity, simulation was carried out 
considering different velocities. The aim was to find a velocity in which the residual velocity 
becomes zero when the projectile was completely perforated into the specimen. The ballistic 
limit velocity of monolithic aluminum plate was found to be 50.5 m/s.
Fig. 11 Perforation of projectile at initial velocity of 129 m/s at (a) 30 (b) 60 (c) 90 and (d) 120 μs time 
interval.
16
Fig. 12 Experimental and numerical comparison of residual velocity versus impact velocity after 
perforating the aluminum plate 
4.2. Impact on aluminum-rubber plate
The aluminum-rubber bilayer samples were setup using two different configurations: first the 
rubber located on the impact face (front face; FF) and second the rubber layer located on the face 
opposite to the impact side (back face; BF). Tests were conducted at four different velocity 
groups of 75 m/s, 101 m/s, 144 m/s and 168 m/s. Each test repeated five times and average 
velocity was calculated for each test group.
The experimental and numerical behavior of the aluminum-rubber composite when rubber 
layer located on the back face (BF) are presented in Figs. 13-15 under the impact velocities of 
75 m/s, 144 m/s and 168 m/s, respectively. A good agreement is shown between numerical and 
experimental results. The damage occurred in case of 75 m/s which is lower than ballistic limit 
velocity is shown in Fig. 13. In this case, Aluminum layer was damaged and projectile 
penetrated the aluminum layer, but rubber panel resisted against projectile impact. Also 
projectile rotation was observed when impact velocity is lower than ballistic limit velocity. In 
17
impact velocities higher than ballistic limit (Figs. 14 and 15), projectile perforated the bilayer 
composite and passed the target without rotation.
(a) (b)
Fig. 13 Failure of composite with the rubber on the back face at the impact velocity of 75 m/s obtained 
Experimentally and numerically (a) Back view (b) Front view
(a) (b)
Fig. 14 Failure of composite with the rubber on the back face at the impact velocity of 144 m/s obtained 
Experimentally and numerically (a) Back view (b) Front view
(a) (b)
Fig. 15 Failure of composite with the rubber on the back face at the impact velocity of 168 m/s obtained 
Experimentally and numerically (a) Back view (b) Front view
18
Response of the composite plate when the rubber layer is located on the front face (FF) under 
the similar impact velocities are shown in Figs. 16-18. It is shown that in this configuration, 
model predictions are in good agreement with experimental results. The response of bilayer 
composite when projectile velocity is lower than ballistic limit velocity is shown in Fig. 16. In 
this case, aluminum layer was deformed but no damage and fracture was observed in composite. 
Similar to BF configuration, projectile rotation was observed when impact velocity is lower than 
ballistic limit velocity. Also in impact velocities higher than ballistic limit (Figs. 17 and 18), 
projectile perforated the bilayer composite and passed the target without rotation.
(a) (b)
Fig. 16 Failure of composite with the rubber on the front face at the impact velocity of 75 m/s obtained 
Experimentally and numerically (a) Back view (b) Front view
(a) (b)
Fig. 17 Failure of composite with the rubber on the front face at the impact velocity of 144 m/s obtained 
Experimentally and numerically (a) Back view (b) Front view
19
(a) (b)
Fig. 18 Failure of composite with the rubber on the front face at the impact velocity of 168 m/s obtained 
Experimentally and numerically (a) Back view (b) Front view
It was assumed that the loss of projectile’s kinetic energy is equal to the energy absorption 
performed by the composite target in at the perforation event. Therefore the energy absorption 
of the composite target can be theoretically calculated by subtracting the residual energy of the 
projectile from its initial energy as presented below.
2 21
( )
2P p i rmE V V 
(6)
Where Ep (J) is dissipated energy during the impact process, mp (kg) is mass of the projectile, 
Vi (m/s) is projectile initial velocity, and Vr (m/s) is residual velocity. Table 4 presents the 
experimental and numerical results performed for BF and FF configurations. In this table, the 
experimental test results, which were performed at velocities of 75, 101, 144 and 168 m/s, are 
presented. A high hardness rubber layer was used for experimental tests. The residual velocity 
of the projectile was measured and the energy absorption is determined using Equation (6). 
Energy absorption is used as criteria to evaluate the ballistic performance of the composite plate. 
Table 4 indicates that the numerical model can be used to estimate the projectile residual velocity 
and energy absorbed by bilayer composite.
20
Table 4 Comparison of energy absorption obtained by experimental and numerical simulation 
Configuration
Impact 
velocity
(m/s)
Experimental 
residual 
velocity (m/s)
Numerical 
residual 
velocity (m/s)
Experimental 
energy 
absorption (J)
Numerical 
energy 
absorption (J)
Error
(%)
75 0 0 26.2 26.2 0
101 58.4 64.5 31.6 28.1 11.1
144 114.6 117.2 35.4 32.6 7.9
Rubber in 
back face
168 138.7 141.5 41.9 38.2 8.8
75 0 0 26.2 26.2 0
101 44.8 53.6 38.2 34.1 10.7
144 104.5 107 45.7 43.3 5.2
Rubber in 
front face
168 133.9 137.6 48 43.3 9.8
5. Parametric study
A parametric study on bilayer aluminum-rubber composite plate under impact loading was 
performed. The main parameters that were investigated are: the relative position of the rubber 
layer with respect to the loading direction, the effect of different impact velocities, the effect of 
rubber hardness, the effect of rubber layer thickness, the strength of rubber-aluminum bonding 
and ballistic performance of aluminum-rubber sandwich panel. The residual velocity of 
projectile was measured in simulations and the energy absorption of bilayer composite was 
calculated and considered as a criterion to compare their impact resistance. The projectile used 
in the finite element models reported in this section has a mass of 9.32 g for all the modeled 
samples. Also, all the aluminum plate models have thickness of 0.5 mm.
5.1. Effect of relative position
As it is mentioned in the previous section, to study the effect of the relative position of rubber, 
two configurations of bilayer composite plate were considered. First configuration, the rubber 
21
layer was located on the impact receiving side, front face (FF), and second, the rubber panel was 
located on the back face of impact (BF). In this section numerical simulation was performed in 
different impact velocities to see the effect of rubber layer position. Also, the ballistic limit of 
the composite target was determined. The high hardness (HH) rubber layer was used and the 
interface shear and normal bonding strength was assumed to be 80 and 50 MPa, respectively. 
Rubber layer with thickness of 2 mm and aluminum layer with thickness of 0.5 mm were used 
to investigate the effect of relative position. Figure 19 shows the residual velocity of projectile 
after perforating the bilayer composite versus impact velocity for two BF and FF configurations. 
Moderate enhancement in ballistic performance in terms of lower residual velocity for bilayer 
composite plate, which rubber layer located in front face, was observed and compared to the 
corresponding back face configuration. It was find out that the composite plate by front face 
rubber layer configuration shows a better penetration resistance compared to the back face 
composite plate.
0 50 100 150 200 250
0
20
40
60
80
100
120
140
160
180
200
HH rubber-BF
HH rubber-FF
R
es
id
ua
l v
el
oc
ity
 (m
/s
)
Initial velocity (m/s)
Fig. 19 Residual velocities versus initial velocities of Al-HH rubber composite for BF and FF 
configuration
22
The ballistic limits obtained by numerical analysis for the composite plate with BF and FF 
configurations are 84.5 and 95 m/s, respectively. While, in Section 4.1, it was observed that the 
ballistic limit for the single-layer aluminum plate is 50.5 m/s. Comparing the ballistic limit of 
the bilayer aluminum-rubber composite with the ballistic limit of the aluminum plate shows the 
rubber layer with high damping properties has a significant effect on the energy absorption of 
the composite target. Increase in the ballistic limit of composite with the BF and FF 
configurations is 67.3% and 88.1%, respectively.
5.2. Effect of rubber hardness
It is known that in rubber material the formulation of its component has influence on its 
mechanical properties and its impact resistance properties. In this section the numerical 
simulation was performed on the bilayer aluminum-low hardness rubber (LH) composite plate 
and compared to the composite plate made by HH rubber. Rubber layer with thickness of 2 mm 
for both LH and HH rubber and aluminum layer with thickness of 0.5 mm were used to 
investigate the effect of rubber hardness. Figs. 20 and 21 show the residual velocity of projectile 
versus initial velocity after perforation of two types composite with HH and LH rubber layer for 
BF and FF configuration. The figures show higher ballistic performance in term of lower 
residual velocity for the composite plate made by HH rubber compared to corresponding LH 
rubber composite plate. This advantage is applicable for both BF and FF configurations. The 
higher energy absorption capacity of HH rubber compare to LH rubber can be referred to its 
stronger molecular chains.
23
0 50 100 150 200 250
0
20
40
60
80
100
120
140
160
180
200
LH rubber-BF
HH rubber-BF
R
es
id
ua
l v
el
oc
ity
 (m
/s
)
Initial velocity (m/s)
Fig. 20 Residual velocities versus initial velocities of Al-rubber composite with LH and HH rubber 
layer for BF configuration
0 50 100 150 200 250
0
20
40
60
80
100
120
140
160
180
200
LH rubber-FF
HH rubber-FF
R
es
id
ua
l v
el
oc
ity
 (m
/s
)
Initial velocity (m/s)
Fig. 21 Residual velocities versus initial velocities of Al-rubber composite with LH and HH rubber 
layer for FF configuration
Fig. 22 illustrates the projectile velocity histories at impact velocity of 150 m/s on the bilayer 
composites plate made by LH and HH rubber layers for BF and FF configurations. As it is shown 
in the figure, the specimens’ perforation and the residual velocities are different. From 
observations of Figure 22, following points are notable:
24
(i) The velocity deceleration rate of the projectile impacting the composite target with HH 
rubber is higher than the LH case, which means the deceleration rate of the composite sample is 
directly related to the hardness of the rubber.
(ii) The residual velocity of the projectile impacting the aluminum-HH rubber layer is lower 
than the aluminum-LH rubber composite sample.
(iii) The first level of composite behavior with BF configuration is affected by the aluminum 
performance causing the intense deceleration of the projectile velocity. After failure of the 
aluminum plate, the gradient is gentle due to low module and large elongation to failure of the 
rubber.
(iv) Time duration of penetration is longer for the composites targets with FF configuration 
compared with the composites targets with BF configuration.
(v) The residual velocity of the projectile impacting on the composite plate with FF 
configuration is lower compared to BF configuration.
0.00 0.05 0.10 0.15 0.20 0.25
100
110
120
130
140
150
160
LH rubber-BF
HH rubber-BF
LH rubber-FF
HH rubber-FF
Time (ms)
V
el
oc
ity
 (m
/s
)
Fig. 22 Comparison of projectile velocity histories at impact velocity of 150 m/s on LH and HH Al-
rubber composite for BF and FF configuration
25
5.3. Effect of rubber layer thickness
A set of simulations was performed to study the effect of the rubber layer thickness on the 
performance of the bilayer Al-rubber composite target. In these simulations the thickness of 
aluminum layer was 0.5 mm and the thicknesses of the rubber layer were 0.5 mm, 1 mm, 1.5 
mm, 2 mm, 2.5 mm, and 3 mm, as shown in Table 5. The simulations were performed for both 
BF and FF configurations. The HH rubber layer was used and the interface shear stress and 
normal stress at bonding interface was assumed to be 80 MPa and 50 MPa, respectively. 
Fig. 23 compares the energy absorption of each composite plate with different rubber layer 
thickness. The comparison reveals that the increase in the thickness of the rubber layer improves 
the overall performance of the bilayer plates for both BF and FF configuration. In the case of 
FF configuration the rubber thickness is more effective, in which increasing the rubber layer 
thickness, significantly increase the energy absorption of the composite target.
Table 5 Numerical modeling of Al-rubber composite with different rubber thickness
26
0 0.5 1 1.5 2 2.5 3 3.5
0
10
20
30
40
50
60
HH rubber-FF
HH rubber-BF
En
er
gy
 a
bs
or
pt
io
n 
(J
)
Rubber layer thickness (mm)
Fig. 23 Energy absorption of Al-HH rubber composite with different rubber layer thickness for BF and 
FF configuration
5.4. Effect of bonding
A simulation was performed to evaluate the effect of the aluminum-rubber interface bonding 
strength on the performance of the bilayer composite plate. The thicknesses of aluminum and 
rubber layer were 0.5 mm and 2 mm, respectively and HH rubber was used in these simulations. 
The initial velocity of projectile was set to be 144 m/s for each simulation. For all simulation 
residual velocity was measured and the energy absorption was calculated. The values of the 
bonding strength used for these simulations are from the low to high bonding values. Six values 
of interface bonding strength were considered in this study and simulation was performed to 
evaluate the ballistic performance of each configuration and bonding interface. The bonding 
values between rubber and aluminum layer were considered to be 0-0 (which means two layers 
does not have any bonding and are separated with each other), 20S-12N, 40S-25N, 60S-38N, 
80S-50N, and Tie (perfect bonding). Fig. 24 shows the energy absorption capacity of the bilayer 
27
composite plate with BF and FF configurations with different interface bonding strength. It can 
be seen that increase bonding has negative effect on ballistic performance of composite for both 
BF and FF configurations. This parameter specially affects the BF configuration. It is shown 
that in BF configuration when there is no bonding, rubber plate can stretch without any 
limitation and have the best performance. By increasing the interface bonding between the 
rubber and aluminum plate, the energy absorption of bilayer composite decreases. It can be seen 
that there is a critical interface bonding point which BF and FF configurations has same 
performance. BF configuration has the better performance for interface bonding values less than 
critical point. On the other hand, by increasing the bonding beyond the critical point, the FF 
configuration has better performance. 
Figs. 25 and 26 show the deformation of bilayer composite for BF and FF configuration, 
respectively. It can be seen that bonding restricts the rubber deformation and doesn’t let the 
rubber layer to present its stretch and damping properties. 
1 2 3 4 5 6
0
10
20
30
40
50
60
70
HH rubber-BF
HH rubber-FF
Aluminum-rubber interface bonding (MPa)
En
er
gy
 a
bs
or
pt
io
n 
(J
)
Fig. 24 Energy absorption of Al-HH rubber composite with different interface bonding for BF and FF 
configuration
28
Fig. 25 Impact behavior of Al-rubber composite at initial velocity of 144 m/s for BF configuration with 
interface boning of (a) 0-0 (b) 20S-12N (c) 80S-50N (d) Tie
Fig. 26 Impact behavior of Al-rubber composite at initial velocity of 144 m/s for FF configuration with 
interface boning of (a) 0-0 (b) 20S-12N (c) 80S-50N (d) Tie
5.5. Aluminum-rubber sandwich panel
A simulation was performed to evaluate the response of an aluminum-rubber sandwich panel 
under impact loading. For this purpose, a 0.5 mm aluminum plate was sandwiched between two 
layers of HH rubber panel with thickness of 1 mm as shown in Fig. 27. The shear and normal 
stress at bonding interfaces were assumed to be 80 MPa and 50 MPa, respectively. Simulations 
were conducted at three initial velocity of projectile which were 101, 144 and 168 m/s. The 
residual velocity of projectile was obtained in each simulation and compared to the results of 
bilayer aluminum-rubber composite which presented in Table 6.
29
Fig. 27 Aluminum-rubber sandwich panel
It can be seen bilayer composite has better performance under impact loading compared to 
aluminum-rubber sandwich panel. The table shows better performance in terms of lower 
residual velocity for both BF and FF configuration of bilayer aluminum-rubber composite 
compared to aluminum-rubber sandwich panel.
Table 6 Comparison of performance of bilayer and sandwich composite 
Initial velocity
(m/s) 101 144 168
Sandwich panel 75.6 125.6 152.1
Bilayer composite with BF configuration 64.5 117.2 141.5
Residual velocity
(m/s)
Bilayer composite with FF configuration 53.6 107 137.6
Perforated specimens under impact velocity of 144 m/s is shown in Fig. 28. Fig. 28 (a) and 
(b) show the bilayer composite in FF and BF configuration, respectively and Fig 28 (c) shows 
the sandwich panel response. Fig. 29 shows the aluminum plate used in specimens after 
perforation. It can be seen that by decreasing the rubber thickness on back face of aluminum 
plate, the larger petals are shown at penetration zone.
30
(a) (b) (c)
Fig. 28 Penetration of projectile in (a) Al-rubber composite with FF configuration (b) Al-rubber 
composite with BF configuration (c) Sandwich panel
(a)
(b)
(c)
Fig. 29 Fracture of aluminum layer in (a) Al-rubber composite with FF configuration (b) Al-rubber 
composite with BF configuration (c) Sandwich panel
6. Conclusion
In this paper, mechanical behavior of an aluminum-rubber bilayer composite target plate under 
impact loading was investigated. A series of experimental tests were conducted using a gas gun 
at projectile velocities of 75 m/s, 101 m/s, 144 m/s and 168 m/s. From the experiments, it was 
focused on the ballistic performance of composite plate considering the relative position of the 
rubber layer with respect to the loading direction. It was found that when rubber layer located on 
the front face of bilayer aluminum-rubber composite, better ballistic performance can be 
31
achieved. A numerical simulation in prallel with experimental tests was developed. The 
simulation was suporrted using a parametric study on bilayer aluminum-rubber composite. 
Rubber mechanical properties, as a strain rate dependent material, were obtaind by SHPB tests 
and assigned to the model. A close agreement was found between numerical and experimental 
results. 
Following conclusions can be highlighted from the parametric study:
1- The ballistic limits of bilayer composite with BF and FF configuration were 84.5 and 95 
m/s, respectively which show 67.3% and 88.1% increase compared to the ballistic limit of the 
monolithic aluminum plate.
2- The ballistic performance of aluminum-rubber composite is highly dependent onto the 
hardness of rubber. The composite sample with higher hardness rubber can resist more 
efficiently against the projectile impact. 
3- Increase the thickness of the rubber layer improves the overall performance of the bilayer 
plates for both BF and FF configurations. On the other hand, the FF configuration is more 
sensitive to the rubber thickness.
4- By increasing the interface bonding between the rubber and aluminum plate, the energy 
absorption of bilayer composite target decreases. There is a critical threshold for the interface 
bonding. For the case of BF configuration, the energy absorption would be higher if the bonding 
values are less than critical point. For the case of FF configuration, by increasing the bonding 
strength beyond the critical point, the ballistic performance would be higher.
32
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