Experimental and numerical evaluation of plasticity model with ductile damage to applied in project of shielding sheet metal.

Bruno Cesar Pockszevnicki Int. Journal of Engineering Research and Applications www.ijera.com
ISSN : 2248-9622, Vol. 5, Issue 5, ( Part -6) May 2015, pp.173-176
www.ijera.com 173 | P a g e
Experimental and numerical evaluation of plasticity model with
ductile damage to applied in project of shielding sheet metal.
Bruno Cesar Pockszevnicki*, Pedro Américo Almeida Magalhães Júnior**
*(Mechanical Engineering Department, Pontifical Catholic University of MinasGerais,BeloHorizonte, Brazil)
**(Mechanical Engineering Department, Pontifical Catholic University of MinasGerais, Belo Horizonte, Brazil)
ABSTRACT
This work aims to develop a methodology for numerical evaluation via finite elements applied to projects shields
sheet metal. To validate the methodology physical test were conduct and were compare with the numerical
simulation. In the simulation, a plasticity material modelit was used at high strain rates, besides the insertion of a
damage model through forming limit diagram (FLD) to capture the initiation of damage and energy criteria for
propagation of the fracture. The tested shielding design is for the II-A protection level using the type 9mm
ammunition.
Keywords- Ballistic Impact, Finite Element, Plasticity, Damage, Blindage.
I. INTRODUCTION
In blindage projects for security and defense
area, different materials are employ to form the
solution of the shield. The understanding what
happens with each material inserted in solution is
necessary and desired. One part of the development
process more usedto evaluate project proposals are
experimental verifications.
Instead of only physical test, the generation of
numerical models using the finite element method
can be a great strategy to reduce time and cost,
optimizing resources and ensuring structural
performance, and enable increased understanding of
the ballistic impact phenomenon.
In the case of shields project the main
phenomenon to be study is structural impact with
perforation. To study perforation, a damage variable
to capture the penetration should be considered in the
model as the other numerous parameters and
mechanical properties that arerequired according [1]
when they usefinite element technique applied to
modeling shield composite.
Therefore, phenomenon and characterization of
materials are the variables to be study in
understanding the art of shielding. To start in the area
using the finite element method, one methodology is
present in this paper aimed at a portion of the
armored project in which only metal components are
used.
The first aim of studying, is start with metallic
materials where is possible to setup models capable
of describing the elastic-plastic behavior and damage,
providing evolution from the initiationof damage,
propagation until final damage, a perforation.
Thus, evaluation parameters are establish to have
a comparison between physical tests and numerical
simulation, the idea is to enable the use of modeling
for cases of shielding levelII-A [2], [3].
First is to present the experimental evaluation
performed using a projectile-type 9mm FMJ (Full
Metal Jacket) with ammo tip ogival.
Following is present equating of the plasticity
model with damage using the finite element
modeling. The results obtained are present in each
operation and a comparison between physical test and
numerical simulation is evaluate.
II. BALLISTIC IMPACT – PHYSICAL TEST
For the assessed physical test, it was consider a
dimension plate of300x300mm.It was tested two steel
plates, one called AISI / SAE 1010 with a thickness
of 2.9mm, and a steel dedicated to the shielding
called Ramor 500 with a thickness of 2.5mm.The test
was perform at a distance of 7m using a 9mm
projectile with ogival tip, mass 10.5g and the
theoretical speed of 384m/s.
From the impact, some measurements were
perform, the final thickness of the plate in impacted
area, also the final displacement caused by the
impact. In these tests, none perforation occur as
shown in Figures 1 and 2.
RESEARCH ARTICLE OPEN ACCESS

Fig. 1 – Steel plate of AISI/SAE 1010
Fig. 2 – Steel plate ofRamor 500
In figure 3 is show the densification obtained in
the thickness of steel AISI / SAE 1010, while the
plate Ramor500 steel did not show any change in
deformation. To evaluate the densification in
thickness, the plate was cut into the impact region to
ensure the final thickness measurement.
At steel Ramor500 only marks on the outer
surface were obtained from impact. No additional
testing was conducted, only the verification that this
steel with 2.5mm of thickness could be used
forblindage level II-A.
Only steel AISI/SAE 1010it was checked the
changing of cross section. The final thickness
measured on the plate was 2.6mm.
Fig. 3 – Cut of plate in region of impact
III. FINITE ELEMENT MODELING
The problem of finite element was develop in the
framework of the dynamics, using explicit integration
algorithm. Elements of shell typewith a high degree
of refinement in the impact area used, 0.1mm of
refinement in the impact region, as shown in Figure
4.
Fig. 4 – Type of mesh refinement adopted
The plasticity model used is the Johnson-Cook
that consider the effect of the strainrate𝜀, the
initiation of damage is governed by the formulation
of the FLD forming limit curve and the propagation
of damage based on the strain energy.The plasticity's
law from is given by equation 1.
𝜎 = 𝐴 + 𝐵 𝜀 𝑝
𝑛
1 + 𝑐𝑙𝑛
𝜀
𝜀𝑜
(1)
where,
𝐴, is yield stress;
𝐵, is hardening coefficient;
𝑛, exponent of hardening;
𝑐, coefficient of strain rate
𝜀0, strain rate of reference.
The initiation criterion of damage, is introduced
via forming limit curve that starts when the
variableFLD =1.The forming limit diagram was
proposedby Keeler andBackofen in 1964 [4], that is
used to measure the amount of material that resists to
deformation before the initiation of necking, as
shown in Figure 5.
Fig. 5 – FLD, FormingLimitDiagram
Where:
FLD, state variable of damage, which increases
with plastic deformation.
𝜔 𝐹𝐿𝐷 =
𝜀 𝑚 á𝑥𝑖𝑚𝑎
𝜀 𝐹𝐿𝐷
𝑚 á𝑥𝑖𝑚𝑎 (𝜀 𝑚 í𝑛𝑖𝑚𝑎 ,𝜃)
(2)

major, maximum deformation limit,
minor, minimum deformation limit;
Already, the formulating of damage
propagationusedis the one proposed by Hillerborg
(1976) [5] where it uses the fracture energyGf given
by:
𝐺𝑓 = 𝜎𝑦 𝑑𝑢𝑓
𝑢 𝑓
0
(3)
At this equation,uf is the plastic equivalent
displacement, and it is consider through following
rule:
 Before damage start:𝑢𝑓 = 0
 After damage start:𝑢𝑓 =
2𝐺 𝑓
𝜎 𝑦
 At fracture:𝑢𝑓 =
2𝐺 𝑓
𝜎 𝑦
𝜎𝑦 , is yield stress,
The graph of Figure 6 shows the relationship
between stress and plastic displacement of fracture.
Fig. 6 - Fracture energy evolution
The law of evolution of the stresses with damage
is given by equation 4.
𝜎 𝑑
= (1 − 𝑑)𝜎 𝑢
(4)
where:
𝜎 𝑑
, is a stress damaged and 𝜎 𝑢
a stress
undamaged.
𝑑,is a damage variable.
IV. RESULTS
According to physical test evaluation, the
maximum displacement measured was 10.5mm in
curvature, presented at figure 7.The final thickness
measured at the center of impact region was 2.6mm.
It was proposed,a form of the measuring the
maximum displacementconsidering thickness.
Fig. 7 - Measurement of the displacement due to the
impact.
In numerical modeling via finite element
method, the maximum displacement was 10.32mm,
figure 8, and final thickness evaluated at impact
region was 2.49mm, figure 9.
To expose the gain of the analysis using the
elastic-plastic model with damage, rather than the
conventional plasticity in Figure 10, is present a
simulation considering a model material without any
law of damage evolution and without the inclusion of
deformation rate effect.
Fig. 8 –Total displacement on the plate, curvature
displacement.
Fig. 9 - Final thickness of plate in simulation
Fig. 10 - Result of curvature displacement for
conventional plasticity
V. CONCLUSION
The shell element used to modeling plasticity
and damage via finite element method presented
values with a higher degree of correlation than 95%,
while the use of the conventional plasticity the
response was quite different. The use of shell
elements to capture the behavior of the material
proposed due to the low computational cost, unlike
the approach using solid elements.

Noted that the deformation rate must be
employed, and to modeling a component in fracture
situation, the inclusion of damage variable is highly
recommended.
The accuracy of the results is drive by the use of
elastic-plastic curves for different strain rates as more
curves are used, better the model describe structural
behavior.
As for the criterion of damage initiation is
necessary a caveat, because a good characterization
of deformation limits, maximum and minimum
deformation to predict beginning of material necking,
as established by the forming limit diagram FLD also
assist in better representation damage.
The energy criterion for the damage propagation
is use due to an attempt to make the nondependent
problem of mesh size even adopting a high
refinement in the impact area.
This methodology becomes possible, after the
calibration of material model, the useof finite element
method for verification of different types of
projectiles. Also provides a prediction of the final
displacement field and with the model is able to study
the structural behavior of metal sheets during an
impact ballistic. Finally, assist the armored project to
avoid perforations.
Acknowledgements
Grateful for professorPedro by support and PUC
Minas, CAPES, and companies Vision Graphic
andRuukki. Also thankful to Brazilian Federal Police
and TiroUrbano Defensive School.
REFERENCES
[1] B. Gama, J. Gillespie, Finite element
modeling of impact, damage evolution and
penetration of thick-section composites,
International Journal of Impact
Engineering, 38(4), 2011, 181-197.
[2] Norma NEB/T E-316, Proteção Balística de
Carros de Passeio, CTEX, 200.
[3] NIJ Standard-0101.06, Ballistic Resistance
of Body Armor,U.S. Department of
Justice,1985.
[4] S. P. Keeler, A. Backofen, Plastic instability
and fracture in sheets stretched over rigid
punches, ASM Transactions Quarterly, 56,
25–48, 1964.
[5] A. Hillerborg, M. Modeer, P.E. Petersson,
Analysis of Crack Formation and Crack
Growth in Concrete by Means of Fracture
Mechanics and Finite Elements, Cement and
Concrete Research, 6, 773–782, 1976.

Experimental and numerical evaluation of plasticity model with ductile damage to applied in project of shielding sheet metal.

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