SELECTION OF THE NUMBER OF THE KEVLAR …yadda.icm.edu.pl/yadda/element/bwmeta1.element.baztech...PENETRATION WITH THE USE OF THE ANSYS - AUTODYN v12 .1.0 PROGRAM Abstract: The article

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prof. dr hab. inż. Adam WIŚNIEWSKI mgr inż. Michał GMITRZUK Wojskowy Instytut Techniczny Uzbrojenia

SELECTION OF THE NUMBER OF THE KEVLAR ARMOUR LAYERS IN THE NUMERICAL ANALYSIS OF THE 5.56 MM PROJECTILE

PENETRATION WITH THE USE OF THE ANSYS - AUTODYN v12 .1.0 PROGRAM

Abstract: The article presents the numerical analysis of the Kevlar armour penetration process with the 5.56 mm type SS109 projectile. The analysis was performed with the use of the Ansys - Autodyn v12.1.0 program by the SPH method (for the projectile) and the Lagrange method (for the Kevlar ar-mour). The influence of the number of the Kevlar layers on the penetration and change of velocity of the projectile was investigated. In the simulation of firing the 5.56 mm type SS109 projectile having a steel-lead core and a brass coat was used. Based on the literature review the validation of the projec-tile was performed using the „bodywork effect” for this purpose. It was shown, that the adopted model of the projectile together with the selected material parameter conforms to the real projectile. For the numerical analysis of the Kevlar armours, four variants of the material layers number in the soft ballistic panel were adopted. As the initial for the panel, the thickness of 12 mm was se-lected, what conformed to 40 layers of the material. For the consecutive variants the 50, 60 and 80 layers were taken respectively. It was shown, that the increase of the number of layers made of Kevlar fabric causes the projectile velocity and length falling down. At the maximum panel thickness the velocity drop-off reached to 663 m/s (72%). The minimum length of the projectile amounted to 8.55 mm (63%). The projectile velocity of braking was also changed. It was proved that the soft panels with the use of Kevlar are not sufficient enough for stopping of the 5.56 mm type SS109 pro-jectile.

DOBÓR LICZBY WARSTW PANCERZA KEVLAROWEGO W NUMERYCZNEJ ANALIZIE PROCESU PENETRACJI

POCISKIEM 5,56 MM Z ZASTOSOWANIEM PROGRAMU ANSYS - AUTODYN v12.1.0

Streszczenie: W artykule przedstawiono analizę numeryczną procesu penetracji pancerza kevlarowe-go pociskiem 5,56 mm SS109. Analizę przeprowadzono za pomocą programu Ansys - Autodyn v12.1.0 z użyciem metody SPH (dla pocisku) i Lagrange’a (dla pancerza kevlarowego). Zbadano wpływ liczby warstw kevlaru na penetrację oraz zmianę prędkości pocisku. W symulacjach ostrzału użyto pocisku 5,56 mm SS109 z rdzeniem stalowo-ołowianym oraz płaszczem wykonanym z brązu. Opierając się na przeglądzie literatury przeprowadzono walidację pocisku wykorzystując w tym celu „efekt karoserii”. Wykazano, że przyjęty model pocisku wraz z dobranymi parametrami materiało-wymi odpowiada rzeczywistemu pociskowi. Do analizy numerycznej pancerzy kevlarowych przyjęto cztery warianty liczby warstw materiału w miękkim panelu balistycznym. Wyjściową grubość panelu przyjęto 12 mm, co odpowiadało 40 warstwom. Do kolejnych wariantów wzięto odpowiednio 50, 60 i 80 warstw. Wykazano, że wzrost liczby warstw tkaniny Kevlaru powoduje spadek prędkości i długo-ści pocisku. Dla maksymalnej grubości panelu uzyskano spadek prędkości o 663 m/s (72%). Minimal-na długość pocisku wyniosła 8,55 mm (63%). Zmianie uległa również prędkość hamowania pocisku.

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Wykazano, że miękkie panele z użyciem Kevlaru są niewystarczające do powstrzymania pocisku 5,56 mm SS109.

1. Introduction

The complexity of the numerical simulations of the projectile impact onto the armour is caused by a large number of variable parameters, as the projectile velocity, relation of stiff-ness to mass, shape and edge conditions, as well as the material properties of the projectile and armour [1]. Additional complication appears in the simulations of penetration of the fab-rics, layered and composite because of their orthotropism and large number of variables in the destruction models.

Projecting of the Kevlar ballistic barriers, based exclusively on the physical models, needs a large number of the experimental data, which are time-consuming and expensive. The progress towards understanding of the mechanism of the layered composite damages al-lows avoiding many experimental tests due to the use of the computer simulations, which are finally confirmed experimentally.

One of the main problem, created when modelling of the respective phenomenon is a complicated structure of the ballistic fabrics, where an important role is played by the weave of the material and the density of fibres [1], what is difficult for modelling in the computer simulations.

2. Computer simulations 2.1. Validation of the model of the 5.56 mm type SS109 projectile

In order to perform an essential simulation of the projectile penetration the initial numeri-cal validation of the 5.56 mm type SS109 projectile impact onto the armour plate was con-ducted on the base of the numerical and experimental results [2, 3].

The 5.56 mm type SS109 projectile consists of a steel-lead core and a brass coat (Fig. 1). For the core, the materials accessible in the Ansys - Autodyn v12.1.0 program database were adopted. The steel 4340 (penetrator), accessible in the program library is characterized by the complete endurance model and the Johnson-Cook (JC) destruction model (Table 1). As a core of the filler the lead was adopted, according to the Stainberg-Guinan endurance model. For the brass coat, the data of material, the endurance (modified JC) and destruction model were adopted basing on the literature [3, 4].

The computer simulation of the 5.56 mm type SS109 projectile, conducted in the Military Institute of Armament Technology (MIAT) covered two variants: with the use of the mild steel metal sheet of 1 mm thickness, preceding the armour plate of 6.5 mm thickness with ten-millimetre gap (Fig. 2-a), and without the use of the mild steel metal sheet (Fig. 2-b).

Fig. 1. The 5.56 mm type SS109 projectile, a – di-mensions of the projec-tile, b – isometric projec-tion with the cross-sec-tion, 1 – brass coat, 2 – core of the 4340 steel (penetrator), 3 – lead filler

1

3

2

a b

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Fig. 2. Model of the projectile and armour system for the initial conditions of the variants: a – with the metal sheet of 1 mm thickness, b – without the thin metal sheet: 1 – mild steel 1006 metal sheet of 1 mm thickness, 2 – projectile, 3 – armour plate of 6.5 mm thickness

Velocity of the projectile in the variant „a” amounted to 1003 m/s [3]. The variant „b” as-sumed the velocity equal to the average projectile velocity after perforation of the thin metal sheet (960 m/s).

The material data and the constitutive model of the armour plate and the low-alloyed 1006 steel metal sheet was adopted according to the literature data [2, 3] and the accessible data in the Ansys - Autodyn v12.1.0 program library.

Table 1. The endurance model and the Johnson-Cook destruction model of the material constants used for the computer simulations

Material Parameter Thin steel plate

(1006) Steel core

(4340) Brass Armour plate

Johnson-Cook destruction model A, MPa 350 792 206 1299 B, MPa 275 510 505 2230

n 0.36 0.26 0.42 0.558530 C 0.022 0.014 0.01 0.044474 m 1.00 1.03 1.68 0.961240

Tm, K 1811 1793 1189 1800 Johnson-Cook destruction model

D1 0.8 0.05 0.54 0.168040 D2 2.1 3.44 4.89 0.034994 D3 -0.5 -2.12 -3.03 -2.44 D3 0.0002 0.002 0.014 -0.045 D5 0.61 0.61 1.12 0.918998

The simulations were performed with the use of the Lagrange method (all the body cov-

ered by the transferring net, deforming together with it). The projectile-armour system was shown in the 2D simulations.

The results of both the experimental and numerical results [2, 3] proved, that the 5.56 mm type SS109 projectile perforates the armour steel plate of 6.5 mm thickness in case of placing the steel 1006 metal sheet in front of it, whereas without this metal sheet the perforation did not occur. The “bodywork effect” was confirmed by the computer simulations, performed in the MIAT (Table 2). It was observed, that the projectile was blunted by a partial suppres-sion of the brass coat forehead after perforation of the thin metal sheet, made of soft steel. During the projectile contact with the armour plate the strain caused by the blunted projectile occurs on the larger area, what results in hammering out of so called „plug”.

The similar appearance and behaviour of the projectile after perforation of the thin metal sheet was described in Figure 3. The deformation of the coat and bulges in the rear part of the projectile, caused by the lead transfer, were observed.

a

2 1

3 b

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Table 2. Graphic presentation of firing simulation results of the armour plate with: a - metal sheet imitating the car bodywork, b - the armour plate only

Time, t, ms Variant

0.0117 0.0310 0.1169

a

b

Damage

Fig. 3. The 5.56 mm SS109 projectile after perforation of the thin metal sheet of the low-alloyed steel for: a – numerical analyses performed in the MIAT , b – experimental tests results obtained in [2]

It was proven, that the projectile model is correct and allows for further performance

of the computer analysis. 2.2. Numerical analysis of the projectile penetration into the armour

This article presents results of the numerical simulations of the 5.56 mm type SS109 pro-jectile penetration (Fig. 1) into the simplified model of the Kevlar armours, consisting of dif-ferent numbers of layers. The numerical analyses were performed with the use of two me-thods of the computational physics: the SPH (Smooth Particle Hydrodynamic), for the projec-tile model, and the Lagrange (the body covered by a deforming net, transferring together with it), for the armour model. In this second case the material parameters were adopted from the data accessible in the Ansys - Autodyn v12.1.0 program (Kevlar-epoxy) (Table 3). For the layer of 0.3 mm thickness and of 220 g/m2 surface density (data received from the Institute of Security Technology MORATEX), the volume density of 0.733 g/ cm3 was achieved, which then was adopted in the simulations for the individual layer of the ar-mour.

The influence of the layers number of the Kevlar armour panel on its protective capability from the impact of the 5.56 mm type SS109 projectile was investigated. The simulations were performed for four variants of the layers numbers (20, 40, 60 and 80), what responds to 6, 12, 18 and 22 millimetres thickness of the panel. For the analysis, the material of the size 400x400 mm (length x width) was selected, conducting the 2D with the use of the axial sym-metry. It allowed for significant time reduction, needed for a single simulation. The edge con-ditions (ties) for all the Kevlar layers were adopted on the edges outermost from the axis of symmetry. As an initial variable, the velocity of the projectile in the point of the impact onto the armour was adopted (923 m/s).

The layers of the armour model are shown in high simplification, what additionally intro-duces some mistakes connected with lower accuracy in relation to the reality. The layers

b a

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of the fabrics in the ballistic panels consist of the fibres woven with a sufficient interleave (Fig. 4), which plays an important role in the absorption of the projectile energy. This phe-nomenon is difficult to express mathematically.

Table 3. List of the parameters of the materials under tests (Kevlar-epoxy), used in the simu-lations of the 5.56 mm type SS109 projectile penetration

Parameter Value Parameter Value Density, g/cm3 0.733 Compressibility module, MPa 3267.29

Equation of state –orthotropic material A2, MPa 40000

Stiffness Stiffness matrix

A3, MPa 0

C11, MPa 3252.166 B0 0 C22, MPa 13058.26 B1 0 C33, MPa 13068.22 T1 3267.29 C12, kPa 0.075610 T2 0 C23, kPa 0.063199 Temperature of relevance, K 373 C31, kPa 0.312318 Specific heat, J/kgK 0

Shear module 12, MPa 1000 Strenght model – elastic Shear module 23, MPa 1000 Shear module, MPa 1000 Shear module 31, MPa 1000 Volume characteristics Polynomial

Destruction model – Material Stress/Strain

Fig. 4. Example of the fabric layer modelled with the use of the Lagrange method [5]

For all the variants, the time of penetration t, length of the steel core (penetrator) L, length

of the entire projectile Lp, bulge of the armour B, diameter of the outlet hole (Fig. 5), number of the perforated layers N and velocity of the projectile Vp and projectile core V after perfora-tion were investigated. The values for every variant are included into Table 4. The average velocity of the projectile (of the coat, steel and lead core) in function of time for each of the variants is shown in Figure 6. The average velocity of the steel core only is shown in Figure 7. The graphic presentation of results is included into Table 5.

Fig. 5. Parameters describing deforma-tion of the projectile and armour after completion of the simulation

Change of the panel thickness together with the increase of the layer number influences

significantly: the armour bulge, length of the projectile and its velocity after the armour perfo-ration.

In all the cases the same phenomena of the armour and projectile deformation were ob-served. The rupture of the first Kevlar layers occurs once the projectile contacts the armour. As a result of the penetration of the projectile, the stress is transferred then consecutively from one layer to another, what causes higher absorption of the kinetic energy. The deforma-tion and blunting of the coat of projectile forehead occurs on the initial stage of penetration.

B

L Lp

R

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The significant slowing down of the steel core for all the variants was observed within ca. 0.00036 ms (Fig. 7-1) after perforation of the five layers of the material. The bulge of the armour for the variant I occurred within 0.005354 ms. Due to the bigger number of lay-ers for the consecutive cases the time, after which the armour bulge occurred, was longer (relatively 0.005981 ms, 0.007071 ms and 0.009902 ms). The braking of the projectile causes deformation of the lead core (filler) on the rear plan of the steel core.

Table 4. Parameters of the particular variants for different numbers of the armour layers

Variant I II III IV Number of the armour layers 40 50 60 80 Thickness of the panel, mm 12 15 18 22 Number of the punctured layers 40 50 60 80 Moment of puncture, ms 0.0200 0.0281 0.0371 0.059 Length of the steel core, L, mm 7.53 7.2 7.09 6.95 Length of the projectile, Lp, mm 16.65 12.35 9.75 8.55 Bulge of the armour, B, mm 24.85 32 35.09 39.5 Velocity of the steel core, V, m/s 775 744 634 444

Velocity of the projectile, Vp, m/s 798 716 589 260

Mom

ent o

f the

sim

ula-

tio

n co

mpl

etio

n

Diameter of the inlet hole, R, mm 10.15 12 14 15

The deformation of lead causes the coat heave, which is broken due to the pressure of the consecutive Kevlar layers. It produces large destructions of the fabric layers inside the panel and increase of the outlet hole diameter. The moment of the armour perforation is characterized by the increase of velocity of the steel core (Fig. 7-2). This increase was caused by the high momentum of the filler, which additionally „pushes” the steel core. The least shortening of the projectile responds to the lowest number of the Kevlar layers and amounts to 6.65 mm (29%). It results from the easiness of the barrier overcome by the projectile. The shortest projectile, obtained in simulations, was observed for the biggest number of the Kevlar layers, and its length amounted to 8.55 mm (decrease of the length about 14.75 mms). The length of the penetrator (steel core) for all the variants does not differ significantly. A low calibre of the projectile and its shape causes effective penetration into the consecutive layers of the fabrics.

Fig. 6. The average velocity of the projectile in function of time for four variants of the panel thickness

Fig. 7. The average velocity of the projectile core in function of time for four variants of the panel thick-ness, 1 – the distinct slow-down of the core of the bullet after the perforation of 5 layers of the fabric, 2 – the moment of the perforation of the armour

Pro

ject

ile v

elo

city

, Vp,

m/s

I

II

III

IV

1

2

Ste

el c

ore

vel

ocity

, V, m

/s

I II

III

IV

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Table 5. List of the graphic results of firing simulation for the four variants of the panel thick-ness, with different number of layers

Time, t, ms Variant

0.01 0.025 0.04 0.06

I

II

III

IV

After perforating all the layers, the average velocity of the projectile for the variant I fell down of 123 m/s (13%) in relation to the initial velocity. The biggest drop-off of the velocity was achieved for the biggest number of layers. The average velocity of the projectile for the variant IV fell down of 663 m/s (72%) The increase of the projectile velocity of brak-ing, together with the increase of the panel thickness was also noticed. It is testified by the increase of inclination of the curves in relation to the axis x.

3. Conclusions

On the base of the literature review and the performed simulations the following conclu-sions one can draw: 1. The 5.56 mm type SS109 projectile causes the „bodywork effect” in the light fighting ve-

hicles because of its characteristic construction. The computer simulations prove the con-formity of the projectile model with the experimental model, adopted from the literature.

2. It results from the computer simulation analysis, that the panels with the use of soft Kevlar insert are of low resistance to penetration by the 5.56 mm type SS109 projectile, even in case of large number of the fabric layers.

3. The main factor in the penetration of the Kevlar armour is the steel core, which does not deform significantly and its shape allows for free penetration deep into the armour.

4. Large deformation of the lead on the steel core results from the bigger inertion of the lead filler and breaking of the steel core during the penetration of the consecutive Kevlar lay-ers. Moreover, the plastic flow of the lead causes large destruction, attaining the repeated value of the caliber and a big diameter R of the outlet hole.

5. The length of the projectile drop-off in relation to the number of the armour layers results from the lead filler deformation. The highest observed shortening of the projectile oc-

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curred for the thickest panel, and amounted to 63% in comparison to the initial length, what corresponds to the length of 23.3 mm.

6. For all the variants the perforating of the armour occurred in the moment of the velocity of the projectile steel core increase. The time needed for penetration of the panel increased with the change of the armour thickness and amounted to the 40, 50, 60 and 80 layers re-spectively: • 0.02 ms, • 0.03 ms, • 0.037 ms, • 0.59 ms.

7. The deepest bulge of the armour B occurred for the largest number of the Kevlar layers amounted about 39.5 mm. The high number of layers for the fourth variant caused the ex-panse of the energy on to the greatest surface, however not stopping the constant penetra-tion of the projectile.

References [1] Silva M.A.G., Cismasiu C., Chiorean C.G.: Numerical simulation of ballistic impact

on composite laminates, International Journal of Impact Engineering, 31, 2005, pp. 289÷306.

[2] Nsiampa N., Coghe F., Dyckmans G.: Numerical investigation of the bodywork effect (K-effect), DYMAT - 2009, 2009, pp. 1561÷1566.

[3] Adams B.: Simulation of ballistic impacts on armoured civil vehicles, PhD thesis, MT 06.03, The Netherlands, 2006.

[4] Borvik T., Dey S., Clausen A.H.: Perforation resistance of five different high-strength steel plates subjected to small-arms projectiles, International Journal of Impact Engineer-ing, 36, 2009, pp. 948÷964.

[5] Duana Y., Keefe M., Bogetti T.A., Powers B.: Finite element modeling of transverse im-pact on a ballistic fabric, International Journal of Mechanical Sciences, 48, 2006, pp. 33÷43.

This work was co-financed by the European Fund for Regional Development in Poland (Pro-ject: “Smart passive body armours with the use of rheological fluids with nanostructures” un-der the contract No. UDA-POIG.01.03.01-00-060/08-00) and carried out in cooperation be-tween The Institute of Security Technology “MORATEX” (Instytut Technologii Bezpie-czeństwa “MORATEX”), Warsaw University of Technology (Politechnika Warszawska) and the Military Institute of Armament Technology (Wojskowy Instytut Techniczny Uzbrojenia).