research article blast-resistant improvement of sandwich

Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2013, Article ID 947571, 8 pageshttp://dx.doi.org/10.1155/2013/947571

Research ArticleBlast-Resistant Improvement of Sandwich Armor Structure withAluminum Foam Composite

Shu Yang and Chang Qi

School of Automotive Engineering, State Key Laboratory of Structural Analysis for Industrial Equipment,Dalian University of Technology, Dalian 116024, China

Correspondence should be addressed to Chang Qi; [email protected]

Received 30 May 2013; Accepted 26 August 2013

Academic Editor: Bin Zhang

Copyright © 2013 S. Yang and C. Qi. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Sandwich armor structures with aluminum foam can be utilized to protect a military vehicle from harmful blast load such asa landmine explosion. In this paper, a system-level dynamic finite element model is developed to simulate the blast event andto evaluate the blast-resistant performance of the sandwich armor structure. It is found that a sandwich armor structure withonly aluminum foam is capable of mitigating crew injuries under a moderate blast load. However, a severe blast load causesforce enhancement and results in much worse crew injury. An isolating layer between the aluminum foam and the vehicle flooris introduced to remediate this drawback. The results show that the blast-resistant capability of the innovative sandwich armorstructure with the isolating layer increases remarkably.

1. Introduction

For ground vehicles, blast loads from landmine explosionsand ballistic loads from a bullet or a missile produce muchdamage to the vehicle structure and result in severe injuriesto crew members. Armor structures are usually employed toprotect a military vehicle and crew from these extreme loads.Metal foam is an ideal choice of sacrificial material for blastprotection due to its low density, a characteristic that is verypreferable for lightweight applications. The porus nature ofthe foam helps in heat dissipation and also provides acousticdamping [1–3]. Besides all these benefits, an undesiredphenomenon observed when using foam material for blastprotection is that, under certain conditions, the peak forcetransmitted to the protected structure can be even higherthan when the foammaterial is not used [4].This unexpectedphenomenon is called “force enhancement” and needs to beconsidered in vehicle armor design. As a result, application offoammaterial for blast protection design is still limited at thepresent time.

In this context, to evaluate the blast-resistant perfor-mances of the aluminum foam sandwich armor structure,as well as to explore design countermeasures against forceenhancement, dynamic finite element models are presented

in this paper for simulating the blast process and the dynamicresponses of the sandwich armor, as well as the crew injuries.An isolating layer is introduced to the sandwich armorstructure, which is demonstrated to be effective in mitigatingthe crew injury based on numerical simulation results.

2. Modeling Description

The blast process is both difficult and expensive to test.Moreover, testing of structural damage processes from theblast loads is no easy task either. Numerical techniques,such as the finite element method, are employed for blastprocess simulation as well as for structural damage analyses.In this study, the finite element models were developed fornumerical simulations based on the LS-DYNA platform.Altair HYPERMESH and LS-PREPOSTwere utilized for pre-and post-processing, respectively.

2.1. Vehicle Model. Finite element (FE) model of a typicalmilitary wheeled vehicle has been developed previously,which is ready to be utilized in this research. The geometricinformation in the FE model was based on the CAD data ofthe vehicle obtained from the Internet. Properties of variousmaterials, such as steel, rubber, and glass, in the model were

2 Advances in Materials Science and Engineering

ox

y

z

Figure 1: Finite element model of a military vehicle for blastprotection design.

imported from other publicly available FE vehicle models.The FE model consists of 56851 elements with 60204 nodes.

2.2. Underbody Armor Model. In order to protect the crewmembers of a military vehicle at the time when dangerousloads, such as landmine blast load, are exerted on the vehiclebody, it is essential to add an armor structure to the vehiclebody. Underbody armor is a kind of protective structuremainly used against threats from the ground. The armor,which consists of a back aluminum panel and a layer ofaluminum foam, is attached to the vehicle floor for enhancedblast-resistant capabilities of the vehicle.

The back facesheet of the two types of sandwich panel ismade of aluminum alloy AA6061-T4 with mechanical prop-erties: mass density 𝜌 = 2700 kg/m3, Young’s modulus 𝐸 =

69GPa, Poisson’s ratio V = 0.28, initial yield strength 𝜎𝑦

=

110.3MPa, and ultimate strength 𝜎𝑢

= 213MPa. To takestrain hardening effects into account, the energy equivalentplateau stress can be calculated as [5]

𝜎 = √𝜎𝑦𝜎𝑢

1 + 𝑛, (1)

where 𝑛 is the strain hardening exponent of the material,𝑛 = 0.2, and the plateau stress for AA6061-T4 adoptedhere is 139.9MPa. Figure 1 shows the typical engineer-ing stress-strain curves for the aluminum alloy AA6061-T4. The sheets are represented by material model 24(∗MAT PIECEWISE LINEAR PLASTIC) provided by LS-DYNA [6]. Since the aluminum is insensitive to the strain ratedependency, here the strain rate dependency is neglected [7].

The yield criterion Γ of aluminum foam is given as follows[8]:

Γ = �� − 𝜎𝑦≤ 0. (2)

Here, 𝜎𝑦is the yield stress and �� is the equivalent stress

which is defined as

��2=

𝜎2

V + 𝜅2𝜎2

𝑚

1 + (𝜅/3)2, (3)

where 𝜎V is the von Mises effective stress defined as

𝜎V = √3

2𝜎dev : 𝜎dev, (4)

o

xy

z

Vehicle floor

Al foam

Al panel

Figure 2: FE model of underbody armor plate for blast protection.

where 𝜎dev is the deviatoric stress tensors and 𝜎

𝑚is the

hydrostatic stress tensors.The yield stress𝜎

𝑦can be expressedwith the plateau stress

𝜎𝑝, the shape parameter of yield surface 𝜅, the equivalent

strain 𝜀, the compaction strain 𝜀𝑑, and the hardening param-

eters 𝜅2, 𝛾, and 𝛽:

𝜎𝑦= 𝜎𝑝+ 𝛾

𝜀

𝜀𝑑

+ 𝜅2ln(

1

1 − (𝜀/𝜀𝑑)𝛽) ,

𝜅2=

9

2

(1 − 2𝜐𝑝)

(1 + 𝜐𝑝),

0 ≤ 𝜅2≤

9

2.

(5)

The equivalent and compaction strain 𝜀, 𝜀𝑑is given as

𝜀2= (1 + (

𝜅

3)

2

) [𝜀2

𝑒+

1

𝜅2𝜀2

𝑚] ,

𝜀𝑑= −

9 + 𝜅2

3𝜅2ln(

𝜌𝑓

𝜌𝑓0

) ,

(6)

where 𝜌𝑓and 𝜌𝑓0

and the density of aluminum foam and thedensity of the base material.

FE model of the underbody armor structure is shown inFigure 2. Both the aluminum panel and the aluminum foamare modeled by reduced one-point integration brick element.The nodes of different components are tied together alongthe material interface. The aluminum foam material [9] inuse has the following material properties: 𝜌

𝑓= 300 kg/cm3,

𝐸 = 1500MPa, 𝜐𝑝 = 0.05, 𝜅 = 2.1, 𝛾 = 6.10, 𝜀𝑑

= 2.2,𝜅2= 38.1MPa, 𝛽 = 3.1, 𝜎

𝑝= 4.41MPa, and 𝜀cr = 0.1.

Material model MODIFIED CRUSHABLE FOAM ofLS-DYNA was used to represent the material behavior of thealuminum foam. This model includes the strain rate effectof the foam material under high speed impact loads. ThePIECEWISE LINEAR PLASTICITY card is used for model-ing of both the front panel and the vehicle floor. Strain rateeffect of aluminum is accounted by using the Cowper andSymonds model [10].

Advances in Materials Science and Engineering 3

Figure 3: A rigid body dummy model representing a crew memberin a blast event simulation.

2.3. Crew Model. Crash dummy models are usually includedin tests and simulations to predict occupant responses underdifferent crash loads; these responses will be used to guide thecrashworthiness design. Various dummy models for vehiclecrashworthiness design have been developed, from verycomplicated FEmodels with thousands of degrees of freedom(DOF) to much simpler rigid body models. Figure 3 showsthe 50th percentile male GEBOD dummy model; the modelis comprised of fifteen rigid bodies that represent the lowertorso, middle torso, upper torso, neck, head, upper arms,forearms, and hands, and upper legs, lower legs, and feet ofthe dummy. The revolutions of the dummy are representedby spring elements with viscous damps. The dummy weighs76 kg, and the model has 1745 finite elements.

At present, specifically developed and validated humansurrogate models suitable for assessing occupant response ina vehicle subject to mine strike are not available. Horst et al.[11] used the standard Hybrid III crash test dummy to studythe lower leg injuries occurring in antitank mine strikes.Williams et al. [12] studied crew member responses in a lightarmored vehicle (LAV) subjected to amine blast load by usingthe GEBOD rigid body dummy model incorporated in LS-DYNA. Due to the computational efficiency of this dummymodel, a total of six GEBOD dummies were put at differentpositions in the LAV for the simulation. In this work, theGEBOD dummy model shown in Figure 3 is employed forblast protection design because of its low computational costand an acceptable error level.

The equation of occupant-seat model can be formulatedas [13]

Mz + Cz + Kz = F𝑘+ F𝑐, (7)

where z = {𝑧𝑠, 𝑧1, 𝑧2, 𝑧3, 𝑧4}𝑇 is the displacement vector, M,

K, and C are the mass matrix, the stiffness matrix, and thedamping matrix, respectively and F

𝑘and F

𝑐are given as F

𝑘=

{𝑘𝑠𝑧0, 0, 0, 0, 0}

𝑇 and F𝑐= {𝑐𝑠��0, 0, 0, 0, 0}

𝑇. Here, 𝑘𝑠and 𝑐𝑠are

Landmine

ox

y

z

Figure 4: Simulation model of a crewed military vehicle under alandmine blast.

the stiffness and the damping coefficient of the suspension.𝑍0is the displacement of the suspension base and can be

obtained from the integral of ��0.

2.4. Blast Load Simulation Model. To simulate the blast loadfrom an explosive device, such as a landmine, one of theempirical models for blast pressure prediction was utilized.This empirical model is based on the CONWEP air blastfunction developed by Kingery and Bulmash [14]. Thismodel, which has been implemented as the ∗LOAD BLASTloading card in LS-DYNA, can predict the blast overpressureunder certain conditions: the free air detonation of a sphericalcharge and the surface detonation of a hemispherical charge;the surface detonation approximates the conditions of amine blast. The model takes into consideration the angle ofincidence of the blast, 𝜃, the incident pressure, 𝑝in, and thereflected pressure, 𝑝ref. The predicted time-dependant blastoverpressure is expressed as

𝑝 (𝑡) = 𝑝ref cos2𝜃 + 𝑝in (1 + cos2𝜃 − 2 cos 𝜃) , (8)

with 𝑝in and 𝑝ref given by

𝑝in = 𝑝𝑖0(1 −

𝑡

𝑡0

) 𝑒−𝑏𝑡/𝑡0 ,

𝑝ref = 𝑝𝑟0

(1 −𝑡

𝑡0

) 𝑒−𝑎𝑡/𝑡0 ,

(9)

where 𝑝𝑖0and 𝑝

𝑟0are the peak incident overpressure and

the peak reflected overpressure, respectively, 𝑡0is the positive

phase duration time, and 𝑎 and 𝑏 are decay coefficients.By selecting appropriate values for 𝑎 and 𝑏, various decaycharacteristics can be indicated.

The solution methodology of wave propagation for thefront and back facesheet of the composite sandwich struc-tures is given by Hoo Fatt and Surabhi [15].

In this paper, the elastic wave speed in the foam 𝑐𝑒is

represented as

𝑐𝑒= √

𝐸

𝜌𝑓

, (10)


(a)

Fringe levels1.687e + 09

1.518e + 09

1.350e + 09

1.181e + 09

1.012e + 09

8.435e + 08

6.748e + 08

5.061e + 08

3.374e + 08

1.687e + 08

5.252e + 03

Contours of effective stress (v-m)Max ipt. value

= 5252.22, at elem. number 58529Min

Max = 1.68698e + 09, at elem. number 55847

(b)

xy

z

o

xy

z

o

Vector of total velocity= 0.0942307, at node number 391= 6.06069, at node number 939

6.061e + 00

5.464e + 00

4.867e + 00

4.271e + 00

3.674e + 00

3.077e + 00

2.481e + 00

1.884e + 00

1.288e + 00

6.909e − 01

9.423e − 02

Fringe levelsMin

Max

(c)

Figure 5: Simulations results. (a) Vehicle deformation. (b) Deformation and effective stress distribution of seat mounts. (c) Resultant velocityof crew.

while the plastic wave speed 𝑐𝑝is

𝑐𝑝= √

𝐸𝑝

𝜌𝑓

, (11)

where 𝐸 is the elastic modulus and 𝐸𝑝is the elastic-plastic

tangent modulus.

Hence, the resistive foam stress acting on the front facesh-eet (vehicle floor) about elastic wave propagation 𝜎

𝑓𝑒is

𝜎𝑓𝑒

= 𝜌𝑐𝑐𝑒(2V1𝑖− V1) , (12)

while the correponding stress about elastic-plastic wavepropagation 𝜎

𝑓𝑒𝑝is

𝜎𝑓𝑒𝑝

= (1 −𝑐𝑝

𝑐𝑒

)𝜎𝑦+ 𝜌𝑓𝑐𝑝V1max + 𝜌

𝑓𝑐𝑒(V1max − V

1) , (13)


0 1 2 3 40

200

400

600

800

1000

1200

1400

1600

Time (ms)

Baseline (no armor)Armored

Resu

ltant

seat

mou

nt fo

rce (

kN)

(a)

0 1 2 3 40

500

1000

1500

2000

2500

3000

3500

4000

Time (ms)

Baseline (no armor)Armored

Resu

ltant

pelv

is ac

cele

ratio

n (m

/s2)

(b)

Figure 6: Comparison of blast-resistant capabilities of various designs with 2 kg TNT equivalent. (a) Seat mounts forces. (b) Crew pelvisacceleration.

0

500

1000

1500

2000

2500

Time (ms)

Baseline (no armor)Armored (foam only)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Resu

ltant

seat

mou

nt fo

rce (

kN)

(a)

Time (ms)


0.0 0.5 1.0 1.5 2.0 2.5 3.00

2000

4000

6000

8000

10000

Resu

ltant

pelv

is ac

cele

ratio

n (m

/s2)

(b)

Figure 7: Comparison of blast-resistant capabilities of various designs with 4 kg TNT equivalent. (a) Seat mounts forces. (b) Crew pelvisacceleration.

where V1is the particle velocity in the foam nearest to the

vehicle floor, and it can be represented with the incident wavevelocities V

1𝑖and the reflected wave velocities V

1𝑟as

V1= V1𝑖+ V1𝑟. (14)

The resistive foam stress acting on the back facesheet (Alpanel, as shown in Figure 2) about elastic wave propagation𝜎𝑏𝑒is given as

𝜎𝑏𝑒

= 𝜌𝑓𝑐𝑒(2V2𝑖− V2) . (15)

And the resistive foam stress acting on the back facesheetabout elastic-plastic wave propagation 𝜎

𝑏𝑒𝑝can be expressed

as

𝜎𝑏𝑒𝑝

= (1 −𝑐𝑝

𝑐𝑒

)𝜎𝑦+

𝑐𝑝

𝑐𝑒

𝜎𝑏𝑒𝑝𝑖max − (𝜎

𝑏𝑒𝑝𝑖max − 𝜎𝑏𝑒𝑝𝑖

)

+ 𝜌𝑓𝑐𝑝V2𝑖max − 𝜌

𝑓𝑐𝑒(V2𝑖max − V

2𝑖)

− 𝜌𝑓𝑐𝑝V2max + 𝜌

𝑓𝑐𝑒(V2max − V

2) ,

(16)

where V2and V2𝑖are the particle and incident wave velocities

in the foam nearest to the Al panel, V2max and V

2𝑖max are


the maximum particle and incident wave velocities, and 𝜎𝑏𝑒𝑝𝑖

and 𝜎𝑏𝑒𝑝𝑖max are the incident andmaximum incident stresses,

respectively.The model uses the following inputs to calculate the

pressure: equivalent mass of TNT; coordinates of the pointof explosion; and the delay time between when the LS-DYNA solution starts and the instant of explosion.Themodeldoes not account for shadowing by the intervening objectsor the effects of confinement. In this work, the CONWEPempirical model is adopted for blast load prediction by virtueof its computational efficiency and acceptable accuracy. The∗LOAD BLAST card in LS-DYNA is utilized to apply blastloads to the frontal panel of the structure, with the optionISURF set to 1 to represent a hemispherical charge situatedon the surface.

3. Simulation, Numerical Results, andDiscussion

Consider a general case when a landmine is denoted directlyunder the left seat mount, at a distance of 0.4m belowthe vehicle floor (Figure 4). Two typical cases are simulatedwith explosive force equivalent to 2 kg and 4 kg of TNT,respectively.The purpose of these simulations is to predict thecrew injuries under such accidents and to evaluate theblast-resistant capabilities of the vehicle underbody armorstructure. The force transmitted to the seat mounts and theresultant acceleration of the crew member’s pelvic under theblast load are the two major indices for the evaluation.

3.1. Blast Load Alleviation Effect. Figure 5 is the simulationresults under the blast load with charge TNT equivalent of2 kg.The vehicle body structure deforms under the blast loadpressure, as shown in Figure 5(a).The transmitted load causesthe passenger seat to deform as illustrated in Figure 5(b).Thecrew body is accelerated due to the momentum transferredfrom the seat cushion as shown in Figure 5(c).

Compared with the baseline design without the under-body armor, the armored vehicle has a much reduced seatmount force, as shown in Figure 6(a), due to the blast energyabsorption of the aluminum foam through plastic deforma-tion.The crew’s resultant pelvis acceleration is thus verymuchreduced in the armored design than in the original design, asshown in Figure 6(b).

According to the simulation results, the aluminum foamsandwich underbody armor is effective in protecting the crewmember of amilitary vehicle in case the blast load is not abovecertain level.

3.2. Blast Load Enhancement Effect. In case the blast loadintensity increases to a certain level (in the example problem,a TNT equivalent of 4 kg) when the aluminum foam in thearmor is fully compacted, the so-called force enhancementphenomenonwill occur. Figure 6 shows that the resultant seatmount force (Figure 7(a)) and the resultant pelvis accelera-tion (Figure 7(b)) are even higher in the armored vehicle thanin the baseline design.

y

Vehicle floor

Al foam

Al panel

o

xy

z Seat mount

Figure 8: An innovative sandwich armor structure for blast-resistant.

Vehicle floor

Al panel

Contact width Contact widthAl foam

Isolating layer

Design I Design II

Figure 9: Design variations of sandwich armor structure withisolating layer.

4. Isolating Structure Design to EliminateForce Enhancement

It is found that the mass density and stiffness mismatchesbetween the metallic foam and the protected structure atthe interface cause an “abrupt” momentum transfer, at themoment of foam densification. This is the physical cause offorce enhancement.

In reality, a vehicle floor is designed for load bearingpurposes; the mass density and stiffness of the floor materialare usually not design variables for blast protection. To avoidpossible force enhancement when the metal foam enablesblast mitigation, alternative solutions are required. Introduc-ing an isolating structure layer between the main structureand the aluminum foam results in an innovative sandwicharmor structure as shown in Figure 8.

Figure 9 depicts the cross-sections of two different designconfigurations of the sandwich armor structure with isolatinglayer. The added weight of the isolating layer is the same forboth Design I and Design II. Note that the isolating layer hasless contact width (area) with the aluminum foam in DesignII than in Design I.

The transmitted seat mount forces in different designsare compared in Figure 10; force enhancement occurs whenthe foam layer was fully compacted under the blast load;Design I eliminates the peak force transmitted to the seatmount, while Design II, which has less contact area betweenthe isolating structure layer and the aluminum foam than


0

500

1000

1500

2000

2500

0.0 0.1 0.2 0.3 0.4 0.5Time (ms)


Armored (Design I)Armored (Design II)

Resu

ltant

seat

mou

nt fo

rce (

kN)

Figure 10: Comparison of seat mount forces in various designs.

0

2000

4000

6000

8000

10000

0.0 0.5 1.0 1.5 2.0 2.5 3.0Time (ms)

Resu

ltant

pelv

is ac

cele

ratio

n (m

/s2)

Baseline (no armor)Armored (foam only)Armored (Design II)

Figure 11: Innovative sandwich armor reduces crew injury.

Design I, reduces the force level even more. These resultswere further verified by examining the crew’s pelvis accel-erations as illustrated in Figure 11; with the introduction ofthe isolating layer in the sandwich armor structure and bydecreasing the contact area between the isolating layer andthe foam, the crew’s pelvis acceleration caused by the forceenhancement with the foam material under a blast load wasgreatly reduced. The crew’s vulnerability to pelvic injury waseffectively mitigated.

It is worth noting that the current work is a qualitativestudy of the proposed sandwich armor structure to show agood design strategy against force enhancement; a detailedstudy with design optimization is believed to give a morethorough and robust picture of the configurations of the blast-resistant armor structure including metal foams.

5. Conclusions

A system-level finite element model is developed whichincludes the blast load, the military vehicle body, the sand-wich armor, and the crew. Structure damages and crewinjuries under a landmine explosion event are successfullypredicted through numerical simulations using the devel-oped model. The blast-resistant performances of the alu-minum sandwich armor are evaluated based on the trans-mitted seat mount force and the resultant crew pelvicacceleration. It was found that a sandwich structure withonly aluminum foam is effective in mitigating a moderateblast load. An isolating layer between the aluminum foamand the protected structure is effective in preventing forceenhancement of foam material under severe blast loading.This innovative sandwich armor structure could be furtherdesigned for improved blast-resistant performances.

Acknowledgments

This work is funded by National Natural Science Foundationof China (nos. 50905024 and 51105053), Liaoning ProvincialNatural Science Foundation of China (no. 20102026), theResearch Fund for the Doctoral Program of Higher Educa-tion of China (nos. 20090041120032 and 20110041120022),and the Fundamental Research Funds for the Central Uni-versities (DUT13Lk47).

References

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