Study of blast load on recyclable empty metal cans

S. Palanivelu1, W. Van Paepegem1, J. Degrieck1, K. De Wolf2,

J. Vantomme2, D. Kakogiannis3, J. Van Ackeren3, D. Van Hemelrijck3

and J. Wastiels3

1 Department of Materials Science and Engineering, Ghent University, Sint-Pietersnieuwstraat41, 9000 Gent, Belgium2 Royal Military Academy, Civil and Materials Engineering Department, Building G, Level 0,8 Av. Hobbema, 1000 Brussels, Belgium3 Department of Mechanics of Materials and Constructions, Vrije Universiteit Brussel,Pleinlaan 2, 1050 Brussels, Belgium

Abstract. Close range free air blast tests have been conducted to understand the energy absorptionbehaviour of a single recyclable empty beverage can (cola can) available in the market. The idea is tomake a macro-foam (sacrificial cladding structure) out of these cans to protect the main load bearingmembers of civil engineering structures from the air blast load. To conduct such an air blast test aspecial small-scale test set-up was designed and manufactured. The effect of inertia of outer skinpanel on the energy absorption of the core members and the influence of finite surface area of outerskin panel on the clearing of reflected pressure waves were studied. The measured blast parametersfrom the experimental tests are compared with CONWEP predicted data. In order to understand thecrushing mechanism and the energy absorption of a single beverage can in detail a numerical simula-tion using Johnson-Cook material model was carried out. The commercially available ABAQUSV6.7-3 Explicit code was used for this study. Finally the results (deformation pattern and the corre-sponding energy absorption) from the numerical study are compared with the experimental results.

1. INTRODUCTION

The entire world faces the security threat of industrial, military and civil engineering structuresdue to terrorist activities. Explosion on these structures can cause tremendous damages andfailures. The failure of the critical load bearing members such as beams, pillars, columns etc., andits debris cause major human casualties. Hence, a preventive solution is needed to safeguard thecivil engineering structures and to avoid human casualties due to explosion. Researchers havebeen working with different approaches to achieve an elegant solution for this problem. Out ofmany proposed solutions, the concept of sacrificial cladding design [1–4] has attracted moreattention in terms of its functionality and its predictable behaviour. A sacrificial structure canhave two parts; an outer skin and an inner core. The function of the outer skin is to distribute theblast pressure more evenly to the inner core so that the inner core can absorb most of the energyfrom the blast by deformation. The proposed sacrificial structure using the recyclable emptybeverage cans and its macro foam assembly are shown in Figure 1. Before designing such a full-scale sacrificial cladding structure the knowledge of the energy absorption of an individualsacrificial member is important. To understand the energy absorption and the deformationpattern of an individual beverage can (cola can), a small-scale blast set-up was designed andmanufactured. Often during the design of a sacrificial cladding structure, the inertia of the outerskin panel is not considered. To transfer the uniform pressure more evenly to the inner core theskin panel should have adequate bending stiffness and low inertia. The effect of inertia of the outerskin panel on the energy absorption of empty cola cans was studied by varying the mass of the

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outer skin panel. Normally during an analysis of a blast event, the blast surfaces are considered tobe an infinite surface, which will not allow any diffraction of pressure waves. Furthermore, topredict the energy absorption of such sacrificial members an equivalent triangular blast wave isconsidered [1–4]. However, this approach will overpredict the efficiency of the sacrificial members.Hence, the effect of clearing of reflected pressure waves was addressed with finite surface area ofouter skin panels. Finally, a tool is necessary to transfer the knowledge of the energy absorption ofindividual sacrificial member to a full-scale sacrificial cladding structure design. The finite elementtool can be adopted for such application [5, 6]. So the finite element model of a blast simulationwas developed using commercially available ABAQUS V6.7-3 Explicit code. In order to capturethe high strain rate loading of test specimens (empty cola cans), the Johnson-Cook material modelwas used and the results are compared with the experimental data.

2. EXPERIMENTAL TESTING AND RESULTS

2.1 Test specimen and experimental test set-up

The used recyclable empty cola cans were utilized for this experimental study. Special care wastaken to choose the cans without major defects such as indents and scratches on those during theusage. The material and geometrical details of an empty cola can are shown in Figure 2(a). Theschematic representative of a small-scale air blast test set-up, which was used for conducting the

Proposed sacrificial cladding structure

Figure 1. Proposed sacrificial cladding structure and macro foam assembly of recyclable empty beveragecans.

a b

Figure 2. (a) Geometry and material details of an empty cola can. (b) Schematic view of experimental set-up.

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air blast tests, is shown in Figure 2(b). An empty cola can was mounted on a resting plate belowwhich a dynamic load cell was connected to measure the transferred impulse to a solid coppertube. The solid copper tube, which represented the non-sacrificial member, was rigidly fixed to aheavy metal panel. The top cover, which represented the outer skin panel of the sacrificial claddingstructure was mounted over the empty cola can. The outer skin panel was fully instrumented withan accelerometer and a pressure sensor to measure the acceleration and the reflected pressure fromthe outer skin panel respectively. The outer skin panels were made with aluminium and sandwichcomposite materials. To simulate the real condition of the blast loading and to avoid the effect ofthe side pressure on the test specimen, a side cover tube made of flexi-glass was attached to theouter skin panel. When a test specimen deforms the side cover tube can slide on the solid coppertube. Semi-circular cross sectional vents were made at three locations on the copper tubethroughout its length to eliminate the resistance offered by the air inside the set-up during the blastloading. In order to understand the crushing behaviour of a single empty cola can, a series of closerange free-air blast tests have been conducted with 20 g of C4 and a stand-off distance of 30 cm.The charge was made spherical in shape. To achieve a zero incidence angle of the pressure waves,the charge was placed perpendicular to the centre of the outer skin panel surface.

2.2 Test results and discussion

2.2.1 Deformation patterns and the effect of outer skin panel inertia on the energy absorption

The deformation patterns of empty cola cans with different masses of outer skin panels (1.382, 0.54and 0.36 kg) are shown in Figure 3. Based on the thickness distribution of an empty cola can it canbe concluded that the initial deformation should occur at the mid-wall location of the can due to itslower thickness. However, it can be noticed from Figure 3 that the initial deformation occurred atdifferent points of the can along its length. The difference in the deformation patterns of the cansmay be influenced by the initial geometry imperfections, which originated during themanufacturing process and during usage. Test specimens with lower inertia of outer skin panels(0.54 and 0.36 kg) showed a uniform formation of lobs around their circumference. The effect ofthe mass of outer skin panel on the deformation length of the specimens can be clearly noticed fromFigure 3 and Figure 4(a). The average maximum deformation length achieved with 1.382 kg ofmass of outer skin panel was 6.435 mm. The sandwich composite skin panels with 0.54 and 0.36 kgyielded average deformation values of 45.2 and 60 mm respectively. Furthermore, the peak crushload of the test specimens was increased with reducing outer skin panel mass (Figure 4(a)).

2.2.2 Reflected pressure profiles and their clearing effect

When the air blast waves encounter an obstacle it is reflected with a magnitude much higher thanthe incident pressure. Few commercial tools (CONWEP, BlastX etc.,) are available to predict the

With outer skin panel mass of 1.382 kg With outer skin panel mass of 0.54 kg With outer skin panel mass of 0.36 kg

Figure 3. Deformation patterns of test specimens with different outer skin panel masses.

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reflected blast parameters for a given set of input values. However, the fully reflected blastparameters given by these programs do not include the effects of clearing associated with a limitedsize of reflected surface. Figure 4(b) shows the experimentally recorded maximum and minimumpressure-time profiles with one of the outer skin panels (mass 0.54 kg; reflected surface area0.05 m2). For the simplification of the analysis only the positive phase is considered. As we statedearlier, these pressure profiles can be approximated as a triangular pressure pulse using Equation(1). However, it can be noticed in Figure 4(b) that the peak reflected pressures suddenly decayed tothe incident peak pressure at time “tc”. The clearance time of pressure waves [7] can be calculatedby using Equation (2), based on the smallest distance at which the pressure wave passes around theedges of the top skin panel (S) and the blast front velocity (Us). This clearing of reflected pressurewaves can be modeled by using Equation (3). Designing a sacrificial member using Equation (1)will overpredict the efficiency of that member. Furthermore, the clearing effect can be understoodfrom the comparison between the CONWEP predicted and the experimentally measured data,which are given in Table 1. It can be noticed that there is a significant difference in the blastparameters.

PðtÞ ¼ PRmax 1� t

tp

� �ð1Þ

tc ¼ 3:5

Us

ð2Þ

a b

Figure 4. (a) Load-deformation histories of empty cola cans with different outer skin panel masses.(b) Clearing of reflected pressure waves with finite surface area of outer skin panel.

Table 1. Comparison of CONWEP predicted and experimentally measured blast parameters.

Mass ofC4 in g(mc)

Stand-offdistance incm (Z)

Reflectedsurface area ofouter skin pa-

nel (m2)

Mass ofouter skinpanel(kg)

MaximumIncident

pressure in bar(Pmax

1 )

Maximumreflected

pressure inbar (Pmax

R )

Positiveduration

(tp)

CONWEP predicted values20 30 - - 8.986 47.55 0.3701

Experimentally measured values20 30 0.0314 1.382 5.25 28.19 0.173820 30 0.0314 1.382 5.42 30.02 0.121220 30 0.05 0.54 5.01 65.53 0.263120 30 0.05 0.54 5.5 69.02 0.291020 30 0.05 0.36 5.9 57.45 0.205020 30 0.05 0.36 Not measured 55.94 0.2291

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PðtÞ ¼ PRmax þ Pt

max � PRmax

tc

� �� t

� �; 0jtjtc

PðtÞ ¼ Ptmax þ �Pt

max

tp � tc

� �� ðt� tcÞ

� �; tc<titp

PðtÞ ¼ 0; otherwise:

ð3Þ

3. NUMERICAL SIMULATION AND COMPARISON OF RESULTS

3.1 Salient features of numerical modeling

In order to understand the crushing phenomena of empty cola cans in detail, a numericalsimulation was carried out in commercially available ABAQUS V6.7-3 Explicit code [8]. Theempty cola can was modeled with shell elements as per the geometry shown in Figure 2(a). Theouter skin panel and the bottom support plate were modeled as rigid plates. The measured time-dependent reflected pressure amplitude was applied to the outer skin panel. A surface-to-surfacecontact algorithm was established between the top rigid surface (outer skin panel) and the emptycola can with a friction coefficient of 0.2. To simulate the blast load only in vertical direction, apartfrom the vertical translation, all degrees of freedom of the top analytical rigid body were arrested.To represent the fixed support plate at the bottom, all the degrees of freedom of the bottom rigidbody were also arrested. The deformation of the can was captured from the displacement of thetop rigid surface and the reaction force was extracted from the interaction force between theempty cola can and the bottom rigid surface. The element length of 1 mm was used for all theanalysis.

3.2 Material model

In order to capture the effect of strain rate on the peak and mean crush load of the empty cola cansthe Johnson-Cook material model was used (Equation (4)). The material of the cola can wasassumed as steel grade 4340 and the corresponding material parameters were adopted from [9].

sy ¼ ðAþ B�epnÞ½1þ C ln _e��½1� T�m� ð4Þ

e ¼ �ep

ep¼ effective plastic strain rate; T� ¼ T� Troom

Tmelt � Troom

A, B, C, n and m are material constants and �ep is effective plastic strain.

3.3 Comparison of results

The numerical deformation patterns of an empty cola can with outer skin panel masses of 1.382and 0.54 kg are shown in Figure 5(a). It can be noticed that these deformation patterns can be verywell compared with the experimental deformation patterns (Figure 3). Similarly a goodcorrelation of the energy absorption was observed. As an example, a comparison of the casewith the outer skin panel mass of 1.382 kg is shown in Figure 5(b).

From Figure 5(b), the peak crush load predicted by the numerical analysis was slightly lowerthan the experimental value. In contrast the total deformation length was 1.5 mm more than theexperimental value. This may be due to the non-consideration of the outside aesthetic and insidelacquer coating in the numerical model. However, to conclude this statement an extensive

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experimental testing may be required on the bright cans (cans without aesthetic and lacquercoatings). The results from these analyses showed that this finite element model can be extendedfor the design of full-scale sacrificial cladding structure with the empty cola cans as inner coremembers.

4. CONCLUSIONS

This paper demonstrated the importance of consideration of inertia of outer skin panel on theenergy absorption of inner core members of a sacrificial cladding structure. Furthermore, itproved that the consideration of clearing of reflected pressure waves to avoid the over predictionof the efficiency of the energy absorption of inner core members. The blast energy absorption of asingle recyclable empty beverage can (cola can) was studied in detail by a small-scale blast test set-up. A finite element model was developed in ABAQUS V6.7-3 Explicit code using Johnson-Cookmaterial model. A good correlation was observed between the numerical and experimental data.Finally, this study concludes that the recyclable beverage cans can be considered as a potentialmember to protect the civil engineering structures from the air blast load.

Acknowledgments

The authors gratefully acknowledge the financial support of the “Fund for Scientific Research” – Flanders(F.W.O) (Grant No: B-07674-02). The authors also wish to thank Luc Van Den Broecke (Ghent University)and Bruno Reymen (Royal Military Academy) for their assistance in manufacturing the test setup andconducting the blast experiments.

References

[1] Guruprasad S. and Mukherjee Abhijit, Layered sacrificial claddings under blast loading Part I– analytical studies. International Journal of Impact Engineering. 24(2000): pp. 957-973.

[2] Guruprasad S. and Mukherjee Abhijit, Layered sacrificial claddings under blast loading PartII – experimental studies. International Journal of Impact Engineering. 24(2000): pp. 975-984.

[3] C. Kotzialis, C. Derdas and V. Kostopoulos, Blast behaviour of plates with sacrificial cladding.5th GRACM International congress on computational mechanics, (2005).

a b

Figure 5. (a) Numerical deformation patterns of test specimens (empty cola cans). (b) Comparison of energyabsorption.

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[4] Hanssen A. G., Enstock L. and Langseth M., Close-range blast loading of aluminium foampanels. International Journal of Impact Engineering. 27(2002): pp. 593-618.

[5] Karagiozova D., Nurick G. N. and Chung Kim Yuen S., Energy absorption of aluminiumalloy circular and square tubes under an axial explosive load. Thin-Walled Structures.43(2005): pp. 956-982.

[6] Nurick G. N., Olson M. D., Fagnan J. R. and Levin A., Deformation and tearing of blast-loaded stiffened square plates. International Journal of Impact Engineering. 16(1995): pp. 273-291.

[7] P. D. Smith, J. G. Hetherington., Blast and ballistic loading of structures. Butterworth-Heinemann Ltd., (1994).

[8] ABAQUS User manual. ABAQUS, Inc. and Dassault Systemes (2007).[9] Gordon R. Johnson, William H. Cook., Fracture characteristics of three metals subjected to

various strains, strain rates, temperatures and pressures. Engineering fracture mechanics.21(1985): pp. 31-48.

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