Materials and Design 45 (2013) 555–560

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Materials and Design

journal homepage: www.elsevier .com/locate /matdes

Short Communication

Dynamic compressive behavior of aluminum matrix syntactic foam and itsmultilayer structure

L.C. Zou a, Q. Zhang a, B.J. Pang b, G.H. Wu a,c,⇑, L.T. Jiang a, H. Su a

a Institute of Metal Matrix Composite Science and Engineering, Harbin Institute of Technology, Harbin 150001, Chinab Hypervelocity Impact Research Center, Harbin Institute of Technology, Harbin 150001, Chinac Space Materials and Environment Engineering Lab., Harbin Institute of Technology, Harbin 150001, China

a r t i c l e i n f o

Article history:Received 9 May 2012Accepted 5 August 2012Available online 16 August 2012

0261-3069/$ - see front matter � 2012 Published byhttp://dx.doi.org/10.1016/j.matdes.2012.08.015

⇑ Corresponding author at: Institute of Metal MaEngineering, Harbin Institute of Technology, Harbin86412164; fax: +86 451 86402375.

E-mail address: Wugh@hit.edu.cn (G.H. Wu).

a b s t r a c t

The dynamic mechanical behavior of aluminum matrix syntactic foam was investigated by Split-Hopkinson pressure bar system in this paper. The aluminum matrix syntactic foam was fabricated bypressure infiltration technique, which had a porosity ratio of 45%. The energy absorption capability ofsyntactic foam exceeded 70% under dynamic loading versus under quasi-static loading. During the defor-mation process, the syntactic foam exhibited a well energy absorption capability due to the reduction oforiginal pores in syntactic foam accused by cenospheres rupture. Base on the excellent energy absorptioncapability of aluminum matrix syntactic foam, a better multilayer structure (consisted of A3 steel, syn-tactic foam and aluminum) as armor than traditional armor was designed in this paper. Besides, thedynamic mechanical behavior of aluminum matrix syntactic foam was also investigated. The results ofdynamic compression indicated that the syntactic foam can resist from the spread of impact wave dueto abundant pores in multilayer structure. Hence, the aluminum matrix syntactic foam is considerablysuit for the varied protective devices aerospace and automobile applications because of their highstrength–density ratio and excellent energy absorption ability.

� 2012 Published by Elsevier Ltd.

1. Introduction

Aluminum foam is a novel class of cellular material prepared byspecial methods. Owing to many pores, it offers potential for shock,impact, energy absorption, thermal management, acoustic absorp-tion [1–3]. Hence, the aluminum foam has been widely used inmodern industry. For example, the protection of multilayer struc-tures with aluminum foam was investigated by US’ Army in 2001[4]. But the aluminum foam is poor in the strength [5]. Recently,people attempt to find a better cellular material to replace it. In1990s, a novel composite was fabricated by mixing cenospheresinto aluminum, which was called ‘‘syntactic foam’’ [6,7]. The ‘‘syn-tactic foam’’ has higher compressive strength and energy absorp-tion than aluminum foams. Hollow Al2O3 ceramic particle and flyash can be used for reinforcement, compounding with differentmatrix such as aluminum, magnesium, plumbum and polymer, re-sults in syntactic foams with various density and strength [8–10].Specially, compared to the traditional metal matrix compositesfabricated by mixing ceramic into metal [11–14], the hollow flyash of syntactic foam is key reinforcement.

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In the past, the studies mostly focused on the quasi-static com-pressive behavior of syntactic foams, only a few work investigatedthe dynamic behavior of syntactic foams [15,16]. Moreover, no lit-erature had reported the application of syntactic foams on energyabsorption today. Hence, the main purpose of this paper was tostudy energy absorption capability of syntactic foams under dy-namic loading. Dynamic compressive and quasi-static tests werecarried out to examine the compressive response of syntacticfoam in this paper. And base on the excellent energy absorptioncapability of syntactic foams a multilayer structure consisting ofthe syntactic foams was designed as an armor and tested by SHPB(Split-Hopkinson pressure bar) system to evaluate the resist ofimpact wave.

2. Materials and experiment

2.1. Materials

In this paper syntactic foams were fabricated by pressure infil-tration technique, described in detail elsewhere [17]. The matrix ofcomposite was 6061 aluminum. The cenospheres, which had aver-age diameters of 200 lm, were extracted from the original fly ashwaste to serve as fillers. Additionally, the porosity ratio of ceno-spheres in the syntactic foams was about 45%.

Fig. 1. Schematic diagram of multilayer structures consist of the syntactic foam.

Table 1Component of the multilayer structure.

A3 steel Syntactic foam Aluminum alloy

Size (mm) u8 � 5 u8 � 3 u8 � 8Density (g/cm3) 7.8 1.6 2.7

556 L.C. Zou et al. / Materials and Design 45 (2013) 555–560

2.2. Multilayer structure

The components of multilayer structure are A3 steel, aluminummatrix syntactic foams and aluminum alloy respectively, in orderto imitate the multilayer structures of armor. The structures ofthe multilayer structures are given in Fig. 1. The size and densityof all components are listed in Table 1.

2.3. Experiment

Quasi-static compressive tests were performed on Zwick uni-versal testing machine according to GB/T 7314-2005 standard[18] for metallic materials compressive test at room temperature.The samples were £15 � 10 mm. The quasi-static compressivetests were carried out with a constant crosshead speed of 1 mm/min. Strain was calculated from the recorded crosshead displace-ment, corrected for deflection of the load frame.

Dynamic compressive tests were performed on SHPB apparatus,as given in Fig. 2. A typical SHPB system consists of two slendercompression bars, a short impact bar, strain gauges and equipmentfor recording the stress wave. The diameters of the incident barand transmission bar are both 12.7 mm, and the length of the inci-dent bar and transmission bar are both 1000 mm. In this paper, theincident and transmission bars were made of 7075 aluminum al-loys when the syntactic foam was examined. The samples in dy-namic compressive test were £7 � 3 mm. However, the incidentand transmission bars were replaced by 45 steel when the multi-layer structure was examined.

Above all tests were accomplished at room temperature. Toguarantee the available of data, three samples are tested for every

Fig. 2. Schematic diagra

strain rate. Besides, we also got two repetitive data in three exper-iments of each sample, at least.

3. Results and discussion

3.1. Microstructures

Representative optical micrographs of the syntactic foams (byOLYMPUS SZX12) are shown in Fig. 3. As displayed in the figure,the microstructure of the syntactic foams is homogeneous, andcenospheres are uniformly distributed in the matrix.

Fig. 4 shows the central parts in cross section of samples aftercompression. The central part mainly consists of ruptured ceno-spheres, matrix and residual pores. For better analyze the dynamicmechanical mechanism of aluminum matrix syntactic foam, re-fined investigation on microstructure in the cross section of sam-ples after compression reveals the following features. All samplesdeform largely, with a distortion of 70% under quasi-static loading.

Syntactic foam has changed dense caused by ruptured ceno-spheres after quasi-static compression, but a few pore still re-mained in the syntactic foam. Some pores joint and form a‘‘crack’’ in some areas near the top surface and bottom surface(pointed by arrows in figure), because the stress in these areas ishigher than other areas. Due to the low deformability of matrix, to-gether with ceramic enhancements and brittle, the syntactic foamcould not deform uniformly.

In detail, Fig. 4b and c shows that the deformation ratio of sam-ple are 50% and 65% respectively at the strain rate of 2650 s�1 and3350 s�1, after dynamic compression. All the pores have been rup-tured lead to dense syntactic foam after deformation at the strainrate of 3350 s�1. However, fortunately, a few pores are still keptin the syntactic foam after deformation at the strain rate of2650 s�1.

3.2. Mechanical properties

Stress–strain curve represents the compressive response of syn-tactic foam under dynamic and quasi-static loading. All curves inFig. 5 shows that the characteristic compressive stress–straincurves of syntactic foam are similar as other metallic foams, com-prising three deformation three stages: (I) elastic deformationstage; (II) plateau stage; (III) densification stage.

At the beginning of the compressive deformation under quasi-static loading, stress ascends linearly with increasing strain, andthe relation of stress–strain absolutely complies with Hooke’slaw. Base on above contents, we can infer that the elastic deforma-tion occurs in this stage. After reaching peak stress, stress is nearlyconstant when a long plateau of plastic deformation occurs underquasi-static loading. In contrast, stress increases a little by the

m of SHPB system.

Fig. 3. Microstructures of the syntactic foam.

L.C. Zou et al. / Materials and Design 45 (2013) 555–560 557

rising of strain until a densification strain under dynamic loading.The reason for this ‘‘plateau region’’ is the original pores inside areoccupied by the matrix after broken cenospheres. When mostpores have been filled by aluminum, syntactic foam deformsmainly by the plastic deformation of the aluminum matrix andthe deformation goes into densification stage. So stress increases

Top surface

Bottom surface

(a)

(c)

(b)

(d)

(e)

Fig. 4. Microstructures after compression at different strain rates. (a) At a strain rate of 1eof 2650 s�1. (e) At a strain rate of 3350 s�1.

rapidly in densification stage like the traditional stress–straincurve of aluminum alloys. The strain corresponding to intersectionof the tangents to the plateau region and densification stage regionwas defined as densification strain.

The strength under dynamic loading is obviously higher thanthat under quasi-static loading as displayed in Fig. 5. Respectively,the peak stress and the plateau stress under quasi-static loadingare 44.9 MPa and 40.5 MPa. Besides, the peak stress at differentstrain rate under dynamic loading is almost no difference. Then,sample deformed at almost a constant stress until densificationstain. The densification strain at a low strain rate is higher thanthat at a high strain rate. The deformation rate is low under qua-si-static loading, so that there is enough time for the plastic defor-mation of the matrix. Moreover, the deformation time for sampleunder different loading method is apparently distinct.

Fig. 6 shows a representative set of incident and transmitter sig-nal collected in SHPB test. The first pulse is from the incident wavewhen stress wave went through the incident bar. After the incidentstage, the stress wave arrived at the surface of syntactic foam. Con-sequently, one part of the stress wave passed into the transmissionbar, this part of stress wave is recorded as the transmitted pulse.The rest of the incident stress wave were reflected in incidentbar and recorded as the reflected pulse. The stress–strain curves,strain rate curves and energy absorption efficiency curves are allcalculated by the three pulses (incident pulse, transmitted pulseand reflected pulse) with a specific method.

It is well known that, the strain rate sensitivity is used to eval-uate the strain rate effect of materials between quasi-static and dy-namic loading [19]. The sensitivity parameter R is defined as:

�3 s�1. (b) At a strain rate of 0.01 s�1. (c) At a strain rate of 0.1 s�1. (d) At a strain rate

Fig. 5. Mechanical response of syntactic foam.

Table 2Strain rate sensitivity of the syntactic foam.

ed eq r⁄ rd rq R

2650 s�1 1e�3 40.5 47.9 44.9 0.0053350 s�1 1e�3 40.5 55.9 44.9 0.018

Table 3Energy absorption of the syntactic foam.

eD W (MJ/m3) gmax (%)

1e�3 0.471 18.30 0.842650 s�1 0.433 28.49 0.9503350 s�1 0.429 32.87 0.917

558 L.C. Zou et al. / Materials and Design 45 (2013) 555–560

R ¼ rdrq

r�1

lnð _ed= _eqÞð1Þ

where r is the stress at a given strain, r⁄ is the stress when thestrain is 0.05 at a strain rate of 10�3 s�1, e is the strain rate, andthe subscripts ‘‘d’’ and ‘‘q’’ refers to the dynamic and quasi-staticloading. The value of parameters in the function is listed below,so as the calculated value of R. Table 2 suggests that the strain ratesensitivity at strain rate 3350 s�1 is higher than that at 2650 s�1,strain rate effect is the reason.

There was studies reported the strain rate sensitivity of 6061aluminum was 0.01 approximately [19]. The strain rate sensitivityof 6061 aluminum means the flow stress alters by the raising ofstrain rate. The R parameters at the strain rate of 2650 s�1 and3350 s�1 are 0.005 and 0.018 respectively, these are so closed tothe R parameters of the 6061 matrix. This result is consistent withthe work of Balch et al. [20], who reported that the strain rate sen-sitivity of syntactic foams was close to that of relevant matrix.Thus, we can infer that the strain rate sensitivity of the matrixmaterial induces the rate sensitivity of the syntactic foams. How-ever, there may be other potential reasons (i.e. the micro-inertia ef-fects) for rate sensitivity of the syntactic foams and this will beinvestigated by future experimentation.

The energy absorption capacity of materials is evaluated by thequantity of energy absorption and the absorption efficiency. Theparameters W and g are defined as follows [21,22]:

Fig. 6. Stress wave

g ¼R e

0 rdermaxe

ð2Þ

W ¼Z eD

0rde ð3Þ

where r is the stress at a given strain, e is the strain rate, rmax is themaximum stress, eD is the densification strain. All results calculatedare listed in Table 3.

Apparently, the energy absorption capacity is higher under dy-namic loading than that under quasi-static loading, as shown in Ta-ble 3. The higher strength results in the better energy absorptioncapacity under dynamic loading. Under dynamic loading, the strainrate is higher; the energy absorption capacity is stronger. Gener-ally, the energy absorption capacity of syntactic foams ranged from15 to 80 MJ/m3 generally [17,21,23,24], while the energy absorp-tion capacity of aluminum foams is usually below 53 MJ/m3 underdynamic loading [25]. In this paper, the higher relative density andload bearing of ceramic cenospheres lead to syntactic foams haveso stronger energy absorption capability than aluminum foams.

3.3. Dynamic compression of multilayer structure

In order to better apply this syntactic foam, a multilayer struc-ture like armor has been designed in this paper base on the greateffective energy absorption capacity of syntactic foam. Themultilayer structure like armor has also been investigated bySplit-Hopkinson pressure bar system. Fig. 7 shows the stress wavecollected by oscillometer during the dynamic compressive test. Thefirst pulse is the wave shape when stress wave went throughthe incident bar. Following that stage the stress wave arrived at

in SHPB test.

Vol

tage

transmitted pluse

reflected pluse

incident pluse

0 500 1000 1500 2000-3

-2

-1

0

1

2

3

Time, us0 500 1000 1500 2000

-3

-2

-1

0

1

2

3

Vol

tage

Time, us

reflected pluse

transmitted pluse

incident pluse

(a) (b)

Fig. 7. Stress wave collected by oscillometer in the dynamic compression test. (a) Multilayer structures. (b) 2024 Aluminum.

Fig. 8. Mechanical response of multilayer structure under dynamic loading.

L.C. Zou et al. / Materials and Design 45 (2013) 555–560 559

the surface of multilayer structure. Part of the stress wave wentthrough the multilayer structure and then passed into the trans-mission bar, recorded as the transmitted wave. The rest of the inci-dent wave were reflected and recorded as the reflected wave.

As shown in Fig. 7a and b, compared to the wave sharp of 2024aluminum, the transmitted wave of multilayer structure is obvi-ously weak, while the reflected wave is strong. It illustrates thatmost impact wave have been reflected, only a short amount wentthrough multilayer structure. Wave impedance is a multiplicationof density and wave velocity. The syntactic foam has weak wave

Fig. 9. Photograph after compression. (a) M

impedance due to the low density. When stress wave arrive atthe surface of A3 steel and syntactic foam, most wave are reflectedback due to the difference of wave impedance. Therefore, only afew waves could arrive the surface of syntactic foam and alumi-num, subsequently go through aluminum.

Fig. 8 shows the stress–strain relation calculated from the pulsein Fig. 7. The stress in Fig. 8 is calculated by the wave pulse col-lected from transmitted bar. Compared to the stress–strain curveof syntactic foam, the wave pulse collected from transmitted baris weaker. It proves that the stress wave was prevented by multi-layer structure.

The whole dynamic compressive process is recorded by the highspeed camera. Fig. 9 is the macromorphology photograph of sam-ples after compression. The syntactic foam has been compressedand is sunk. It suggests that the syntactic foam absorbs impact en-ergy and resists the impact wave by the deformation itself, so thatthe syntactic foam can effective prevent the back plate fromdamage.

4. Conclusions

The dynamic mechanical behavior of aluminum matrix syntac-tic foam and its designed multilayer structure has been analyzedusing SHPB system. The main conclusions of this study are asfollows:

1. Both peak stress and plateau stress are over 50 MPa underdynamic loading. In contrast, both peak stress and plateaustress are below 50 MPa under quasi-static loading. The energyabsorption capability of syntactic foam exceeds 70% underdynamic loading versus under quasi-static loading.

ultilayer structures. (b) Syntactic foam.

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2. The cenospheres were ruptured and condensed, so that the ori-ginal pores were occupied by aluminum under a constant load-ing. It is the reason that the syntactic foam has high energyabsorption.

3. Designed multilayer structure in this paper can be used asarmor in future. Because it can resist from impact wave andprotect the back plate under the dynamic compression.

Acknowledgement

The authors gratefully acknowledge financial support from Na-tional Natural Science Foundation of China (No. 51001037).

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