A Study on Dynamic Properties of Composite Metal Foams

by Afsaneh Rabiei and Vincent H. Hammond

ARL-CR-706 December 2012

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Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5069

ARL-CR-706 December 2012

A Study on Dynamic Properties of Composite Metal Foams

Vincent H. Hammond

Weapons and Materials Research Directorate, ARL

Afsaneh Rabiei

North Carolina State University

Approved for public release; distribution is unlimited.
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A Study on Dynamic Properties of Composite Metal Foams

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Afsaneh Rabiei* and Vincent Hammond

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U.S. Army Research Laboratory

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Department of Mechanical and Aerospace Engineering, Raleigh, NC 27695

14. ABSTRACT

Composite metal foam is a new class of closed cell metal foam offering higher energy absorption compared to the monolithic 

material at approximately one-third of the density. The superior energy absorption capability of composite metal foam makes 

this material a potential candidate for structural components such as automotive crash box and crumple zones. However, a 

systematic study on the dynamic properties of composite metal foams is necessary before they can be utilized in vehicular 

structures. It has been reported that the energy absorption of other metal foams will increase by increasing the strain rate up to 

4× at impact speeds of up to 200 m/s. In this study, the dynamic properties of composite metal foams under various impact 

speeds are obtained through a variety of experimental techniques. Subsequently, there results are compared to those published 

in the literature for other foams tested at various impact rates.

15. SUBJECT TERMS

composite metal foams, steel, high strain rate testing

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Vincent H. Hammond

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iii

Contents

List of Figures iv

List of Tables v

1. Introduction 1

2. Experimental Procedure/Materials and Method 1

2.1 Materials and Processing.................................................................................................1

2.2 Structural Analysis..........................................................................................................2

2.3 Quasi-Static and Dynamic Compression Testing Procedure ..........................................2

2.4 Split Hopkinson Pressure Bar..........................................................................................3

3. Results and Discussion 3

3.1 Structural Properties........................................................................................................3

3.2 Mechanical Properties.....................................................................................................6

4. Conclusions 7

5. References 8

List of Symbols, Abbreviations, and Acronyms 9

Distribution List 10
iv

List of Figures

Figure 1. (A), (B) and (C) Digital images of the composite foam with 2.2-, 4- and 5.2-mm 

sphere diameters, (D), (E) and F) corresponding SEM images of the area highlighted with 

the box in each image respectively. ...........................................................................................4

Figure 2. Surface features of hollow spheres (A), and the presence of limited debonding at 

the interface (B). ........................................................................................................................5

Figure 3. Stress-strain curve for samples with various sphere sizes under various loading 

speeds.........................................................................................................................................6

Figure 4. Comparison of the yield strength (left) and elastic modulus (right) of CMF at 

various speeds. ...........................................................................................................................7
v

List of Tables

Table 1. Physical properties and chemical composition of hollow spheres and the matrix 

material. .....................................................................................................................................2

Table 2. Composition of various features in 2.2-mm sphere CMF before and after 

processing. .................................................................................................................................4

Table 3. Compositional changes for selected elements after processing for foams with 

indicated sphere size. .................................................................................................................5
vi

INTENTIONALLY LEFT BLANK.
1

1. Introduction

Composite Metal Foam (CMF) is a cellular material developed at North Carolina State 

University consisting of a random loose arrangement of hollow metal spheres embedded in a 

metallic matrix. Quasi-static compression testing shows a uniform deformation behavior without 

the presence of localized collapse bands, resulting in a high plateau strength in the range of 

50–150 MPa depending on the material and processing methods (1). Composite metal foams 

have also been tested extensively under compression-compression fatigue, loading-unloading 

compression, and 4 point bending (2–8). However, their behavior at higher loading rates has 

never been tested. 

Excluding a singular exception (9), most of the studies on high strain rate loading of metal foams 

in the literature report an increased yield and plateau strength as the loading rate increases for 

foams made from aluminum (10–12), steel (13), and hollow sphere structures (14, 15). The 

strain rate effect on metal foams is explained as a combination of compressed air trapped inside 

the cells during impact, micro-inertial effects of the cell walls, and strain rate sensitivity of the 

cell wall material (12), although some differences exist in the literature as to which of these 

effects is dominant. A few studies have tested Al foams at higher speeds up to 210 m/s, and 

report an additional shock phenomena that dominates the foam’s behavior above a critical 

velocity (16, 17). This “critical” velocity appears to depend on foam properties, such as density 

(17), and for some foams it is reported as low as 50–55 m/s (18). In this study, steel composite 

foams, processed using powder metallurgy technique with a variety of spheres sizes, are tested at 

several loading rates up to 28 m/s using servo-hydraulic machine and Split Hopkinson Pressure 

Bar apparatus.

2. Experimental Procedure/Materials and Method

2.1 Materials and Processing

Steel composite foam specimens with various sphere sizes of 2.2-, 4- and 5.2-mm outer diameter 

were produced by powder metallurgy technique. The hollow spheres were produced at Hollomet 

in Dresden, Germany and their chemical composition and wall thickness are shown in table 1

along with chemical properties of the matrix powder. The matrix material for all static and 

dynamic compression specimens is 316L stainless steel powder produced by North American 

Hoganas High Alloys LLC with particle size sieved to –325 mesh (95%) and –200/+325 mesh 

(5%).
2

Table 1. Physical properties and chemical composition of hollow spheres and the matrix material.

Composite 

Foam 

Component

Chemical Composition Weight-Percent Sphere Properties

Fe C Mn Si Cr Ni Mo Wall thickness 

(μm)

5.2 mm spheres balance 0.87 0.07 0.34 17.09 12.60 2.12 275

4.0 mm spheres balance 0.58–

0.69

0.15–

0.07

1.14–

0.32

17.34–

16.48

12.28–

12.42

2.28–

2.11 225

2.2 mm spheres balance 0.68 0.13 0.82 16.11 11.53 2.34 110

Matrix material balance 0.030 2.00 1.00 16.00–

18.00

10.00-

14.00

2.00–

3.00 NA

Both spheres and powder are placed into a mold and vibrated at a frequency of 20 Hz for ~50 

min to achieve a dense packing between the spheres and matrix followed by sintering at 1200 °C 

in a vacuum hot press. More details of the powder metallurgy processing are available elsewhere 

(6). All sintered specimens were cut into blocks with a square cross section (24 × 24 mm or 

36 × 36 mm) using a Buehler Isomet 4000 linear precision saw. Sample sizes for mechanical 

testing were chosen such that at least 6 spheres are present in the cross-section to avoid any edge 

effects and all samples have a width to length aspect ratio of 1.75. Thin slices of foam samples 

were also cut for scanning electron microscopy imaging (SEM).

2.2 Structural Analysis

The surfaces of thin slice samples were ground using progressive sanding paper followed by 

polishing with a progression of diamond slurries. Digital images were taken of the foam surface 

macrostructure as well as individual hollow spheres from each batch. A Hitachi S-3200 

Environmental SEM equipped with Energy Dispersive Spectroscopy (EDS) was used to take 

higher magnification images of the composite foam’s microstructure using accelerating voltage 

of 20–30 keV, and EDS was performed at several locations to determine the chemical 

composition of various components in the microstructure. Image analysis software (Image-J) 

was used to calculate the percentage of the matrix area occupied by porosity. 

2.3 Quasi-Static and Dynamic Compression Testing Procedure

Both quasi-static and low speed dynamic compression tests were performed on a servo-hydraulic 

high rate test system with 300-mm crush depth at velocities up to 8 m/s and load capacity from 

60,000–100,000 lb dependent on the velocity. Crosshead displacement is recorded by a Linear 

Variable Differential Transformer (LVDT) that indicates the actuator position. Tests were 

performed on multiple specimens of each composite foam at four different crosshead 

displacement speeds: 0.01, 0.1, 1, and 8 m/s. One test at each speed was also performed on a 

steel hollow sphere foam (HSF) made by Hollomet with average cell diameter of 2.45 mm and 
3

specimen dimensions 23 × 23 × 40 mm for comparison. High speed video was recorded from 

each test at 500–10000 fps depending on impact speed. 

2.4 Split Hopkinson Pressure Bar

Another batch of stainless steel composite foam samples was tested using Split Hopkinson 

Pressure Bar (SHPB) technique. Cylindrical specimens were manufactured through the powder 

metallurgy technique described previously using 2.2-mm diameter stainless steel spheres and 

316L stainless steel powder, similar to those explained in section 2.1. The diameter of the SHPB 

specimens was 19 mm with 9.53-mm length for an L/D ratio of 0.5. The SHPB used 19-mm 

diameter aluminum incident and transmitted bars and a striker bar 177.8-mm long. Air pressure 

on the gas gun ranged from 50–100 psi to achieve strain rates up to 2770 s-1 (26.4 m/s). Data 

was reduced to calculate the stress-strain material response through the 2-wave analysis method 

and high speed video was recorded at 96,000 fps. 

3. Results and Discussion

3.1 Structural Properties

Figure 1 shows digital and SEM images of the composite foam samples used in this study. As 

can be seen, there is a uniform distribution of spheres in the matrix in all samples with some 

leftover pores in the matrix which is the nature of composite foams processed using powder 

metallurgy technique. Using image analysis, the porosity was measured to be in the range of 

10% to 13% for all samples.

The (SEM-EDS) of the sphere wall and the matrix showed the diffusion of the various alloying

elements in the 316L matrix such that sharp compositional gradients are minimized (table 2). 

Similarly, although not directly measured, the sharp gradient in carbon content for the spheres 

(minimum of 0.6 weight-percent) relative to the matrix (0.03 weight-percent) supports the 

outward diffusion of carbon from the spheres into the matrix. This combined elemental diffusion 

helped balance the compositional differences between the spheres and the matrix and lowered

the carbon content of spheres. As a result, the hardness of the composite foam was reduced 

while their ductility and energy absorption was improved.
4

Figure 1. (A), (B) and (C) Digital images of the composite foam with 2.2-, 4- and 5.2-mm sphere diameters, 

(D), (E) and F) corresponding SEM images of the area highlighted with the box in each image 

respectively.

Table 2. Composition of various features in 2.2-mm sphere CMF before and after processing.

Feature Fe C Mn Si Cr Ni Mo

Before 

processing

Hollow spheres Balance 0.68 0.13 0.82 16.11 11.53 2.34

316L powder Balance 0.03 2.00 1.00 16–18 10–14 2–3

After 

processing

Sphere wall Balance NA 0.21 0.77 15.54 12.49 1.92

Sphere/matrix interface Balance NA 0.23 0.283 16.46 13.85 1.57

Matrix Balance NA 0.07 0.31 16.73 14.17 1.12

Figure 2A presents a digital image of the outer shell of all spheres. As can be seen, the 2.2-mm 

spheres have a very smooth surface while some features can be seen on the surface of 4- and 

5.2-mm spheres. Despite the increased contact area between the matrix and spheres due to the 

greater surface roughness, these spheres have not bonded well to the matrix (figure 2B). This 

effect caused some spheres to be debonded from the matrix during mechanical loading. Further 

investigation is ongoing to change the matrix powder size for better filling those surface features 

on sphere walls, and to provide optimum bonding at the interface of the matrix with the sphere 

wall.
5

Figure 2. Surface features of hollow spheres (A), 

and the presence of limited debonding at 

the interface (B).

Table 3. Compositional changes for selected elements after processing for foams with indicated sphere size. 

Sphere Size Feature Fe Mn (Δ%) Cr (Δ%) Mo (Δ%)

5.2 mm spheres

Sphere wall Balance 0.10 (42.9%) 16.56 (–3.2%) 1.71 (–19.4%)

Sphere/matrix 

interface

Balance 0.19 15.99 1.62

Matrix Balance 0.26 (–87%) 15.59 (–8.3%) 0.84 (–66.4%)

4 mm spheres

Sphere wall Balance 0.30 (172.8%) 16.25 (–4.0%) 1.75 (–20.3%)

Sphere/matrix 

interface

Balance 0.30 15.93 1.80

Matrix Balance 0.38 (–81%) 15.07 (–11.4%) 2.18 

(–12.8%)

2.2 mm spheres

Sphere wall Balance 0.18 (38.5%) 16.05 (–0.4%) 1.95 (–16.7%)

Sphere/matrix 

interface Balance 0.23 15.57 1.87

Matrix Balance 0.31 (–84.5%) 15.4 (–9.5%) 2.14 (–14.4%)
6

3.2 Mechanical Properties

Figure 3 presents the stress-strain curve of composite foams with various sphere sizes tested 

under various loading speeds. Figures 4A and B compare the yield strength and modulus of 

elasticity of all samples at various speeds. As can be seen, both the yield and plateau strength of 

all composite foams is decreased by increasing the spheres diameter whereas an increase in the 

loading speed increased the yield and plateau strength of the material. As the result, the highest 

strength belonged to the composite foams made out of the 2-mm spheres tested under 25 m/s 

loading speed. In addition, a distinct increase is also observed in the plateau stress when the

response of the composite foams is compared with a sintered sphere-only perform of similar 

material and sphere dimensions. This shows the importance of the matrix in bonding the spheres 

together and reinforcing the sphere walls under loading. 

Figure 3. Stress-strain curve for samples with various sphere sizes under various loading speeds.

It is notable that an improvement of 80% to 140% is observed in the yield strength of the 

composite foam samples made with 2-mm hollow spheres and tested under SHPB. This is in 

agreement with previous reports that an increase in the loading speed can increase the yield and 

plateau strength of metal foams. Comparing the modulus of elasticity of the samples tested in 

servo-hydraulic testing machine shows a decrease in modulus with increasing loading speed 

from 0.01 to 8 m/s. However, results from those tests using Hopkinson Bar techniques show a 

completely opposite relationship, namely an increase in modulus with increasing loading speed. 
7

Figure 4. Comparison of the yield strength (left) and elastic modulus (right) of CMF at various speeds.

This can be related to the large compliance in the servo-hydraulic testing machine, particularly at 

the higher loading speeds. The compliance is also responsible for the wavy stress-strain curve in 

figure 3 at higher impact rates.

4. Conclusions

• Composite metal foams consisting of hollow steel spheres in 316L stainless steel matrix 

have been produced using powder metallurgy techniques.

• Microstuctural analysis indicated that the hollow spheres were uniformly distributed in the 

matrix. However, some residual porosity (10%–13%), as well as some matrix-sphere 

debonding, was also observed. 

• Elemental analysis indicated that manganese and nickel diffused from the matrix into the 

sphere wall. In addition, the large difference in carbon content for the spheres compared to 

the matrix suggests the outward diffusion of carbon from the spheres into the matrix. As a 

result, the hardness of the foam is reduced while the ductility and energy absorption was 

improved.

• High strain rate testing indicated that an increase in sphere diameter resulted in a reduction 

in the yield and plateau strength for the composite foam. For constant sphere diameter, 

increasing the loading speed acted to increase the yield and plateau strengths.

• In keeping with other studies reported in the literature, an increase in the yield and plateau 

strength was observed with increasing loading speed for the metal foam. In addition, the 

elastic modulus was also found to increase with loading speed in the Hopkinson Bar 

experiments.
8

5. References

1. Rabiei, A.; Vendra, L. J. Mater. Lett. 2009, 63, 533. 

2. Rabiei, A.; Neville, B.; Reese, N.; Vendra, L. Mater. Sci. Forum 2007, 1868.

3. Rabiei, A.; Vendra, L.; Reese, N.; Young, N.; Neville, B. P. Mater. Trans. 2006, 47, 2148.

4. Rabiei, A.; O’Neill, A. T. Mater. Sci. Eng. A. 2005, 404, 159.

5. Vendra, L. J.; Rabiei, A. Mater. Sci. Eng. A. 2010, 527, 1784.

6. Neville, B. P.; Rabiei, A. Mater. Des. 2008, 29, 388.

7. Brown, J. A.; Vendra, L. J.; Rabiei, A. Met. Mater. Trans. A. 2010, 41A, 2784.

8. Vendra, L. J.; Brown, J. A.; Rabiei, A. J. Mat. Sci. 2011, 46, 4574.

9. Deshpande, V. S.; Fleck, N. A. Int. J. Impact. Eng. 2000, 24, 277.

10. Mukai, T.; Kanahashi, H.; Miyoshi, T.; Mabuchi, M.; T. Nieh, G.; Higashi, K. Scripta 

Mater. 1999, 40.

11. Sadot, O.; Antegy, I.; Harush, S.; Levintant, O.; Nizri, E.; Ostraich, B.; Schenker, A.;

Gal, E.; Kivity, Y.; Ben-Dor, G. J. Struct. Eng. 2005, 131, 1226.

12. Dannemann, K. A.; Lankford, J. Jr. Mater. Sci. Eng. A. 2000, 293, 157.

13. Park, C.; Nutt, S. R. Mater. Sci. Eng. A. 2002, 323, 358.

14. Tasdemirci, A.; Ergonenc, C.; Guden, M. Int. J. Impact. Eng. 2010, 37, 250.

15. Zhao, H.; Elnasri, I.; Abdennadher, S. Int. J. Mech. Sci. 2005, 47, 757.

16. Lopatnikov, S. L.; Gama, B. A.; Haque, M. J.; Krauthauser, C.; Gillespie, J. W. Jr.;

Guden, M.; Hall, I. W. Comp. Struct. 2003, 61, 61.

17. Tan, P. J.; Reid, S. R.; Harrigan, J. J.; Zou, Z.; Li, S. J. Mech Phys.Solids. 2005, 53, 2174.

18. Elnasri, I.; Pattofatto, S.; Zhao, H.; Tsitsiris, H.; Hild, F.; Girard, Y. J. Mech. Phys. Solids.

2007, 55, 2652.
9

List of Symbols, Abbreviations, and Acronyms

CMF Composite Metal Foam

EDS Energy Dispersive Spectroscopy

HSF hollow sphere foam

LVDT Linear Variable Differential Transformer

SEM scanning electron microscopy imaging

SHPB Split Hopkinson Pressure Bar
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