Specific properties and fracture toughness of syntactic foam: Effect of foam microstructures

COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 65 (2005) 1840–1850 www.elsevier.com/locate/compscitech

Specific properties and fracture toughness of syntactic foam: Effect of foam microstructures Erwin M. Wouterson a, Freddy Y.C. Boey a, Xiao Hu a, Shing-Chung Wong a

b,*

School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b Department of Mechanical Engineering, University of Akron, Akron, OH 44325-3903, USA Received 14 March 2005; accepted 23 March 2005 Available online 28 April 2005

Abstract Studies were performed on the specific strength, moduli and fracture toughness of varied microstructures of syntactic foam. The different microstructures were created by using three different types of microspheres, namely 3M Scotchlite K15 and K46 glass bubbles, and Phenoset BJO-093 hollow phenolic microspheres, and by changing the volume fractions of microspheres from 0 to 50 vol%. Tension, compression, flexural and fracture tests were performed. Results showed that the tensile and flexural strengths decreased with increasing filler content. The behavior of the tensile and flexural strength was not affected by the component microspheres. Interestingly, the tensile and flexural moduli showed different trends for each type of microspheres with increasing filler content. Results of the compression tests revealed superior behavior of the high density microspheres. The specific fracture toughness data yielded maximum values at 30 vol% for each type of microspheres investigated. Scanning electron microscope studies were performed to determine the failure mode for each loading condition.
2005 Elsevier Ltd. All rights reserved. TM

Keywords: Syntactic foam; Mechanical properties; Fracture toughness

1. Introduction Syntactic foam is a ternary material system made in a mechanical way by mixing hollow particles (the filler) with a resin system (the binder). The hollow particles may be made of polymer, ceramic, carbon, or metal. Most often thermoset resins are used as the binder. Dispersion of the hollow particles creates a porous material with closed cells. By changing the amount of hollow filler particles, different densities and thus microstructures of syntactic foam can be created. Syntactic foams are known to possess low density, high stiffness, excellent compressive and hydrostatic strength, and good impact behavior [1–6]. Unlike most *

Corresponding author. Tel.: +1 330 972 8275; fax: +1 330 972 6027. E-mail address: [email protected] (S.-C. Wong). 0266-3538/$ - see front matter
2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2005.03.012

other foams, syntactic foam is a material whose density before curing is the same as that after curing. Such predictability is advantageous in the manufacturing process in aerospace structures. Using hollow particles, having a lower density compared to the binder material, allows for the manufacturing of light-weight materials with the increase of the filler content. This type of syntactic foam with a filler density that is lower compared to the binder can be considered as a special type of particulate-filled polymer composite (PFPC). Generally, the weight of a PFPC increases with increasing filler content as solid filler particles are most often used. The mechanical and fracture behaviors of the PFPC were studied extensively [7]. The moduli and fracture properties often improve with increased solid filler content, given an intrinsically brittle matrix system and good interfacial bonding between the filler and the matrix.

E.M. Wouterson et al. / Composites Science and Technology 65 (2005) 1840–1850

Most studies on the mechanical and fracture properties of syntactic foams are based on the maximum filler content of microspheres as this elicits the lowest possible weight of the composites [1–3]. This study investigates the influence of different compositions of syntactic foam on its specific mechanical and fracture properties. A comprehensive understanding of the structure–property relationship is lacking. Different compositions will be created by varying the type and volume fraction of microspheres. This route is different from that reported by Gupta et al. [4] whereby different microstructures were created by using different types of microspheres with similar size distributions. However, only the volume fraction of microspheres was kept constant for each type of microspheres. Bunn and Mottram [5] also reported on the effect of the volume fraction on phenolic microspheres on the compressive properties. The present paper compares the results derived from phenolic microspheres to those made of glass shells. It is expected the interfacial adhesion between phenolic microspheres and polymers could be stronger. It is essential that specific properties be discussed in this paper because the density of syntactic foam changes with the amount of microspheres introduced. Specific properties allow for the comparison of the performance of syntactic foams to other potential foam materials in sandwich composites such as PU, PVC and aluminum foams. Furthermore, specific properties aid the comparison between the different microspheres used in this study. By normalizing mechanical and fracture properties against the density, it is believed the results would be fruitful for guiding future design for syntactic foams based on specific properties. Results from tension, compression, flexure and fracture tests will be discussed in relation to their microstructures.

2. Experimental work 2.1. Materials and equipment The syntactic foams for this research were produced by mechanical dispersion of hollow microspheres in epoxy resin. Three different types of hollow microspheres, namely 3M Scotchlite Glass Bubbles K15 and K46 and Phenoset BJO-093 phenolic microspheres TM

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were used for the filler. Properties of the different types of microspheres are listed in Table 1. The values for the mean diameter were obtained with a Fritsch particle size analyzer. The values presented are an average of three measurements. The values for the average wall thickness were calculated based on the true density of the microsphere, the density of the microsphere wall material and the mean sphere diameter. The two types of hollow glass microspheres were chosen to study the possible differences in mechanical behavior between low and high density hollow glass microspheres with an increasing volume fraction. The phenolic microspheres were also investigated to examine any effects arising from the different nature and interfacial adhesion between filler and binder. Although the microspheres exhibit different material parameters, it is reasonable to examine the specific properties, which can be quantified and compared readily. In this paper, we address other material parameters, which form complex interactions with the mechanisms of failure and fracture, qualitatively. For all the specimens Epicote 1006 epoxy resin was used as the binder. Epicote 1006 is a combination of liquid bisphenol-A, epichlorohydrin epoxide resin, amine and polymeric additives. The microspheres were added to the epoxy while slowly stirring the mixture to minimize gas bubbles in the resin. The microspheres were added in multiple steps to the epoxy resin to avoid agglomeration. Due to the low density of the microspheres compared to the binder, the microspheres showed a tendency to float to the top surface. This effect was minimized by stirring the mixtures close to the gel time of the resin, which was about 60 min for the epoxy. Scanning electron microscope (SEM) photomicrographs confirmed the homogeneity of the syntactic foam. After dispersion, the syntactic foam was compression molded using an aluminum mold coated with a silicone release agent. The syntactic foam was left under the press at a pressure of 1.6 MPa for 18–22 h to cure at room temperature. By adding different amounts of microspheres to the matrix, syntactic foams with various densities were thus created. Fig. 1 illustrates the general microstructure of syntactic foam. Fig. 2 shows the measured densities of syntactic foams with increasing hollow microsphere content. The measured densities presented in Fig. 2 are an average of about 30 samples. The density was measured by

Table 1 Physical properties of the studied microspheres Type of microsphere

True density (g/cc)

Static pressure (MPa)

Mean diameter (lm)

Average wall thickness (lm)

Thickness-to-radius ratio

BJO-093 K15 K46

0.25 0.15 0.46

3.44 2.07 41.37

71.5 70a 43.6

1.84b 0.70 1.37

0.052 0.02 0.063

a b

From [17]. From SEM measurement.

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2.2. Mechanical tests

Fig. 1. General microstructures under SEM for syntactic foam.

1.25

Theoretical density

Density (g/cm3)

Measured density 1.00

0.75

0.50 0

10

20

30

40

50

vol% microspheres Fig. 2. Density variation with microsphere content for syntactic foam containing (j) K15, (m) K46 and (d) phenolic microspheres.

dividing the volume of a specimen by its weight. Fig. 2 shows that all three types of microspheres show a linear decreasing trend in the density with increasing filler content. The linear trend is expected from the rule of mixtures. The theoretical density of each composition, see Fig. 2, is calculated according to the rule of mixtures. The measured density of syntactic foam is lower compared to the theoretical density. The lower value in density is caused by the presence of voids in the composite which are created during mechanical mixing of the components of syntactic foam. The voids will affect the mechanical properties. The void content for the different microstructures has been estimated by the following equation according to ASTM D-2734 [8]: V v ¼ 100
ðqtheoretical  qmeasured Þ=qtheoretical ;

ð1Þ

where Vv is the void volume fraction, qtheoretical is the theoretical density, and qmeasured is the measured density. The void volume fraction is provided in Table 2.

Three types of mechanical tests, namely tension, flatwise compression and flexure were performed with different microstructures of syntactic foam. For the tensile tests, the syntactic foam was machined into a standard Ôdog-boneÕ bar by means of a TensilKut machine. The specimens were loaded at an ambient temperature using an Instron Model 5567 at a cross-head speed of 5 mm/min. For each test the tensile strain was recorded with a clip-on strain gauge. The YoungÕs modulus, Et, was measured from the initial region of deformation. The results are based on an average of five tests. The error bar is the standard deviation for five measured values. Andrews et al. [9] report about the effect of the specimen size on the mechanical properties. According to Andrews et al., for closed-cell aluminum foam having a cell diameter of 2–3 mm, the length of a tensile specimen should be at least six times the cell diameter to avoid the specimen dimensions from affecting the mechanical properties. Because of the small cell size of syntactic foams, see Fig. 1, all specimens comply with the requirements set by Andrews et al. and the specimen sizes used do not affect the mechanical properties. For the flatwise compression test, syntactic foam was machined to blocks of 25 · 25 · 12 mm3. The length and width were chosen according to ASTM C365-00 [10]. Due to the short of supply for K46 microspheres, compression tests for K46 specimens were conducted from 0 to 30 vol% microspheres only. In our future work, the syntactic foam would be used as the core material in sandwich composites. The height of the specimen was chosen based on earlier studies [1]. Low aspect ratio specimens were used to minimize the effect of the shear stress. The tests were carried out at room temperature using an Instron Model 4206 with a maximum capacity of 100 kN. The cross-head speed was 0.5 mm/min. The flatwise compressive yield strength of syntactic foam was calculated by [10]: rc ¼

P ; A

ð2Þ

where rc is the compressive yield strength, P the load at yield, and A is the cross-sectional area. The flatwise compression modulus, Ec, was calculated by Ec ¼

mt ; A

ð3Þ

where m is the slope of the initial linear region of the load–deflection curve, and t is the thickness of the syntactic foam. The results presented are an average of five tests. The error bars are derived from the standard deviation.

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Table 2 Mechanical properties and void content of the studied compositions of syntactic foam Tensile strength (MPa)

YoungÕs modulus (GPa)

Compression yield strength (MPa)

Compression modulus (GPa)

Flexure strength (MPa)

Flexure modulus (GPa)

Fracture toughness p (MPa m)

Epoxy 10% K15 20% K15 30% K15 40% K15 50% K15

29.81 37.32 32.95 23.68 18.96 17.45

2.68 2.76 2.55 2.23 2.07 1.99

85.54 52.95 54.18 44.73 37.96 31.17

1.04 0.88 0.68 0.66 0.63 0.63

78.61 56.61 43.63 27.67 25.59 22.51

2.82 2.31 2.07 1.81 2.06 1.99

0.83 0.95 1.20 1.16 0.94 0.71

2.91 5.14 6.86 9.18 8.85 7.02

10% 20% 30% 40% 50%

K46 K46 K46 K46 K46

43.29 32.87 26.21 23.25 23.23

2.82 3.12 3.29 3.41 3.78

84.61 80.64 76.63 NA NA

0.95 1.14 1.14 NA NA

53.32 36.04 31.38 33.39 33.99

2.97 3.13 3.22 3.58 3.86

1.17 1.39 1.27 0.95 NA

4.87 6.07 8.44 10.21 10.02

10% 20% 30% 40% 50%

BJO BJO BJO BJO BJO

40.84 32.16 25.75 22.29 15.50

2.39 1.88 1.58 1.33 1.17

62.87 51.08 38.11 31.39 25.95

0.80 0.71 0.63 0.57 0.53

60.47 46.70 38.91 31.52 27.22

2.14 1.90 1.52 1.25 1.09

0.87 0.99 1.15 0.92 0.66

3.95 3.69 8.76 8.61 8.03

For the flexural tests, syntactic foam was machined to specimens of 127 · 12.7 · 3 mm3 in dimensions. The tests were performed by an Instron 5567. The span of the support, S, was chosen to be 48 mm to achieve a span-to-depth ratio of 16 as recommended by the ASTM D790-00 [11]. The strain rate was maintained at 0.01/min. The cross-head speed, z, was calculated by [11] z¼

R
S2 ; 6d

ð4Þ

where R is the strain rate, S is the span of the support, and d is the depth of the sample. The bending modulus, Eb, was calculated by [11] Eb ¼

S3m ; 4Wt3

ð5Þ

where S is the span of the support, and W is the width of the testing sample. All specific properties were calculated by normalizing the measured strength and moduli against the measured density, q, of the sample. 2.3. Fracture toughness In the fracture toughness assessment under quasi-static loading, single-edge notched bend (SENB) specimens were loaded in a three-point bend (3PB) geometry. The tests were performed by an Instron 5567 at a cross-head speed of 5 mm/min. Due to the short of supply for K46 microspheres, fracture toughnesses for K46 specimens were assessed from 0 to 40 vol% microspheres only. The specimen dimensions were 60 · 12.7 · 6.35 mm3. This specimen geometry satisfies the requirement for plane strain conditions [12],


t > 2:5

K Ic ry

Void (%)

2 ;

ð6Þ

where t is the sample thickness, KIc is the critical stress intensity factor, and ry is the yield strength. For all the specimens, a constant crack-to-width ratio, a/W, of 0.5 was prepared by a vertical band saw. A sharp crack was introduced by tapping a fresh razor blade into a notch. The critical stress intensity factor, KIc, can be estimated from the following equations [13]: pffiffiffi 3PS a ; ð7Þ K Ic ¼ Y 2tW 2 a  a 2  a 3 Y ¼ 1:93  3:07  25:11 þ 14:53 W W W  a 4 þ 25:80 ; ð8Þ W where Y is a shape factor, P is the peak load at the onset of crack growth in a linear elastic fracture, and a is the crack length. After testing, the fracture surface was cut from the specimen. A thin layer of gold was sputter coated onto the fracture surface by means of a gold coater (SPI module). The fracture behavior of syntactic foam was then characterized by means of a Jeol JSM 5410LV, low vacuum SEM.

3. Results and discussion 3.1. Tensile test Syntactic foam behaves like a linear-elastic material up to failure when loaded in tension. The material

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experiences catastrophic failure across a plane perpendicular to the tensile axis. Table 2 presents mechanical properties and void content of the different compositions studied. The trends in the tensile strength, rt, with increasing microsphere content are rather similar for the three types of microspheres, except for 50 vol% phenolic microspheres. Luxmoore and Owen [14] concluded that a crack will initiate from an oversized void when a composite is subjected to tensile loading. Indeed, numerous specimens fractured at a cross-section containing a void. The specific tensile strength, rt/q, for various compositions of syntactic foam are shown in Fig. 3(a). Fig. 3(a) shows that rt/q increases upon inclusion of a small amount of microspheres when compared to neat epoxy resin. The presented data for neat epoxy are obtained experimentally. However, beyond 10 vol%, a decreasing trend in rt/q is observed for all types of microspheres. The decreasing trend in rt/q with increasing filler content indicates that the relative reduction in strength is larger than the relative reduction in density. Indeed, a decrease in rt with increasing filler content is observed, see Table 2. The comparable trends and values suggest that rt/q is independent of the microsphere type and size, but only varies with the microsphere volume fraction. The following curve-fitting relation between the rt/q and the volume fraction of microspheres, x, has been derived for 0.1 < x < 0.5: rt =q ¼ 2157x4 þ 2712x3  1050x2 þ 93x þ 40:768:

50

40

30

20

10

0 0

(a)

5

3

-1

Specific Young's Modulus (GPa.cm .g )

Specific Tensile Strength (MPa.cm3.g-1)

ð9Þ Nevertheless, clear advantages regarding other mechanical properties arise upon introduction of hollow microsphere as shown in the discussion that follows. Luxmoore and Owen suggested [14] that the failure of the foam is attributed to the failure of the resin matrix. The non-linearity in rt/q is caused by the reduction in the area of the epoxy matrix in a cross-sectional area.

Introduction of hollow microspheres reduces the epoxy volume fraction and increases the inhomogeneity content, consequently reducing the tensile strength of the composite as a result of poor interfacial strength between the matrix and the filler. The authors assume that the matrix serves as the load-bearing phase in the composite whereas the hollow microspheres only provide light weight and minimal strengthening effect. The reduction in load-bearing volume outweighs the increase in stiff microsphere shells. Around 40 vol% of filler content, a minimum is observed for all the microspheres. We believe that 40 vol% of microspheres is to be the maximum amount of filler which can be fully wetted by the epoxy matrix in this system. The change in properties and behavior of syntactic foam around 40 vol% was also observed by Bunn and Mottram [5]. Nevertheless, clear advantages regarding other mechanical properties arise upon introduction of hollow microsphere as shown in the discussion that follows. It has been reported that the YoungÕs modulus, Et, generally decreases with increasing content of hollow microspheres [15,16]. Only Bardella and Genna [17] report an increasing trend in Et for syntactic foams containing K37 (q = 0.37 g cm3) microspheres. The results from the current research, as presented in Fig. 3(b) shows similarity to those in [17] in the sense that the trend for the specific YoungÕs modulus, Et/q, with increasing filler content depends on the type of microspheres used. K46 microspheres show a rather linear increase in Et/q, whereas K15 show a constant trend. Based on the data presented in Table 2, it is known that the constant trend in Et/q is caused by a proportionate decrease in the YoungÕs modulus and the density. For hollow phenolic microspheres, a decrease in Et with increasing filler content is observed. The value obtained for neat epoxy resin is too low considering the trends in Et for syntactic foams containing the three different

10

20

30

vol % microspheres

40

50

4

3

2

1

0 0

(b)

10

20

30

40

50

vol % microspheres

Fig. 3. Specific tensile properties of syntactic foam containing (j) K15, (m) K46 and (d) phenolic microspheres: (a) specific tensile strength; (b) specific YoungÕs modulus.

E.M. Wouterson et al. / Composites Science and Technology 65 (2005) 1840–1850

types of microspheres. The lower value for neat epoxy resin could be caused by the presence of voids. The differences between the results for K46 and K15 microspheres can be attributed to their size and thickness. It has been shown by several researchers that, in general, for microspheres of the same material composition, improved mechanical properties are obtained for the microspheres with a higher density. Higher density for microspheres of the same material is often associated with a larger thickness-to-radius ratio, t/R, see Table 1. Syntactic foam containing microspheres of the same material having a larger t/R often exhibit improved mechanical properties [18]. The difference between the phenolic and K15 glass microspheres explains that apart from microsphere size, the material composition of the microsphere is of equal importance for the mechanical properties. Phenol–formaldehyde has an Et of about 6.8 GPa whereas soda lime glass has an Et of about 77 GPa. The difference in Et is believed to be an important factor to the difference in the trends of hollow glass and phenolic microspheres. This relationship between the material of the matrix and the inclusion was studied by Pawlak and Galeski [19]. It was found that the higher the YoungÕs modulus of the inclusion compared to the epoxy used, the higher the stresses developed in the material. Further they observed that the position of the maximum stress at the spherical inclusion changed from the pole for hard inclusion to the equator for soft inclusion. Fig. 4 shows an SEM fractograph of a tensile specimen; the micrograph confirms the role of the binder in the failure of syntactic foam under tensile loading. The micrograph shows a rough surface of the epoxy resin after brittle fracture. Numerous step structures can be identified in Fig. 4 which indicates the plastic yielding of the epoxy resin. Besides plastic yielding, intact microspheres are observed on the fracture surface. The fact

1845

that the crack has grown over the interface between the matrix and the microsphere indicates the presence of debonding. Debonding of hard inclusions was also reported by Pawlak and Galeski [19]. The debonding is caused by the complex stress state around the particle– matrix interface which will not be further discussed in this paper. 3.2. Compression test Fig. 5 shows the stress–strain curves of flatwise compression tests of syntactic foams containing various amounts of K15 microspheres. The compression curves of syntactic foam were previously discussed by other investigators [1,2,5]. For each individual curve, three different regions can be identified in Fig. 5. The first region (I) is characterized by an almost linear-elastic behavior of the syntactic foam. The region ends when the material starts to yield and reaches its compressive yield strength. Upon yielding, the microspheres become crushed under the compression load and severe damage occurs. Yielding and inelastic damage occur during the test as the sides of the specimen barrel outward under compression loading. Barreling of the specimen is caused by friction between the contact surfaces. It affects the stress of the syntactic foam under loading. The second region (II) of the flatwise compression curves in Fig. 5 is characterized by relatively horizontal plateaus. The horizontal plateau is attributed to the implosion of the hollow microspheres under the increasing compression load. Syntactic foams with higher fractions of hollow microspheres show a larger horizontal plateau and thus a larger strain. The third region (III) is characterized by a steep increase in the load–displacement curve. The steep increase is caused by a large number of microspheres being crushed and compacted, and the maximum den-

Compressive Strength, σc (MPa)

160 140 120 100

III

80

I

60

II

40 20

increasing volume fraction

0 -20 -0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Engineering strain (mm/mm) Fig. 4. Fracture surface of a syntactic foam specimen perpendicular to the direction of applied tensile loading.

Fig. 5. Compression stress–strain curves of syntactic foam with various amounts of K15 microspheres content. Regions I–III are sequentially observed and indicated.

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(a)

3.3. Flexure testing of syntactic foam Figs. 7(a) and (b) show typical stress–strain curves for K46 and BJO-093 microspheres, respectively. K15 microspheres show a similar flexure behavior as BJO093 microspheres. For all specimens, it is observed that the strain is reduced with increasing filler content. Especially, the syntactic foams containing K46 microspheres show a larger reduction in failure strain. The larger reduction in strain for K46 microspheres is attributed to the higher strength of the K46 microspheres, see Table 1. Fewer microspheres will fracture under the applied load which prevents stress relief in the material [20]. On the contrary BJO-093 and K15 microspheres fracture more easily, relieving the stress in the material, and thus showing a larger strain. Upon fracture some plastic deformation is observed for most compositions of syntactic foam with up to 30 vol% of microspheres. The amount of plastic deformation decreases with increasing filler content since the plastic deformation is attributed to the behavior of the epoxy binder. Syntactic foams containing hollow phenolic microspheres show larger plastic deformation compared to K46 and K15 microspheres. The latter might be attributed to ductile deformation of hollow phenolic microspheres as shown in Fig. 8; an observation that has not been seen for hollow glass microspheres. The plastic deformation of syntactic foam containing hollow phenolic microspheres can be clearly seen on the fracture surface of specimens due to stress whitening. The stress whitening cannot be distinguished on syntactic foam containing hollow glass microspheres since the resulting syntactic foam is white in color. During flexure loading, the specimen is subjected to compressive stresses on the top part of the specimen and to tensile stresses at the lower part of the specimen. From the tension and compression data presented in the

1.50

3

-1

Specific Compression Modulus (GPa.cm .g )

100

3

-1

Specific Compression Strength (MPa.cm .g )

sity is being reached. The point, where region III starts, is considered to be point of failure for the syntactic foam as this is the point where most of the load bearing microspheres have been crushed. The overall results of the flatwise compression tests are given in Fig. 6 and the void content is summarized in Table 2. Fig. 6(a) shows that the specific compressive yield strength, ryc/q, decreases with increasing filler content for phenolic and K15 microspheres. The microspheres act as voids and weaken the structure. Nevertheless, an upward trend in ryc/q is observed for K46 microspheres. The upward trend is attributed to a relatively minor decrease in the compressive yield strength, see Table 2, compared to the decrease in density. From the difference in trends between the K15 and K46 microspheres, it is induced that the specific compressive yield strength is influenced by the wall thickness of microspheres. The results for the specific compressive modulus, Ec/q, are shown in Fig. 6(b). Again K46 microspheres perform better as shown. The performance of K46 microspheres is also attributed to their t/R ratio. For all microspheres investigated, it appears a densification process takes place at low filler content (10 vol%). The densification is caused by a compaction process of the matrix polymer in the presence of voids in curing. As microsphere content increases and matrix volume decreases, the microsphere takes up more load under compression and, as a result, the specific compressive moduli increase. The trend under compression is in contrast to that under tension. K15 and phenolic microspheres show a rather similar behavior in Ec/q. Both show a minimum around 20 vol%, after which the Ec/q slightly increases. This increase is higher for K15 microspheres. The specific compressive stiffness levels off as the increase in stiffness is counter-balanced by the increases in density of the composite.

80

60

40

20

0 0

10

20

30

40

vol% microspheres

50

60

(b)

1.25

1.00

0.75

0.50

0.25

0.00 0

10

20

30

40

50

60

vol% microspheres

Fig. 6. Specific compressive properties of syntactic foam containing (j) K15, (m) K46 and (d) phenolic microspheres: (a) specific compressive yield strength; (b) specific compressive modulus. Due to the short of supply for K46 microspheres, compression tests for K46 specimens were conducted from 0 to 30 vol% microspheres only.

80

80

60

60

Stress (MPa)

Stress (MPa)

E.M. Wouterson et al. / Composites Science and Technology 65 (2005) 1840–1850

40

20

1847

40

20 Increasing volume fraction

0

0 0.00

(a)

0.01

0.02

0.03

0.04

0.05

0.06

Strain (mm/mm)

Increasing volume fraction

0.00

(b)

0.01

0.02

0.03

0.04

0.05

0.06

Strain (mm/mm)

Fig. 7. Flexure stress strain curves of syntactic foam containing various amounts of (a) K46 and (b) phenolic microspheres. The arrow indicates the direction of increasing filler content.

Fig. 8. A deformed hollow phenolic microsphere.

previous paragraphs, it is expected that syntactic foam subjected to flexure loading will fail at the side under tensile stresses. Indeed, failure at the lower specimen side under tensile stresses is observed during experimental work. All specimens fail at the center of the support span. No indentation of the indenter is observed. The comparable failure mode under tensile loading and three-point-bending explains why there is some resemblance between Figs. 3 and 9. The only difference observed is the behavior of syntactic foams with 0–10 vol% of microspheres. For the specific flexural strength, rf/q, see Fig. 9(a), some differences between the different compositions are observed. The differences are mainly caused by the difference in density between the different compositions as the values and trends are rather similar for the maximum flexural strengths, see Table 2. All types of microspheres show a decreasing trend in the maximum specific flexural strength with increasing filler content. Similar to the tensile test results, the specific strength approaches a

minimum around 40–50 vol% of filler content. Based on the data presented in Table 2, it can be said that the horizontal trend for the hollow phenolic and K15 microspheres at high volume fractions, is caused by a proportionate decrease in the flexure strength and density. The trends and values in the specific flexural modulus, Ef/q, see Fig. 9(b), are consistent with the results presented in Fig. 3(b). The similarity is again attributed to the tensile failure mode of syntactic foam under three point bending. The K46 microspheres show an increase in Ef/q with increasing filler content. K15 microspheres show a constant value for Ef/q whereas phenolic microspheres show a decrease with increasing filler content. The constant value in Ef/q from K15 microspheres is attributed to a similar relative decrease in the flexure modulus and density, see Table 2. SEM fractographs of the specimens tested under three point bending reveal the different loading modes at the upper and lower parts of the cross-sectional area of the specimen. Fig. 10(a) shows the fracture surface of the lower part of the cross-section, which is loaded in tension. The fractograph is consistent with that in Fig. 4. The surface is characterized by a rough surface. In between the tension and compression sides of the specimen, an area with relative smooth features is observed. The transition between the tensile surface and the compressive surface is shown in the right side of Fig. 10(a). A typical fracture surface of the upper part of the cross-section, loaded in compression, is shown in Fig. 10(b). The compressive surface shows step structures behind the microspheres which we previously observed [21]. The step structures diminish with increasing filler content and are less pronounced for phenolic microspheres. Besides the step structures, debonded microspheres can be identified. Debonded microspheres indicate a poor interface between the matrix and the filler.

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E.M. Wouterson et al. / Composites Science and Technology 65 (2005) 1840–1850 6 -1 3

70 60 50 40 30 20 10 0

(a)

Specific Flexure Modulus (GPa.cm .g )

3

-1

Specific Flexure Strength (MPa.cm .g )

80

4

2

0

0

10

20

30

40

50

vol % microspheres

0

10

20

30

40

50

vol % microspheres

(b)

Fig. 9. Specific flexural properties of syntactic foam containing (j) K15, (m) K46 and (d) phenolic microspheres: (a) specific flexure strength; (b) specific flexure modulus.

Fig. 10. SEM micrograph of the fracture surface of syntactic foam containing 10 vol% K15 microspheres under (a) tension, and (b) compression. The black arrows denote the direction of crack propagation.

0.5

Fig. 11 shows the specific fracture toughness, K1c/q, for various compositions of syntactic foam. The actual fracture toughnesses, KIc, together with the void content are presented in Table 2. The most distinctive features in Fig. 11 are the significant increase in K1c/q for filler content up to 30 vol% for all types of microspheres, and the decrease in K1c/q beyond 30 vol% of filler content. The maximum in the trend for the fracture toughness has been reported for other particulate composites [7,13,22]. The increase in K1c/q for 0–30 vol% filler content suggests the presence of a toughening mechanism which increases the fracture energy compared to neat epoxy resin. Again the hollow glass microspheres outperform the hollow phenolic microspheres with K46 microspheres resulting in the highest value for K1c/q, suggesting that the wall thickness and density of the microspheres affect the fracture toughness of the syntactic foam.

3

-1

Specific Fracture Toughness (MPa.m .cm .g )

3.4. Fracture toughness of syntactic foam 1.5

1.0

0.5

0.0 0

10

20

30

40

50

vol % microspheres Fig. 11. Specific fracture toughness vs. filler content for syntactic foam containing (j) K15, (m) K46 and (d) phenolic microspheres. Due to the short of supply for K46 microspheres, fracture toughnesses for K46 specimens were assessed from 0 to 40 vol% microspheres only.

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Fig. 12. SEM micrographs of the fractured surface of SENB specimens of syntactic foam containing 10 vol% microspheres: (a) K15 microspheres; (b) hollow phenolic microspheres.

We believe that the type of major toughening mechanism has changed at the transition point of 30 vol% of microsphere content. The toughening from 0 to 30 vol% microspheres is influenced by a combination of the filler stiffening effect and crack bowing mechanisms in the presence of round-shaped fillers. The change in toughening mechanism is conjectured to be caused by the inter-particle spacing theory as discussed by Lee and Yee [13] for glass bead/epoxy composites. Increase in microsphere content will decrease interparticle separation between microspheres. The increase of microsphere content beyond the complete wetting ability of epoxy also introduces inter-sphere sliding and stress concentration. Higher volume fractions of microspheres allow more microspheres to debond from the matrix. Debonding is accompanied by premature cracks. If the direction of these cracks is parallel to the crack growth direction, the subcritical cracks act as precursors and facilitate crack propagation. SEM was used to elucidate the toughening mechanisms in the various microstructures of syntactic foam, see Figs. 12 and 13. Fig. 12 shows the fracture surface of syntactic foams containing low volume fractions of

microspheres, where Fig. 13 shows the fracture surface of syntactic foams containing high volume fractions of microspheres. Clearly, Figs. 12(a) and (b) show that step structures prevail for the microstructures containing low volume fractions of microspheres. These step structures were previously observed [21]. These step structures are considered to be the evidence of the existence of the crack front bowing mechanism. This mechanism was first observed experimentally by Lange in 1970 [23]. Lange suggested that an approaching crack front is pinned by a rigid particle. Secondary crack fronts will be formed and these secondary crack fronts will bend/ bow between the rigid particles. There will be a point when the crack front breaks away from the rigid particle. At this point, the arms of the secondary crack fronts will come together and form a characteristic step structure as both secondary crack fronts propagate at a different crack plane. These characteristic step structures are also called ÔtailsÕ or ÔlancesÕ. Many studies on the fracture toughness of particulate composites consider the crack bowing as the main toughening mechanism. As hollow glass microspheres exhibit higher fracture toughness compared to hollow phenolic microspheres,

Fig. 13. SEM micrograph of the fractured surface of SENB specimens of syntactic foam containing large amounts of microspheres: (a) 40 vol% K46 microspheres; (b) 50 vol% hollow phenolic microspheres.

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with K46 microspheres outperforming K15 microspheres, it is surmised that the thickness-to-radius ratio and the wall material of the microsphere affect the size of the step structure and thus K1c/q. Figs. 13(a) and (b) show the microstructures of syntactic foam containing 40 vol% K46 and 50 vol% hollow phenolic microspheres, respectively. Compared to Fig. 12, step structures are absent in Fig. 13. Instead, the dominant fracture mechanism that is observed is debonding of microspheres. The existence of excessive debonding introduces stress concentration and premature cracks leading to reduced fracture toughness. 4. Conclusions From the results presented in this paper, it can be concluded that the specific properties of syntactic foam depend on the types and volume fractions of microspheres utilized in the syntactic foam. Increase in microsphere density (K46 vs. K15) and the thickness-to-radius ratio led to an increase in specific tensile stiffness. The results for the tensile and flexural tests were comparable due to the fact that both types of tests exhibited the same failure mode. Both tests elicited a decreasing trend in specific strength with increasing filler content. A distinct trend in compressive behavior was noted in contrast to the tensile failure. The compression tests revealed the excellent compressive properties of syntactic foam and in particular the superior performance of K46 microspheres, giving rise to higher compressive yield strengths and moduli compared to K15 and phenolic microspheres. From the fracture toughness tests, it was concluded that all types of studied microspheres show a similar trend in the specific fracture toughness with increasing filler content. For lower filler content an increase in the specific fracture toughness was observed. The increase reached a maximum after which a decrease in the specific fracture toughness was seen. The change in behavior was attributed to a change in the dominant toughening mechanisms from filler stiffening, crack front bowing to excessive debonding of microspheres in reduced matrix volume. This work, however, demonstrated the usefulness of a combination of desired properties for syntactic foam such as light-weight high stiffness, high compression and high toughness. Acknowledgments The authors thank Nanyang Technological University and DSO National Laboratories Singapore for the support of this work. One of us (SCW) acknowledges the support of NSF Grant# CMS 0335390 administered by the Mechanics and Materials Program.

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