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Manufacturing and failure mechanisms of syntactic foam under compression☆
Composites: Part A 35 (2004) 1009–1015 www.elsevier.com/locate/compositesa
Manufacturing and failure mechanisms of syntactic foam under compressionq Ho Sung Kim*, Pakorn Plubrai Department of Mechanical Engineering, School of Engineering, The University of Newcastle, Callaghan, Newcastle, NSW 2308, Australia
Abstract Failure mechanisms of syntactic foams under compression were studied. The syntactic foams produced with a new manufacturing method involving buoyancy were made of glass hollow microspheres and epoxy resin. The foam density was varied from 0.09 to 0.15 g/cc, which resulted from variation of resin concentration. Two different failure modes were identified and these were attributed to different failure initiation processes. One was initiated by longitudinal splitting and the other by layered crushing. The former occurred in the lowest foam density and the latter in higher densities used in the study. The failure sequence associated with the former mode was: (a) formation of multiple longitudinal cracks along the circumference of the compression specimen, (b) widening of longitudinal cracks and then (c) failure at one end of the specimen resulting in further lateral expansion. The sequence for the latter failure mode was deduced on the basis of the proposed schematic model: (a) the first failure initiation occurrence at a weakest microsphere, (b) failure of adjacent microspheres due to stress concentration resulting in propagation of microsphere failure laterally until it reaches the end of specimen wall, and (c) thickening of the crushed layer. The thickening of the crushed layer is due to accumulation of broken microspheres. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Foams; B. Fracture; C. Micro-mechanics; D. Mechanical testing
1. Introduction Syntactic foams, which are made of hollow microspheres and binder, have been used as structural components [1 – 5]. Various consolidating techniques for binder and microspheres have also been used, which includes coating microspheres [6], extrusion [7,8] and ones that use inorganic binder solution and firing [9], dry resin powder [10 – 13] for sintering, compaction [14 – 15], and liquid resin as binder [16] for in situ reaction injection moulding. The variety of manufacturing techniques can widen the applicability of syntactic foams. When syntactic foams are used as core materials for sandwich composites, they contribute to increase in specific stiffness of composites [17]. If they are used for protective structural components, they would contribute to reduction of damage and prevent failure of other components by inducing their own failure. Failure of syntactic foams in the literature has been sparsely covered in the 1980s, while most investigations on q
This paper is based on a presentation made at the International Composites Conference ACUN-4, held at the University of New South Wales, Sydney, July 2002. * Corresponding author. Tel.: þ 61-2-4921-6211; fax: þ61-2-4921-6946. E-mail address: [email protected] (H.S. Kim). 1359-835X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2004.03.013
syntactic foams have been confined to manufacturing and physical properties [6,9 – 12,16]. Narkis et al. [6,11] found that failure of syntactic foams with low concentration resin is mainly by disintegration under compression. It has been reported, however, that relatively high-density foams under compression failed by the formation of 458 shear planes [18,19]. Recently, Kim and Oh [15], and Gupta et al. [20] have also reported that failure mode of syntactic foams with relatively high-concentration of resin under uniform compression was by shear on planes inclined approximately 458 to the loading direction. Also, Gupta et al. [20,21] highlighted that the shear failure is affected by specimen aspect ratio. Thus, the failure mode may depend upon various factors. In general, composites can be tailored for optimum performance. Syntactic foam itself is also a composite. Their contribution towards desired behaviour depends how they are designed and used. It is important to understand failure mechanisms of syntactic foam prior to development of syntactic foams to the needs of structural component design. The present paper focuses on compression loading to investigate failure mechanisms of syntactic foam produced with a new manufacturing method involving buoyancy.
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2. Experimental 2.1. Constituent materials The batch of hollow microspheres used in the study was made of soda-lime-borosilicate glass (Scotchlite Glass Bubbles K1) and manufactured by 3 M. A size distribution of microspheres was obtained using a Malvern 2600C laser particle size analyser as shown in Fig. 1. Particle densities were measured using an air comparison pycnometer (Beckman 930) and an average of three measurements was found to be 0.125 g/cc for microspheres. Bulk density of the microspheres was also measured, using a measuring cylinder with a tapping device (800 taps were conducted), to be 0.0762 g/cc. A resin system, used for binding microspheres, consisted of Epoxy 105 (West System—a blend of Bisphenol A and Bisphenol F), Slow Hardener 206 (West System—a blend of aliphatic amines and aliphatic amine adducts based on diethylene triamine and triethylenetetramine) and acetone (Septone, ASA20). All measured densities are listed in Table 1. Viscosities were also measured at 21 8C by a Brookfield syncro-electric viscometer (LVF 18705) for epoxy and hardener and a Cannon-Fenske Routine viscometer for acetone and its mixture. Viscosity values were found to be 940, 195 and 0.32 cP for epoxy, hardener and acetone, respectively. The viscosity of the mixture consisting of epoxy, hardener and acetone measured as a function of resin system ( ¼ epoxy þ hardener, at a mixing ratio of 5/1 for epoxy/ hardener by volume) is shown in Fig. 2. 2.2. Manufacturing of syntactic foams 2.2.1. Preliminary observation and packing time measurements Different stages of microspheres in a test tube after the mixture being shaken were observed as shown in Fig. 3: at the first stage (Fig. 3(a)), microspheres were fully dispersed; at the next stage, microspheres started to be packed by squeezing themselves in as shown in Fig. 3(b); and finally packing is completed as shown in Fig. 3(c). To measure the packing time taken between the first stage and the last stage,
Fig. 1. Size distribution of microspheres used.
Table 1 Densities of constituent materials for syntactic foam Material
Density (g/cc)
Microspheres (bulk) Epoxy105/slow hardener206 (volume ratio: 5/1) Acetone
0.0762 1.073 0.775
mixing of acetone and microspheres was conducted in a test tube. The times measured were 2 min 20 s and 2 min 40 s for a low acetone content (6/1/2-acetone/microspheres/ (epoxy þ hardener) by mass) and a high acetone content (40/1/2-acetone/microspheres/(epoxy þ hardener) by mass), respectively. 2.2.2. Manufacturing For each mould, a cylindrical tube made of wax with an inside diameter of 16 and 5 mm in wall thickness was produced and then cut to a length of 30 mm. The end opening was sealed by wrapping with HDPE plastic sheet and secured with an elastic band. The HDPE plastic sheet and wax had been tested for chemical reaction with the acetone and it was found that there was no visible chemical attack on the surfaces. Epoxy and hardener placed separately in two glass beakers were conditioned until their temperatures reach 38 8C in an oven, which was set to 38 8C before mixing at a ratio of 5/1 for epoxy/hardener by volume. Wax moulds were also conditioned at 38 8C for a minimum of 1 h prior to use. The temperature, 38 8C, was chosen because it is always a little higher than room temperature so that it allows us to keep it constant. For mixing epoxy, hardener and acetone, the following procedure was adopted: a clean empty mixing container was placed on an electronic scale and a pre-mixed epoxy/hardener was injected into the container using a plastic syringe until a required mass was reached; acetone was added; the container was then sealed and shaken vigorously for 2 min; the container was opened and the microspheres were added
Fig. 2. Viscosity of mixture consisting of epoxy, hardener and acetone as a function of volume fraction of resin system ( ¼ epoxy þ hardener, at a mixing ratio of 5/1 for epoxy/hardener by volume).
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Fig. 3. Mixture stages: (a) dispersed microspheres right after being shaken; (b) partially packed microspheres by buoyancy; and (c) fully packed microspheres.
through a glass funnel; and the container was sealed and shaken again vigorously for further 5 min for dispersion of microspheres. For subsequent casting, the container was kept shaken to maintain a constant mixture ratio, while the mixture was being poured through a tube into a mould. After casting, a duration of 3 min was allowed for self-packing of microspheres by self-buoyancy and then the hole at the bottom of the mould was opened by removing a sealing tape to drain the bottom layer of liquid. Subsequently, the top layer consisting packed microspheres and liquid phase slid down to the bottom of the mould. After 30 min, the foam in the mould was sufficiently dry and structurally self-supporting so that it was demoulded. Test specimens after demoulding were left for at least 3 days before testing-note curing time for epoxy used is 24 h at room temperature. Volume fractions were calculated assuming no acetone was left in foams and listed in Table 2. The void fraction ðvv Þ was calculated using v v ¼ 1 2 rf
mr m þ m rr rm
where r is the density, m is the mass fraction, and subscripts (f ; r and m) indicate foam, resin/hardener and microsphere, respectively. It is noted that volume fraction of voids increases with increasing volume fraction of epoxy/ hardener. The current manufacturing method is capable of producing a wide density range depending upon amount of diluents for binder and allows constituent materials to be easily mixed without damaging hollow microspheres. It is possible though that fabricated samples may not be
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homogeneous and isotropic but rather ‘functionally graded’ due to the difference in the rising/settling velocity of microspheres caused by the buoyancy effect. However, we have not found significant microstructure variation over the height of test specimens (Fig. 9(a), (c) and (d)). Probably, the microstructure have been affected by the initial microsphere locations as well before rising—if microspheres are close to the top surface, some of them would be packed early even though their rising velocity is low (if they were at the same depth, they would have more been ‘functionally graded’). A total of five different mixing ratios as listed in Table 2 were used to manufacture syntactic foams. 2.3. Test specimen preparation and compression tests Test specimens, 15 mm in length, were cut out from the moulded cylinders. Parallelism of top and bottom surfaces was ensured by measuring the lengths of three different locations along the circumference of the specimen using a pair of Vernier callipers—parallelism tolerance was ^ 2.58. Compression tests were conducted at a crosshead speed of 1.0 mm/min and an ambient temperature range of 22 – 25 8C using a universal testing machine (Shimadzu 5000) with a Hounsfield compression cage. Platens of Hounsfield compression cage were lubricated using a Multipurpose WD 40 to minimise friction between specimen and each platen. Five samples were tested for each mixing ratio—three of specimens were compressed to about 10% of the initial height, around which the initial breakage occurred, and the other two compressed until about 50% of the initial height of the specimen to observe crushing/densification process after the initial breakage.
3. Results and discussion Typical stress – strain curves for syntactic foams with five different mixing ratios are shown in Fig. 4. Two stages in each curve are seen—the initial high slope is followed by a low slope/plateau regime in ‘(II)’ to ‘(V)’ or followed by a negative slope in ‘(I)’. The initial high slope obviously corresponds to the elastic deformation of the foam and
Table 2 Foam compositions and properties Foam designation
Foam I Foam II Foam III Foam IV Foam V
Volume fractions for mixing
Properties
Volume fractions in foam calculated assuming no acetone exists in foams
Acetone
Epoxy and hardener
Micro-spheres
Density (g/cc)
Compressive strength (MPa) ^ SD
Elastic modulus (MPa)
Voids
Epoxy and hardener
Micro-spheres
0.656 0.622 0.606 0.550 0.503
0.019 0.037 0.053 0.126 0.172
0.325 0.325 0.325 0.325 0.325
0.093 0.105 0.120 0.134 0.145
0.164 ^ 0.074 0.460 ^ 0.060 0.551 ^ 0.037 0.952 ^ 0.080 1.447 ^ 0.089
8.19 18.48 31.12 38.03 71.41
0.476 0.527 0.535 0.656 0.680
0.0290 0.0484 0.0652 0.0960 0.1107
0.495 0.425 0.400 0.248 0.209
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Fig. 5. Longitudinal splitting between platens under compression of Foam I.
Fig. 4. Typical stress strain displacement curves for different mixing volume ratios of acetone/epoxy þ hardener/microspheres: (I) 0.656/0.019/0.325; (II) 0.622/0.037/0.325; (III) 0.606/0.053/0.325; (IV) 0.550/0.126/0.325; and (V) 0.503/0.172/0.325.
the low slope/plateau regime is easily related to crushing/ densification of microspheres (see crushing/densification mechanism below). The plateau regime is obvious in foam II, but as resin content increases (Table 2) it appears less obvious. The performance difference in crushing/densification may be due partly to the resin content—when the resin content is high, it is possible that smearing of resin over microspheres occurs and thus direct microsphere contact resulting in easy transferring the load for crushing is prevented. This is compared to those of different syntactic foams [20,22] in which the plateau region is due to crushing of microspheres after the initial failure by shear planes inclined 458 to the loading direction [20]. The transitional points between initial slopes and other slopes (Fig. 4) appear to occur at about a strain range of 0.03– 0.05 without a noticeable trend dependant upon specimen density. In general, the strength and the elastic modulus of the foam steadily increases as expected with increasing volume fraction of epoxy and hardener. These trends were also found elsewhere [22]. Mean values of compressive strength with standard deviation and elastic moduli of syntactic foam with various densities are listed in Table 2. Two different modes of failure under compressive loading for the five different resin concentrations were observed: one was characterised by longitudinal splitting; and the other by layered crushing. The details are given below. The longitudinal splitting occurred only in Foam I, which has the lowest foam density. Note the lowest foam density (0.096) is close to the microsphere bulk density of 0.076. The foam easily crumbled under a slight finger squeeze. This nature appears to be similar to that of low resin content foams which were manufactured by a coating method [6] or a powder mixing method [11]. The following failure sequence under compression was observed—(a) formation
of 3 – 6 longitudinal cracks along the compression specimen, (b) widening of longitudinal cracks and then (c) failure at one end of the specimen resulting in further lateral expansion as shown in Fig. 5. This failure is similar in terms of cracking direction to that of a foam containing high volume fraction of glass microspheres in carbon/glass/resin syntactic foam [23]. Layered crushing mode of failure occurred in Foams II – V (Table 2). It was initially observed that as the compression load increases the height of specimen decreases without substantial lateral volume expansion as shown in Fig. 6. This observation indicates that during compression, hollow microspheres were broken and subsequently densification occurred. There can be two possibilities for the breakage and densification. One is that the initiation might have occurred locally at the weakest location and then propagated. The other is that it might have occurred globally in a uniform manner throughout the specimen prior to densification. In order to test the two possibilities, photographs of cross sections as schematically shown in Fig. 7 were prepared to examine internal characteristics after compressive testing. Fig. 8 shows two different cross sections from different specimens but for the same Foam V. Fig. 8(a) shows three layers in which top and bottom layers appear to be the same and Fig. 8(b) two layers on each cross section. The different appearances are caused by the light reflection. The light reflection of the middle layer in the case of Fig. 8(a) appears to be different from those of top and bottom layers. The different reflections were found to be due to different microstructures. SEM photographs were taken using the same specimen used for Fig. 8(a) and are shown in Fig. 9 to examine the difference in microstructure. The features of top (Fig. 9(a)) and bottom (Fig. 9(c)) layers are in contrast with those of middle layer (Fig. 9(b)) in which most of microspheres are seen to be crushed and densified. In addition, Fig. 9(d) shows top and middle layers in which the dashed line indicates the interface between the two layers. These observations indicate that the initiation of microsphere crushing in this case occurred at the middle section of the specimen.
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Fig. 6. Typical deformation stages of Foam V under compression: (a) before loading; (b) after being subjected to a compressive loading; and (c) further compressive loading.
From the evidence in SEM photos (Fig. 9), the latter possibility suggested above that the initiation might have occurred globally and in a uniform manner throughout the specimen can be rejected. Also, it is interesting to note that the progressive nature of layered failure is similar to deformation of honeycomb [24]. Now a question arises as to how the layer can be formed by the initiation occurred locally at a weakest location and then thickening of the layer occurs. The answer to the question may be given using the sequence shown schematically in Fig. 10. Fig. 10(a) shows the first failure initiation of a weakest microsphere. Even though no evidence was found, the failure initiation site would be multiple if the specimen is large (higher probability). Fig. 10(b) shows failure of adjacent microspheres due to stress concentration. At this stage, the stress concentration sites would be located rather on the lateral sides of the first initiation site than top or bottom side because of the loading direction. Thus, the fracture of microspheres propagates laterally until it reaches the end of specimen wall in this small specimen unless there are higher stress concentration sites on top and bottom sides due to deformation and protrusion. Next stage is thickening of the layer as shown Fig. 10(c). The thickening of the layer is accomplished by the accumulation of broken microspheres. The breakage at this stage may be due to
Fig. 7. Cross section of compressive specimen used for examination of deformation.
Fig. 8. Cross sections of Foam V specimens mounted in epoxy exhibiting a layer of crushing: (a) occurred in the middle of the specimen—the superimposed dashed line indicates the border between crushed and uncrushed layers; and (b) occurred at the top of the specimen.
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Fig. 9. A cross section of a specimen Foam V—the same specimen used for Fig. 8(a) showing internal structure and deformation after compressive loading: (a) top layer; (b) middle layer; (c) bottom layer; and (d) interface between top and middle layers in which the superimposed dashed line indicates the border between undensified and densified layers. The initiation of microsphere crushing occurred at a specimen midsection in this case.
another stress concentration caused by the contact of protruding microspheres with the layer of broken spheres. The grey microspheres shown in Fig. 10(c) represent the protruding microspheres. The process shown in Fig. 10(c) can thus be a major mechanism of thickening of the layer prior to the stage of a thickened layer shown in Fig. 10(d). Further loading at the stage of Fig. 10(d) repeats the same mechanism shown in Fig. 10(c). Obviously, all the sequence described for the layered crushing would begin at the start of plateau regime following the initial high slope deformation (Fig. 4).
4. Conclusions Syntactic foams of various densities were produced with a new method involving buoyancy. Failure mechanisms for produced syntactic foams have been studied. Two different failure modes were found—one is characterised by longitudinal splitting and the other by layered crushing. The former occurred at a low density of foam and latter at relatively high densities of foam. A schematic model for the layered crushing is proposed to explain its mechanism. Fig. 10. Suggested sequence of microsphere crushing: (a) initiation at a weakest microsphere; (b) lateral propagation due to stress concentration on adjacent microspheres; (c) saturation of lateral propagation in which grey microspheres indicate ones that are to be crushed after saturation due to stress concentration caused by contact of protruding microspheres with densified layer; and (d) thickened layer as a result of process ‘(c)’.
Acknowledgements The authors thank Hatexio Pty Ltd for the financial support for this work.
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References [1] Watkins L. Syntactic foam buoyancy for production risers. The Seventh International Conference on Offshore Mechanical and Artic Engineering, Houston, Texas, Feb. 7–12; 1988. pp. 403– 410. [2] Seamark MJ. Use of syntactic foam for subsea buoyancy. Cell Polym 1991;10(4):308–21. [3] Hinves JB, Douglas CD. The development of a hybrid advanced composite-syntactic foam structural component for use in undersea vehicles. IEEE 1993;III-468 –72. [4] Jize NN, Hiel C, Ishai O. In: Deo RB, Saff CR, editors. Mechanical performance of composite sandwich beams with syntactic foam cores. ASTM STP 1274; 1996. p. 125 –38. [5] Cvitkovich MK, Jackson WC. Compressive failure mechanisms in composite sandwich structures. J Am Helicop Soc October 1999; 260–8. [6] Narkis M, Gerchcovich M, Puterman M, Kenig S. Syntactic foams III. Three-phase materials produced from resin coated microballoons. J Cell Plast July/August 1982;230–2. [7] Lawrence E, Wulfsohn D, Pyrz R. Microstructural characterisation of a syntactic foam. Polym Polym Compos 2001;9(7):449–57. [8] Lawrence E, Pyrz R. Viscoelastic properties of polyethylene syntactic foam with polymer microballoons. Polym Polym Compos 2001;9(4): 227–37. [9] Verweij H, De With G, Veeneman D. Hollow glass microsphere composites: preparation and properties. J Mater Sci 1985;20: 1069–78. [10] Puterman M, Narkis M. Syntactic foams I. Preparation, structure and properties. J Cell Plast July/August 1980;223 –9. [11] Narkis M, Puterman M, Kenig S. Syntactic foams II. Preparation, and characterization of three-phase systems. J Cell Plast Nov/Dec 1980; 326–30. [12] Kenig S, Raiter I, Narkis M. Three-phase silicone based syntactic foams. J Cell Plast Nov/Dec 1984;423–9.
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[13] Meter C. Syntactic foam core material for composite structures, International patent classification: B29C, 65/00, B29D 9/00, B32B 3/ 26, 5/18. [14] Kim HS, Oh HH. Impact behaviour of syntactic foam. The first ACUN International Composites Meeting on Composites: Innovation and Structural Applications, 23–25 February, Sydney, Australia: University of New South Wales; 1999. [15] Kim HS, Oh HH. Manufacturing and impact behavior of syntactic foam. J Appl Polym Sci 2000;76:1324–8. [16] te Nijenhuis K, Addink R, van der Vegt AK. A study on composites of nylon-6 with hollow glass microspheres. Polym Bull 1989;21: 467–74. [17] Tsai SW, Hahn HT. Introduction to Composite Materials. Lancaster, Pennsylvania: Technomic; 1980. [18] Luxmoore AR, Owen DRJ. Chapter 9—syntactic foam. In: Hilyard NC, editor. Mechanics of Cellular Plastics. London: Applied Science Publishers Ltd; 1982. p. 359 –91. [19] Rizzi E, Papa E, Corigliano A. Mechanical behavior of a syntactic foam: experiments and modeling. Int J Solids Struct 2000;37: 5773– 94. [20] Gupta N, Kishore, Woldesenbet E, Sankaran S. Studies on compressive failure features in syntactic foam material. J Mater Sci 2001;36(18):4485–91. [21] Gupta N, Woldesenbet E, Kishore. Compressive fracture features of syntactic foams-microscopic examination. J Mater Sci 2002;37: 3199– 209. [22] Bunn P, Mottram JT. Manufacture and compression properties of syntactic foams. Composites 1993;24(7):565–71. [23] Kenig S, Raiter I, Narkis M. Three-phase carbon microballoon syntactic foam composites. Polym Compos 1985;6(2):100–4. [24] Gibson LJ, Ashby MF. Cellular Solids—Structure and Properties, 2 nd ed. Cambridge, New York: Cambridge University Press; 1997. p. 115.
Manufacturing and failure mechanisms of syntactic foam under compressionq Ho Sung Kim*, Pakorn Plubrai Department of Mechanical Engineering, School of Engineering, The University of Newcastle, Callaghan, Newcastle, NSW 2308, Australia
Abstract Failure mechanisms of syntactic foams under compression were studied. The syntactic foams produced with a new manufacturing method involving buoyancy were made of glass hollow microspheres and epoxy resin. The foam density was varied from 0.09 to 0.15 g/cc, which resulted from variation of resin concentration. Two different failure modes were identified and these were attributed to different failure initiation processes. One was initiated by longitudinal splitting and the other by layered crushing. The former occurred in the lowest foam density and the latter in higher densities used in the study. The failure sequence associated with the former mode was: (a) formation of multiple longitudinal cracks along the circumference of the compression specimen, (b) widening of longitudinal cracks and then (c) failure at one end of the specimen resulting in further lateral expansion. The sequence for the latter failure mode was deduced on the basis of the proposed schematic model: (a) the first failure initiation occurrence at a weakest microsphere, (b) failure of adjacent microspheres due to stress concentration resulting in propagation of microsphere failure laterally until it reaches the end of specimen wall, and (c) thickening of the crushed layer. The thickening of the crushed layer is due to accumulation of broken microspheres. q 2004 Elsevier Ltd. All rights reserved. Keywords: A. Foams; B. Fracture; C. Micro-mechanics; D. Mechanical testing
1. Introduction Syntactic foams, which are made of hollow microspheres and binder, have been used as structural components [1 – 5]. Various consolidating techniques for binder and microspheres have also been used, which includes coating microspheres [6], extrusion [7,8] and ones that use inorganic binder solution and firing [9], dry resin powder [10 – 13] for sintering, compaction [14 – 15], and liquid resin as binder [16] for in situ reaction injection moulding. The variety of manufacturing techniques can widen the applicability of syntactic foams. When syntactic foams are used as core materials for sandwich composites, they contribute to increase in specific stiffness of composites [17]. If they are used for protective structural components, they would contribute to reduction of damage and prevent failure of other components by inducing their own failure. Failure of syntactic foams in the literature has been sparsely covered in the 1980s, while most investigations on q
This paper is based on a presentation made at the International Composites Conference ACUN-4, held at the University of New South Wales, Sydney, July 2002. * Corresponding author. Tel.: þ 61-2-4921-6211; fax: þ61-2-4921-6946. E-mail address: [email protected] (H.S. Kim). 1359-835X/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2004.03.013
syntactic foams have been confined to manufacturing and physical properties [6,9 – 12,16]. Narkis et al. [6,11] found that failure of syntactic foams with low concentration resin is mainly by disintegration under compression. It has been reported, however, that relatively high-density foams under compression failed by the formation of 458 shear planes [18,19]. Recently, Kim and Oh [15], and Gupta et al. [20] have also reported that failure mode of syntactic foams with relatively high-concentration of resin under uniform compression was by shear on planes inclined approximately 458 to the loading direction. Also, Gupta et al. [20,21] highlighted that the shear failure is affected by specimen aspect ratio. Thus, the failure mode may depend upon various factors. In general, composites can be tailored for optimum performance. Syntactic foam itself is also a composite. Their contribution towards desired behaviour depends how they are designed and used. It is important to understand failure mechanisms of syntactic foam prior to development of syntactic foams to the needs of structural component design. The present paper focuses on compression loading to investigate failure mechanisms of syntactic foam produced with a new manufacturing method involving buoyancy.
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2. Experimental 2.1. Constituent materials The batch of hollow microspheres used in the study was made of soda-lime-borosilicate glass (Scotchlite Glass Bubbles K1) and manufactured by 3 M. A size distribution of microspheres was obtained using a Malvern 2600C laser particle size analyser as shown in Fig. 1. Particle densities were measured using an air comparison pycnometer (Beckman 930) and an average of three measurements was found to be 0.125 g/cc for microspheres. Bulk density of the microspheres was also measured, using a measuring cylinder with a tapping device (800 taps were conducted), to be 0.0762 g/cc. A resin system, used for binding microspheres, consisted of Epoxy 105 (West System—a blend of Bisphenol A and Bisphenol F), Slow Hardener 206 (West System—a blend of aliphatic amines and aliphatic amine adducts based on diethylene triamine and triethylenetetramine) and acetone (Septone, ASA20). All measured densities are listed in Table 1. Viscosities were also measured at 21 8C by a Brookfield syncro-electric viscometer (LVF 18705) for epoxy and hardener and a Cannon-Fenske Routine viscometer for acetone and its mixture. Viscosity values were found to be 940, 195 and 0.32 cP for epoxy, hardener and acetone, respectively. The viscosity of the mixture consisting of epoxy, hardener and acetone measured as a function of resin system ( ¼ epoxy þ hardener, at a mixing ratio of 5/1 for epoxy/ hardener by volume) is shown in Fig. 2. 2.2. Manufacturing of syntactic foams 2.2.1. Preliminary observation and packing time measurements Different stages of microspheres in a test tube after the mixture being shaken were observed as shown in Fig. 3: at the first stage (Fig. 3(a)), microspheres were fully dispersed; at the next stage, microspheres started to be packed by squeezing themselves in as shown in Fig. 3(b); and finally packing is completed as shown in Fig. 3(c). To measure the packing time taken between the first stage and the last stage,
Fig. 1. Size distribution of microspheres used.
Table 1 Densities of constituent materials for syntactic foam Material
Density (g/cc)
Microspheres (bulk) Epoxy105/slow hardener206 (volume ratio: 5/1) Acetone
0.0762 1.073 0.775
mixing of acetone and microspheres was conducted in a test tube. The times measured were 2 min 20 s and 2 min 40 s for a low acetone content (6/1/2-acetone/microspheres/ (epoxy þ hardener) by mass) and a high acetone content (40/1/2-acetone/microspheres/(epoxy þ hardener) by mass), respectively. 2.2.2. Manufacturing For each mould, a cylindrical tube made of wax with an inside diameter of 16 and 5 mm in wall thickness was produced and then cut to a length of 30 mm. The end opening was sealed by wrapping with HDPE plastic sheet and secured with an elastic band. The HDPE plastic sheet and wax had been tested for chemical reaction with the acetone and it was found that there was no visible chemical attack on the surfaces. Epoxy and hardener placed separately in two glass beakers were conditioned until their temperatures reach 38 8C in an oven, which was set to 38 8C before mixing at a ratio of 5/1 for epoxy/hardener by volume. Wax moulds were also conditioned at 38 8C for a minimum of 1 h prior to use. The temperature, 38 8C, was chosen because it is always a little higher than room temperature so that it allows us to keep it constant. For mixing epoxy, hardener and acetone, the following procedure was adopted: a clean empty mixing container was placed on an electronic scale and a pre-mixed epoxy/hardener was injected into the container using a plastic syringe until a required mass was reached; acetone was added; the container was then sealed and shaken vigorously for 2 min; the container was opened and the microspheres were added
Fig. 2. Viscosity of mixture consisting of epoxy, hardener and acetone as a function of volume fraction of resin system ( ¼ epoxy þ hardener, at a mixing ratio of 5/1 for epoxy/hardener by volume).
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Fig. 3. Mixture stages: (a) dispersed microspheres right after being shaken; (b) partially packed microspheres by buoyancy; and (c) fully packed microspheres.
through a glass funnel; and the container was sealed and shaken again vigorously for further 5 min for dispersion of microspheres. For subsequent casting, the container was kept shaken to maintain a constant mixture ratio, while the mixture was being poured through a tube into a mould. After casting, a duration of 3 min was allowed for self-packing of microspheres by self-buoyancy and then the hole at the bottom of the mould was opened by removing a sealing tape to drain the bottom layer of liquid. Subsequently, the top layer consisting packed microspheres and liquid phase slid down to the bottom of the mould. After 30 min, the foam in the mould was sufficiently dry and structurally self-supporting so that it was demoulded. Test specimens after demoulding were left for at least 3 days before testing-note curing time for epoxy used is 24 h at room temperature. Volume fractions were calculated assuming no acetone was left in foams and listed in Table 2. The void fraction ðvv Þ was calculated using v v ¼ 1 2 rf
mr m þ m rr rm
where r is the density, m is the mass fraction, and subscripts (f ; r and m) indicate foam, resin/hardener and microsphere, respectively. It is noted that volume fraction of voids increases with increasing volume fraction of epoxy/ hardener. The current manufacturing method is capable of producing a wide density range depending upon amount of diluents for binder and allows constituent materials to be easily mixed without damaging hollow microspheres. It is possible though that fabricated samples may not be
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homogeneous and isotropic but rather ‘functionally graded’ due to the difference in the rising/settling velocity of microspheres caused by the buoyancy effect. However, we have not found significant microstructure variation over the height of test specimens (Fig. 9(a), (c) and (d)). Probably, the microstructure have been affected by the initial microsphere locations as well before rising—if microspheres are close to the top surface, some of them would be packed early even though their rising velocity is low (if they were at the same depth, they would have more been ‘functionally graded’). A total of five different mixing ratios as listed in Table 2 were used to manufacture syntactic foams. 2.3. Test specimen preparation and compression tests Test specimens, 15 mm in length, were cut out from the moulded cylinders. Parallelism of top and bottom surfaces was ensured by measuring the lengths of three different locations along the circumference of the specimen using a pair of Vernier callipers—parallelism tolerance was ^ 2.58. Compression tests were conducted at a crosshead speed of 1.0 mm/min and an ambient temperature range of 22 – 25 8C using a universal testing machine (Shimadzu 5000) with a Hounsfield compression cage. Platens of Hounsfield compression cage were lubricated using a Multipurpose WD 40 to minimise friction between specimen and each platen. Five samples were tested for each mixing ratio—three of specimens were compressed to about 10% of the initial height, around which the initial breakage occurred, and the other two compressed until about 50% of the initial height of the specimen to observe crushing/densification process after the initial breakage.
3. Results and discussion Typical stress – strain curves for syntactic foams with five different mixing ratios are shown in Fig. 4. Two stages in each curve are seen—the initial high slope is followed by a low slope/plateau regime in ‘(II)’ to ‘(V)’ or followed by a negative slope in ‘(I)’. The initial high slope obviously corresponds to the elastic deformation of the foam and
Table 2 Foam compositions and properties Foam designation
Foam I Foam II Foam III Foam IV Foam V
Volume fractions for mixing
Properties
Volume fractions in foam calculated assuming no acetone exists in foams
Acetone
Epoxy and hardener
Micro-spheres
Density (g/cc)
Compressive strength (MPa) ^ SD
Elastic modulus (MPa)
Voids
Epoxy and hardener
Micro-spheres
0.656 0.622 0.606 0.550 0.503
0.019 0.037 0.053 0.126 0.172
0.325 0.325 0.325 0.325 0.325
0.093 0.105 0.120 0.134 0.145
0.164 ^ 0.074 0.460 ^ 0.060 0.551 ^ 0.037 0.952 ^ 0.080 1.447 ^ 0.089
8.19 18.48 31.12 38.03 71.41
0.476 0.527 0.535 0.656 0.680
0.0290 0.0484 0.0652 0.0960 0.1107
0.495 0.425 0.400 0.248 0.209
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Fig. 5. Longitudinal splitting between platens under compression of Foam I.
Fig. 4. Typical stress strain displacement curves for different mixing volume ratios of acetone/epoxy þ hardener/microspheres: (I) 0.656/0.019/0.325; (II) 0.622/0.037/0.325; (III) 0.606/0.053/0.325; (IV) 0.550/0.126/0.325; and (V) 0.503/0.172/0.325.
the low slope/plateau regime is easily related to crushing/ densification of microspheres (see crushing/densification mechanism below). The plateau regime is obvious in foam II, but as resin content increases (Table 2) it appears less obvious. The performance difference in crushing/densification may be due partly to the resin content—when the resin content is high, it is possible that smearing of resin over microspheres occurs and thus direct microsphere contact resulting in easy transferring the load for crushing is prevented. This is compared to those of different syntactic foams [20,22] in which the plateau region is due to crushing of microspheres after the initial failure by shear planes inclined 458 to the loading direction [20]. The transitional points between initial slopes and other slopes (Fig. 4) appear to occur at about a strain range of 0.03– 0.05 without a noticeable trend dependant upon specimen density. In general, the strength and the elastic modulus of the foam steadily increases as expected with increasing volume fraction of epoxy and hardener. These trends were also found elsewhere [22]. Mean values of compressive strength with standard deviation and elastic moduli of syntactic foam with various densities are listed in Table 2. Two different modes of failure under compressive loading for the five different resin concentrations were observed: one was characterised by longitudinal splitting; and the other by layered crushing. The details are given below. The longitudinal splitting occurred only in Foam I, which has the lowest foam density. Note the lowest foam density (0.096) is close to the microsphere bulk density of 0.076. The foam easily crumbled under a slight finger squeeze. This nature appears to be similar to that of low resin content foams which were manufactured by a coating method [6] or a powder mixing method [11]. The following failure sequence under compression was observed—(a) formation
of 3 – 6 longitudinal cracks along the compression specimen, (b) widening of longitudinal cracks and then (c) failure at one end of the specimen resulting in further lateral expansion as shown in Fig. 5. This failure is similar in terms of cracking direction to that of a foam containing high volume fraction of glass microspheres in carbon/glass/resin syntactic foam [23]. Layered crushing mode of failure occurred in Foams II – V (Table 2). It was initially observed that as the compression load increases the height of specimen decreases without substantial lateral volume expansion as shown in Fig. 6. This observation indicates that during compression, hollow microspheres were broken and subsequently densification occurred. There can be two possibilities for the breakage and densification. One is that the initiation might have occurred locally at the weakest location and then propagated. The other is that it might have occurred globally in a uniform manner throughout the specimen prior to densification. In order to test the two possibilities, photographs of cross sections as schematically shown in Fig. 7 were prepared to examine internal characteristics after compressive testing. Fig. 8 shows two different cross sections from different specimens but for the same Foam V. Fig. 8(a) shows three layers in which top and bottom layers appear to be the same and Fig. 8(b) two layers on each cross section. The different appearances are caused by the light reflection. The light reflection of the middle layer in the case of Fig. 8(a) appears to be different from those of top and bottom layers. The different reflections were found to be due to different microstructures. SEM photographs were taken using the same specimen used for Fig. 8(a) and are shown in Fig. 9 to examine the difference in microstructure. The features of top (Fig. 9(a)) and bottom (Fig. 9(c)) layers are in contrast with those of middle layer (Fig. 9(b)) in which most of microspheres are seen to be crushed and densified. In addition, Fig. 9(d) shows top and middle layers in which the dashed line indicates the interface between the two layers. These observations indicate that the initiation of microsphere crushing in this case occurred at the middle section of the specimen.
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Fig. 6. Typical deformation stages of Foam V under compression: (a) before loading; (b) after being subjected to a compressive loading; and (c) further compressive loading.
From the evidence in SEM photos (Fig. 9), the latter possibility suggested above that the initiation might have occurred globally and in a uniform manner throughout the specimen can be rejected. Also, it is interesting to note that the progressive nature of layered failure is similar to deformation of honeycomb [24]. Now a question arises as to how the layer can be formed by the initiation occurred locally at a weakest location and then thickening of the layer occurs. The answer to the question may be given using the sequence shown schematically in Fig. 10. Fig. 10(a) shows the first failure initiation of a weakest microsphere. Even though no evidence was found, the failure initiation site would be multiple if the specimen is large (higher probability). Fig. 10(b) shows failure of adjacent microspheres due to stress concentration. At this stage, the stress concentration sites would be located rather on the lateral sides of the first initiation site than top or bottom side because of the loading direction. Thus, the fracture of microspheres propagates laterally until it reaches the end of specimen wall in this small specimen unless there are higher stress concentration sites on top and bottom sides due to deformation and protrusion. Next stage is thickening of the layer as shown Fig. 10(c). The thickening of the layer is accomplished by the accumulation of broken microspheres. The breakage at this stage may be due to
Fig. 7. Cross section of compressive specimen used for examination of deformation.
Fig. 8. Cross sections of Foam V specimens mounted in epoxy exhibiting a layer of crushing: (a) occurred in the middle of the specimen—the superimposed dashed line indicates the border between crushed and uncrushed layers; and (b) occurred at the top of the specimen.
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Fig. 9. A cross section of a specimen Foam V—the same specimen used for Fig. 8(a) showing internal structure and deformation after compressive loading: (a) top layer; (b) middle layer; (c) bottom layer; and (d) interface between top and middle layers in which the superimposed dashed line indicates the border between undensified and densified layers. The initiation of microsphere crushing occurred at a specimen midsection in this case.
another stress concentration caused by the contact of protruding microspheres with the layer of broken spheres. The grey microspheres shown in Fig. 10(c) represent the protruding microspheres. The process shown in Fig. 10(c) can thus be a major mechanism of thickening of the layer prior to the stage of a thickened layer shown in Fig. 10(d). Further loading at the stage of Fig. 10(d) repeats the same mechanism shown in Fig. 10(c). Obviously, all the sequence described for the layered crushing would begin at the start of plateau regime following the initial high slope deformation (Fig. 4).
4. Conclusions Syntactic foams of various densities were produced with a new method involving buoyancy. Failure mechanisms for produced syntactic foams have been studied. Two different failure modes were found—one is characterised by longitudinal splitting and the other by layered crushing. The former occurred at a low density of foam and latter at relatively high densities of foam. A schematic model for the layered crushing is proposed to explain its mechanism. Fig. 10. Suggested sequence of microsphere crushing: (a) initiation at a weakest microsphere; (b) lateral propagation due to stress concentration on adjacent microspheres; (c) saturation of lateral propagation in which grey microspheres indicate ones that are to be crushed after saturation due to stress concentration caused by contact of protruding microspheres with densified layer; and (d) thickened layer as a result of process ‘(c)’.
Acknowledgements The authors thank Hatexio Pty Ltd for the financial support for this work.
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