Composites: Part B 43 (2012) 1024–1029

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Composites: Part B

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Stochastic honeycomb sandwich cores

M. Hostetter a, B. Cordner b, G.D. Hibbard a,⇑a Department of Materials Science and Engineering, University of Toronto, 184 College Street, Toronto, Ontario, Canada M5S 3E4b Faculty of Architecture, University of Toronto, 230 College Street, Toronto, Ontario, Canada M5T 1R2

a r t i c l e i n f o a b s t r a c t

Article history:Received 5 July 2011Received in revised form 28 October 2011Accepted 10 December 2011Available online 20 December 2011

Keywords:A. HoneycombB. BucklingB. Mechanical PropertiesSandwich structures

1359-8368/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.compositesb.2011.12.002

⇑ Corresponding author. Tel.: +1 416 946 0437; faxE-mail address: glenn.hibbard@utoronto.ca (G.D. H

This study presents a new type of cellular material that is simpler to make than conventional honey-combs and foams, while possessing many of the exceptional mechanical properties that make foamsand honeycombs valuable as low density structural materials. The stochastic honeycombs had relativedensities ranging from 7% to 13% with compressive strengths between 1 and 3 MPa, and compressivestiffnesses between 60 and 130 MPa. These values are comparable to those seen for commercial polypro-pylene honeycombs and significantly higher than those of commercial polypropylene foams. Rigid sand-wich panels could be fabricated by reinforcing the stochastic honeycomb cores with externalpolypropylene face sheets. The panels could be assembled without adhesives, creating a rigid sandwichmade entirely of a single material.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Polymeric cellular materials have enabled important advancesin the aerospace, automotive and packaging industries [1,2]. Themost common uses of foams and honeycombs are as the core insandwich panels, where two relatively thin and stiff sheets are sep-arated by a thicker low-density cellular core [3]. Historically, poly-mer core sandwich panels have primarily had metal face sheets [4],but over the past 15 years it has become more and more commonfor the face sheets to be a polymer matrix composite adhesivelybonded to the core [5,6]. There are many types of adhesives, themost common being epoxies or other thermosets [5,7].

Various methods have been used to improve the mechanicalproperties of polymer foams and honeycombs. Fiber addition hasbeen used to improve the properties of bulk polymers [8] andhas recently been used to improve the properties of honeycombsandwich structures as well [9]. Clay nanocomposites are alsobeing used to improve the properties of polymer foams [10]. Whilethese methods can improve the mechanical properties of the cellu-lar material, they typically do so at the expense of recyclability[11].

The greatest potential for recycling sandwich materials is if asingle material could be used for the core, face sheets and adhesivelayers [12]. This would allow the entire structure to be recycledwithout the need for additional (and costly) steps to separate thedifferent materials. In the present study, high melt strength poly-

ll rights reserved.

: +1 416 978 4155.ibbard).

propylene (HMS PP) was used to create a novel polymeric cellulararchitecture having a honeycomb-like structure (Fig. 1), in which anetwork of interconnected webs are oriented perpendicular to apair of polymeric skins (effectively creating a sandwich structurewith built-in skin layers). This starting stochastic honeycomb corecan be reinforced by thermally welding conventional PP sheets tothe outer faces in order to create a rigid sandwich structure(Fig. 1b) that is composed entirely of a single material.

2. Experimental

The fabrication method presented here is a new, simple, low-cost process that requires no blowing agents, additives or adhe-sives (PCT Application date June 2011). The operating temperaturein the furnace was 180 �C, and the press consisted of a simpleframe with adjustable height settings to guide the position of twinaluminum platens. The internal temperature of the furnace and thetemperature of the Al platens were measured by thermocouple andan infrared laser thermometer was used to monitor the tempera-ture of the polymer in the furnace and in the press.

The platens were preheated in the furnace until they reached atemperature between 60 and 100 �C. High melt strength PP pellets(0.31 N melt strength, measured with a capillary rheometer andforce transducer recording the force required to fracture the melt[13]) were then placed on the lower platen and left in the furnaceuntil they formed a viscous melt (approximately 10–12 min). Atthis point, the platens along with the PP were removed from thefurnace and placed in the press. The platens were compressed forapproximately 30 s to ensure hot tack adhesion between the upperplaten and the PP. The upper platen was then raised vertically in

Fig. 1. Top view of an as-fabricated stochastic honeycomb core showing the cellularweb structure (a). Also shown is a three point bend coupon 4.5 cm � 20 cm, 20 mmcore thickness, reinforced with conventional polypropylene and an as-fabricatedstochastic honeycomb core compression 3 cm � 3 cm test coupon, 20 mm corethickness (b).

Table 1Mean (±standard deviation) of web thickness and length for four density ranges.

Density (%) Thickness (mm) Length (mm)

7.9–8.0 0.547 ± 0.138 4.20 ± 2.298.0–8.5 0.483 ± 0.120 4.27 ± 2.589.4–9.7 0.477 ± 0.130 4.21 ± 3.1410.5–11.0 0.544 ± 0.156 4.47 ± 2.46

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the press, expanding the polymer uniaxially and spontaneouslycreating the webbed structure. It was locked in place at the desiredsample height and the press was left to cool. Once the pressreached a temperature of approximately 35 �C (after 6–7 min)the PP stochastic honeycomb separated itself from the aluminumplatens.

The current fabrication window spans expansion heights of 10–35 mm, with relative densities ranging from 5% to 14%. For thisstudy, a height of 20 mm was selected with relative densities rang-ing from 7% to 12%. Given the relatively large variability in localcell structure, at least five samples were tested for each densityrange. The relative density of each core was estimated accordingto Eq. (1), where m is the sample mass, ts is the average skin thick-ness (over approximately 50 measurements), w is the width of thesquare sample, V is the volume, qcore is the relative density of thecore, and qPP is the density of the bulk PP:

qcore ¼m� ð2ts �w2 � qppÞ

V � ð2ts �w2Þ � 1qpp

ð1Þ

30 mm � 30 mm coupons were cut from the as-fabricated samplesand used for compression testing at a cross-head speed of 1 mm/min (after [14]). Web collapse mechanisms during compressiontesting were investigated by pre-loading samples to characteristicuniaxial strain values and studying the deformed web structure ina scanning electron microscope (SEM). Three point bend sampleswere also made in the same manner, and cut into samples200 mm long and 45 mm wide (after [15,16]). The three-point bendtests were conducted at a cross-head speed of 1 mm/min and spanlength of 100 mm.

PP face sheets were purchased commercially, having thick-nesses of tf = 0.73 mm, 0.94 mm, 1.19 mm and 1.75 mm. The sheets

were cut to size and joined to a subset of the three point bend sam-ples using thermal welding. The reinforcing sheets were heated onthe upper and lower Al platens in the furnace until they formed apartial melt (i.e., the PP was soft but not flowing). The lower platenwas then fixed in the press as before, with the three point bendsample placed on top, and the upper Al platen fixed in the pressat a height of 20 mm. This provided sufficient pressure to ensureadhesion between the PP face sheets and the as-fabricated samplewhile not affecting the web structure.

3. Cellular architecture

Since the cellular architecture spontaneously forms as theopposing Al platens are separated, there is large variation in theas-fabricated web thickness, web length and angle between adja-cent webs. This variation was characterized by imaging thecross-section of the samples at mid-height and averaging thethickness over the web length in order to give a single value perweb. Given the relatively large dispersions of structures at a partic-ular density and the relatively narrow range of densities consid-ered (�5%), it was difficult to establish a clear trend withdensity. This is clearly illustrated in Table 1, where the mean andstandard deviation in web thickness and length for four densityranges is presented. There was a large coefficient of variance (stan-dard deviation normalized by the mean) for both the thickness andlength parameters; the coefficient of variance was between 25%and 30% for the thickness values, and for the length values, thecoefficient of variance was found to be between 55% and 75%, overall densities. Fig. 2 presents histograms of web thickness and weblength as global histograms covering all densities. Measurementsfrom approximately 500 webs are included here. The thickness his-togram can be seen to be almost Gaussian, while the length and as-pect ratio histograms are positively skewed. The inset on Fig. 2billustrates this variation in web length.

There is a complex network of partial webs that connect theslender webs of the stochastic core to the built-in skin. This canbe seen in Fig. 3, which presents a series of SEM images of stochas-tic honeycombs cut at mid-height. While much of the mass of thestructure is contained in either the skin or the complete webs (i.e.those webs extending continuously from lower to upper skin),there are also numerous incomplete webs. These webs may havegaps which extend to the built-in skin (Fig. 3a) or be fully con-tained within the web (indicated by an arrow in Fig. 3b).

There are three key properties of the high melt strength poly-propylene used in this study that allows the architecture to beformed: hot tack adhesion, a low melt flow rate, and the low sur-face energy of PP. The first allows the PP to form an immediatebond with the Al when the platen is hot [17]. The PP adheresstrongly to the Al in the melt state, allowing it to be expandeduniaxially in the press. Secondly, the PP has a low melt flow rate(MFR) of 2.4 g/10 min (as measured by ASTM D1238 [18]) whichlimits the flow of PP in the melt state [19]; the high melt strengthof the PP used in this study (0.31 N following [13]), was higher thanthat of conventional PP, which is typically between 0.01 N and0.20 N [20]. Lastly, while PP has good hot tack adhesion to Al, ithas very low surface energy, making it a poor adhesive. Special sur-face treatments are required, or polar compounds need to begrafted onto the polymer backbone, to increase the surface energy

Fig. 2. Variation in web thickness, t (a) and web length, l (b) over all densities.

Fig. 4. Typical uniaxial compression curves for stochastic honeycomb cores.

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enough for it to function as an adhesive. The low surface energy isan asset in this application as it causes the sample to spontane-ously separate from the Al plates when it has cooled.

Finally, because the sandwich core is composed of a singlematerial, the recyclability of stochastic honeycombs is high; itcan be ground up and pelletized without need for separating theface sheet from the core. In certain cases, it may also be possibleto refabricate the samples without the need for repelletization.For example, preliminary studies have indicated that it is possibleto fabricate a stochastic honeycomb core, crush it in uniaxial com-pression, and then re-expand the crushed core without any signif-icant loss in the refabricated mechanical properties. Thischaracteristic of stochastic honeycombs may become importantin energy absorption applications.

4. Mechanical properties

4.1. Uniaxial compression

Fig. 4 shows typical uniaxial compression stress–strain curvesfor three different densities of stochastic honeycombs. The curves

Fig. 3. SEM images showing the internal structure of the stochastic honeycomb architecmid-height gap indicated by the arrow (b), and the complex buttressing of the webs (c)

consist of an initial peak stress, followed by a broad valley in whichthe stress is relatively constant until final densification. The overallform of the curves resembles that seen for out-of-plane compres-sion in conventional honeycombs (e.g. [4]). To determine how thewebs collapsed during compression, deformation in the samemid-height location of a q = 11.8% sample was tracked from theas-prepared state through to final densification; Fig. 5 presentsSEM images of the as-fabricated structure (Fig. 5a) and after pre-loading to strains of e � 0.05 (Fig. 5b), e � 0.25 (Fig. 5c) ande � 0.35 (Fig. 5d). The slenderness of the web structure can be seenin the as-prepared state (Fig. 5a). Local plastic buckling (indicatedby arrows) can be seen in samples pre-loaded to the peak stress(Fig. 5b). Large scale global buckling had occurred by the timethe valley stress had been reached (Fig. 5c) and continued throughdensification (Fig. 5d).

Fig. 6 shows the peak strength and the stiffness plotted againstthe relative density. While the strength increased from 1.0 MPaat 7.2% relative density to 2.4 MPa at 11.5% relative density,the specific strength values were approximately constant at21.8 ± 2.0 kPa m3/kg. Similarly, the stiffness varied from 60 MPato 130 MPa over the same density range, and the specific stiffnesswas nearly constant at 1.18 ± 0.09 MPa m3/kg.

Comparing these specific strength and stiffness values to thoseseen for commercial foams and honeycombs over a similar densityrange yields encouraging results. The specific strength for stochastichoneycombs (21.8 kPa m3/kg) is approximately 3–5 times higherthan that of conventional, commercial PP foams (e.g. 3–4 kPa m3/kg [21] and 3–8 kPa m3/kg [22]); likewise the specific stiffness(1.18 MPa m3/kg) was approximately 4–10 times higher (e.g.0.10–0.30 MPa m3/kg [23]) than that of commercial PP foam. Whencompared to PP honeycombs of comparable densities, the specificstrength was on par with those found for commercial honeycombs(16–28 kPa m3/kg [24] and 10–22 kPa m3/kg [25]). Similarly, thespecific stiffness of the stochastic honeycombs was comparable to

ture. An incomplete web with a gap extending to the skin is shown (a), along with a.

Fig. 5. SEM images of the web structure in a q = 11.8% sample (a) and afterpreloading to strains of e � 0.05 (b), e � 0.25 (c) and e � 0.35 (d).

Fig. 6. Stochastic honeycomb compressive strength (a) and stiffness (b) over arange of core densities.

Fig. 7. Typical load–deflection curves for as-prepared stochastic honeycomb inthree point bending.

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that of one commercial honeycomb (0.78–1.21 MPa m3/kg [24]) andslightly higher than that of another commercial PP honeycomb(0.19–0.46 MPa m3/kg [25]).

4.2. Three-point bend testing

The stochastic honeycombs were first tested in the as-fabricatedcondition. Fig. 7 shows typical load–deflection curves for sampleshaving relative densities of 8.8%, 9.8%, and 10.8%. As the density in-creased, the samples failed at a lower deflection and a higher load.The built-in face sheets meant that there was a continuous transition

from core to outer skin, making it difficult to distinguish between thenearly simultaneous face sheet and core fracture mechanisms. Fig. 8plots the flexural strength and flexural stiffness of the stochastichoneycomb cores as a function of relative density. The flexuralstrength and flexural stiffness increased from 0.53 MPa to0.95 MPa and from 61.4 MPa mm2 to 131.0 MPa mm2 respectivelyas the relative density increased from 8.4% to 10.6%. The significantlygreater flexural strength and stiffness of the higher relative densitystochastic honeycomb cores is partly explained by differences in thebuilt-in skin thickness. For example, the average skin thickness on aq = 11.0% sample was �0.24 mm while the average skin thicknesson a q = 8.7% sample was �0.14 mm.

The stochastic honeycomb cores were also reinforced withpolypropylene sheets having thicknesses varying from 0.73 mm

Fig. 8. Flexural strength (a) and flexural stiffness (b) of stochastic honeycomb coresin three point bending.

Fig. 9. Failure map plotting regions of dominant sandwich panel failure mecha-nism. The dominant failure mechanism at low sheet thickness was face sheetwrinkling, core shearing dominated at intermediate thickness, and face sheetyielding and fracture dominated at the greatest thickness.

Fig. 10. Typical three point bending load deflection curves illustrating face sheetyield and fracture (tf = 1.75 mm, a), core shearing (tf = 0.94 mm, b) and face sheetwrinkling, (tf = 0.73 mm, c).

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to 1.75 mm. Sandwich panels under three-point bend testing typ-ically fail by one of four mechanisms: delamination of the facesheet, wrinkling of the face sheet, core shear failure, and face sheetyield and fracture [4]. All four of these failure mechanisms wereobserved in the reinforced sandwich structures of the presentstudy. However, it was found that through careful sample prepara-tion, delamination of the face sheet could essentially be eliminated.Fig. 9 summarizes the failure mechanisms in the form of a failuremap for these samples, where the dashed lines show the transitionbetween dominant failure modes. It is apparent that at the lowerface sheet thicknesses and lower densities, wrinkling was the dom-inant failure mechanism. As the face sheet thickness increased,along with the density, core shearing and then face sheet yield

and fracture consecutively became the dominant failure mecha-nisms. Typical load–displacement curves for each of these failuremechanisms are shown in Fig. 10.

The samples that failed primarily through face sheet yield andfracture failed at the highest applied loads (Fig. 10a). These sam-ples generally deformed to between 4 and 6 mm deflection andthen snapped, failing abruptly. For tf = 1.75 mm, the flexuralstrength and stiffness varied from 5.46 MPa and 544 MPa mm2 at7.6% relative density to 6.12 MPa and 614 MPa mm2 at a relativedensity of 8.7%, respectively.

Core shearing (Fig. 10b) occurred mostly in the samples with facesheet thicknesses of 0.94 mm and 1.19 mm. In both cases, crackingnoises could be heard as the core progressively sheared, corre-sponding to periodic load drops in the load–displacement curves.For both face sheet thicknesses, the flexural strength and stiffnessincreased with core density. When tf = 0.94 mm, the flexuralstrength and stiffness increased from 4.32 MPa and 370 MPa mm2

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at 7.4% relative density to 5.24 MPa and 418 MPa mm2 at 9.4% rela-tive density, respectively. Similarly, when tf = 1.19 mm, the flexuralstrength and stiffness increased from 4.48 MPa and 455 MPa mm2

at 7.5% relative density to 4.65 MPa and 629 MPa mm2 at 9.4%relative density, respectively.

Wrinkling of the face sheet (Fig. 10c) caused the samples to foldin on themselves, and most did not fracture but rather slid off ofthe supports after reaching displacements of more than 65 mm.Samples that failed by face sheet wrinkling had the thinnest PPsheet reinforcement and failed at the lowest peak loads. Whentf = 0.73 mm, the flexural strength and stiffness increased from1.88 MPa and 301 MPa mm2 at 7.0% relative density to 2.73 MPaand 333 MPa mm2 at 8.9% relative density, respectively.

In the reinforced sandwich panels, failure was either largelycontrolled by the face sheet (in face sheet yield and fracture) orby the core (in core shearing and wrinkling). When sample failurewas controlled by the face sheet (for tf = 1.75 mm), the specificflexural strength and stiffness varied by less than 2% over all thesamples, at 78.3 ± 0.9 kPa m3/kg and 7.88 ± 0.14 Pa m5/kg respec-tively. Alternatively, when sample failure occurred primarily inthe core, much larger variation in the specific flexural strengthand stiffness was seen. For tf = 1.19 mm (failure by core shearing),the specific flexural strength was 60.8 ± 5.5 kPa m3/kg, a variationof 9%, and the specific flexural stiffness was 6.60 ± 0.81 Pa m5/kg,a variation of 12%. For tf = 0.73 mm (failure by wrinkling), the spe-cific flexural strength and stiffness were 30.9 ± 3.1 kPa m3/kg and4.22 ± 0.54 Pa m5/kg, variations of 10% and 13% respectively.

At constant core density, the peak load increased with increas-ing face sheet thickness. Likewise, at constant face sheet thickness,the peak load increased with increasing core density. It should benoted that relatively larger sample-to-sample variability was seenin the flexural properties of the as-prepared samples than the rein-forced samples. For example, at a core relative density of 8.8%, theflexural strength values for the as-prepared samples were within15%, while at the same density, the reinforced samples varied by5% on average. In flexure, fracture occurs at the outer surface andis more sensitive to changes in the local built-in or reinforced facesheet thicknesses. The reinforced sheets were not only much thick-er, but also more uniform than the built-in skins, accounting forthe reduction in variability in the reinforced samples. Furtherdevelopment of stochastic honeycomb sandwich panels could in-clude using alternative materials for the face sheets. Glass-fiberreinforced PP sheets, for example, would have higher tensilestrength and stiffness, increasing the flexural properties of thesandwich panels, but could still be thermally welded in place.

5. Conclusions

This study has shown that with minimal equipment, and using asimple, one step process, it is possible to fabricate a PP stochastichoneycomb core that has comparable compressive performanceto conventional PP honeycomb cores and can exceed the perfor-mance of conventional PP foams. The built-in skin allows externalface sheets to be joined to the core without using an adhesive.Employing a single thermoplastic material allows the stochastichoneycombs to be recycled without any special preparation, andallows for the possibility of refabrication (melting down the sam-ples and re-expanding them uniaxially in the press) withoutrepelletization.

When the as-fabricated stochastic honeycomb cores weretested in three point bending, the panels failed by face sheet yield

and fracture, and the flexural strength and stiffness varied with thecore relative density. When the cores were reinforced with exter-nal PP sheets, all four of the typically observed sandwich panel fail-ure mechanisms were observed: face sheet yield and fracture, coreshearing, face sheet wrinkling and delamination. With carefulsandwich preparation it was possible to eliminate delaminationin the reinforced samples. For all of the sandwich panels, the flex-ural strength and stiffness increased with core relative density at aconstant face sheet thickness, and also increased with increasingface sheet thickness at constant core relative density.

Acknowledgment

Financial support from the Natural Sciences and EngineeringResearch Council of Canada (NSERC) is gratefully acknowledged.

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