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Materials Science and Engineering A 528 (2011) 5621–5626

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

lexural creep behavior of sandwich panels containing Kraft paper honeycombore and wood composite skins

heng Chena,1, Ning Yana,∗, James Dengb, Greg Smithc,2

Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ont. M5S 3B3, CanadaComposite Products, FPInnovations, 319 rue Franquet, Quebec, QC, G1P 4R4, CanadaDepartment of Wood Science, University of British Columbia, 2935-2424 Main Mall, Vancouver, BC V6 T 1Z4, Canada

r t i c l e i n f o

rticle history:eceived 4 February 2011ccepted 23 March 2011

a b s t r a c t

Flexural creep behavior is an important performance related characteristic for sandwich panels usedas products, such as kitchen bench tops, table legs, and bookshelves. In order to characterize the creepbehavior of the sandwich panels with Kraft paper honeycomb core and wood composite skins, a series

vailable online 29 March 2011

eywords:reepandwichoneycomb

of creep tests were carried out under a constant three-point bending. The sandwich panels containeddifferent types of core and skin materials as well as various core and skin thicknesses. The flexural creepdeflection as a function of time for each type of sandwich panel was measured. The results show thatthe flexural creep behavior of the sandwich panel is affected by honeycomb core shape, core and skinthickness, and skin material type.

raft paperood composite

. Introduction

Sandwich panels containing Kraft paper honeycomb cores andood composite skins are of increasing interest in the furniture

ndustry due to their lightweight and lower cost. It is known thattiffness and deformation of sandwich panels with paper-basedoneycomb cores are time dependent and can be significantly influ-nced by the loading rate [1]. For sandwich panels intended to beoaded for long durations, at elevated temperatures, or high relativeumidity, their creep performance is an important quality.

Unfortunately there is little published work related to the creepehavior of paper honeycomb core sandwich panels in the liter-ture. Some published work on the creep of sandwich panels hasocused on using numerical approach to analyze and predict thereep behavior of the sandwich panels [2]. However, these studiesere done on aluminum honeycomb, solid wood panels, connec-

ors, or concrete [3–7] and may not be applicable to Kraft paper

ollow core panels.

Research dealing with the creep rate of wood and wood compos-tes has focused on the influence of moisture content, temperature,tress level, species, and adhesive type [8–10]. These studies have

∗ Corresponding author. Tel.: +1 416 946 8070; fax: +1 416 978 3834.E-mail addresses: cxc620@yahoo.ca (Z. Chen), ning.yan@utoronto.ca (N. Yan),

ames.deng@fpinnovations.ca (J. Deng), greg.smith@ubc.ca (G. Smith).1 Tel.: +1 416 304 1309; fax: +1 416 978 3834.2 Tel.: +1 604 822 0081.

921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2011.03.092

© 2011 Elsevier B.V. All rights reserved.

shown that creep rate increases with increasing temperature, mois-ture content, and stress level. Resin types also affect creep rate dueto their differing degrees of hydrophobicity [11,12]. Creep rate isalso found to be a function of the size of the wood component withthe product of large size wood components [8,13,14], i.e., veneer,having much smaller creep rates than the products made of smallsize fibers, as indicated in Table 1.

There is no published data on the influence of Kraft paper honey-comb core type, cell shape, thickness ratio of core to skin (shellingratio), or material properties of the skin on the creep behavior ofthe sandwich panels. However, an early study on the stiffness of thesandwich panels with honeycomb paper core and wood compositeskins shows that shelling ratio and large cell size are correlated withthe panel stiffness [15]. As a panel undergoes creep, deformationcan be exacerbated. Undesirable changes in the stress level of thepanel may be produced and lead to instabilities resulting in failureof the panel. It is the aim of this study to measure and report thecreep behavior of Kraft paper honeycomb core panels made fromvarious core configurations and composite skin types.

2. Quantitative analysis

In this study, a series of flexural creep test of sandwich pan-

els with varied Kraft paper honeycomb cores and wood compositeskins were carried out under a constant load. Temperature and rela-tive humidity of the test environment were kept constant followingASTM standard C393 [16]. The creep of the constituent componentsas well as the assembled panels were measured as functions of time.

5622 Z. Chen et al. / Materials Science and Eng

Table 1Comparison of the relative creep rate of different wood composites types.

Relative creep rate References

T

ı

wud

osdocob

ε

ı

ı

wpmoctc

dg

ı

wd

ı

Ft

Hardboard > particle board > waferboard Dinwoodie et al. [13]Fibreboard > particleboard > plywood Perkitny and Perkintny [14]MDF > OSB or MDF > chipboard Pritchard et al. [8]

he relative total flexural creep deflection was defined as:

t−l = (ıtt − ıte)ıte

(1)

here ıt−l is relative total flexural creep deflection; ıtt is the flex-ral creep deflection at time t; ıte is the instantaneous elasticeflection under load.

A sample undergoing creep goes through three distinct stagesf deformation: primary, secondary, and tertiary creep as shownchematically in Fig. 1. Primary creep is characterized by a steadilyecreasing creep rate. Secondary creep refers to the linear portionf the curve where the creep rate is essentially constant. Tertiaryreep is characterized by a rapidly increasing creep rate and occursver a relatively short time frame. The behavior is usually describedy the following Eqs. (2)-(4):

c = f (�, T, t) (2)

t−1 = A1tN1 (t ≤ tp) (3)

t−1 = ı0 + A2t (tp < t ≤ ts) (4)

here εc is the creep deformation, � is the stress level, T is the tem-erature and t is time. A1, and N1 are constants that may vary withaterial type, temperature and humidity; tp is the time at the end

f the primary creep stage. ts is the time at the end of the secondaryreep stage. ı0 is the intercept of the secondary creep stage line withhe total relative deflection axis, and A2 is the relative total flexuralreep deflection rate during the secondary creep [17,18,19].

Based on Timoshenko’s beam theory [20], the relative totaleflection is also dependent on the bending and shear deflectionsiven by the following equation:

t−1 = ıb + ıs (5)

here ıb and ıs is the relative bending deflection and relative sheareflection, respectively.

The relative bending deflection can be expressed as:

b = PL3

48D(6)

ig. 1. A typical creep curve for a viscoelastic material. εc is the creep strain and εc0

he instantaneous elastic deformation when instantaneous load is applied.

ineering A 528 (2011) 5621–5626

where L is the sample span, P is the applied flexural constant load,D is the flexural creep stiffness and is defined as:

D = Ef (t)(d3 − c3)b12

(7)

where d is the specimen thickness, c is the core thickness, and b isthe specimen width. Ef (t) is flexural stiffness of the skin materialexpressed as a function of time, t, and can be calculated using thefollowing equation with the flexural creep data obtained throughtesting of the skin material:

Ef (t) = Pf Lf3

4bf df3ıf (t)

(8)

where Pf is the constant load using in the creep test of the skinmaterial, ıf (t) is the relative deflection in the creep test for theskin material at time t, Lf is the sample span, bf is the specimenwidth, and the specimen thickness is denoted as df [16]. Thus thecurves for bending deflection of sandwich panels with differentcore shapes versus time can be plotted using Eqs. (6)–(8).

The relative shear deflection can be expressed as:

ıs = PL

4U(9)

where P is the applied flexural constant load in the creep test andU the shear rigidity which is defined as

U = Gc(t)(d − c)3b

4c(10)

where Gc(t) is the honeycomb core shear modulus as a function oftime.

3. Methods and materials

3.1. Samples

The core materials used for flexural creep specimens were twoshapes of Kraft paper honeycomb core: an expanded core with a31.75 mm size cell and a corrugated core with a 19.05 mm size cell,both shown in Fig. 2 (which were provided by the Casewell Prod-ucts Company in Vancouver, BC, Canada and Pregis Company inDeerfield, IL, USA, respectively). Three kinds of wood compositewere used as the skins: hardboard (H), medium density fibreboard(M), and plywood (P). All sandwich panel specimens were manu-factured by the Wood Composites Group at the University of BritishColumbia [21].

3.2. Flexural creep test

In order to carry out the flexural creep test for the sandwichpanels a test frame was made according to ASTM C480 [22]. Thelayout of the set up is shown schematically in Fig. 3. The size of thespecimens used in flexural creep testing of each type of sandwichpanels (Table 2) was selected following ASTM standard C393 [16].A static load was applied to each sandwich panel at the mid span.A linear variable differential transformer (LVDT) was positioned atthe center of the span through the load head placed on the top skinof the specimen where the load head was connected to the staticweight as shown in Fig. 4; 32 specimens under constant flexuralloading can be tested simultaneously provided they are of the samesample span. The test temperature was kept constant at 21 ± 1 ◦Cand the relative humidity (RH) was at 70 ± 2%.

The various combinations of skin and core materials were usedto make the sandwich panels from which the beam sections werecut; their dimensions and testing parameters are listed in Table 2.The static load for each sandwich panel type and thickness was setto be 1/3 of static ultimate flexural load of those specimens. During

Z. Chen et al. / Materials Science and Engineering A 528 (2011) 5621–5626 5623

Fig. 2. Two types of Kraft paper honeycomb cores used for creep testing. The left is the corrugated honeycomb core and the right is expanded honeycomb core. The ribbondirection in both cases is parallel to the X-axis.

exura

tt3lpr

4

4

t

Fc

Fig. 3. Schematics of the fl

he creep experiments, the total flexural creep deflection of eachest piece was recorded every 5 s for the first 5 min and then every0 min thereafter. Each test piece was loaded at a constant stress

evel for 167 h, and then the specimen was unloaded for 119 h torovide a measure of the amount of irrecoverable deflection. Twoeplicates were tested for each specimen type.

. Results and discussion

.1. Creep failure determination:

Examination of Table 3 shows that the irrecoverable deflec-ion of all but the panel with the corrugated core oriented in the

ig. 4. Photograph of the test frame used for flexural creep testing of the honeycombore sandwich panels.

l creep testing apparatus.

x-direction (H3C26X) was a little larger than the instantaneousdeflection. Based on this result it was judged that all of these sam-ples did indeed creep. Failure in creep is defined as the time atwhich the transition from secondary creep to tertiary creep occurs.

4.2. Effect of honeycomb core structure and orientation:

The most evident example of this transition in Fig. 5 is for theexpanded core panel oriented in the x-direction (H3E26X), whichfailed at approximately 28 h. The failure times for the other ori-entation and core type was similar and indicated that there wasno influence of core type or orientation on the creep failure time.The effect of core orientation was evident from Fig. 5. Both the cor-rugated and expanded cores oriented in the y-orientation (wherethe ribbon direction was perpendicular to the sample length, i.e.,H3E26Y and H3C26Y) had much higher relative deflections in thesecondary creep stage with values of about 1.7–1.8 compared with avalue of only 0.7–0.8 for those panels with the cores oriented in thex-direction. It was also evident that the corrugated cores enteredtertiary creep slightly later than the expanded cores. This difference

is likely due to the higher stiffness of the corrugated core than theexpanded core [23].

It is interesting to compare the overall deflections of flexuralcreep of the samples that were the results of shear and bendingdeflections with the contribution from bending alone. This was the

5624 Z. Chen et al. / Materials Science and Engineering A 528 (2011) 5621–5626

Table 2Panel constituents, sample dimensions and sample ID. Note that the width was constant for all samples at 75 mm.

Skin Core Beam sample Sample ID

Materiala Thickness Typeb Thickness (mm) Orientationc Span (mm) Load (N) Shelling ratio

H 3 E 26 X 744 54.25 8.7 H3E26XH 3 C 26 X 744 54.25 8.7 H3C26XH 3 E 26 Y 744 54.25 8.7 H3E26YH 3 C 26 Y 744 54.25 8.7 H3C26YM 6 E 12.7 Y 600 43.75 2.1 M6E13YM 3 E 12.7 Y 456 33.25 4.3 M3E13YM 6 E 26 Y 864 66.55 4.3 M6E26YM 3 E 26 Y 744 54.25 8.7 M3E26YP 6 E 26 Y 864 70.19 4.3 P6E26YH 6 E 26 Y 864 63.00 4.3 H6E26Y

a H = hardboard, M = medium density fiberboard, P = plywood.b E = expanded core, C = corrugated core.c X = ribbon direction is oriented parallel to the long edge of the beam sample (Fig. 2), Y = ribbon direction is oriented perpendicular to the long edge of the beam sample.

Ffo

sisc

epcoco

TIm

Table 4Values for parameters in Eqs. (7) and (8) by fitting the measurement from the creepcurves for each specimen type.

Specimen ID Eq. (3) Eq. (4)

A1 N1 ı0 A2

H3E26X 0.1989 0.7054 0.7817 0.0019H3C26X 0.3385 0.3919 0.7876 0.0033H3E26Y 0.4106 0.7309 2.0679 0.0032H3C26Y 0.6939 0.4083 1.7017 0.0028M6E13Y 0.1624 0.8775 1.4832 0.0358M3E13Y 0.2672 0.6542 1.5404 0.0135

stage of creep than that with 12.7 mm thick core, but also the thickercores had a higher relative total flexural creep deflection rate in thisstage than the panel with the thinner honeycomb core when theirshelling ratio was similar (i.e., samples M6E26Y and M3E13Y inFig. 6 both had a shelling ratio of 4.3). The panels with thicker cores

ig. 5. Relative total flexural creep deflection as a function of time of panels withour types of core. The lowest line is the contribution of deflection due to bendingnly.

ame for all four types of sample and is shown in Fig. 5. By compar-ng the magnitude of that curve with the other samples, it clearlyhows that shear was the predominant mode of deformation duringreep testing of the samples made with both core types.

Fitting Eqs. (3) and (4) to the creep results in Fig. 5 producedstimates for A1, N1, ı0 and A2, which are listed in Table 4. Thisermits one to compare the primary and secondary creep rates. Theonclusions in this case are the same as those made from inspection

f Fig. 5; rates of primary creep were higher for the panels with theores oriented in the y-direction than the x-direction. Comparisonf the values for ı0 shows the same trend.

able 3nstantaneous elastic and irrecoverable deflection, and creep failure time of speci-

ens with different Kraft paper honeycomb cores and wood composite skins.

Specimen ID Instantaneouselastic deflection(mm)

Irrecoverabledeflection afterunloaded (mm)

Creep failuretime (h)

H3E26X 0.55 0.63 32H3C26X 1.29 1.28 32H3E26Y 0.41 2.27 32H3C26Y 2.34 4.20 32M6E13Y 0.50 1.62 145M3E13Y 1.05 2.41 145M6E26Y 2.50 13.73 65M3E26Y 0.45 16.90 65P6E26Y 2.24 2.76 65H6E26Y 15.94 42.20 65

M6E26Y 0.4576 0.6248 2.6598 0.0345M3E26Y 0.3088 0.8714 0.3516 0.2476P6E26Y 0.2069 0.6167 1.043 0.0196H6E26Y 0.0380 0.7721 0.3326 0.0057

4.3. Effect of thickness of panel’s core and skin:

As is seen in Fig. 6, the sandwich panels with the 26 mm thickhoneycomb core not only took less time to complete the secondary

Fig. 6. Curves of relative total flexural creep deflections of sandwich panels withdifferent thick cores and as a function of time. The 2-digit number to the right of thesample name is the shelling ratio for that sample. The load level and specimen sizediffered for each panel type (Table 1).

Z. Chen et al. / Materials Science and Engineering A 528 (2011) 5621–5626 5625

Fds

aMhhsctctcmstoci1r

cchtwnpwwr

wcrtl1

wt

4s

e

ig. 7. Curves of the relative total flexural creep deflection of sandwich panels withifferent skin materials vs. time. Both specimen’s dimension and load level wereimilar.

lso had higher creep rates in the primary stage (i.e., M6E26Y and3E26Y in Fig. 6). Among the sandwich panels with 26 mm thick

oneycomb cores, the specimens with higher shelling ratios hadigher relative total flexural creep deflection rate in the secondarytage of the creep. But they had the same relative total flexuralreep deflection rate as the samples with lower shelling ratios inhe primary stage. Among the sandwich panel with 12.7 mm thickore, the specimen with higher shelling ratio had a lower relativeotal flexural creep deflection rate in the secondary stage of thereep although their total flexural creep deflection rate in the pri-ary stage of the creep was similar to the specimen with lower

helling ratio. The sandwich panel with the same core thicknessook the same time to complete the primary and secondary stagef creep regardless of shelling ratio. Sandwich panels with 26 mmores completed primary creep in about 35 h and secondary creepn about 29 h. Sandwich panels with 12.7 mm thick core spent about6 and 124 h to complete the primary and secondary stage of creep,espectively.

All these results mentioned above confirm that the flexuralreep of honeycomb sandwich panel is sensitive to the honeycombore thickness. However since both the test span and load level ofoneycomb sandwich panels with 26 mm thick core was larger thanhe panel with 12.7 mm thick core (Table 2) it is difficult to judgehether this sensitivity was caused by differences in core thick-ess, sample span, or load level. The possible explanation for thehenomenon above is that the creep deformation rate of a sand-ich panel with thicker cores was higher than that of the panelith thinner core because normally higher level of load (Table 2)

esults in more creep [17].The effects of shelling ratio on the creep behavior of the sand-

ich panels were different between the panels with 26 mm thickores and the panels with 12.7 mm cores. The higher shellingatio made the secondary creep rate higher when the panel’s corehickness was 26 mm. The higher shelling ratio resulted in theower secondary creep deflection rate lower when panel’s core was2.7 mm.

The creep failure time was affected by the core thickness. Thereas no influence of the shelling ratio on the creep failure time when

he core thicknesses were similar.

.4. Flexural creep behavior of sandwich panels with differentkin materials

The results indicate that in comparison with the sandwich pan-ls containing plywood skin, the sandwich panels with hardboard

Fig. 8. Curves of relative total flexural creep deflection of sandwich panels withdifferent skin materials vs. time. Both specimen’s dimension and load level weresimilar.

skins had a lower relative total flexural creep deflection rate (Fig. 7).However both panel types completed the primary and secondarystage of creep in the same time. It is worth noting that there wasno distinct transition between the primary and secondary stage ofcreep of the panels with MDF skins. Comparing with the sandwichpanels with hardboard skins with those with MDF skins (Fig. 8)shows that the relative total flexural creep deflection rates for pri-mary creep were similar for both panel types. Interestingly, thepanel containing MDF skins had higher relative total flexural creepdeflection rate in the secondary creep stage than those containinghardboard skins. Since the authors’ early study indicated that thestiffness of MDF was lower than hardboard [23], this implies thatthere is a relationship between the flexural creep of sandwich paneland skin stiffness of sandwich under similar conditions such as thesame core structure, etc.

5. Conclusions

A series of creep tests on sandwich panels consisting of Kraftpaper honeycomb cores with composite skins loaded in three-pointbending load were carried out. Material factors such as skin typeand core type and thickness were found to have a large influenceon the assemblies creep behavior. For example, the creep rate forthick cores was found to be much higher than for thinner cores,showing that although much stiffer panels can be made by increas-ing the core thickness, there may be cost for doing that in terms ofdecreased service life.

Based on the testing results and data analysis, the followingconclusions can be made:

1. Honeycomb core shape and orientation mainly affected the rela-tive total flexural creep deflection rate of sandwich panels in theprimary stage of creep. When the ribbon direction of the sand-wich was perpendicular to its length, its flexural creep deflectionrate was higher than when the ribbon direction was parallel.The expanded honeycomb core panel had higher creep flexu-ral deflection rates than the corrugated honeycomb core panels.

There was no influence of core shape on the flexural creep ratein the secondary stage and creep failure time when the corethicknesses were the same.

2. The influence of the shelling ratio on the flexural creep deflectionrate of the sandwich panels was dependent on the core thickness

5 d Eng

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York, 1955.[21] S. Sam-Brew, K. Semple, G.D. Smith, Journal of Forest Product, in press.[22] American Society for Testing Materials (ASTM), Standard Test Methods for Flex-

626 Z. Chen et al. / Materials Science an

and was mainly on the creep rate in the secondary stage. Whenthe core thickness was 26 mm, a higher shelling ratio lead to ahigher creep rate in the secondary stage of creep. The panels witha higher shelling ratio had a lower creep rate in the secondarystage if the core thickness was 12.7 mm. The creep failure of thethicker honeycomb core panel occurred at a shorter time thanthat of the thinner core panels. Shelling ratios did not show influ-ence on the creep time to failure and creep rate in the primarystage if the core thickness was similar. However the panels witha thicker core showed higher creep rates in the secondary stageand shorter creep time to failure.

. The panels with MDF skins showed the highest creep rate in thesecondary stage, longest creep time to failure but a similar creeprate in the primary stage to that of the panel with hardboardskins.

. The panel with plywood skins had a higher creep rate thanthat with hardboard skins in either primary or secondary stage.However both types of panel completed the creep at the sametime.

cknowledgements

Authors would like to acknowledge NRCan Value to Wood pro-ram for the financial support of the project, and thank Francineote from the Material Evaluation Lab of FPInnovations in Quebecity, Canada for her works on creep testing.

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[

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