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Flexural creep behavior of sandwich panels containing Kraft paper honeycomb core and wood composite skins
Materials Science and Engineering A 528 (2011) 5621–5626
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Flexural creep behavior of sandwich panels containing Kraft paper honeycomb core and wood composite skins Zheng Chen a,1 , Ning Yan a,∗ , James Deng b , Greg Smith c,2 a b c
Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ont. M5S 3B3, Canada Composite Products, FPInnovations, 319 rue Franquet, Quebec, QC, G1P 4R4, Canada Department of Wood Science, University of British Columbia, 2935-2424 Main Mall, Vancouver, BC V6 T 1Z4, Canada
a r t i c l e
i n f o
Article history: Received 4 February 2011 Accepted 23 March 2011 Available online 29 March 2011 Keywords: Creep Sandwich Honeycomb Kraft paper Wood composite
a b s t r a c t Flexural creep behavior is an important performance related characteristic for sandwich panels used as products, such as kitchen bench tops, table legs, and bookshelves. In order to characterize the creep behavior of the sandwich panels with Kraft paper honeycomb core and wood composite skins, a series of creep tests were carried out under a constant three-point bending. The sandwich panels contained different types of core and skin materials as well as various core and skin thicknesses. The flexural creep deflection as a function of time for each type of sandwich panel was measured. The results show that the flexural creep behavior of the sandwich panel is affected by honeycomb core shape, core and skin thickness, and skin material type. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Sandwich panels containing Kraft paper honeycomb cores and wood composite skins are of increasing interest in the furniture industry due to their lightweight and lower cost. It is known that stiffness and deformation of sandwich panels with paper-based honeycomb cores are time dependent and can be significantly influenced by the loading rate [1]. For sandwich panels intended to be loaded for long durations, at elevated temperatures, or high relative humidity, their creep performance is an important quality. Unfortunately there is little published work related to the creep behavior of paper honeycomb core sandwich panels in the literature. Some published work on the creep of sandwich panels has focused on using numerical approach to analyze and predict the creep behavior of the sandwich panels [2]. However, these studies were done on aluminum honeycomb, solid wood panels, connectors, or concrete [3–7] and may not be applicable to Kraft paper hollow core panels. Research dealing with the creep rate of wood and wood composites has focused on the influence of moisture content, temperature, stress 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: [email protected] (Z. Chen), [email protected] (N. Yan), [email protected] (J. Deng), [email protected] (G. Smith). 1 Tel.: +1 416 304 1309; fax: +1 416 978 3834. 2 Tel.: +1 604 822 0081. 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.03.092
shown that creep rate increases with increasing temperature, moisture content, and stress level. Resin types also affect creep rate due to their differing degrees of hydrophobicity [11,12]. Creep rate is also found to be a function of the size of the wood component with the product of large size wood components [8,13,14], i.e., veneer, having much smaller creep rates than the products made of small size fibers, as indicated in Table 1. There is no published data on the influence of Kraft paper honeycomb core type, cell shape, thickness ratio of core to skin (shelling ratio), or material properties of the skin on the creep behavior of the sandwich panels. However, an early study on the stiffness of the sandwich panels with honeycomb paper core and wood composite skins shows that shelling ratio and large cell size are correlated with the panel stiffness [15]. As a panel undergoes creep, deformation can be exacerbated. Undesirable changes in the stress level of the panel may be produced and lead to instabilities resulting in failure of the panel. It is the aim of this study to measure and report the creep behavior of Kraft paper honeycomb core panels made from various core configurations and composite skin types. 2. Quantitative analysis In this study, a series of flexural creep test of sandwich panels with varied Kraft paper honeycomb cores and wood composite skins were carried out under a constant load. Temperature and relative humidity of the test environment were kept constant following ASTM standard C393 [16]. The creep of the constituent components as well as the assembled panels were measured as functions of time.
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Table 1 Comparison of the relative creep rate of different wood composites types.
where L is the sample span, P is the applied flexural constant load, D is the flexural creep stiffness and is defined as:
Relative creep rate
References
Hardboard > particle board > waferboard Fibreboard > particleboard > plywood MDF > OSB or MDF > chipboard
Dinwoodie et al. [13] Perkitny and Perkintny [14] Pritchard et al. [8]
The relative total flexural creep deflection was defined as: ıt−l =
(ıtt − ıte ) ıte
(2) (t ≤ tp ) (tp < t ≤ ts )
Pf Lf 3 3
4bf df ıf (t)
(8)
where Pf is the constant load using in the creep test of the skin material, ıf (t) is the relative deflection in the creep test for the skin material at time t, Lf is the sample span, bf is the specimen width, and the specimen thickness is denoted as df [16]. Thus the curves for bending deflection of sandwich panels with different core 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 and U the shear rigidity which is defined as 3
Gc (t)(d − c) b 4c
(4)
where Gc (t) is the honeycomb core shear modulus as a function of time.
(5)
where ıb and ıs is the relative bending deflection and relative shear deflection, respectively. The relative bending deflection can be expressed as: PL3 48D
where d is the specimen thickness, c is the core thickness, and b is the specimen width. Ef (t) is flexural stiffness of the skin material expressed as a function of time, t, and can be calculated using the following equation with the flexural creep data obtained through testing of the skin material:
U=
is the creep deformation, is the stress level, T is the temperature and t is time. A1 , and N1 are constants that may vary with material type, temperature and humidity; tp is the time at the end of the primary creep stage. ts is the time at the end of the secondary creep stage. ı0 is the intercept of the secondary creep stage line with the total relative deflection axis, and A2 is the relative total flexural creep deflection rate during the secondary creep [17,18,19]. Based on Timoshenko’s beam theory [20], the relative total deflection is also dependent on the bending and shear deflections given by the following equation:
ıb =
(7)
(3)
where εc
ıt−1 = ıb + ıs
12
Ef (t) =
εc = f (, T, t)
ıt−1 = ı0 + A2 t
Ef (t)(d3 − c 3 )b
(1)
where ıt−l is relative total flexural creep deflection; ıtt is the flexural creep deflection at time t; ıte is the instantaneous elastic deflection under load. A sample undergoing creep goes through three distinct stages of deformation: primary, secondary, and tertiary creep as shown schematically in Fig. 1. Primary creep is characterized by a steadily decreasing creep rate. Secondary creep refers to the linear portion of the curve where the creep rate is essentially constant. Tertiary creep is characterized by a rapidly increasing creep rate and occurs over a relatively short time frame. The behavior is usually described by the following Eqs. (2)-(4):
ıt−1 = A1 t N1
D=
(6)
Fig. 1. A typical creep curve for a viscoelastic material. εc is the creep strain and εc0 the instantaneous elastic deformation when instantaneous load is applied.
(10)
3. Methods and materials 3.1. Samples The core materials used for flexural creep specimens were two shapes of Kraft paper honeycomb core: an expanded core with a 31.75 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 Products Company in Vancouver, BC, Canada and Pregis Company in Deerfield, IL, USA, respectively). Three kinds of wood composite were used as the skins: hardboard (H), medium density fibreboard (M), and plywood (P). All sandwich panel specimens were manufactured by the Wood Composites Group at the University of British Columbia [21]. 3.2. Flexural creep test In order to carry out the flexural creep test for the sandwich panels a test frame was made according to ASTM C480 [22]. The layout of the set up is shown schematically in Fig. 3. The size of the specimens used in flexural creep testing of each type of sandwich panels (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 at the center of the span through the load head placed on the top skin of the specimen where the load head was connected to the static weight as shown in Fig. 4; 32 specimens under constant flexural loading can be tested simultaneously provided they are of the same sample span. The test temperature was kept constant at 21 ± 1 ◦ C and the relative humidity (RH) was at 70 ± 2%. The various combinations of skin and core materials were used to make the sandwich panels from which the beam sections were cut; their dimensions and testing parameters are listed in Table 2. The static load for each sandwich panel type and thickness was set to be 1/3 of static ultimate flexural load of those specimens. During
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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 ribbon direction in both cases is parallel to the X-axis.
Fig. 3. Schematics of the flexural creep testing apparatus.
the creep experiments, the total flexural creep deflection of each test piece was recorded every 5 s for the first 5 min and then every 30 min thereafter. Each test piece was loaded at a constant stress level for 167 h, and then the specimen was unloaded for 119 h to provide a measure of the amount of irrecoverable deflection. Two replicates were tested for each specimen type.
x-direction (H3C26X) was a little larger than the instantaneous deflection. Based on this result it was judged that all of these samples did indeed creep. Failure in creep is defined as the time at which the transition from secondary creep to tertiary creep occurs.
4. Results and discussion
The most evident example of this transition in Fig. 5 is for the expanded core panel oriented in the x-direction (H3E26X), which failed at approximately 28 h. The failure times for the other orientation and core type was similar and indicated that there was no influence of core type or orientation on the creep failure time. The effect of core orientation was evident from Fig. 5. Both the corrugated and expanded cores oriented in the y-orientation (where the ribbon direction was perpendicular to the sample length, i.e., H3E26Y and H3C26Y) had much higher relative deflections in the secondary creep stage with values of about 1.7–1.8 compared with a value of only 0.7–0.8 for those panels with the cores oriented in the x-direction. It was also evident that the corrugated cores entered tertiary creep slightly later than the expanded cores. This difference is likely due to the higher stiffness of the corrugated core than the expanded core [23]. It is interesting to compare the overall deflections of flexural creep of the samples that were the results of shear and bending deflections with the contribution from bending alone. This was the
4.1. Creep failure determination: Examination of Table 3 shows that the irrecoverable deflection of all but the panel with the corrugated core oriented in the
Fig. 4. Photograph of the test frame used for flexural creep testing of the honeycomb core sandwich panels.
4.2. Effect of honeycomb core structure and orientation:
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Table 2 Panel constituents, sample dimensions and sample ID. Note that the width was constant for all samples at 75 mm. Skin
Core a
Beam sample b
c
Sample ID
Material
Thickness
Type
Thickness (mm)
Orientation
Span (mm)
Load (N)
Shelling ratio
H H H H M M M M P H
3 3 3 3 6 3 6 3 6 6
E C E C E E E E E E
26 26 26 26 12.7 12.7 26 26 26 26
X X Y Y Y Y Y Y Y Y
744 744 744 744 600 456 864 744 864 864
54.25 54.25 54.25 54.25 43.75 33.25 66.55 54.25 70.19 63.00
8.7 8.7 8.7 8.7 2.1 4.3 4.3 8.7 4.3 4.3
a b c
H3E26X H3C26X H3E26Y H3C26Y M6E13Y M3E13Y M6E26Y M3E26Y P6E26Y H6E26Y
H = hardboard, M = medium density fiberboard, P = plywood. E = expanded core, C = corrugated core. 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. Table 4 Values for parameters in Eqs. (7) and (8) by fitting the measurement from the creep curves for each specimen type. Specimen ID
H3E26X H3C26X H3E26Y H3C26Y M6E13Y M3E13Y M6E26Y M3E26Y P6E26Y H6E26Y
Eq. (3)
Eq. (4)
A1
N1
ı0
A2
0.1989 0.3385 0.4106 0.6939 0.1624 0.2672 0.4576 0.3088 0.2069 0.0380
0.7054 0.3919 0.7309 0.4083 0.8775 0.6542 0.6248 0.8714 0.6167 0.7721
0.7817 0.7876 2.0679 1.7017 1.4832 1.5404 2.6598 0.3516 1.043 0.3326
0.0019 0.0033 0.0032 0.0028 0.0358 0.0135 0.0345 0.2476 0.0196 0.0057
4.3. Effect of thickness of panel’s core and skin: Fig. 5. Relative total flexural creep deflection as a function of time of panels with four types of core. The lowest line is the contribution of deflection due to bending only.
same for all four types of sample and is shown in Fig. 5. By comparing the magnitude of that curve with the other samples, it clearly shows that shear was the predominant mode of deformation during creep testing of the samples made with both core types. Fitting Eqs. (3) and (4) to the creep results in Fig. 5 produced estimates for A1 , N1 , ı0 and A2 , which are listed in Table 4. This permits one to compare the primary and secondary creep rates. The conclusions in this case are the same as those made from inspection of Fig. 5; rates of primary creep were higher for the panels with the cores oriented in the y-direction than the x-direction. Comparison of the values for ı0 shows the same trend.
As is seen in Fig. 6, the sandwich panels with the 26 mm thick honeycomb core not only took less time to complete the secondary stage of creep than that with 12.7 mm thick core, but also the thicker cores had a higher relative total flexural creep deflection rate in this stage than the panel with the thinner honeycomb core when their shelling ratio was similar (i.e., samples M6E26Y and M3E13Y in Fig. 6 both had a shelling ratio of 4.3). The panels with thicker cores
Table 3 Instantaneous elastic and irrecoverable deflection, and creep failure time of specimens with different Kraft paper honeycomb cores and wood composite skins. Specimen ID
Instantaneous elastic deflection (mm)
Irrecoverable deflection after unloaded (mm)
Creep failure time (h)
H3E26X H3C26X H3E26Y H3C26Y M6E13Y M3E13Y M6E26Y M3E26Y P6E26Y H6E26Y
0.55 1.29 0.41 2.34 0.50 1.05 2.50 0.45 2.24 15.94
0.63 1.28 2.27 4.20 1.62 2.41 13.73 16.90 2.76 42.20
32 32 32 32 145 145 65 65 65 65
Fig. 6. Curves of relative total flexural creep deflections of sandwich panels with different thick cores and as a function of time. The 2-digit number to the right of the sample name is the shelling ratio for that sample. The load level and specimen size differed for each panel type (Table 1).
Z. Chen et al. / Materials Science and Engineering A 528 (2011) 5621–5626
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Fig. 7. Curves of the relative total flexural creep deflection of sandwich panels with different skin materials vs. time. Both specimen’s dimension and load level were similar.
also had higher creep rates in the primary stage (i.e., M6E26Y and M3E26Y in Fig. 6). Among the sandwich panels with 26 mm thick honeycomb cores, the specimens with higher shelling ratios had higher relative total flexural creep deflection rate in the secondary stage of the creep. But they had the same relative total flexural creep deflection rate as the samples with lower shelling ratios in the primary stage. Among the sandwich panel with 12.7 mm thick core, the specimen with higher shelling ratio had a lower relative total flexural creep deflection rate in the secondary stage of the creep although their total flexural creep deflection rate in the primary stage of the creep was similar to the specimen with lower shelling ratio. The sandwich panel with the same core thickness took the same time to complete the primary and secondary stage of creep regardless of shelling ratio. Sandwich panels with 26 mm cores completed primary creep in about 35 h and secondary creep in about 29 h. Sandwich panels with 12.7 mm thick core spent about 16 and 124 h to complete the primary and secondary stage of creep, respectively. All these results mentioned above confirm that the flexural creep of honeycomb sandwich panel is sensitive to the honeycomb core thickness. However since both the test span and load level of honeycomb sandwich panels with 26 mm thick core was larger than the panel with 12.7 mm thick core (Table 2) it is difficult to judge whether this sensitivity was caused by differences in core thickness, sample span, or load level. The possible explanation for the phenomenon above is that the creep deformation rate of a sandwich panel with thicker cores was higher than that of the panel with thinner core because normally higher level of load (Table 2) results in more creep [17]. The effects of shelling ratio on the creep behavior of the sandwich panels were different between the panels with 26 mm thick cores and the panels with 12.7 mm cores. The higher shelling ratio made the secondary creep rate higher when the panel’s core thickness was 26 mm. The higher shelling ratio resulted in the lower secondary creep deflection rate lower when panel’s core was 12.7 mm. The creep failure time was affected by the core thickness. There was no influence of the shelling ratio on the creep failure time when the core thicknesses were similar. 4.4. Flexural creep behavior of sandwich panels with different skin materials The results indicate that in comparison with the sandwich panels containing plywood skin, the sandwich panels with hardboard
Fig. 8. Curves of relative total flexural creep deflection of sandwich panels with different skin materials vs. time. Both specimen’s dimension and load level were similar.
skins had a lower relative total flexural creep deflection rate (Fig. 7). However both panel types completed the primary and secondary stage of creep in the same time. It is worth noting that there was no distinct transition between the primary and secondary stage of creep of the panels with MDF skins. Comparing with the sandwich panels with hardboard skins with those with MDF skins (Fig. 8) shows that the relative total flexural creep deflection rates for primary creep were similar for both panel types. Interestingly, the panel containing MDF skins had higher relative total flexural creep deflection rate in the secondary creep stage than those containing hardboard skins. Since the authors’ early study indicated that the stiffness of MDF was lower than hardboard [23], this implies that there is a relationship between the flexural creep of sandwich panel and skin stiffness of sandwich under similar conditions such as the same core structure, etc. 5. Conclusions A series of creep tests on sandwich panels consisting of Kraft paper honeycomb cores with composite skins loaded in three-point bending load were carried out. Material factors such as skin type and core type and thickness were found to have a large influence on the assemblies creep behavior. For example, the creep rate for thick cores was found to be much higher than for thinner cores, showing that although much stiffer panels can be made by increasing the core thickness, there may be cost for doing that in terms of decreased service life. Based on the testing results and data analysis, the following conclusions can be made: 1. Honeycomb core shape and orientation mainly affected the relative total flexural creep deflection rate of sandwich panels in the primary stage of creep. When the ribbon direction of the sandwich was perpendicular to its length, its flexural creep deflection rate was higher than when the ribbon direction was parallel. The expanded honeycomb core panel had higher creep flexural deflection rates than the corrugated honeycomb core panels. There was no influence of core shape on the flexural creep rate in the secondary stage and creep failure time when the core thicknesses were the same. 2. The influence of the shelling ratio on the flexural creep deflection rate of the sandwich panels was dependent on the core thickness
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and was mainly on the creep rate in the secondary stage. When the core thickness was 26 mm, a higher shelling ratio lead to a higher creep rate in the secondary stage of creep. The panels with a higher shelling ratio had a lower creep rate in the secondary stage if the core thickness was 12.7 mm. The creep failure of the thicker honeycomb core panel occurred at a shorter time than that of the thinner core panels. Shelling ratios did not show influence on the creep time to failure and creep rate in the primary stage if the core thickness was similar. However the panels with a thicker core showed higher creep rates in the secondary stage and shorter creep time to failure. 3. The panels with MDF skins showed the highest creep rate in the secondary stage, longest creep time to failure but a similar creep rate in the primary stage to that of the panel with hardboard skins. 4. The panel with plywood skins had a higher creep rate than that with hardboard skins in either primary or secondary stage. However both types of panel completed the creep at the same time. Acknowledgements Authors would like to acknowledge NRCan Value to Wood program for the financial support of the project, and thank Francine Cote from the Material Evaluation Lab of FPInnovations in Quebec City, Canada for her works on creep testing. References [1] W.N. Findley, J.S. Lai, K. Onaran, Creep and Relaxation of Nonlinear Viscoelastic Materials, Dover, New York, 1976.
[2] J.Y. Lin, J.S. Huang, Composite Structures 67 (2005) 477–484. [3] S.B. Taylor, H.B. Manbeck, J.J. Janowiak, D.R. Hiltunen, Journal of Structural Engineering (December) (1997) 1658–1665. [4] H. Epmeier, M. Johansson, R. Kliger, M. Westin, Holz als Roh-und Werkstoff 65 (5) (2007) 343–351. [5] J.Y. Lin, J.S. Huang, Composites Science and Technology 66 (2006) 51–60. [6] W. Nuguib, A. Mirmiran, Journal of Composites for Construction November (2002) 272–279. [7] J.S. Huang, L.J. Gibson, Materials Science and Engineering A 339 (2003) 220–226. [8] J. Pritchard, M.P. Ansell, R.J.H. Thompson, P.W. Bonfield, Wood Science and Technology 35 (2001) 405–423. [9] A.P. Schniewind, R. Cal, Wood Science and Technology 2 (1968) 188–206. [10] T.L. Laufenberg, Creep testing of structural composite panels: a literature review and proposed standard, in: Proceedings 21st International Particle/Composite Materials Symposium, Pullman, WA, USA, March 24–26, 1987, 1987. [11] V.P. Gressel, Holz als Roh-und Werkstoff 30 (1972) 259–488. [12] J.M. Dinwoodie, B.H. Paxton, C.B. Pierce, Wood Science and Technology 15 (1981) 125–144. [13] J.M. Dinwoodie, D.J. Robson, B.H. Paxton, J.S. Higgins, Wood Science and Technology 25 (1991) 225–238. [14] J. Perkitny, P. Perkitny, Holztechnologie 4 (1966) 265–270. [15] Z. Chen, N. Yan, unpublished results. [16] American Society for Testing Materials (ASTM), Standard Test Methods for Flexural Properties of Sandwich Construction, C393-00, American Society for Testing and Materials, West Conshohocken, PA, 2000. [17] J. Bodig, B. Jayne, Mechanics of Wood and Wood Composites, Van Nostrand Reinhold Company Inc., New York, 1982. [18] R.K. Penny, D.L. Marriott, Design for Creep, Chapman & Hall, London, 1995. [19] H. Kraus, Creep Analysis, Wiley, New York, 1980. [20] S. Timoshenko, Strength of Materials, Litton Educational Publishing Inc., New 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 Flexural Creep of Sandwich Constructions, C480-99, American Society for Testing and Materials, West Conshohocken, PA, 2005. [23] Z. Chen, N. Yan, G. Smith, S. Sambrew, J. Deng, unpublished results.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Flexural creep behavior of sandwich panels containing Kraft paper honeycomb core and wood composite skins Zheng Chen a,1 , Ning Yan a,∗ , James Deng b , Greg Smith c,2 a b c
Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, Ont. M5S 3B3, Canada Composite Products, FPInnovations, 319 rue Franquet, Quebec, QC, G1P 4R4, Canada Department of Wood Science, University of British Columbia, 2935-2424 Main Mall, Vancouver, BC V6 T 1Z4, Canada
a r t i c l e
i n f o
Article history: Received 4 February 2011 Accepted 23 March 2011 Available online 29 March 2011 Keywords: Creep Sandwich Honeycomb Kraft paper Wood composite
a b s t r a c t Flexural creep behavior is an important performance related characteristic for sandwich panels used as products, such as kitchen bench tops, table legs, and bookshelves. In order to characterize the creep behavior of the sandwich panels with Kraft paper honeycomb core and wood composite skins, a series of creep tests were carried out under a constant three-point bending. The sandwich panels contained different types of core and skin materials as well as various core and skin thicknesses. The flexural creep deflection as a function of time for each type of sandwich panel was measured. The results show that the flexural creep behavior of the sandwich panel is affected by honeycomb core shape, core and skin thickness, and skin material type. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Sandwich panels containing Kraft paper honeycomb cores and wood composite skins are of increasing interest in the furniture industry due to their lightweight and lower cost. It is known that stiffness and deformation of sandwich panels with paper-based honeycomb cores are time dependent and can be significantly influenced by the loading rate [1]. For sandwich panels intended to be loaded for long durations, at elevated temperatures, or high relative humidity, their creep performance is an important quality. Unfortunately there is little published work related to the creep behavior of paper honeycomb core sandwich panels in the literature. Some published work on the creep of sandwich panels has focused on using numerical approach to analyze and predict the creep behavior of the sandwich panels [2]. However, these studies were done on aluminum honeycomb, solid wood panels, connectors, or concrete [3–7] and may not be applicable to Kraft paper hollow core panels. Research dealing with the creep rate of wood and wood composites has focused on the influence of moisture content, temperature, stress 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: [email protected] (Z. Chen), [email protected] (N. Yan), [email protected] (J. Deng), [email protected] (G. Smith). 1 Tel.: +1 416 304 1309; fax: +1 416 978 3834. 2 Tel.: +1 604 822 0081. 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.03.092
shown that creep rate increases with increasing temperature, moisture content, and stress level. Resin types also affect creep rate due to their differing degrees of hydrophobicity [11,12]. Creep rate is also found to be a function of the size of the wood component with the product of large size wood components [8,13,14], i.e., veneer, having much smaller creep rates than the products made of small size fibers, as indicated in Table 1. There is no published data on the influence of Kraft paper honeycomb core type, cell shape, thickness ratio of core to skin (shelling ratio), or material properties of the skin on the creep behavior of the sandwich panels. However, an early study on the stiffness of the sandwich panels with honeycomb paper core and wood composite skins shows that shelling ratio and large cell size are correlated with the panel stiffness [15]. As a panel undergoes creep, deformation can be exacerbated. Undesirable changes in the stress level of the panel may be produced and lead to instabilities resulting in failure of the panel. It is the aim of this study to measure and report the creep behavior of Kraft paper honeycomb core panels made from various core configurations and composite skin types. 2. Quantitative analysis In this study, a series of flexural creep test of sandwich panels with varied Kraft paper honeycomb cores and wood composite skins were carried out under a constant load. Temperature and relative humidity of the test environment were kept constant following ASTM standard C393 [16]. The creep of the constituent components as well as the assembled panels were measured as functions of time.
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Table 1 Comparison of the relative creep rate of different wood composites types.
where L is the sample span, P is the applied flexural constant load, D is the flexural creep stiffness and is defined as:
Relative creep rate
References
Hardboard > particle board > waferboard Fibreboard > particleboard > plywood MDF > OSB or MDF > chipboard
Dinwoodie et al. [13] Perkitny and Perkintny [14] Pritchard et al. [8]
The relative total flexural creep deflection was defined as: ıt−l =
(ıtt − ıte ) ıte
(2) (t ≤ tp ) (tp < t ≤ ts )
Pf Lf 3 3
4bf df ıf (t)
(8)
where Pf is the constant load using in the creep test of the skin material, ıf (t) is the relative deflection in the creep test for the skin material at time t, Lf is the sample span, bf is the specimen width, and the specimen thickness is denoted as df [16]. Thus the curves for bending deflection of sandwich panels with different core 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 and U the shear rigidity which is defined as 3
Gc (t)(d − c) b 4c
(4)
where Gc (t) is the honeycomb core shear modulus as a function of time.
(5)
where ıb and ıs is the relative bending deflection and relative shear deflection, respectively. The relative bending deflection can be expressed as: PL3 48D
where d is the specimen thickness, c is the core thickness, and b is the specimen width. Ef (t) is flexural stiffness of the skin material expressed as a function of time, t, and can be calculated using the following equation with the flexural creep data obtained through testing of the skin material:
U=
is the creep deformation, is the stress level, T is the temperature and t is time. A1 , and N1 are constants that may vary with material type, temperature and humidity; tp is the time at the end of the primary creep stage. ts is the time at the end of the secondary creep stage. ı0 is the intercept of the secondary creep stage line with the total relative deflection axis, and A2 is the relative total flexural creep deflection rate during the secondary creep [17,18,19]. Based on Timoshenko’s beam theory [20], the relative total deflection is also dependent on the bending and shear deflections given by the following equation:
ıb =
(7)
(3)
where εc
ıt−1 = ıb + ıs
12
Ef (t) =
εc = f (, T, t)
ıt−1 = ı0 + A2 t
Ef (t)(d3 − c 3 )b
(1)
where ıt−l is relative total flexural creep deflection; ıtt is the flexural creep deflection at time t; ıte is the instantaneous elastic deflection under load. A sample undergoing creep goes through three distinct stages of deformation: primary, secondary, and tertiary creep as shown schematically in Fig. 1. Primary creep is characterized by a steadily decreasing creep rate. Secondary creep refers to the linear portion of the curve where the creep rate is essentially constant. Tertiary creep is characterized by a rapidly increasing creep rate and occurs over a relatively short time frame. The behavior is usually described by the following Eqs. (2)-(4):
ıt−1 = A1 t N1
D=
(6)
Fig. 1. A typical creep curve for a viscoelastic material. εc is the creep strain and εc0 the instantaneous elastic deformation when instantaneous load is applied.
(10)
3. Methods and materials 3.1. Samples The core materials used for flexural creep specimens were two shapes of Kraft paper honeycomb core: an expanded core with a 31.75 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 Products Company in Vancouver, BC, Canada and Pregis Company in Deerfield, IL, USA, respectively). Three kinds of wood composite were used as the skins: hardboard (H), medium density fibreboard (M), and plywood (P). All sandwich panel specimens were manufactured by the Wood Composites Group at the University of British Columbia [21]. 3.2. Flexural creep test In order to carry out the flexural creep test for the sandwich panels a test frame was made according to ASTM C480 [22]. The layout of the set up is shown schematically in Fig. 3. The size of the specimens used in flexural creep testing of each type of sandwich panels (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 at the center of the span through the load head placed on the top skin of the specimen where the load head was connected to the static weight as shown in Fig. 4; 32 specimens under constant flexural loading can be tested simultaneously provided they are of the same sample span. The test temperature was kept constant at 21 ± 1 ◦ C and the relative humidity (RH) was at 70 ± 2%. The various combinations of skin and core materials were used to make the sandwich panels from which the beam sections were cut; their dimensions and testing parameters are listed in Table 2. The static load for each sandwich panel type and thickness was set to be 1/3 of static ultimate flexural load of those specimens. During
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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 ribbon direction in both cases is parallel to the X-axis.
Fig. 3. Schematics of the flexural creep testing apparatus.
the creep experiments, the total flexural creep deflection of each test piece was recorded every 5 s for the first 5 min and then every 30 min thereafter. Each test piece was loaded at a constant stress level for 167 h, and then the specimen was unloaded for 119 h to provide a measure of the amount of irrecoverable deflection. Two replicates were tested for each specimen type.
x-direction (H3C26X) was a little larger than the instantaneous deflection. Based on this result it was judged that all of these samples did indeed creep. Failure in creep is defined as the time at which the transition from secondary creep to tertiary creep occurs.
4. Results and discussion
The most evident example of this transition in Fig. 5 is for the expanded core panel oriented in the x-direction (H3E26X), which failed at approximately 28 h. The failure times for the other orientation and core type was similar and indicated that there was no influence of core type or orientation on the creep failure time. The effect of core orientation was evident from Fig. 5. Both the corrugated and expanded cores oriented in the y-orientation (where the ribbon direction was perpendicular to the sample length, i.e., H3E26Y and H3C26Y) had much higher relative deflections in the secondary creep stage with values of about 1.7–1.8 compared with a value of only 0.7–0.8 for those panels with the cores oriented in the x-direction. It was also evident that the corrugated cores entered tertiary creep slightly later than the expanded cores. This difference is likely due to the higher stiffness of the corrugated core than the expanded core [23]. It is interesting to compare the overall deflections of flexural creep of the samples that were the results of shear and bending deflections with the contribution from bending alone. This was the
4.1. Creep failure determination: Examination of Table 3 shows that the irrecoverable deflection of all but the panel with the corrugated core oriented in the
Fig. 4. Photograph of the test frame used for flexural creep testing of the honeycomb core sandwich panels.
4.2. Effect of honeycomb core structure and orientation:
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Table 2 Panel constituents, sample dimensions and sample ID. Note that the width was constant for all samples at 75 mm. Skin
Core a
Beam sample b
c
Sample ID
Material
Thickness
Type
Thickness (mm)
Orientation
Span (mm)
Load (N)
Shelling ratio
H H H H M M M M P H
3 3 3 3 6 3 6 3 6 6
E C E C E E E E E E
26 26 26 26 12.7 12.7 26 26 26 26
X X Y Y Y Y Y Y Y Y
744 744 744 744 600 456 864 744 864 864
54.25 54.25 54.25 54.25 43.75 33.25 66.55 54.25 70.19 63.00
8.7 8.7 8.7 8.7 2.1 4.3 4.3 8.7 4.3 4.3
a b c
H3E26X H3C26X H3E26Y H3C26Y M6E13Y M3E13Y M6E26Y M3E26Y P6E26Y H6E26Y
H = hardboard, M = medium density fiberboard, P = plywood. E = expanded core, C = corrugated core. 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. Table 4 Values for parameters in Eqs. (7) and (8) by fitting the measurement from the creep curves for each specimen type. Specimen ID
H3E26X H3C26X H3E26Y H3C26Y M6E13Y M3E13Y M6E26Y M3E26Y P6E26Y H6E26Y
Eq. (3)
Eq. (4)
A1
N1
ı0
A2
0.1989 0.3385 0.4106 0.6939 0.1624 0.2672 0.4576 0.3088 0.2069 0.0380
0.7054 0.3919 0.7309 0.4083 0.8775 0.6542 0.6248 0.8714 0.6167 0.7721
0.7817 0.7876 2.0679 1.7017 1.4832 1.5404 2.6598 0.3516 1.043 0.3326
0.0019 0.0033 0.0032 0.0028 0.0358 0.0135 0.0345 0.2476 0.0196 0.0057
4.3. Effect of thickness of panel’s core and skin: Fig. 5. Relative total flexural creep deflection as a function of time of panels with four types of core. The lowest line is the contribution of deflection due to bending only.
same for all four types of sample and is shown in Fig. 5. By comparing the magnitude of that curve with the other samples, it clearly shows that shear was the predominant mode of deformation during creep testing of the samples made with both core types. Fitting Eqs. (3) and (4) to the creep results in Fig. 5 produced estimates for A1 , N1 , ı0 and A2 , which are listed in Table 4. This permits one to compare the primary and secondary creep rates. The conclusions in this case are the same as those made from inspection of Fig. 5; rates of primary creep were higher for the panels with the cores oriented in the y-direction than the x-direction. Comparison of the values for ı0 shows the same trend.
As is seen in Fig. 6, the sandwich panels with the 26 mm thick honeycomb core not only took less time to complete the secondary stage of creep than that with 12.7 mm thick core, but also the thicker cores had a higher relative total flexural creep deflection rate in this stage than the panel with the thinner honeycomb core when their shelling ratio was similar (i.e., samples M6E26Y and M3E13Y in Fig. 6 both had a shelling ratio of 4.3). The panels with thicker cores
Table 3 Instantaneous elastic and irrecoverable deflection, and creep failure time of specimens with different Kraft paper honeycomb cores and wood composite skins. Specimen ID
Instantaneous elastic deflection (mm)
Irrecoverable deflection after unloaded (mm)
Creep failure time (h)
H3E26X H3C26X H3E26Y H3C26Y M6E13Y M3E13Y M6E26Y M3E26Y P6E26Y H6E26Y
0.55 1.29 0.41 2.34 0.50 1.05 2.50 0.45 2.24 15.94
0.63 1.28 2.27 4.20 1.62 2.41 13.73 16.90 2.76 42.20
32 32 32 32 145 145 65 65 65 65
Fig. 6. Curves of relative total flexural creep deflections of sandwich panels with different thick cores and as a function of time. The 2-digit number to the right of the sample name is the shelling ratio for that sample. The load level and specimen size differed for each panel type (Table 1).
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Fig. 7. Curves of the relative total flexural creep deflection of sandwich panels with different skin materials vs. time. Both specimen’s dimension and load level were similar.
also had higher creep rates in the primary stage (i.e., M6E26Y and M3E26Y in Fig. 6). Among the sandwich panels with 26 mm thick honeycomb cores, the specimens with higher shelling ratios had higher relative total flexural creep deflection rate in the secondary stage of the creep. But they had the same relative total flexural creep deflection rate as the samples with lower shelling ratios in the primary stage. Among the sandwich panel with 12.7 mm thick core, the specimen with higher shelling ratio had a lower relative total flexural creep deflection rate in the secondary stage of the creep although their total flexural creep deflection rate in the primary stage of the creep was similar to the specimen with lower shelling ratio. The sandwich panel with the same core thickness took the same time to complete the primary and secondary stage of creep regardless of shelling ratio. Sandwich panels with 26 mm cores completed primary creep in about 35 h and secondary creep in about 29 h. Sandwich panels with 12.7 mm thick core spent about 16 and 124 h to complete the primary and secondary stage of creep, respectively. All these results mentioned above confirm that the flexural creep of honeycomb sandwich panel is sensitive to the honeycomb core thickness. However since both the test span and load level of honeycomb sandwich panels with 26 mm thick core was larger than the panel with 12.7 mm thick core (Table 2) it is difficult to judge whether this sensitivity was caused by differences in core thickness, sample span, or load level. The possible explanation for the phenomenon above is that the creep deformation rate of a sandwich panel with thicker cores was higher than that of the panel with thinner core because normally higher level of load (Table 2) results in more creep [17]. The effects of shelling ratio on the creep behavior of the sandwich panels were different between the panels with 26 mm thick cores and the panels with 12.7 mm cores. The higher shelling ratio made the secondary creep rate higher when the panel’s core thickness was 26 mm. The higher shelling ratio resulted in the lower secondary creep deflection rate lower when panel’s core was 12.7 mm. The creep failure time was affected by the core thickness. There was no influence of the shelling ratio on the creep failure time when the core thicknesses were similar. 4.4. Flexural creep behavior of sandwich panels with different skin materials The results indicate that in comparison with the sandwich panels containing plywood skin, the sandwich panels with hardboard
Fig. 8. Curves of relative total flexural creep deflection of sandwich panels with different skin materials vs. time. Both specimen’s dimension and load level were similar.
skins had a lower relative total flexural creep deflection rate (Fig. 7). However both panel types completed the primary and secondary stage of creep in the same time. It is worth noting that there was no distinct transition between the primary and secondary stage of creep of the panels with MDF skins. Comparing with the sandwich panels with hardboard skins with those with MDF skins (Fig. 8) shows that the relative total flexural creep deflection rates for primary creep were similar for both panel types. Interestingly, the panel containing MDF skins had higher relative total flexural creep deflection rate in the secondary creep stage than those containing hardboard skins. Since the authors’ early study indicated that the stiffness of MDF was lower than hardboard [23], this implies that there is a relationship between the flexural creep of sandwich panel and skin stiffness of sandwich under similar conditions such as the same core structure, etc. 5. Conclusions A series of creep tests on sandwich panels consisting of Kraft paper honeycomb cores with composite skins loaded in three-point bending load were carried out. Material factors such as skin type and core type and thickness were found to have a large influence on the assemblies creep behavior. For example, the creep rate for thick cores was found to be much higher than for thinner cores, showing that although much stiffer panels can be made by increasing the core thickness, there may be cost for doing that in terms of decreased service life. Based on the testing results and data analysis, the following conclusions can be made: 1. Honeycomb core shape and orientation mainly affected the relative total flexural creep deflection rate of sandwich panels in the primary stage of creep. When the ribbon direction of the sandwich was perpendicular to its length, its flexural creep deflection rate was higher than when the ribbon direction was parallel. The expanded honeycomb core panel had higher creep flexural deflection rates than the corrugated honeycomb core panels. There was no influence of core shape on the flexural creep rate in the secondary stage and creep failure time when the core thicknesses were the same. 2. The influence of the shelling ratio on the flexural creep deflection rate of the sandwich panels was dependent on the core thickness
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and was mainly on the creep rate in the secondary stage. When the core thickness was 26 mm, a higher shelling ratio lead to a higher creep rate in the secondary stage of creep. The panels with a higher shelling ratio had a lower creep rate in the secondary stage if the core thickness was 12.7 mm. The creep failure of the thicker honeycomb core panel occurred at a shorter time than that of the thinner core panels. Shelling ratios did not show influence on the creep time to failure and creep rate in the primary stage if the core thickness was similar. However the panels with a thicker core showed higher creep rates in the secondary stage and shorter creep time to failure. 3. The panels with MDF skins showed the highest creep rate in the secondary stage, longest creep time to failure but a similar creep rate in the primary stage to that of the panel with hardboard skins. 4. The panel with plywood skins had a higher creep rate than that with hardboard skins in either primary or secondary stage. However both types of panel completed the creep at the same time. Acknowledgements Authors would like to acknowledge NRCan Value to Wood program for the financial support of the project, and thank Francine Cote from the Material Evaluation Lab of FPInnovations in Quebec City, Canada for her works on creep testing. References [1] W.N. Findley, J.S. Lai, K. Onaran, Creep and Relaxation of Nonlinear Viscoelastic Materials, Dover, New York, 1976.
[2] J.Y. Lin, J.S. Huang, Composite Structures 67 (2005) 477–484. [3] S.B. Taylor, H.B. Manbeck, J.J. Janowiak, D.R. Hiltunen, Journal of Structural Engineering (December) (1997) 1658–1665. [4] H. Epmeier, M. Johansson, R. Kliger, M. Westin, Holz als Roh-und Werkstoff 65 (5) (2007) 343–351. [5] J.Y. Lin, J.S. Huang, Composites Science and Technology 66 (2006) 51–60. [6] W. Nuguib, A. Mirmiran, Journal of Composites for Construction November (2002) 272–279. [7] J.S. Huang, L.J. Gibson, Materials Science and Engineering A 339 (2003) 220–226. [8] J. Pritchard, M.P. Ansell, R.J.H. Thompson, P.W. Bonfield, Wood Science and Technology 35 (2001) 405–423. [9] A.P. Schniewind, R. Cal, Wood Science and Technology 2 (1968) 188–206. [10] T.L. Laufenberg, Creep testing of structural composite panels: a literature review and proposed standard, in: Proceedings 21st International Particle/Composite Materials Symposium, Pullman, WA, USA, March 24–26, 1987, 1987. [11] V.P. Gressel, Holz als Roh-und Werkstoff 30 (1972) 259–488. [12] J.M. Dinwoodie, B.H. Paxton, C.B. Pierce, Wood Science and Technology 15 (1981) 125–144. [13] J.M. Dinwoodie, D.J. Robson, B.H. Paxton, J.S. Higgins, Wood Science and Technology 25 (1991) 225–238. [14] J. Perkitny, P. Perkitny, Holztechnologie 4 (1966) 265–270. [15] Z. Chen, N. Yan, unpublished results. [16] American Society for Testing Materials (ASTM), Standard Test Methods for Flexural Properties of Sandwich Construction, C393-00, American Society for Testing and Materials, West Conshohocken, PA, 2000. [17] J. Bodig, B. Jayne, Mechanics of Wood and Wood Composites, Van Nostrand Reinhold Company Inc., New York, 1982. [18] R.K. Penny, D.L. Marriott, Design for Creep, Chapman & Hall, London, 1995. [19] H. Kraus, Creep Analysis, Wiley, New York, 1980. [20] S. Timoshenko, Strength of Materials, Litton Educational Publishing Inc., New 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 Flexural Creep of Sandwich Constructions, C480-99, American Society for Testing and Materials, West Conshohocken, PA, 2005. [23] Z. Chen, N. Yan, G. Smith, S. Sambrew, J. Deng, unpublished results.