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Experimental research of compressive responses of multi-layered woven textile sandwich panels under quasi-static loading
Composites: Part B 42 (2011) 1151–1156
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Experimental research of compressive responses of multi-layered woven textile sandwich panels under quasi-static loading Hualin Fan a,b,⇑, Wei Yang c,⇑, Qing Zhou d a
State Key laboratory of Hydrology-Water Resources and Hydraulic Engineering & College of Mechanics and Materials, Hohai University, Nanjing 210098, PR China State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, PR China c University Office, Zhejiang University, Hangzhou 310058, PR China d State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, PR China b
a r t i c l e
i n f o
Article history: Received 10 May 2010 Received in revised form 1 March 2011 Accepted 20 March 2011 Available online 9 April 2011 Keywords: A. Fabrics/textiles B. Mechanical properties D. Mechanical testing
a b s t r a c t Multi-layered panels were manufactured by stacking thin monolayer panels to improve the energy absorption ability of the woven textile sandwich. Quasi-static compression experiments were conducted to get the stress–strain curves and to reveal the energy absorption mechanism. For multi-layered panels staked by monolayer of 3 mm thickness, the core piles experience the strength failure; that renders the panel high load resistance and a strain-hardening curve before the densification. The thickening of the monolayer would trigger the monolayer stack to buckle one after the other, which decreases the load resistance of the panel and renders a prolonged zigzag load–deformation curve. The peak load of the multi-layered structure is found to be close to its constituting monolayer, but much greater than a monolayer panel of the same thickness. The deformation of the multi-layered panel then becomes a multiple process of monolayer collapses. Energy absorption of the multi-layered panel is greatly enhanced and far exceeds that of the monolayer panel of the same thickness. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Woven textile sandwich panels provide a class of integrated sandwich material with a high skin-core debonding resistance. The strength and the stiffness of the composite material are comparable to honeycombs [1,2]. In quasi-static compression tests, the material possesses a long stable deformation plateau and densification after initial buckling failure, which suggests that the material is ideal to serve as an energy absorbing core [3]. The buckling strength of the core piles puts a stringent constraint to their size. At present, the maximum thickness of the woven sandwich panel manufactured by Nanjing Fiberglass Research & Design Institute (NFRDI) of China is no more than 20 mm [3]. The strength of the material will diminish when the panel becomes thicker. The compression strength is about 7.2 MPa for a panel of 3 mm thickness and reduces to about 1.0 MPa for a panel of 20 mm thickness [3]. Though undergone much larger deformation, the energy absorption of a thicker panel cannot be obviously improved. On the contrary, the thinner panel possesses much better efficiency of energy absorption. For example, the energy absorption is 5 J/cm3 and 0.5 J/cm3 for panels of 3 mm and 20 mm thickness ⇑ Corresponding authors. Address: College of Mechanics and Materials, Hohai University, Nanjing 210098, PR China (H. Fan). E-mail addresses: [email protected] (H. Fan), [email protected] (W. Yang). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.03.008
[3], respectively. Accordingly, the thickening of sandwich panels does not present a valid way to improve the energy absorption of the structure. The multi-layered sandwich structure, on the other hand, is an effective way to improve the energy absorption of a panel of larger thickness. Dharmasena et al. [4], and Wadley et al. [5] found that a multi-layered sandwich structure is effective at dispersing highintensity impulses, as well as reducing the peak pressure transmitted to the underlying structure. In the present paper, multi-layered glass fiber reinforced composite (GFRC) sandwich panels were designed and manufactured by stacking thinner layers as shown in Fig. 1. The piles in the core in the warp direction resemble the 8-shape as shown in Fig. 1a. The distance between the neighboring warp piles is greater than that between weft piles as shown in Fig. 1b. The fiber piles tilt due to the woven manufacturing method. Supplied by NFRDI, the monolayer textile materials are woven by E-glass fibers and solidified by epoxy resins (618). Monolayer panels of 3 mm and 8 mm thickness were selected to stack the multi-layered composite panels. In the warp direction, the distance between the neighboring piles is about 5 mm, as shown in Fig. 1c. The skin of the monolayer is about 0.5 mm thick. The monolayer panels of 3 mm and 8 mm thickness have densities of 1558 g/m2 and 2046 g/m2, respectively. These thin monolayer panels were stacked and then bonded by cyanoacrylate adhesives (502) to form the multi-layered panel. The multi-layered panel was solidified at room temperature under a
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(a)
(b)
(c) 63 mm
30 mm
Fig. 1. Structure of the multi-layered woven textile sandwich in (a) warp direction, (b) weft direction and (c) fabricated 10-layer sample of 30 mm thickness.
pressure for 10 h. Both the length and the width of the panel are 63 mm. A 10-layer sample of 30 mm thickness was shown in Fig. 1c. 2. Compressive responses of multi-layered panels stacked by 3 mm layers 2.1. Experiments Compressing the panel quasi-statically normal to its plane is a convenient way to test the energy absorbing ability of the structure. The compression tests were conducted at a loading rate of 0.5 mm/min on DNS 300 machine with a loadcell capacity of 300 kN, made by Changchun Research Institute for Testing Machine Co., LTD (CRITM). In tests, the displacements were measured directly by the crosshead displacement of the test machine. Loading was controlled by the closed-loop control of the displacement, whose precision accuracy is controlled within ±0.5%. The stress–strain curves of multi-layered panels composed of 3 mm thick monolayers are depicted in Fig. 2. These curves exhibit three stages: the elastic deformation, the strain-hardening and the densification. No abrupt stress drop was found in the curves. After the first failure, the curves keep climbing till densification. The panel could deform up to a half of its thickness. The strength and compressibility of multi-layered panels are comparable to the constituting monolayers [3] while the deformability is multiplied, rendering greater energy absorption. 2.2. Failure mechanism The failure modes of the multi-layered panel stacked by 3 mm monolayer were depicted in Fig. 3. The panel contains 10-layers. The multi-layered sandwich panel has a total thickness of 30 mm, but the piles in the core of each monolayer are just of 2 mm height. Strength failure determines the load capacity of the panel. All layers jointly resist the load. No monolayer was found crushed
individually. Deformation of each monolayer is reasonably uniform. Wave deformation pattern in skins was observed, as shown in Fig. 4. In compression, the core piles supported by intra-cell skins will be compressed into the skin. The skin is crumpled to a wave pattern, as shown in Fig. 4. Wave crests locate at the core piles, while wave troughs at the intra-cell skins. Wave pattern of the skin confirms the strength failure mode of the multi-layered panel stacked by layers of 3 mm thickness. All layers were densified and the total displacement is nearly 15 mm. The compression ratio of the 10-layer panel is nearly 50%.
3. Compressive responses of multi-layered panels stacked by thicker layers 3.1. Experiments Compression tests of multi-layered panels stacked by 8 mm thick monolayer were carried out at a loading rate of 1.0 mm/ min on DNS 100 machine with a loadcell capacity of 100 kN. The load–deformation curves are shown in Fig. 5. After the peak stress, the pile buckling leads to stress drop till the core is densified. The compressibility of the monolayer is about 0.6. Corresponding to the successive failure of each layer, the load– displacement curves are featured by zigzags. Number of the zigzag is equal to that of the panel layers. In each stage, the curve first rises up elastically and then drops due to the pile buckling. Rotation of the piles renders a residual load capacity for the layer till it is completely crushed and compacted. The curve then climbs up to form the next zigzag when the load is transmitted to other layers. When all layers are crushed, the densification of the panel raises the curve steeply, as shown in Fig. 5. The strength and the compressibility of the multi-layered panel maintain the same level as its constituting monolayer, while the deformation is multiplied by multiple zigzags, which is typical for the multi-layered cellular composites and indictive for the potential to absorb large amounts of energy.
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(d)
Displacement (mm) 0.9
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3.6 198.5
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0.15
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Strain
0 0.00
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Strain
Fig. 2. Stress–strain curves of panels stacked by (a) two, (b) three, (c) four, (d) five, (e) seven and (f) 10-layers of 3 mm thickness under compression.
3.2. Failure mechanism Figure 6 delineates the failure mechanism of the multi-layered panel composed of six layers. The total thickness of the panel is 48 mm. Failure started at the bottom layer due to the pile buckling, and then the load–displacement curve began to drop, as shown in Fig. 5d. The piles continued tilting and finally fell down. The layer was compressed in accompany with severe shear deformation. After initial pile buckling, the load was supported by the rotation resistance of the joints that link the piles and the skin. When the piles were completely fell down, the densification of the bottom layer transmitted the load to other layers and the load–displacement curve climbed up again. Piles tiling and falling down were then observed in the third layer. This failure mechanism was successively passed to the other layers, as depicted in Figs. 5d and 6. Finally, all layers were crushed and the total displacement is above 20 mm. The compression ratio of the six-layer panel is about 60%.
For multi-layered panels stacked by thicker layers, pile buckling controls the failure mode. The strength of the multi-layer panel maintains the same level as its constituting monolayer but less than that of the panel stacked by thinner layers.
4. Energy absorption mechanisms Two types of stress–strain curves for multi-layered panels were found in the experiments. Stacked by thinner layers, the panel is featured by a strain-hardening curve. Stacked by thicker layers, the panel is featured by a zigzag load–deformation curve. Three energy absorption mechanisms are revealed from the tests. Each curve demonstrates a prolonged deformation process before the densification. The compressibility is nearly 0.5 for panels stacked by 3 mm thick layers and enlarged to above 0.6 for panels stacked by 8 mm thick layers. Compressibility of the multi-layered
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Wave pattern
Wave trough
Wave crest
Fig. 4. Wave pattern of the multi-layered panel stacked by 3 mm layers.
Fig. 3. Failure modes of the multi-layered panel stacked by layers of 3 mm thickness.
Displacement (mm) 0.00 5
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panel is identical to its thin constituting monolayers. With multilayers, the deformation of the panel is multiplied, and consequently the energy absorption. The peak stress of the multi-layered panel maintains the same level as its constituting thin monolayers. Compared with a monolayer of the same thickness, the multi-layered panel enjoys far greater peak stress. As shown in Fig. 7, the peak stress of a 4-layer panel of 12 mm thickness is 8.52 MPa, comparatively larger than 1.61 MPa, the peak stress of a 12 mm thick monolayer panel. The difference is enlarged to 7.90 MPa for a 7-layer panel of 21 mm thickness to 1.16 MPa for a 20 mm thick monolayer panel. Enhancement of the peak stress promotes the energy absorption. If the thickness of the constituting layers is sufficiently thin, e.g. smaller than 3 mm, the failure mode of the multi-layered panel
14.40
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Elastic deformation 0 0.0
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Fig. 5. Load–displacement curves of panels stacked by (a) two (b) three, (c) four and (d) six layers of 8 mm thickness under compression.
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Compression
Shear
Stress (MPa)
8 mm
7-layer panel of 21 mm thickness
25
63 mm
20
4-layer panel of 12 mm thickness 15
10
monolayer of 20 mm thickness 5
monolayer of 12 mm thickness
0 0.0
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[3]
[3]
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Strain Fig. 7. Comparison between the multi-layered panel and the monolayer.
25
Stress (MPa)
20
4-layer GFRC textile panel 15
Monolayer steel textile [7] panel with facesheets
10
Monolayer nichrome textile panel 5
4-layer prismatic steel panel
[6]
[4]
Monolayer GFRC textile panel 0 0.0
0.2
0.4
0.6
0.8
Strain Fig. 8. Comparisons between the GFRC textile panels and the metallic textile panels.
300
2 layers 3 layers 4 layers 6 layers 9 layers
250
Energy (J)
200
150
100
50
0
0
5
10
15
20
25
30
35
40
45
Displacement (mm) Fig. 9. Energy absorptions of the multi-layered panels stacked by panels of 8 mm thickness.
Densification Fig. 6. Failure modes of the multi-layered panel stacked by layers of 8 mm thickness.
will be converted from buckling to strength failure. Zigzag curves are replaced by the strain-hardening curves. The energy absorption of the multi-layered panel can be further enhanced by the strainhardening.
5. Comparisons The GFRC woven textile panels were compared with woven or multi-layered metallic textile panels in Fig. 8. Obviously, the multilayered woven GFRC panel stacked by layers of 3 mm thickness is stronger, while the monolayer panel of 20 mm thickness is substantially weaker. In Fig. 8, the density is 0.672 g/cm3 for a four-layer 304 stainless steel prismatic truss core (1.416 g/cm3 for the panel including the face sheets) [4], 1.43 g/cm3 for a nichrome textile panel [6] and 1.8 g/cm3 for a 304 stainless steel
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6. Conclusions
10
2 layers 3 layers 4 layers 5 layers 7 layers 10 layers
Energy density (J/cm3)
8
6
Steel textile panel with face sheets
GFRC woven textile panel
4
Nichrome textile panel
2
4-layer prismatic steel panel 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Strain Fig. 10. Energy absorptions of the multi-layered panels stacked by panels of 3 mm thickness.
textile panel with face sheets (1.36 g/cm3 for the core) [7]. With a density of 0.519 g/cm3, the four-layer GFRC textile panel is lighter. The monolayer panel of 20 mm thickness is only 0.215 g/cm3 [3]. The specific strength of the multi-layered woven GFRC panel is even greater. Appropriate multi-layered design, by reducing the thickness of its constituting layers, enhances the load capacity of the woven textile panels. The energy absorption of the multi-layered woven textile sandwich panel is shown in Figs. 9 and 10, for panels stacked by monolayers of 8 mm and 3 mm thicknesses. In Fig. 9, the energy absorption of the multi-layered panel stacked by layers of 8 mm thickness varies almost linearly with respect to the displacement. The energy absorptions of each panel, though in different layers, are all very close to 1.0 J/cm3 or 3.9 J/g. It is concluded that the energy absorption efficiency of the multi-layered panel keeps constant and is uniformly distributed within all layers. The multi-layered panel has much better energy absorbing ability than that of a monolayer of the same combining thickness. Specific energy absorption of the woven textile truss is 4.8 J/g at 55% strain for nichrome [6] and 4.6 J/g at 70% strain for steel [7]. With face sheets, specific energy absorption of the woven steel textile sandwich is 7.2 J/g at 70% strain when densified [7]. The specific energy absorption of the referenced four-layer prismatic steel panel is about 3.3 J/g. Compared with these metallic panels, the tested multi-layered woven textile composite panels stacked by thicker layers possess comparable specific energy absorptions. Stacked by thinner layers, the GFRC panel should possess better absorbing ability. Specific energy absorption of panels stacked by 3 mm thick monolayers is above 4.0 J/cm3 or 8.0 J/g in average. Compared with the referenced metallic panels, the GFRC woven textile sandwich has stronger specific energy absorbing ability, as shown in Fig. 10.
Multi-layered GFRC textile panels were manufactured by stacking monolayer panels. Two types of the stress–strain curves and failure modes were demonstrated by the experiments. Stacked by layers of 3 mm thickness, the GFRC panel has a strain-hardening curve and the peak load of the panel is determined by core pile strength. Stacked by thicker layers, e.g. layers of 8 mm thickness, the multi-layered panel has a zigzag stress–strain curve induced by pile buckling. The peak load of the multi-layered panel maintains a comparable level as its constituting monolayers, which considerably exceeds the monolayer panel of the same combining thickness. Multi-layers also make the panel capable of larger deformation at the same compressibility ratio as the constituting monolayer. Energy absorption of the panel is greatly improved when compared with the monolayer of the same combining thickness. Even compared with steel textile panels and multi-layered prismatic steel panels, the multi-layered GFRC woven textile panels have comparable or possibly superior energy absorbing abilities. It is concluded from the tests that the multi-layered structure is an effective way to improve the energy absorption of the GFRC woven textile sandwich panels. Acknowledgements Supports from the National High Technology Research and Development Program (‘‘863’’ Program) of China (2007AA03Z547), National Natural Science Foundation of China (10702033), State Key Laboratory of Automotive Safety and Energy of Tsinghua University (KF11031, KF09132) and State Key Laboratory of Explosion Science and Technology (KFJJ10-16) are gratefully acknowledged. The authors would also wish to acknowledge NFRDI for supplying the monolayer woven textile material in this study. References [1] van Vuure AW, Ivens JA, Verpoest I. Mechanical properties of composite panels based on woven sandwich-fabric preforms. Composites Part A 2000;31:671–80. [2] van Vuure AW, Pflug J, Ivens JA, et al. Modeling the core properties of composite panels based on woven sandwich-fabric performs. Compos Sci Technol 2000;60:1263–76. [3] Fan HL, Zhou Q, Yang W, et al. An experiment study on the failure mechanisms of woven textile sandwich panels under quasi-static loading. Composites: Part B 2010;41:686–92. [4] Dharmasena KP, Queheillalt D, Wadley HNG, et al. Dynamic response of a multilayer prismatic structure to impulsive loads incident from water. Int J Impact Eng 2009;36:632–43. [5] Wadley HNG, Dharmasena KP, Chen Y, et al. Compressive response of multilayered pyramidal lattices during underwater shock loading. Int J Impact Eng 2008;35:1102–14. [6] Sypeck DJ, Wadley HNG. Multifunctional microtruss laminates: textile synthesis and properties. J Mater Res 2001;16:890–7. [7] Caulfield J, Karlsson AM, Sypeck DJ. Crushing of a textile core sandwich panel. AIAA J 2006;44:1339–44.
Contents lists available at ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Experimental research of compressive responses of multi-layered woven textile sandwich panels under quasi-static loading Hualin Fan a,b,⇑, Wei Yang c,⇑, Qing Zhou d a
State Key laboratory of Hydrology-Water Resources and Hydraulic Engineering & College of Mechanics and Materials, Hohai University, Nanjing 210098, PR China State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, PR China c University Office, Zhejiang University, Hangzhou 310058, PR China d State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, PR China b
a r t i c l e
i n f o
Article history: Received 10 May 2010 Received in revised form 1 March 2011 Accepted 20 March 2011 Available online 9 April 2011 Keywords: A. Fabrics/textiles B. Mechanical properties D. Mechanical testing
a b s t r a c t Multi-layered panels were manufactured by stacking thin monolayer panels to improve the energy absorption ability of the woven textile sandwich. Quasi-static compression experiments were conducted to get the stress–strain curves and to reveal the energy absorption mechanism. For multi-layered panels staked by monolayer of 3 mm thickness, the core piles experience the strength failure; that renders the panel high load resistance and a strain-hardening curve before the densification. The thickening of the monolayer would trigger the monolayer stack to buckle one after the other, which decreases the load resistance of the panel and renders a prolonged zigzag load–deformation curve. The peak load of the multi-layered structure is found to be close to its constituting monolayer, but much greater than a monolayer panel of the same thickness. The deformation of the multi-layered panel then becomes a multiple process of monolayer collapses. Energy absorption of the multi-layered panel is greatly enhanced and far exceeds that of the monolayer panel of the same thickness. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Woven textile sandwich panels provide a class of integrated sandwich material with a high skin-core debonding resistance. The strength and the stiffness of the composite material are comparable to honeycombs [1,2]. In quasi-static compression tests, the material possesses a long stable deformation plateau and densification after initial buckling failure, which suggests that the material is ideal to serve as an energy absorbing core [3]. The buckling strength of the core piles puts a stringent constraint to their size. At present, the maximum thickness of the woven sandwich panel manufactured by Nanjing Fiberglass Research & Design Institute (NFRDI) of China is no more than 20 mm [3]. The strength of the material will diminish when the panel becomes thicker. The compression strength is about 7.2 MPa for a panel of 3 mm thickness and reduces to about 1.0 MPa for a panel of 20 mm thickness [3]. Though undergone much larger deformation, the energy absorption of a thicker panel cannot be obviously improved. On the contrary, the thinner panel possesses much better efficiency of energy absorption. For example, the energy absorption is 5 J/cm3 and 0.5 J/cm3 for panels of 3 mm and 20 mm thickness ⇑ Corresponding authors. Address: College of Mechanics and Materials, Hohai University, Nanjing 210098, PR China (H. Fan). E-mail addresses: [email protected] (H. Fan), [email protected] (W. Yang). 1359-8368/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2011.03.008
[3], respectively. Accordingly, the thickening of sandwich panels does not present a valid way to improve the energy absorption of the structure. The multi-layered sandwich structure, on the other hand, is an effective way to improve the energy absorption of a panel of larger thickness. Dharmasena et al. [4], and Wadley et al. [5] found that a multi-layered sandwich structure is effective at dispersing highintensity impulses, as well as reducing the peak pressure transmitted to the underlying structure. In the present paper, multi-layered glass fiber reinforced composite (GFRC) sandwich panels were designed and manufactured by stacking thinner layers as shown in Fig. 1. The piles in the core in the warp direction resemble the 8-shape as shown in Fig. 1a. The distance between the neighboring warp piles is greater than that between weft piles as shown in Fig. 1b. The fiber piles tilt due to the woven manufacturing method. Supplied by NFRDI, the monolayer textile materials are woven by E-glass fibers and solidified by epoxy resins (618). Monolayer panels of 3 mm and 8 mm thickness were selected to stack the multi-layered composite panels. In the warp direction, the distance between the neighboring piles is about 5 mm, as shown in Fig. 1c. The skin of the monolayer is about 0.5 mm thick. The monolayer panels of 3 mm and 8 mm thickness have densities of 1558 g/m2 and 2046 g/m2, respectively. These thin monolayer panels were stacked and then bonded by cyanoacrylate adhesives (502) to form the multi-layered panel. The multi-layered panel was solidified at room temperature under a
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(a)
(b)
(c) 63 mm
30 mm
Fig. 1. Structure of the multi-layered woven textile sandwich in (a) warp direction, (b) weft direction and (c) fabricated 10-layer sample of 30 mm thickness.
pressure for 10 h. Both the length and the width of the panel are 63 mm. A 10-layer sample of 30 mm thickness was shown in Fig. 1c. 2. Compressive responses of multi-layered panels stacked by 3 mm layers 2.1. Experiments Compressing the panel quasi-statically normal to its plane is a convenient way to test the energy absorbing ability of the structure. The compression tests were conducted at a loading rate of 0.5 mm/min on DNS 300 machine with a loadcell capacity of 300 kN, made by Changchun Research Institute for Testing Machine Co., LTD (CRITM). In tests, the displacements were measured directly by the crosshead displacement of the test machine. Loading was controlled by the closed-loop control of the displacement, whose precision accuracy is controlled within ±0.5%. The stress–strain curves of multi-layered panels composed of 3 mm thick monolayers are depicted in Fig. 2. These curves exhibit three stages: the elastic deformation, the strain-hardening and the densification. No abrupt stress drop was found in the curves. After the first failure, the curves keep climbing till densification. The panel could deform up to a half of its thickness. The strength and compressibility of multi-layered panels are comparable to the constituting monolayers [3] while the deformability is multiplied, rendering greater energy absorption. 2.2. Failure mechanism The failure modes of the multi-layered panel stacked by 3 mm monolayer were depicted in Fig. 3. The panel contains 10-layers. The multi-layered sandwich panel has a total thickness of 30 mm, but the piles in the core of each monolayer are just of 2 mm height. Strength failure determines the load capacity of the panel. All layers jointly resist the load. No monolayer was found crushed
individually. Deformation of each monolayer is reasonably uniform. Wave deformation pattern in skins was observed, as shown in Fig. 4. In compression, the core piles supported by intra-cell skins will be compressed into the skin. The skin is crumpled to a wave pattern, as shown in Fig. 4. Wave crests locate at the core piles, while wave troughs at the intra-cell skins. Wave pattern of the skin confirms the strength failure mode of the multi-layered panel stacked by layers of 3 mm thickness. All layers were densified and the total displacement is nearly 15 mm. The compression ratio of the 10-layer panel is nearly 50%.
3. Compressive responses of multi-layered panels stacked by thicker layers 3.1. Experiments Compression tests of multi-layered panels stacked by 8 mm thick monolayer were carried out at a loading rate of 1.0 mm/ min on DNS 100 machine with a loadcell capacity of 100 kN. The load–deformation curves are shown in Fig. 5. After the peak stress, the pile buckling leads to stress drop till the core is densified. The compressibility of the monolayer is about 0.6. Corresponding to the successive failure of each layer, the load– displacement curves are featured by zigzags. Number of the zigzag is equal to that of the panel layers. In each stage, the curve first rises up elastically and then drops due to the pile buckling. Rotation of the piles renders a residual load capacity for the layer till it is completely crushed and compacted. The curve then climbs up to form the next zigzag when the load is transmitted to other layers. When all layers are crushed, the densification of the panel raises the curve steeply, as shown in Fig. 5. The strength and the compressibility of the multi-layered panel maintain the same level as its constituting monolayer, while the deformation is multiplied by multiple zigzags, which is typical for the multi-layered cellular composites and indictive for the potential to absorb large amounts of energy.
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(d)
Displacement (mm) 0.9
1.8
2.7
3.6 198.5
Displacement (mm) 0.00 50
119.1
20
79.4
10
39.7
0.30
0.45
Stress (MPa)
30
Load (kN)
Stress (MPa)
40
158.8
0.15
4.50
0.0 0.60
158.8
30
119.1
20
79.4
10
39.7
0 0.00
0.15
0.30
1.35
2.70
4.05
(e)
5.40 198.5
119.1
20
79.4
10
39.7
0.30
0.45
Stress (MPa)
30
Load (kN)
Stress (MPa)
3.15
6.30
0.0 0.60
158.8
30
119.1
20
79.4
10
39.7
0 0.00
0.15
0.30
1.8
3.6
0.45
0.0 0.60
Strain
(f)
Displacement (mm) 0.0 50
12.60 198.5
40
Strain
(c)
9.45
seven layers of 3 mm 158.8
0.15
0.0 0.60
Displacement (mm) 0.00 50
three layers of 3 mm 40
0 0.00
0.45
Strain
Displacement (mm) 0.00 50
9.00 198.5
40
Strain
(b)
6.75
five layers of 3 mm
two layers of 3 mm
0 0.00
2.25
Load (kN)
0.0 50
Load (kN)
(a)
5.4
7.2 198.5
Displacement (mm) 0.0 50
4.5
9.0
13.5
18.0 198.5
ten layers of 3 mm
four layers of 3 mm
158.8
40
158.8
40
20
79.4
119.1
30
79.4
20
Load (kN)
119.1
Stress (MPa)
30
Load (kN)
Stress (MPa)
Densification
Strain-strengthening 39.7
10
39.7
10
Elastic deformation 0 0.00
0.15
0.30
0.45
0.0 0.60
Strain
0 0.00
0.15
0.30
0.45
0.0 0.60
Strain
Fig. 2. Stress–strain curves of panels stacked by (a) two, (b) three, (c) four, (d) five, (e) seven and (f) 10-layers of 3 mm thickness under compression.
3.2. Failure mechanism Figure 6 delineates the failure mechanism of the multi-layered panel composed of six layers. The total thickness of the panel is 48 mm. Failure started at the bottom layer due to the pile buckling, and then the load–displacement curve began to drop, as shown in Fig. 5d. The piles continued tilting and finally fell down. The layer was compressed in accompany with severe shear deformation. After initial pile buckling, the load was supported by the rotation resistance of the joints that link the piles and the skin. When the piles were completely fell down, the densification of the bottom layer transmitted the load to other layers and the load–displacement curve climbed up again. Piles tiling and falling down were then observed in the third layer. This failure mechanism was successively passed to the other layers, as depicted in Figs. 5d and 6. Finally, all layers were crushed and the total displacement is above 20 mm. The compression ratio of the six-layer panel is about 60%.
For multi-layered panels stacked by thicker layers, pile buckling controls the failure mode. The strength of the multi-layer panel maintains the same level as its constituting monolayer but less than that of the panel stacked by thinner layers.
4. Energy absorption mechanisms Two types of stress–strain curves for multi-layered panels were found in the experiments. Stacked by thinner layers, the panel is featured by a strain-hardening curve. Stacked by thicker layers, the panel is featured by a zigzag load–deformation curve. Three energy absorption mechanisms are revealed from the tests. Each curve demonstrates a prolonged deformation process before the densification. The compressibility is nearly 0.5 for panels stacked by 3 mm thick layers and enlarged to above 0.6 for panels stacked by 8 mm thick layers. Compressibility of the multi-layered
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Wave pattern
Wave trough
Wave crest
Fig. 4. Wave pattern of the multi-layered panel stacked by 3 mm layers.
Fig. 3. Failure modes of the multi-layered panel stacked by layers of 3 mm thickness.
Displacement (mm) 0.00 5
3.20
6.40
9.60
(c)
12.80 19.8
Displacement (mm) 0.00 5
3
11.9
2
7.9
1
4.0
0.2
0.4
0.6
Stress (MPa)
15.9
0 0.0
12.80
15.9
3
11.9
2
7.9
1
4.0
0 0.0
0.0 0.8
0.2
0.4
4.80
9.60
0.6
0.0 0.8
Strain
Displacement (mm) 0.00 5
25.60 19.8
4
Strain
(b)
19.20
four layers of 8 mm
4
Load (kN)
Stress (MPa)
two layers of 8 mm
6.40
Load (kN)
(a)
panel is identical to its thin constituting monolayers. With multilayers, the deformation of the panel is multiplied, and consequently the energy absorption. The peak stress of the multi-layered panel maintains the same level as its constituting thin monolayers. Compared with a monolayer of the same thickness, the multi-layered panel enjoys far greater peak stress. As shown in Fig. 7, the peak stress of a 4-layer panel of 12 mm thickness is 8.52 MPa, comparatively larger than 1.61 MPa, the peak stress of a 12 mm thick monolayer panel. The difference is enlarged to 7.90 MPa for a 7-layer panel of 21 mm thickness to 1.16 MPa for a 20 mm thick monolayer panel. Enhancement of the peak stress promotes the energy absorption. If the thickness of the constituting layers is sufficiently thin, e.g. smaller than 3 mm, the failure mode of the multi-layered panel
14.40
(d)
19.20 19.8
Displacement (mm) 0.00 5
three layers of 8 mm
9.60
19.20
six layers of 8 mm
4
15.9
28.80
38.40 19.8
Densification
4
15.9
2
7.9
1
4.0
0 0.0
0.2
0.4
Strain
0.6
0.0 0.8
3
11.9
2
7.9
1
4.0
Load (kN)
11.9
Stress (MPa)
3
Load (kN)
Stress (MPa)
Buckling
Elastic deformation 0 0.0
0.2
0.4
0.6
0.0 0.8
Strain
Fig. 5. Load–displacement curves of panels stacked by (a) two (b) three, (c) four and (d) six layers of 8 mm thickness under compression.
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Compression
Shear
Stress (MPa)
8 mm
7-layer panel of 21 mm thickness
25
63 mm
20
4-layer panel of 12 mm thickness 15
10
monolayer of 20 mm thickness 5
monolayer of 12 mm thickness
0 0.0
0.2
0.4
[3]
[3]
0.6
0.8
Strain Fig. 7. Comparison between the multi-layered panel and the monolayer.
25
Stress (MPa)
20
4-layer GFRC textile panel 15
Monolayer steel textile [7] panel with facesheets
10
Monolayer nichrome textile panel 5
4-layer prismatic steel panel
[6]
[4]
Monolayer GFRC textile panel 0 0.0
0.2
0.4
0.6
0.8
Strain Fig. 8. Comparisons between the GFRC textile panels and the metallic textile panels.
300
2 layers 3 layers 4 layers 6 layers 9 layers
250
Energy (J)
200
150
100
50
0
0
5
10
15
20
25
30
35
40
45
Displacement (mm) Fig. 9. Energy absorptions of the multi-layered panels stacked by panels of 8 mm thickness.
Densification Fig. 6. Failure modes of the multi-layered panel stacked by layers of 8 mm thickness.
will be converted from buckling to strength failure. Zigzag curves are replaced by the strain-hardening curves. The energy absorption of the multi-layered panel can be further enhanced by the strainhardening.
5. Comparisons The GFRC woven textile panels were compared with woven or multi-layered metallic textile panels in Fig. 8. Obviously, the multilayered woven GFRC panel stacked by layers of 3 mm thickness is stronger, while the monolayer panel of 20 mm thickness is substantially weaker. In Fig. 8, the density is 0.672 g/cm3 for a four-layer 304 stainless steel prismatic truss core (1.416 g/cm3 for the panel including the face sheets) [4], 1.43 g/cm3 for a nichrome textile panel [6] and 1.8 g/cm3 for a 304 stainless steel
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6. Conclusions
10
2 layers 3 layers 4 layers 5 layers 7 layers 10 layers
Energy density (J/cm3)
8
6
Steel textile panel with face sheets
GFRC woven textile panel
4
Nichrome textile panel
2
4-layer prismatic steel panel 0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Strain Fig. 10. Energy absorptions of the multi-layered panels stacked by panels of 3 mm thickness.
textile panel with face sheets (1.36 g/cm3 for the core) [7]. With a density of 0.519 g/cm3, the four-layer GFRC textile panel is lighter. The monolayer panel of 20 mm thickness is only 0.215 g/cm3 [3]. The specific strength of the multi-layered woven GFRC panel is even greater. Appropriate multi-layered design, by reducing the thickness of its constituting layers, enhances the load capacity of the woven textile panels. The energy absorption of the multi-layered woven textile sandwich panel is shown in Figs. 9 and 10, for panels stacked by monolayers of 8 mm and 3 mm thicknesses. In Fig. 9, the energy absorption of the multi-layered panel stacked by layers of 8 mm thickness varies almost linearly with respect to the displacement. The energy absorptions of each panel, though in different layers, are all very close to 1.0 J/cm3 or 3.9 J/g. It is concluded that the energy absorption efficiency of the multi-layered panel keeps constant and is uniformly distributed within all layers. The multi-layered panel has much better energy absorbing ability than that of a monolayer of the same combining thickness. Specific energy absorption of the woven textile truss is 4.8 J/g at 55% strain for nichrome [6] and 4.6 J/g at 70% strain for steel [7]. With face sheets, specific energy absorption of the woven steel textile sandwich is 7.2 J/g at 70% strain when densified [7]. The specific energy absorption of the referenced four-layer prismatic steel panel is about 3.3 J/g. Compared with these metallic panels, the tested multi-layered woven textile composite panels stacked by thicker layers possess comparable specific energy absorptions. Stacked by thinner layers, the GFRC panel should possess better absorbing ability. Specific energy absorption of panels stacked by 3 mm thick monolayers is above 4.0 J/cm3 or 8.0 J/g in average. Compared with the referenced metallic panels, the GFRC woven textile sandwich has stronger specific energy absorbing ability, as shown in Fig. 10.
Multi-layered GFRC textile panels were manufactured by stacking monolayer panels. Two types of the stress–strain curves and failure modes were demonstrated by the experiments. Stacked by layers of 3 mm thickness, the GFRC panel has a strain-hardening curve and the peak load of the panel is determined by core pile strength. Stacked by thicker layers, e.g. layers of 8 mm thickness, the multi-layered panel has a zigzag stress–strain curve induced by pile buckling. The peak load of the multi-layered panel maintains a comparable level as its constituting monolayers, which considerably exceeds the monolayer panel of the same combining thickness. Multi-layers also make the panel capable of larger deformation at the same compressibility ratio as the constituting monolayer. Energy absorption of the panel is greatly improved when compared with the monolayer of the same combining thickness. Even compared with steel textile panels and multi-layered prismatic steel panels, the multi-layered GFRC woven textile panels have comparable or possibly superior energy absorbing abilities. It is concluded from the tests that the multi-layered structure is an effective way to improve the energy absorption of the GFRC woven textile sandwich panels. Acknowledgements Supports from the National High Technology Research and Development Program (‘‘863’’ Program) of China (2007AA03Z547), National Natural Science Foundation of China (10702033), State Key Laboratory of Automotive Safety and Energy of Tsinghua University (KF11031, KF09132) and State Key Laboratory of Explosion Science and Technology (KFJJ10-16) are gratefully acknowledged. The authors would also wish to acknowledge NFRDI for supplying the monolayer woven textile material in this study. References [1] van Vuure AW, Ivens JA, Verpoest I. Mechanical properties of composite panels based on woven sandwich-fabric preforms. Composites Part A 2000;31:671–80. [2] van Vuure AW, Pflug J, Ivens JA, et al. Modeling the core properties of composite panels based on woven sandwich-fabric performs. Compos Sci Technol 2000;60:1263–76. [3] Fan HL, Zhou Q, Yang W, et al. An experiment study on the failure mechanisms of woven textile sandwich panels under quasi-static loading. Composites: Part B 2010;41:686–92. [4] Dharmasena KP, Queheillalt D, Wadley HNG, et al. Dynamic response of a multilayer prismatic structure to impulsive loads incident from water. Int J Impact Eng 2009;36:632–43. [5] Wadley HNG, Dharmasena KP, Chen Y, et al. Compressive response of multilayered pyramidal lattices during underwater shock loading. Int J Impact Eng 2008;35:1102–14. [6] Sypeck DJ, Wadley HNG. Multifunctional microtruss laminates: textile synthesis and properties. J Mater Res 2001;16:890–7. [7] Caulfield J, Karlsson AM, Sypeck DJ. Crushing of a textile core sandwich panel. AIAA J 2006;44:1339–44.