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Mechanically Efficient Cellular Microstructures in Plants
Lorna J. GibsonMaterials Science & Engineering
MIT
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Introduction
• Plants are typically loaded in bending by wind and in compression by self-weight
• Minimizing mass reduces metabolic cost to grow material
• Examine strategies used in plants to reduce mass
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Introduction
• Wood: Uniform honeycomb-like structure
• Palm stem: Radial density gradient• Plant stem: Cylindrical shell with
compliant core• Monocotyledon leaves: Sandwich
structures
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Wood:Honeycomb-Like Microstructure
Cedar
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Wood: Honeycomb Models
Cell wall:FiberCompositeModelE*
Esalong
=ρ*
ρs
E*
Es across
=ρ*
ρs
⎛
⎝⎜⎞
⎠⎟
3
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Wood in Bending: E1/2/ρ E*( )1/2
ρ* =Es( )1/2
ρs
ρs
ρ*
⎛⎝⎜
⎞⎠⎟
1/2
Stiffness performance index for wood inbending is similar to that for bestengineering composites
Wood cell wall
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Wood in Bending: σf2/3/ρ
σ f*( )2 /3
ρ* =σ ys( )2 /3
ρs
ρs
ρ*
⎛⎝⎜
⎞⎠⎟
1/3
Strength performance index for wood inbending is similar to that for best engng composites
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Wood
• Tree in bending loaded as cantilever• Radius decreases with distance away
from the ground• Further increases mechanical
performance of the tree• E = constant, r = r (z)
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Palm Stem: Radial Density Gradient
(Also Bamboo)
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Palm Stem:A Different Strategy
• Stem has constant diameter: r = constant
• As palm grows taller, it increases the density of the material towards its periphery
• Cell wall thickness increases towards periphery of stem and towards the base of the stem E = E (r, z) Coconut Palm
http://en.wikipedia.org/wiki/Image:Palmtree_Curacao.jpg
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Palm: Microstructure ofPeripheral Stem Tissue
Young Old
6 μm 10 μm
Rich, 1987
Kuo-Huang et al., 2004
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Palm Stem: Density Gradient
Rich, PM (1987) Bot.Gazette 148, 42-50.
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Palm Stem: Density at Breast Height
A single mature palm has a similar range of density as nearly all species of wood combined
Rich, PM (1987) Bot.Gazette 148, 42-50.
Densitiesof commonwoods
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Palm Stem: Density Gradient
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Palm Stem: Mechanical Properties vs. Density
Rich, PM (1987) Bot.Gazette 148, 42-50.
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Density Gradient:Iriartea gigantea
ρ =rro
⎛⎝⎜
⎞⎠⎟
n
ρmax
E = Cρ
ρmax
⎛
⎝⎜⎞
⎠⎟
m
= Crro
⎛
⎝⎜⎞
⎠⎟
mn
EI( )gradient =Cπro
4
mn + 4
EI( )gradient
EI( )uniform
=4
mn + 4n + 2
2⎛⎝⎜
⎞⎠⎟
m
Iriartea palm: n = 2, m = 2.5, (EI)gradient/(EI)uniform = 2.5
Similar calculation for Welfia georgii, gives(EI)gradient/(EI)uniform = 1.6
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Palm Stem:Bending Stress Distribution
σ (y) = Eε = Eκ y
σ (r,θ ) = Crro
⎛
⎝⎜⎞
⎠⎟
mn
κ r cosθ ∝ rmn+1
Iriartea gigantea: m = 2.5, n =2
σ ∝ r6
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Palm Stem:Bending Strength Distribution
σ * ∝ρ
ρmax
⎛⎝⎜
⎞⎠⎟
q
∝rro
⎛⎝⎜
⎞⎠⎟
nq
Iriartea gigantea: n = 2, q = 2
σ * ∝ r4
Strength matchesbending stress distribution
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Plant Stems:Cylindrical Shells with
Compliant Cores
(Also in Animal Quills,Toucan Beak)
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Plant Stems
Milkweed Grassy stem
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Milkweed Stem
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Grassy Stem
Hollow struts
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Plant stems
• Circular tube cross-section• Resists bending (wind loads)• Maximize shape factor
• Maximize a/t, but limited by local buckling and ovalization
• Plant stems have compliant core (“core-rind structure”)
Φ =4π IA2 =
at
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Plant Stems: Bending
• Core resists ovalization and increases local buckling resistance
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Plant Stems: Bending
• Local buckling occurs when normal stress in compressive side of cylinder equals critical stress for axisymmetric buckling under uniaxial stress
• Hollow cylinder:
• Cylinder with compliant core:
Mlb =0.939Eat 2
1− ν 2
Mlb =πEa2t1− ν 2
fδr
,Ecore
Eshell
,at
⎧⎨⎩
⎫⎬⎭
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Plant Stems• Foam-like core can act like elastic foundation supporting outer
shell, increasing local buckling moment, Mlb, reducing buckling λ
Hollow tube
Ec increasing
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Plant Stems
Hollow tube
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Plant Stems
• Within the core stress decays as move radially inward, away from the shell
• Stresses less than 5% of maximum at aradial distance of 5λcr
• Can remove inner core, leaving core thickness, c = 5λcr
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Plant Stems
25
50
59
a/t
3.270.77YesSedge grass, common barley
3.811.26YesOat, rye grasses
3.811.37YesTall blue lettuce
c/λcrMlb/MeqElastic foundation
Species
Core increases buckling resistance for high a/t
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Monocotyledon Leaves:Sandwich Structures
(Also in Skulls, Cuttlefish Bone, Horseshoe Crab Shell)
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31Iris Bulrush
Monocotyledon Leaves: Sandwich Beams
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Sandwich Structures: Leaves
Iris leaf
Bulrush leaf1mm
0.5 mm
SclerenchymaParenchyma
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Sandwich Structures: Leaves
Lolium perenne(Rye grass)
Stipa gigantea(Giant feather
grass)
Vincent, 1982,1991
Black = sclerenchymaWhite = parenchyma
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Monocotyledon Leaves
• Fibers (sclerenchyma) along outer surface of leaves
• Foam-like cells (parenchyma) or ribs in core • Acts like structural sandwich panel• Increase in moment of inertia by separating
stiff “faces” by a lightweight “core”• Large surface area for photosynthesis
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Sandwich Beam Deflection
δ = δb + δ s =Pl 3
B1 EI( )eq
+Pl
B2 AG( )eq
EI( )eq ≈Ef btc2
2AG( )eq ≈ bcGcFlexural rigidity: Shear rigidity:
Cantilever: B1 = 3 B2 = 1
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Iris Leaves
t = 30 μm
c = 0.5 to 3.0 mm
Ef = 8.2 GPa
Gc = 2 MPa
Measured stiffnesses (N/mm): 0.66 0.54 0.41 0.25Calculated stiffnesses (N/mm): 1.21 0.78 0.51 0.29
Calculated/measured: 1.83 1.44 1.24 1.16
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Conclusion
• Wood– Uniform honeycomb increases E1/2/ρ, σf
2/3/ρ– Constant ρ, E, σf, vary r(z) in tree
• Palm stem– Radial density gradient– Constant r, vary ρ(r), E(r), σf(r) in palm stem– Increases (EI) relative to uniform distribution of
solid– Stress distribution across radius matches strength
distribution
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Conclusion
• Plant stems– Cylindrical shell with compliant core– Increases buckling resistance over equivalent
hollow circular tube for large a/t
• Monocotyledon leaves– Sandwich structure, efficient in bending– Leaves provide own structural support as well as
area for photosynthesis– Rectangular cross-section maximizes surface area
for photosynthesis
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Acknowledgements
• Mike Ashby, Ken Easterling, Hugh Shercliff• Gebran Karam, Phoebe Cheng, Ulrike Wegst, Ros Olive, Tessa
Shercliff• Justin Breucop, Don Galler, Beth Beighlie
• National Science Foundation• Matoula S. Salapatas Professorship at MIT
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Image References
• Connor S (1994) New England Natives: A celebration of people and trees. Harvard University Press.
• Dinwoodie J (1981)Timber: Its nature and behaviour. Van Nostrand Reinhold.
• Rich PM (1987) Mechanical structure of the stem of arborescent palms. Bot. Gazette 148, 42.
• Rich PM (1987) Developmental anatomy of the stem of Welfia Georgii , Iriartea Gigantea and other arborescent palms:Implications for mechanical support. Am. J. Botany 74, 792.