material and structural design of biomimetic composites ... · material and structural design of...

10
Material and structural design of biomimetic composites learning from the quadrangular plant stem K. Kurashiki 1 , S. Hisatsugu 2 , Q.-Q. Ni 1 & M. Iwamoto 1 1 Advanced Fibro-Science in Graduate School, Kyoto Institute of Technology, Japan 2 Graduate Student, Kyoto Institute of Technology, Japan Abstract Organisms come in various forms in nature. A circular cross section can be found in many structures, such as the stems of many plants, however, plants with a stem of a noncircular cross section also exist. It seems that a stem with a quadrangular cross section has structural advantages and characteristics unlike the circular cross section stem. The structure and mechanical characteristics of a stem of quadrangular cross section of Perilla (beefsteak plant) are examined in order to obtain new ideas for designing the optimum composite material. The stem of the plant is mainly composed of the epidermis and cortex, vascular bundle and pith. The cortex is the organization that provides the firmness of the stem. It was found that the stiffness of the stem could be increased in Perilla by a change in the proportion of the cortex and a change of the cross-sectional geometry. An increase of 15% is being attempted in the moment of inertia of area by a geometry change. Keywords: biomimetics, optimum design, plant stem, natural fiber, Perilla. 1 Introduction The stem of the plant is the transfer line for important nutrients and water, and supports the upper leaves and flowers. In the fiber that constitutes the stem and other plant body, it is known that there are some plants that have over half the strength of glass fiber. In recent years, these plant fibers have been utilized as a reinforcement of the FRP [1,2]. It appropriately places various tissues, including those such as fiber, where the stem of the plant seems to have done the rational High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors) © 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

Upload: dohanh

Post on 15-Dec-2018

227 views

Category:

Documents


0 download

TRANSCRIPT

Material and structural design of biomimetic composites learning from the quadrangular plant stem

K. Kurashiki1, S. Hisatsugu2, Q.-Q. Ni1 & M. Iwamoto1 1Advanced Fibro-Science in Graduate School, Kyoto Institute of Technology, Japan 2Graduate Student, Kyoto Institute of Technology, Japan

Abstract

Organisms come in various forms in nature. A circular cross section can be found in many structures, such as the stems of many plants, however, plants with a stem of a noncircular cross section also exist. It seems that a stem with a quadrangular cross section has structural advantages and characteristics unlike the circular cross section stem. The structure and mechanical characteristics of a stem of quadrangular cross section of Perilla (beefsteak plant) are examined in order to obtain new ideas for designing the optimum composite material. The stem of the plant is mainly composed of the epidermis and cortex, vascular bundle and pith. The cortex is the organization that provides the firmness of the stem. It was found that the stiffness of the stem could be increased in Perilla by a change in the proportion of the cortex and a change of the cross-sectional geometry. An increase of 15% is being attempted in the moment of inertia of area by a geometry change. Keywords: biomimetics, optimum design, plant stem, natural fiber, Perilla.

1 Introduction

The stem of the plant is the transfer line for important nutrients and water, and supports the upper leaves and flowers. In the fiber that constitutes the stem and other plant body, it is known that there are some plants that have over half the strength of glass fiber. In recent years, these plant fibers have been utilized as a reinforcement of the FRP [1,2]. It appropriately places various tissues, including those such as fiber, where the stem of the plant seems to have done the rational

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

structural form. Many reports on the structure of plants have been made from a biomechanical standpoint, such as the maize leaf mechanics that will concern the modeling as a composite tapered beam [3], the iris leaf resembles a sandwich beam [4] and the hemp palm branch was constructed of the functionally graded structure [5]. The plants seem to have done the work of building structures that are optimized dynamically. They have the ability of showing a considerably flexible response for changes according to their situation and the forces that they are subjected to. They seem to have the optimality by their evolution over a long time, and it is expected that they may offer many hints on the design method of composite material. The purpose of this study is to examine the structure and mechanical characteristic of the stem of the Perilla, a stem of quadrangular cross section, and to obtain hints for the design of composite material.

2 Material

The sample is the stem of the Perilla frutescens. Perilla (beefsteak plant) of labiatae is an annual herb of dicotyledoneae. The stem is quadrangular, and it grows to 50-60cm height. The Perilla is shown in figure 1. The sample was set in water before the test. .

Figure 1: Perilla frutescens.

3 Experimental procedure

The fine structures of the cross section of the stem were observed by scanning using electron microscopes (Nikon, ESEM-2700) and digital high fidelity microscope (Keyence, VH-8000). It was observed, after using the sample to make a slice, by the microtome, and it is fixed in a Carnoy fluid and dyed. Tensile test of the stem and the tissues were carried out using a Universal Testing Instrument (JTtohsi, LSC-1) at the crosshead speed of 1mm/min. The specimens are stem(ts) and the tissues that are collected by the razor for the raw stem, or by hand picking for the fiber specimen of the retted stem. Tissue

100㎜

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

402 High Performance Structures and Materials II

specimens are epidermis(te), cortex(tc), pith(tp) and fiber(tf). These were cut from each internode by the razor. For measuring tensile properties, a gauge length of 25mm of each tissue was mounted on cardboard as shown in figure 2. In the experiment of the stem itself(ts), rubber was used in order to avoid damage in the chuck division, where the stem was fixed.

Figure 2: Tensile specimen of the tissues.

The bending test was also carried out using the same equipment of the tensile test by three-point bending. The specimens are stem(bs), stem except for the epidermis(be) and stem except for epidermis and fiber(bf). The schemata of the cross section of these specimens are shown in table 1, these specimens are dried. The span length is 64mm, and the crosshead speed is 10mm/min. Bending and tensile tests were done under a constant temperature and humidity (20±2°C, 60±5%RH).

Table 1: The schemata of the cross section of the stem.

4 Results and discussion

4.1 Structure of the stem

The cross-section of the stem of Perilla is shown in figure 3, and the enlarged view of each tissue is shown in figure 4. The stem contains ordinary the vascular bundles which xylem and phloem becomes a set are radially distributed between epidermis and center of stem. The fundamental system has filled the part of the

bf bs be

epidermis cortex pith fiber without epidermis & fiber without epidermis stem

25

25

specimen

glue

cardboard

45

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

High Performance Structures and Materials II 403

remainder. Though it is the similar composition in the Perilla, the cross section is quadrangular, and the arrangement of the vascular bundle would become also a quadrangle. Main organization is epidermis, cortex and pith, and there is slight fiber. The cortex, called a collenchyma, has been thickly placed in the quadrangular corner. The cross sections of the stem from the root to the tip are shown in figure 5, the numeral shows the number of the internode. From figure 5, it is found that the proportion of each tissue and cross-sectional geometry change according to the No. of the internode. The result of measuring the proportion of each tissue from the root to the tip of the stem by the microscope is shown in figure 6. The cortex decreases, as it goes to the tip from the root. Reversely, the pith increases as it goes to the tip. The change of the proportion of epidermis is slight. The cortex is the organization that occupies the stem and leaf of the herbaceous plant firm generally, and there are many closer to the root where it is proven that the flexural stiffness is increased. The fibers exist in the boundary of cortex and epidermis, as it is shown in figure 4, and about 10 have been collected in the one bundle, and the cell wall is very thick. It seems that the fibers reinforced four corners and middle points of four sides. In the quadrangular stem, it is indicated that it may change to the extremal value from the geometry [6], it agrees with the point.

Figure 3: Cross-section of the stem of Perilla.

Figure 4: Enlarged view of the tissues.

1㎜

� �

� cortex

pith

epidermis

� vascula

r

� fiber �

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

404 High Performance Structures and Materials II

Though the underground root is circular, cross-sectional geometry of the stem changes from the round shape with the roundness to square and cruciform, as it goes to the tip from the root as shown in figures 5. The result that calculated moment of inertia of area of these geometries, as the area was same, is shown in table 2. It becomes 115 near the tip of the stem, when circular moment of inertia of area was made to be 100. It is found that the stiffness has been increased by the geometry changed, though the strengthening tissues are few for the tip of the stem. Figure 5: Cross section of the stem from the root to the tip. (The numeral shows

the internode No.)

Figure 6: Proportion of the tissues.

2 3 4

6 7 8

1 2 3 4 5 6 7 80

20

40

60

80

100

Are

a ra

tio o

f tis

sue

(%)

Internode No.

epidermis cortex pith

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

High Performance Structures and Materials II 405

Table 2: Moment of inertia of area of cross-sectional geometry of the stem.

4.2 Mechanical properties

4.2.1 Tensile test Figure 7(a),(b) show the stress-strain curves of the stem and cortex by tensile test. The stress-strain curves of the stem show the linearity to the strain of about 1.5%, then nonlinear relation was shown, and reached the maximum stress in the strain of 3~4% and came to the rupture. However, the tensile strength is greatly different by the position of the stem, that relation is shown in figure 8. The tensile strength decreases, as it goes to the tip. This is based on the increase of the pith with the low strength. This tendency was similar at the Young’s modulus as shown in figure 9. Though the change by the position of the stem is observed in each organization, the change is small. The tensile strength and the Young’s modulus of the stem and each tissue are shown in figure 10 and 11, these values show the average of all data without considering the position of the stem. Though the dispersion is observed at the tensile strength, the fiber is the strongest, and it becomes the order of the following, cortex, stem, epidermis and pith. (a) Stem (b) Cortex

Figure 7: Stress-strain curves of the stem and cortex. The Young’s modulus tends to be also similar to tensile strength. The value of the fiber is the highest, however the proportion occupied for the stem is little, the

0 1 2 3 4 50

10

20

30

40

50

Stre

ss

(MPa

)

Strain (%)0 1 2 3 4 5

0

20

40

60

80

100

Stre

ss

(MPa

)

Strain (%)

1st internode tip

100 102 1155 11001055

Part

Geometry

(same area)

root

value

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

406 High Performance Structures and Materials II

cortex mainly undertakes the stiffness of the stem. Tensile strength and Young’s modulus of the fiber show the highest value, and it is respectively about 5 times and 10 times of the cortex. Though the proportion occupied in the cross section of the fiber is about 3%, it seems to greatly contribute to the strength of the stem.

Figure 8: Relationship between tensile strength and internode No.

Figure 9: Relationship between Young’s modulus and internode No.

Figure 10: Tensile strength of the stem and the tissues.

1 2 3 4 5 6 7 80

10

20

30

40

50

Tens

ile st

reng

th

(MPa

)

Internode No.

1 2 3 4 5 6 7 80.0

0.5

1.0

1.5

2.0

You

ng's

mod

ulus

(G

Pa)

Internode No.

ts te tc tp tf0

100

200

300

400

500

600

Tens

ile st

reng

th

(MPa

)

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

High Performance Structures and Materials II 407

Figure 11: Young’s modulus of the stem and the tissues. 4.2.2 Bending test Because the bending test of only of the tissue is difficult, it was tested in 3 kinds of specimen of the stem, the stem that removed the epidermis and stem that removed epidermis and fiber, as was shown in table 1. The specimen was chosen whose geometry and composition proportion of the tissue resembled each other well.

Figure 12: Bending stress-deflection curves of the stem.

Figure 13: The bending strength of the stem.

bf be bs0

20

40

60

80

Ben

ding

stre

ss

(MPa

)

0 2 4 6 80

20

40

60

80

100

Ben

ding

stre

ss

(MPa

)

Deflection (mm)

ts te tc tp tf012345

1020304050

You

ng's

mod

ulus

(G

Pa)

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

408 High Performance Structures and Materials II

The bending stress-deflection curves of the stem as figure 12, the bending strength is shown in figure 13. From the comparison of stem(be) and stem(bf), the bending strength of with the fiber is higher than other one at about 20%. It is found that the fiber has strengthened the structure of the stem and that it shows the effect for the bending. For the effect of the epidermis, because of the bending strength of stem(bs) is higher than stem(bf) about 20% and is higher than stem(be) about 40%, the effect of the epidermis has appeared. From the above results, by summarizing the performance of each tissue, it is shown in table 3.

Table 3: Performance of the tissues.

5 Conclusion

The microstructure and its tensile test of the stem of Perilla with quadrangular cross section were conducted and the reinforcement mechanism was investigated. As a result, for the strengthening mechanism of the stem, the role of each tisuue was clarified. That is, it was found that the stem of Perilla was attempting structure strengthening by cross-sectional geometry changes. To increase flexural stiffness, mechanically supporting tissues (cortex and fiber) are stationed outside and those proportions change. The tissue (epidermis) with the flexibility in the outside of the stem fulfils the role that arranges each organization and protection of the stem.

epidermis

fiber

stem

cortex

epidermis

・ reinforcement of structural tissue & weak point

・ decreasing differences of mechanical properties of the stem from the root to the tip

・ integrating the individual structural tissue into an aggregation

・ fills role of mechanical support of the stem

・ arrangement and geometry make a contribute

・ protection of four corners &internal tissue

・ high adaptability and bending stiffness to loading from outside environment

Performance of the tissue

Tissue

・ protection of internal tissue

Schema of arrangement of the tissue

to high stiffness

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

High Performance Structures and Materials II 409

References

[1] Karus, M., Kaup, M. & Ortmann, S., Use of natural fibres in composites-market survey 2002:status, analysis and trends, EcoComp 2003, 2003(CD-ROM).

[2] Thomas, S., Natural fiber composites - The new millennium material, Proc. of Second International Workshop on “Green” Composites, pp.1-7, 2004.

[3] Moulia, A. Fournier, M. & Guitard, D., Mechanics and form of the maize leaf: in vivo qualification of flexural behaviour, Journal of Materials Science, 29, pp.2359-2366. 1994.

[4] Gibson, . ,L.J Ashby, M.F. & Easterling K.E., Structure and mechanics of the iris leaf, Journal of Materials Science, 23, pp.3041-3048. 1988.

[5] Amada, S., & Terauchi, Y., Mechanical characteristics of hemp palm-branch with triangular cross-section and functionally graded structure, Transactions of the Japan Society of Mechanical Engineers(A), 65, pp.2418-2423, 1999.(in Japanese)

[6] Tanaka, K., The design of the plant, p.82. Kyouritu kagaku books, 1984. (in Japanese)

High Performance Structures and Materials II, C.A. Brebbia & W.P. De Wilde (Editors)© 2004 WIT Press, www.witpress.com, ISBN 1-85312-717-5

410 High Performance Structures and Materials II