Mechanical Properties of Natural Versus Manmade Fibers

Kara Phillips, Daria Monaenkova, and Konstantin G. Kornev* School of Materials Science and Engineering, Clemson University, Clemson, SC

Contact Author: kkornev@clemson.edu; Speaker: kjphill@clemson.edu

OBJECTIVE The purpose of this study is to evaluate the differences in the mechanical properties of natural and manmade fibers under compressive and tensile loads with particular interest being paid to the properties of spider silk. INTRODUCTION The significance of the mechanical properties of a material depends on the types of stresses it will be undergoing in its typical working environment. Because they are anisotropic, fibers stressed in tension and those in compression often display very different properties [1]. Many fibrous materials work under tensile loads where the longitudinal Young’s Modulus of the material is of particular interest like in the case of ropes, fishing nets, or masts. The Young’s Modulus of the fibers can, in most cases, be easily obtained using the standard Instron tensile testing machine. However, in some situations, certain fibrous materials work under compression. For example, in the use of Kevlar for bulletproofing, the properties of the fiber under compression and the compression modulus are significant. There are very few methods to determine the compression modulus, and it is usually assumed to be in the order of the tensile modulus. In this work, we study the difference between the compression and tensile properties of natural and artificial materials. EXPERIMENTAL The compression modulus of Kevlar, PET, Nylon, glass fiber, and N. Clavipes spider silk were obtained using the Kawabata Compression tester for single fibers (KatoTech, Japan). The reversibility of the deformation from the compression testing was analyzed using microscopy with polarized light before and after testing. The tensile testing of the samples was conducted on the Instron 1125 tensile testing machine. Kawabata Single Fiber Compression Tester The fiber diameters ranged from five to seventy micrometers, and because of the smaller diameters, the common compression procedures were not applicable. To measure the compression modulus of the samples, we have employed instrumentation presented by Kawabata [1]. The single fiber compression tester had a .2mm by .2mm square tipped probe to press down on the fiber that was laid across a flat surface as displayed in Figure 1. An example of the force versus deformation curve obtained is shown in Figure 2.

FIGURE 1: The setup of the Kawabata compression tester showing the change in radius of the fiber with F being the compressive force, R the radius of the fiber, and U the deformation.

FIGURE 2: Typical loading and unloading compression curve for the N. Clavipes spider silk. The compression modulus of the fiber was then found using “Eq. (1 & 2)” which are based on the equations derived by Ward and Jawad [2]:

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U =4σπET

* (0.19 + sinh−1(Rb)) (1)

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b2 =8σR2

πET (2)

Where

R: radius of fiber

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σ: stress

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U : strain = deformation/diameter

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ET : transverse modulus RESULTS Table I shows the compression modulii of the chosen fibers calculated using “Eq. (1)” as well as the tensile modulii found using the Instron. From the experimental data, it can be seen that the transverse modulus for the manmade fibers is significantly less than the tensile modulus values showing a high degree of anisotropy. However, in the case of the

F

sq. probe tip

Nephila Clavipes spider silk, the tensile and compression modulus are close enough to indicate that it is an isotropic material. This specific property is of particular interest because a fiber that has high strength along both axes would prove to be superior to one that only has high strength in the longitudinal direction as most fibers do. TABLE I. Transverse and Longitudinal Modulii of Fibers

Material

Diameter (microns)

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ET (GPa)

Tensile Modulus

(GPa)

Kevlar 12.3 1.5 17.627 PET 26.5 1.4 1.978

Glass Fiber 8 .687 30.628 N. Clavipes Spider Silk

5.8 3.68 4.05

Nylon 68 .2 .45 After testing the fibers in compression, the extent of deformation was evaluated under polarized light. The photos under higher loading conditions (50 gram-forces) showed permanent deformation, as displayed in Figure 3. To pinpoint the transition area between the elastic and plastic region, the spider silk was tested under two, five, and ten gram-forces and then evaluated under polarized light directly after and twenty hours after testing. The spider silk showed no smash marks from the probe under two and five gram-forces but did under ten gram-forces showing that the transition area between the elastic and plastic region for the N. Clavipes spider silk is between five and ten gram-forces. The compression modulus results for these experiments also further confirm that the spider silk transitions out of the elastic region and into the plastic region between five and ten gram-forces. Table II shows the compression modulus being higher at 3.68 GigaPascals under the five gram-force loading conditions because it is nearing the end of the elastic region of the material. The modulus decreases under ten gram-force loading conditions because the properties of the already damaged fiber are being tested, which would show weaker properties.

FIGURE 3: Pictures of the spider silk fiber shown before, directly after, and 20 hours after testing (shown from left to right).

TABLE II: Compression Modulus Under Different Force Conditions

Force Conditions 2 g-f 5 g-f 10 g-f Compression Modulus (GPa) 0.523 3.68 0.258

CONCLUSION & FUTURE WORK It can be seen from our experiments that the mechanical properties of spider silk are different from those of manmade fibers. The isotropic properties of the silk are desirable because fibers often see both compressive and tensile loads in their typical working conditions. A fiber that shows high strength under loads in both directions is superior to the current manmade fibers that only show high strengths along the longitudinal axis. In future work, we plan to investigate how the mechanical properties of spider silk change with differing chemical composition. We want to find the strongest silk and the combination of amino acids of which it is comprised with future hopes of artificially producing spider silks. We also wish to investigate further the mechanism in the silk fiber production that causes the disordering of the amino acid chains which accounts for the isotropic properties of spider silks. ACKNOWLEDGMENT The funding for this research was provided by the National Science Foundation, Grant No. EFRI 0937985, and the Clemson REU program with an NSF Grant No. 1062873, as well as the Department of Materials Science and Engineering at Clemson University. The authors wish to thank Dr. Michael Ellison of Clemson University for his assistance in this research, specifically with the spider silks. REFERENCES [1] Kawabata, S. “Measurement of the Transverse Mechanical Properties of High-Performance Fibres,” Department of Polymer Chemistry, Kyoto University, Kyoto, Japan. 6.7.1990. 442-447. [2] Ward, I. M., Jawad, S. Abdul; “The Transverse Compression of Oriented Nylon and Polyethylene Extrudates,” Journal of Materials Science, v13. 1979, 1381-1387. [3] Ko, Frank K., et al. “Engineering Properties of Spider Silk,” Materials Research Center. 1-7. [4] Yuan, Xeupei, et al. “Synthesis and Characterization of Poly(ethylene terephthalate)/Attapulgite Nanocomposites,” Journal of Applied Polymer Science, v103, 15 January 2007, 1279-1286.

200  microns