Mechanical properties of silkworm cocoon pelades

Engineering Fracture Mechanics 74 (2007) 1953–1962 www.elsevier.com/locate/engfracmech

Mechanical properties of silkworm cocoon pelades Hong-Ping Zhao a, Xi-Qiao Feng a b

a,*

, Wei-Zheng Cui b, Feng-Zhu Zou

b

Department of Engineering Mechanics, Tsinghua University, Beijing 100084, PR China Department of Sericulture, Shandong Agriculture University, Tai’an 271018, PR China

Received 4 January 2006; received in revised form 13 June 2006; accepted 13 June 2006 Available online 28 July 2006

Abstract The pelade, the innermost layer of silkworm cocoon next to the chrysalis, has special microstructures, mechanical properties and protective functions distinctly different from those of all the other layers. In the present paper, a series of static tensile tests and dynamic mechanical thermal analysis were performed for the first time to measure the mechanical properties of pelades, including Young’s modulus, tensile strength and thermomechanical parameters. The fracture process of precracked pelade specimens was observed by in-situ scanning electron microscopy under tension. It is found that the Young’s modulus, tensile strength, storage modulus and loss modulus of cocoon pelades are superior to the corresponding thickness-averaged values of a complete silkworm cocoon. The damage and fracture process of pelades involve delamination, silk breaking and damage localization band. The results indicate that silkworm caterpillars can be appreciated as sophisticated sewers to make anisotropic and optimized structures of cocoons with both protective functions and mechanical properties varying in their thickness direction. The present study might be helpful to guide biomimetic design of novel safe-guarding materials and structures from both the viewpoints of microstructures and spatial functional gradients. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Silkworm cocoon; Pelade; Mechanical property; Biomaterial; Biomimetics

1. Introduction Many lepidopteron and other insects can construct protective structures against potential predators [1–4]. From the standpoint of biomimetics, it is interesting to study the interrelations among the properties, compositions, functions and structures of natural protective systems for developing advanced materials and structures with superior performances. Silkworm caterpillars are representative to spin cocoons in order to protect the moth pupae against possible attacks from the outside during metamorphosis [5,6]. At the end of the fifth instar, silkworm larvae stop eating and start to spin cocoons. Generally, a larva completes the construction of cocoon within 3–4 days. A silkworm cocoon contains mainly three parts, the outermost floss, the middle compact layers and the innermost pelade, which have different microstructures and functions. The floss, also named cocoon coating or frison, is incompact, brittle and unreelable and, therefore, is always peeled *

Corresponding author. Tel.: +86 10 6277 2934; fax: +86 10 6278 1824. E-mail address: [email protected] (X.-Q. Feng).

0013-7944/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfracmech.2006.06.010

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off before the reeling of silk in textile industry [7]. Its weight is about 3.5–4.0% of the total weight of the cocoon. The middle compact shell part, consisting of many sublayers, plays the most significant role for the cocoon to achieve the excellent protective function. This part has a weight of about 91% of the total cocoon weight and provides almost all the silks for reeling. The pelade layer, which is also referred to as the pupal shirt, pelette, wadding, tetelette and neri, has a weight about 5–5.5% of the total cocoon and makes a special contribution to the protective function of cocoon [8,9]. The most crucial function of silkworm cocoons is to shield the moth pupae during metamorphosis from possible dangers and bacteria. As the innermost layer of cocoon, the pelade plays a significant and multifunctional role in the metamorphosis process. Some researches have been done on the construction process, chemical and physical properties of silkworm cocoons. Miura et al. [10,11] suggested a statistical model of body movement and cocoon shape of Bombyx mori in silk spinning behavior and studied the main behavioral factors affecting cocoon shape formation by comparing three different strains. Zhang et al. [12] studied the color, size and shape of B. mori cocoon shells after heat treatment at different temperatures. Recently, Zhao et al. [13] investigated the properties of Chinese silkworm cocoon, B. mori. Interestingly, they found that, on one hand, the overall or thickness-averaged mechanical properties of a complete cocoon are excellent and, on the other hand, the elastic modulus, strength as well as thermomechanical parameters vary along the thickness direction of the cocoon in an apt manner, leading to a further enhancement in its ability to resist possible attacks from the outside. To date, however, there is no research on the mechanical properties and microstructures of the pelade of silkworm cocoon in spite of its special functions and microstructures. Therefore, an attempt is made here to investigate via systematic experiments the mechanical properties (e.g., tensile modulus E, tensile strength ru, and tensile elongation eu) of cocoon pelades constructed by Chinese silkworm larvae, B. mori. The dependence relationship of these mechanical properties on microstructure is examined. We find that both the elastic modulus and strength of the pelades are superior to the corresponding thickness-averaged values of the complete cocoons. In addition, the fracture mechanisms of the pelades are studied by tensile tests of precracked pelade specimens with in-situ scanning electron microscopy (SEM) observations. 2. Experiments 2.1. Materials Each male silkworm, B. mori, of euryphagous strain was placed in a closed paper, where it spun a complete cocoon at the end of the fifth larval instar under controlled conditions at 70–75% relative humidity and 24 °C temperature. The length ratios between the major and minor axes of the obtained cocoons are about 1.8. The silk were reeled first from the obtained cocoons according to the following procedure. Firstly, the cocoons were cooked in boiling water for 2 min to vent air inside and then immerged into water of 65 °C for 2 min to saturate all cocoon layers. Secondly, the cocoons were macerated in boiling water for 2 min again to soften the silk gum and then cooled gradually down to 87 °C in air within about 15 min. Finally, the cocoons were quenched in water of 18 °C and moved into warm water of 60 °C to reel the silk. Subsequently we can get a residual cocoon layer, i.e., the pelade. Three types of rectangular specimens of 3–3.5 mm in width and 20–30 mm in length were cut from the pelades, along the longitudinal direction, the transverse direction and the 45° direction measured from the major axis, respectively. 2.2. Methods First, the thickness and widths at 5–6 different positions of each rectangular specimen were measured to calculate its average cross-sectional area. Quasi-static uniaxial tension tests were performed at ambient temperature of 23 °C and under a constant humidity of 65% RH using a computer controlled AGS-10KN universal test machine to study the deformation and failure behavior of the three types of pelade specimens described above. The crosshead speed was adjusted at a loading rate of 2 mm/min in the specimen gauge section of length 10 mm. The load cell was set at 50 N. The load-displacement curve was recorded automatically by computer, and the corresponding stress–strain curve was obtained by dividing the load and displacement by the cross-sectional areas and the gauge length of the specimens, respectively. Due to the possible dispersion

H.-P. Zhao et al. / Engineering Fracture Mechanics 74 (2007) 1953–1962

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of the experimental data, six specimens of each type were measured under the same condition to yield the average results. The results were compared with those of the samples cut from an as-spun complete silkworm cocoon. Thermomechanical properties of the pelade samples were determined with a dynamic thermomechanical analyzer (TA instrument DMA 2980-USA). The storage modulus E 0 , the loss modulus E00 and the loss factor tand = E00 /E 0 were obtained with three pelade specimens. A small static force of 0.01 N was applied along the axial direction of the sample, and the displacement amplitude was set to 20 lm. The measurements were performed at a frequency of 1 Hz within a temperature range from 35 to 320 °C. In-situ tensile tests were performed using SS-550 SEM in order to observe the fracture process of the pelades. The tensile specimens of 3.5 mm width and 30 mm length were cut from the pelades along the longitudinal direction, and then gold coated. A precrack of 0.5 mm length was made on one side of each specimen with a razor blade in order to clearly observe the crack propagation path during in-situ tensile test. The specimen gauge section was 10 mm in length, and the loading speed was set as 0.01 mm/s. 3. Results and discussions 3.1. Mechanical properties of pelades Several representative tensile stress–strain curves of rectangular specimens of the pelades obtained from quasi-static tension tests are given in Fig. 1, in comparison with a typical stress–strain curve of a sample cut from a complete silkworm cocoon [13]. Young’s moduli E, tensile strengths ru and ultimate tensile strains eu in different directions of pelades and complete cocoons [13] are summarized in Table 1. Evidently, both the mean Young’s modulus and tensile strength of cocoon pelades are much higher than those of complete cocoons. In the longitudinal direction, the average Young’s moduli and tensile strength of pelades are 1135 ± 40 MPa and 50 ± 8 MPa, which are about 3.4 and 2.5 times those (337 ± 51 MPa and 20 ± 2 MPa) of complete cocoons, respectively [13]. Such greatly enhanced mechanical properties in the innermost layer are important for a cocoon to protect the moth pupa efficiently. However, the ultimate tensile strain of the pelade in the longitudinal direction is lower than that of complete cocoon. This is attributed to the relatively higher content of the thinner silk fiber in the pelades, as will be discussed in the sequel. In addition, it is seen from Fig. 1 and Table 1 that the mechanical properties of cocoon pelades show a pronounced anisotropy similar to those of complete cocoons [13]. Young’s modulus, tensile strength and ultimate tensile strain of the pelades in the 45° direction are 1648 ± 357 MPa, 62 ± 6 MPa and 13 ± 3%, respectively, which are evidently higher than those in the longitudinal and transverse directions. This is attributed to the anisotropic distribution of the silk orientations in the pelades, resulting from the manner in which silkworm caterpillars spin silks [10].

longitudinal direction transverse direction 45º direction

72

Stress (MPa)

60 48 36

complete cocoon [13]

24 12 0

0

5

10 15 Strain (%)

20

25

Fig. 1. Typical tensile stress–strain curves of rectangular specimens cut from cocoon pelades and a complete cocoon.

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Table 1 Mean tensile modulus, mean ultimate tensile strength, mean ultimate tensile strain and their standard deviations of pelades and complete cocoons Specimen

Tensile direction

Mean tensile modulus, E (MPa)

Mean ultimate tensile strength, ru (MPa)

Mean ultimate tensile strain, eu (%)

Cocoon pelade

Longitudinal direction 45° direction Transverse direction

1135 ± 40 1648 ± 357 1003 ± 121

50.3 ± 7.88 61.7 ± 6.19 45.8 ± 11.30

10.44 ± 0.96 12.61 ± 2.83 10.54 ± 1.94

Complete cocoon [13]

Longitudinal direction

337 ± 50.6

20.1 ± 2.18

25.42 ± 3.61

The SEM images in Fig. 2(a) and (b) show the microstructures of the pelade layer and the middle layer of a cocoon, respectively. Both the layers may be considered as a porous matrix of sericin reinforced by randomly oriented continuous fibroin fibers. As a requirement for its special protective functions, the pelade has a much

Fig. 2. SEM images of the microstructures of a cocoon: (a) the innermost pelade layer and (b) the middle shell layer.

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lower porosity and a smaller average diameter of the silk than those in the outer layers of the cocoon [9]. The microstructures of the pelade and the cocoon shell vitally affect their different mechanical properties, though they are made of the same materials, namely, fibroin and sericin. Both the complete cocoon and its pelade layer can be thought as silk foam or non-woven materials with different porosities. Here the properties of porous materials obey approximately the following Gibson–Ashby relation [14,15] E rb   Es rys

 2 q qs

ð1Þ

where E*, rb* and q* denote Young’s modulus, tensile strength and mass density of the cellular material (the complete cocoon or its pelade layer), Es, rys and qs denote Young’s modulus, yield stress and mass density of the silk, respectively. As aforementioned, both the Young’s modulus and tensile strength of a pelade are much higher than those of the complete cocoon. This can be attributed to two reasons. First, in comparison with the complete cocoon, the pelade layer has a lower porosity and a higher q*/qs. From Eq. (1), therefore, the pelade should have a higher Young’s modulus and a higher tensile strength. Second, the tensile modulus, strength and yield stress of silk show distinct size effects [16,17], that is, they increase with the decrease in the silk diameter. The silk diameter in the pelade is pronouncedly smaller than the average silk diameter of the complete cocoon, and hence the mechanical properties of cocoon pelade are better. It is interesting to note that the large differences in the microstructures and mechanical properties in the pelade layer and the cocoon shell layer are not accidental but requisite for a cocoon to efficiently protect the pupa. First, the porosity of a silkworm cocoon structure should meet the requirements of temperature, humidity and ventilation for the moth pupa to live in it. Through evolution of a very long history, silkworm caterpillars have formed the ability to construct cocoons with different microstructures (e.g., porosity, random orientation distribution of silk) in different layers in order to optimize all aspects of their functions. The pelade layer and the shell layer of a cocoon have different microstructures and different protective functions. Stress analysis shows that in order to bear the same externally applied pressure force without occurrence of failure, both a higher Young’s modulus and a higher strength are required in the innermost, tensioned layer. Furthermore, the lower porosity of the pelade can effectively prevent minute organisms and ambient water from intruding and destroying the pupa. The specific fracture energies of the pelades and the complete cocoons, determined from the areas under the corresponding stress–strain curves, are calculated as about 3.96 and 3.81 MJ/m3, respectively, while the mean thickness of complete cocoons is 20–30 times larger than that of pelades. Therefore, the cocoon shell layer is the most significant part to resist an external attack force and to absorb the external impact energy. A silkworm caterpillar can be appreciated as a sophisticated sewer to make an anisotropic and optimized microstructure of the cocoon from the outer to the inner with different protective functions. 3.2. Dynamic thermomechanical properties of pelades Dynamic mechanical thermal analysis (DMTA) was also performed in order to characterize the mechanical and thermal properties of cocoon pelades. Fig. 3 shows the variations of the storage moduli, the loss moduli and the mechanical loss factors of three representative cocoon pelades and a complete cocoon as a function of temperature in the range of 35–315 °C. The dynamic thermomechanical properties of both the pelades and the cocoon shown in Fig. 3 are typical of polymers. It is seen that the storage modulus E 0 of the pelades is evidently higher than that of the complete cocoon, but the loss modulus E00 of the former has little difference from that of the latter. However, the mechanical loss factor tan d = E00 /E 0 of the pelades is smaller than that of the cocoon. The storage modulus is related to the elastic properties, the loss modulus is related to the internal friction and energy dissipation, and the mechanical loss factor characterizes the damping properties of the material. Therefore, the pelade layer has a higher stiffness modulus than the complete cocoon, as is consistent with the results of static tensile tests in Section 3.1, but the cocoon has a better damping property than the pelade. These differences of dynamic thermomechanical properties are due to that both the porosity and the sericin content of pelades are lower than the thickness-averaged values of complete cocoons [9].

H.-P. Zhao et al. / Engineering Fracture Mechanics 74 (2007) 1953–1962

a

9.4

Storage modulus log (E') (Pa)

1958

9.2 9.0

8.6 8.4 8.2

Loss modulus log (E'') (Pa)

b

cocoon [13] cocoon pelades

8.8

0

50

100

150 200 250 Temperature ( ºC)

300

350

8.4 cocoon [13] cocoon pelades

8.2

8.0

7.8

7.6

c

-0.4

Mechanical loss factor log (tan(δ))

7.4

-0.6

0

50

100

150 200 250 Temperature ( ºC)

300

350

300

350

cocoon [13] cocoon pelades

-0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0

0

50

100

150 200 250 Temperature ( ºC)

Fig. 3. Dynamic thermomechanical properties of a pelade and a complete cocoon: (a) dynamic storage moduli, (b) loss moduli, and (c) mechanical loss factors.

The onset temperature, corresponding to the endothermic peak of the loss modulus, is assigned as the glass transition temperature (Tg) [18]. From Fig. 3(b), Tg of the pelades and cocoons are about 202 and 204 °C, respectively. Since both the pelades and cocoons are mixtures of fibroin and sericin, their glass transition temperatures Tg may be estimated by the following Fox equation [19,20]

H.-P. Zhao et al. / Engineering Fracture Mechanics 74 (2007) 1953–1962

1 w1 w2 ¼ þ ; T g T g;1 T g;2

1959

ð2Þ

where wa and Tg,a are the weight fraction and the glass transition temperature of the ath phase, with a = 1 and 2 standing for the fibroin and sericin phases, respectively. The average weight contents of fibroin and sericin in cocoons are about w1 = 76% and w2 = 24%, respectively [9]. Taking the glass transition temperature of sericin as Tg,2 = 215 °C [18], we derive from the Fox relation of cocoons the glass transition temperature of fibroin to be about 200 °C. Then using the Fox relation again, the weight fractions of the fibroin and sericin phases in pelades are deduced to be about 86% and 14%, respectively. Therefore, the variation in the glass transition temperature from the cocoon shell to the pelade layer is caused by the change in the contents of sericin and fibroin. 3.3. In-situ SEM observation of fracture process of pelades under tension Fig. 4 shows a load-displacement curve of a notched pelade specimen under tension with in-situ SEM observations. The test was performed in the SS-550 SEM under displacement-controlled loading. The loaddisplacement curve can be divided into three stages separated by B and C, as shown in Fig. 4. Stage AB is almost purely elastic. From B to C, the load drops rapidly due to damage. Typical damage mechanisms of the pelade at the mesoscale include laminate delamination of the sublayers, debonding, breaking and pullout

b

18

In-situ SEM observation

B

Load (N)

15 12 9 c

6 3

e

d

a

D

C

0 A 0.0

0.5

1.0 1.5 Displacement (mm)

2.0

Fig. 4. Load-displacement curve of a cocoon pelade specimen recorded by in-situ SEM observation.

1.6 e

1.4 1.2 COD (mm)

d

1.0 0.8 0.6 c

0.4 0.2 0.0 a 0.0

b

0.2

0.4

0.6 0.8 1.0 1.2 Displacement (mm)

1.4

1.6

Fig. 5. Crack opening displacement of a precracked pelade specimen under tension.

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Fig. 6. Snapshots of the fracture process of a precracked pelade sample under tension, which correspond to the points a–e in Fig. 5, respectively.

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of the fibroin silk filaments from their sericin sheaths. A damage localization band forms ahead of the crack. In stage CD, breaking and pullout of silks continuously happen in the damage localization band until the final rupture of the specimen. The crack opening displacements (COD) [21,22] of the pelade specimen at different time intervals from point a–e were obtained by directly measuring the distance between the two crack faces at the original crack tip from in-situ SEM images and given in Fig. 5 as a function of tensile displacement. The snapshots of the fracture process corresponding to the points a–e in Fig. 5 are shown in Fig. 6. Fig. 6(a) depicts the precrack specimen before tension. Before the rapid drop of stress, damage and large deformation occurs mainly around the crack tip due to the high stress concentration, as shown in Fig. 6(b). With the further increase in the externally applied displacement, a damage localization band (the white area in Fig. 6(c)) forms in front of the crack. Delaminations occur at the interfaces of the silks in the damage zone. Some silk fibers in the localization band still bridge the upper and the lower parts of the specimen, while some others are broken and pulled out from one side of the specimen. In Fig. 6(d) and (e), most bridging silk fibers in the damage localization band have been pulled out or broken, followed by the final rupture of the pelade specimen. In addition, it is observed that, as is similar to the fracture process of complete cocoon [13], the tensile fracture process of a cocoon pelade involves several different energy dissipation mechanisms at different scales, including the damage localization band at the macroscale, separation of the two originally bonded strands of a silk, silk debonding, fracture and pullout of the silk core from the sericin coating at the mesoscale, and crazes at nanoscale. These fracture mechanisms at different scales can effectively dissipate the impact energy of attacks from the outside and, therefore, lead to the superior protective function of cocoon. The study of fracture behaviors of biomaterials may provide some inspirations for the development of advanced impact energy dissipation materials from the viewpoint of multiscale designs. 4. Conclusions A series of experiments have been performed to study the microstructures and mechanical properties (elastic modulus, strength, elongation and thermomechanical properties) of cocoon pelades spun by silkworm caterpillars, B. mori. It is found that the pelades have denser microstructures and finer silks than the complete cocoons, and therefore both the static and the dynamic properties of the former are superior to the average properties of cocoons. These particular properties of the pelade layer yield a further enhancement of the cocoon’s ability to resist possible attacks from the outside and protect the minute organism from invading. Our experiments indicate that silkworm caterpillars can be appreciated as sophisticated sewers to make an anisotropic and optimized microstructure of the different layers of a cocoon, which have different protective functions. This study is a primary effort to understand the mechanical behavior and its dependence on the microstructures of cocoon pelades. The obtained results might be helpful to guide biomimetic designs of novel safe-guarding materials from both the viewpoints of microstructures and spatial functional gradients. The present paper has considered merely the influences of some macroscopic and empirical factors (porosity, density, silk diameter) on the mechanical properties of silkworm cocoons and their pelades. It is also of importance to study such microscopic structural features as the crystalline and molecular structures of fibroin cores and sericin sheathes and to establish the interrelations of compositions, structures, properties and functions of silks and cocoons at different scales. Acknowledgements The supports from the National Natural Science Foundation of China (Grant Nos. 10525210, 10402017 and 10121202) and the 973 project of MOST (Grant No. 2003CB615603) are acknowledged. References [1] Geier PW. The life history of codling moth, Cydia pomonella (L.) (Lepidoptera: Tortricidae), in the Australian capital territory. Aust J Zool 1963;11:323–67.

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