JOURNAL

OF M A T E R I A L S

S C I E N C E L E T T E R S 5 (1986) 1077

1080

Tensile properties and fracture of ivory A. R A J A R A M *

Biophysics Division, Central Leather Research Institute, Adyar, Madras 600 020, India

coating. Chemical analyses were performed for estimation of colageha content from hydroxyproline determination [5], nitrogen [6] and phosphorus [7]. The tensile data are given in Table I along with the density values for ivory which is compared with antler bone and mammalian compact bone samples of comparable dimensions. Antler bone has more collagen than compact bone though less than that for ivory. Table II shows the chemical composition of the ivory compared with the other calcified tissues. Ivory has the least tensile strength and modulus. These values are less than those obtained by Bonfield and Li [3]. Presumably, they may have tested ivory from African elephants which are reported to have higher mineral content [2]. The value of the modulus obtained here agrees with that of Serizawa et al. [8] who tested ivory in bending. The modulus is a reflection of the degree of mineralization that has taken place in the tissue and is expressed by the ratio of phosphorus to nitrogen [9]. This is very low for ivory. Even though this value compares well with antler bone, ivory also has a much higher collagen content and, the modulus of collagen being only one hundredth that of the mineral phase [10], the low degree of mineralization together with the high collagen content can account for the very low modulus of ivory. The modulus values can be higher for all the tissues if microstrain measuring techniques are employed [11]. The low tensile strength is compatible with the low density. Carter and Hayes [12] have observed this in the case of compact bone. The work to fracture is comparable to that for bovine bone and this can be due to the considerable plastic flow before fracture. This is borne out by the nature of the fractured surfaces. Fig. la and b and Fig. 2a and b show the surface of the sample tested dry. Fig. l a shows a region where the fracture has taken place mainly along the dentinal tubules and Fig. 1b, a region where the fracture is across the tubules

Ivory is a calcified substance which is capable of being shaped into beautiful pieces of art by skilled artisans and has a high economic value. The factors contributing to the demand for ivory are its workability, fine grain pattern and ability to take up a high polish by physical abrasion in the natural state itself. Ivory is a development of the incisor teeth and so is a dentine matrix which can be obtained large enough for testing in tension according to standard specifications. However, it has little in common with normal tooth dentine, being much softer than the latter and differing in structure as well [1]. There has been a report that the ivory from African elephants has a higher mineral content than the ivory from Indian elephants [2]. Initial studies on the deformation and fracture of ivory have been made by Bonfield and Li [3]. The present study examines the tensile fracture of ivory obtained from the Indian elephant with respect to its composition and structure. Dry samples of ivory were obtained from the tusks of male Indian elephants. Specimens oriented along the length of the tusk were shaped according to standard specifications [4] by manual abrasion with various grades of emery paper, the final abrasion being done with 600 grit silicon carbide. The samples had a length of 5.7 cm with a waisted section of 2.54 cm. The maximum and minimum widths were 9.5 and 2.54 mm (at the central section), respectively. The thickness was 1.65 mm. Twelve specimens were tested at a strain rate of 3.3 x 10 4sec i in an Instron machine using a LVDT for extension measurement. Samples to be tested wet were immersed in Ringer's saline for 48 h (which resulted in maximum uptake of water) at 4°C before equilibrating and testing at a room temperature of 24 -t- 1° C. The density of ivory was determined after drying samples of finite dimensions (2cm x 1 cm x 0.3 cm) over anhydrous silica gel in a desiccator at 70°C for 5 days. Fractured surfaces were scanned in a Cambridge Stereoscan S 150 SEM after sputter

T A B L E I Density and tensile properties of ivory compared with antler bone and bovine compact bone Sample

Dry density ( g c m 3)

Condition

No. of samples

Ultimate tensile strength ( M N m 2)

Elastic modulus (GN m -2)

Work to fracture ( 1 0 - S J m 3)

Ivory

1.70 _ 0.02

dry wet dry wet wet

6 6 10 10 10

l l 0 4- 8 36 _+ 4 188 + 12 108 ___ 5 99.2 4- 4.7

12.5 3.5 17.1 7.5 17.7

8.7 4.9 13.5 14.6 4.0

Antler bone* Bovine femure*

1.86 _+ 0.016 1.94 to 2.04

_+ 0.8 _+ 0.37 4- 0.8 _+ 0,9 _+ 0.8

_+ + 44+

0.92 0.54 1.2 1.6 0.4

*[14l. t[171. * Present address: Department of Textiles and Apparel, New York State College of H u m a n Ecology, M a r t h a van Rensselaer Hall, Cornell

University, Ithaca, N Y 14853, USA.

0261-8028/86 $03.00 + .12 © 1986 Chapman and Hall Ltd.

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T A B L E I I Chemical composition of ivory compared with antler bone and bovine femure (per cent dry weight of six determinations) Sample

Collagen*

Phosphorus (P)

Nitrogen (N)

P/N

Ivory Antler bone? Bovine femoral osteon*

37.1 + 2.3 29.1 ___ 0.9 23.6 + 1.0

12.3 __+ 0.26 9.56 ± 0.21 10.11 + 0.26

7.6 ___ 0.26 5.83 _ 0.21 4.42 _ 0.16

1.62 1.64 2.287

*[9] Collagen content = per cent hydoxyproline x 7.14. t[141.

as well. From these it can be observed that the dentinal tubules follow a sinusoidal pattern. The stereo-pair (Fig. 2a, b) also bring out this feature. The fine grain pattern on a cross-section of polished ivory when viewed in reflected light which is somewhat reminiscent of the structure observed in certain flower patterns like the sunflower has been explained by Miles and Boyde [13] as being due to the interference of light caused by the dentinal tubules lying in a sinusoidal pattern with subsequent groups of tubules being in and out of phase with each other. They observe that the tubules in sinusoidal pattern have their planes perpendicular to the cross-section of the tusk but from the SEM pictures shown here, it is evident that the plane of the tubules is parallel to the cross-section. The earlier study [13] was based on optical microscope methods which showed up tubules below the surface also, but the SEM showing the surface alone indicates that the plane of the dentinal tubules lies in the cross-sectional plane. More intensive study is needed to explain the generation of the grain pattern. The high work to fracture resulting from plastic flow before fracture is evidenced in the high magnification picture of the sample tested dry (Fig. 3) which shows that fibrillar segments are torn out of the matrix. This kind of fibrillar pullout has been observed for ivory by Bonfield and Li [3]. This has also been observed in antler bone [14]. This fibrillar pullout can be attributed to the lower degree of mineralization in these tissues. Bovine compact bone does not seem to exhibit this feature. The nature of the surfaces tested wet (Fig. 4a, b) also reflects the poor degree of calcification in ivory. From Fig. 4a it can be seen that whole segments of the calcified matrix have been pulled out and the strength is also low. A segment of the calcified matrix contain-

ing dentinal tubules is shown in Fig. 4b. In bone, osteons are found to be pulled out, because the interosteonal regions can be weak [10]. Here, there does not seem to be any organization like the osteons in bone. Of all the calcified substances for which the tensile properties have been studied, the ivory tested here has the lowest strength and modulus. The low degree of mineralization may, however, be a positive aspect in the comparative ease with which it can be shaped into pieces of art. The level of hydration plays an important part in the mechanical properties as is evident from the data. To what extent hydration affects the functional properties of the tusk in vivo is worth discussing. Unfortunately, no detailed study has been made on the level of hydration or the mechanical loading of the tusk in normal use. There has, however, been a report that the tusk loses only about 0.9% of its fresh weight on continued storage [15], but when dry samples of ivory are soaked in water it gains almost 15% of its weight. So in the functional state, the tusk is probably more dry than wet, enabling it to have more desirable properties in carrying out its many functions such as in fighting, in debarking trees, as trunk rests and as status symbols in displays. Also, for all normal functions, the trunk may not be loaded to the breaking point, and the properties exhibited, especially the high work to fracture, may be quite adequate, but it is noteworthy that tuskers with broken tusks are frequently seen in Indian jungles [16]. In the light of the observations made in this study, it would be of great interest to compare the tensile properties in relation to the structure, composition and function o f tusks of the Indian elephant with ivory from the male and female African elephants.

Figure 1 Surface of sample tested dry. (a) The dentinal tubules follow a sinusoidal pattern ( x 98). (b) Higher magnification than (a) showing plane of dentinal tubules lying in the cross-sectional plant ( × 238).

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Figure 2 (a and b) Stereo pair of fractured surface ( x 198).

Figure 3 High magnification picture of sample fractured dry. Arrow shows fibrillar pullout (x 1288).

Figure 4 (a) Surface of sample tested wet. Note pulled out segment ( x 49). (b) Higher magnification of a pulled out segment showing dentinal tubules ( x 440).

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References G. H. SPERBER, J. Can. Dent. Assoc. 5 (1976) 257. S. RAO and K. V. SUBBAIAH, J. Archeol. Chem. 1 (1983) h 3. w. BONFIELD and C. H. LI, J. Appl. Phys. 36 (1965) 3181. 4. ASTM, "Plastic - General methods of testing, Nomenclature part 27" (American Society for Testing and Materials, Philadelphia, Pennsylvania, 1967) p. 585. 5. 1. BERGMANN and R. LOXLEY, Anal. Chem. 35 (1963) 1961. 6. O. M I N A R I and D. B. ZILVERSMIT, Anal. Biochem. 6 (1963) 320. 7. O. L I N D B E R G and L. ERNSTER, Meth. Bioehem. Anal. 3 (1956) 1. 8. M. SERIZAWA, Y. T A K E M U R A , H. WAKANO and T. T A K A H A S H I , Gypsum Lime 165 (1980) 67 (abstract only). 9. M. C. P U G L I A R E L L O , E. VITTUR, B. DE BERNARD, E. BONUCCI and A. ASCENZI, Calcif Tiss. Res. 5 (1980) 108.

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K. R. P I E K A R S K I , "Fracture", Vol. 1. ICF4, Waterloo, Canada (1977) p. 607. 11. W. B O N F I E L D and P. O ' C O N N O R , J. Mater. Sci. 13 (1978) 202. 12. D. R. CARTER and W. C. HAYES, J. Biomechanics l0 (1977) 325. 13. A. E. W. MILES and A. BOYDE, J. Anat. (Lond.) 95 (1961) 450. 14. A. RAJARAM and N. R A M A N A T H A N , Calc. Tiss. Int. 34 (1982) 301. 15. S. K. E L T R I N G H A M , "Elephants" (Blanford Press, Poole, Dorset) p. 6. 16. M. K R I S H N A N , "India's Wildlife in 1959 70", (Bombay Natural History Society, Bombay, 1975) p. 88. 17. T. M. W R I G H T and W. C. HAYES, Med. Biol. Engng Comput. 14 (1967) 671.

Received 3 June and accepted 5 June 1986