# An Instrumented Macro-Indentation Method for Determining the Mechanical Properties of Coconut Shell (Coco Nucifera of Cameroon)

E. Njeugna, M.B.K. Ganou, D. Ndapeu, J.N.T. Foba, N.R.T. Sikame, P.W.M. Huisken

► To cite this version:

E. Njeugna, M.B.K. Ganou, D. Ndapeu, J.N.T. Foba, N.R.T. Sikame, et al. An Instrumented Macro-Indentation Method for Determining the Mechanical Properties of Coconut Shell (Coco Nucifera of Cameroon). Mechanics, Materials Science & Engineering Journal, 2016, 10.13140/RG.2.1.1447.3846. hal-01367491

HAL Id: hal-01367491

https://hal.science/hal-01367491

Submitted on 16 Sep 2016

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# An Instrumented Macro-Indentation Method for Determining the Mechanical Properties of Coconut Shell (Coco Nucifera of Cameroon)

E. Njeugna¹,², M.B.K. Ganou¹,³,a, D. Ndapeu⁴,⁵, J.N.T. Foba⁶,⁷, N.R.T. Sikame⁴,⁵, P.W.M. Huisken ¹,⁸

1. Laboratory of Mechanics and Production (LMP)-UFD, University of Douala-Cameroon

2. Professor, Senior researcher, Head of LAMMA, Laboratory of Mechanics and Appropriate Materials (LAMMA)-ENSET, University of Douala-Cameroon

3. Master's Degree Student, researcher in LAMMA, Laboratory of Mechanics and Appropriate Materials (LAMMA)-ENSET, University of Douala-Cameroon

4. PhD, Senior researcher in LAMMA, Head of LAMMA, Laboratory of Mechanics and Appropriate Materials (LAMMA)-ENSET, University of Douala-Cameroon

5. Laboratory of Mechanics and Modelling of Physical System (L2MSP)-University of Dschang, Cameroon

6. Professor, Senior researcher in LAMMA, Laboratory of Mechanics and Appropriate Materials (LAMMA)-ENSET, University of Douala-Cameroon

7. Laboratory of Chemistry-University of Buea-Cameroon

8. PhD Student, Senior researcher in LAMMA, Laboratory of Mechanics and Appropriate Materials (LAMMA)-ENSET, University of Douala-Cameroon

a. morinoganou@yahoo.fr

<figure><img src="image_2.png" /></figure>

DOI 10.13140/RG.2.1.1447.3846

**Keywords:** macro-indentation, coconut shell, hardness, young modulus, creep ratio, abrasives.

**ABSTRACT.** An instrumented macro-indentation test was used to determine the viscoelastic parameters and hardness of the shell of Coco Nucifera from Cameroon in order to promote their use in the manufacture of abrasives. Samples measuring 10 mm x 10 mm x 3 mm were cut out from the bottom of the fruit, close to the natural indentations (the eye) of an approximately round-shaped fruit. The indentation load ranged from 50 to 500 N with an increment of 50 N. A comparator with digital display and a camera attached to the machine measured the total penetration and rate of penetration. The Oliver and Pharr indentation model and Hertz contact theory were used to determine the Young's modulus, hardness and creep ratio of coconut shells. Young's modulus determined in the polar zone was 3.52x10³ MPa and 1.45x10³ MPa in the equatorial zone. The hardness was 1.16x10² MPa in the polar zone and 0.7x10² MPa in the equatorial zone. The creep ratio was 16 in the polar region and 8 in the equatorial zone. Validation of the procedure and results will be done with indentation tests on a kind of wood: Azobé (Lophira alata).

**Introduction.** Coconut Shell of Coco Nucifera (CSCN) is a product of the coconut tree (Coco nucifera) which is exploited for the valuable flesh (copra) enclosed in the hard shell. Important quantities of CSCN are generated during the production of copra [1]. The CSCN alone occupies about 25% by weight of the nuts and with about 54 million tons of coconuts produced annually worldwide [2], an estimated 13.5 million tonnes of CSCNs is generated. A small part of these shells is used in the production of activated carbon. [3]. However, most of CSCN is not used, presenting a challenge for solid waste management [4]. Usually the CSCN is simply discarded in nature with negative environmental impact (occupying land that could otherwise be used for agriculture, breeding sites for mosquitos). In recent years, the instrumented indentation test has been used as an alternative method for determining mechanical properties of materials [5]. The hard nature and the abrasiveness of the shell makes it a potential food processing material. A good knowledge of the properties of this hull allows it to be used as a green alternative in agro-processing tools to plump goods such as beans, maize and groundnuts, otherwise like abrasives for woods. Preliminary characterisation of this shell
by Njeugna et al. [5] was bend tests and beam theory to determine Young's modulus. The rectangular test pieces used in their study showed an unavoidable curvature due to shape of the coconut shell. Young's modulus calculated by this method is expected to be strongly influenced by the curved shape of the test pieces. We propose in this work to use an instrumented macro-indentation testing as Oliver and Pharr method to determine some mechanical properties of the shell. The validation of the procedure and results will be done with indentation tests on a kind of wood: Azobé (Lophira alata) part of the harder and dense wood species.

## Materials and methods.

**Materials.** Coconut shell used in this work came from the Littoral region of Cameroon. To prepare prismatic test pieces, we chose mature round hulls because of ease of cutting out specimens 10 mm x 10 mm x 3 mm by grinding of the faces. Sampling was done in two zones: Equatorial and Polar zones as shown in Fig. 1. In a test rig, we performed 5 indentation tests after conditioning the shells at ambient temperature in the laboratory for 2 months.

<figure><img src="image_3.png" /><figcaption>Fig. 1. Localisation of indentation points: a) Coconut tree b) and Sampling from a shell c) indication of indentation d) plates samples indented.</figcaption></figure>

A Dial gauge to the thousandth and a Facom brand digital display was used to measure displacement. A Canon camera provided video monitoring of comparator variations throughout the duration of the test. The indentation was done with an adapted macro-indentation device designed and constructed in our laboratory [11] (Figs. 2, 3).

<figure><img src="image_4.png" /><figcaption>0 – Built, 1– Specimen of CSCN, 2 – Indenter Ball, 3 – indenter rod, 4 – Slide the rod, 5 – Loading platform, 6 – load, 7 – Dial gauge to the thousandth, 8 – Canon camera</figcaption></figure>

Fig. 2. Image Test device.

Fig. 3. Device block diagram.
**Methods.** The instrumented macro-indentation test takes place in three phases: a charging phase; a hold phase and an unloading phase [10]. Loading is achieved by the masses placed on the tray 5. The loads applied ranged from 50N to 500N with an increment of 50N. For Each load there are equivalent penetration of the steel ball in the sample was measured. The comparator measured and displayed the value of penetration. When the maximum load of 500N was reached, a hold time of 30 minutes was maintained and the flow of the CSCN was observed as the continuous penetration of the indentation ball in the shell sample. The camera recorded the display of comparator over the entire hold time. During the unloading phase, we reduced the load from 500N to 50N in stepwise reduction of 50N. The elastic recovery of the material was displayed by the comparator. The pairs of load vs penetration depth during loading and unloading were plotted to obtain the characteristic curve of the CSCN material covering the three phases as shown in figure 4. 50 samples were tested.

To exploit these curves, we adopted the method of Oliver and Pharr [6, 18, 21], this method describes the upper limit of the discharge curve by the law in Eq. (1):

$$P = P_{max} \left( \frac{h-h_{f}}{h_{max}-h_{f}} \right)^{m}, \quad (1)$$

where P – is the force;

$P_{max}$ – is the maximum applied force;

h –is the depth of indentation;

$h_f$ – is the indentation depth unloading;

$h_{max}$ – is the maximum indentation depth at $F_{max}$;

m – is a function of the geometry of the indenter [12-15]. For a spherical indenter m = 1.

<figure><img src="image_5.png" /><figcaption>Fig. 4. Load vs depth plot.</figcaption></figure>

**Modulus of elasticity (Young's modulus).** The determination of Young's modulus is based on the assumption that during the unloading curve the removal of the indenter is accompanied by a spring back due to the elasticity of the material [7]. Thus the slope of the unloading curve provides a measure Young's modulus. The method is based on the recovery theory of Hertz and Bulychev, later developed by Snedon [8, 20]. It provides for the modelling of the unloading slope with a function that relates the contact area to the reduced Young's modulus according to the Eq. (2):
$$S = \left(\frac{dP}{dh}\right)_{h=h_{max}} = \frac{2}{\sqrt{\pi}} E_r \sqrt{A_c}, \qquad (2)$$

Where S – is the stiffness of the contact between the intention and the material when unloading, measured at the maximum penetration ($h_{max}$) recorded;

$A_c$ – is the projected contact area given by the Eq. (3):

$$A_c = \pi \cdot a^2 = 2 \cdot \pi \cdot R \cdot h_c - \pi \cdot h_c^2 \qquad (3)$$

where a – projected contact radius;

R – radius of the indenter;

$h_c$ – penetration depth given by the Eq. (4):

$$h_c = h_{max} - \varepsilon \frac{P_{max}}{S}, \qquad (4)$$

where $\varepsilon$ – a constant related to the geometry of the indenter, which is 0.75 for a spherical indenter [12, 19];

$E_r$ – Reduced Young's modulus from the theory of elastic contact of Hertz [8], given by Eq. (5).

$$\frac{1}{E_r} = \frac{(1 - v_m^2)}{E_m} + \frac{(1 - v_i^2)}{E_i}. \qquad (5)$$

In this equation the indices i and m refer to the characteristics of the indenter and the material respectively.

Since the indenter is made of stainless steel, we consider that: $v_i=0.3$ and $E_i = 210.000$ MPa.

Thus, Young's modulus of the CSCN is given by Eq. (6):

$$E_m = (1 - v_m^2) \cdot \left[ \frac{2}{\sqrt{\pi}} \frac{1}{S} \sqrt{A_c} - \frac{(1 - v_i^2)}{E_i} \right]^{-1} \qquad (6)$$

**Hardness.** Once the method for the calculation of the contact area is available, it is possible to calculate hardness. [9, 15, 17]. This is done using the formula from Eq. (7) and the contact area $A_c$ is obtained by Eq. (3):

$$H = \frac{P}{A_c}. \qquad (7)$$

**Creep ratio.** $C_{IT}$ denoted indentation creep ratio is defined by the relative change in depth of
corresponding penetration, according to Eq. (8):

$$C_{IT} = \frac{h_2 - h_1}{h_1} \times 100 \%. \qquad (8)$$

**Results and discussion.** Figure 5 is an example of the instrumented macro-indentation curve of CSCNs.

<figure><img src="image_6.png" /><figcaption>Fig. 5. Sample of macro-indentation curve CSCNs.</figcaption></figure>

The Oliver and Pharr method and Hertz's theory were applied to curves obtained by testing samples from polar and equatorial zones. The values of Young's modulus, hardness and creep ratio obtained are presented in Table 1.

<table><caption>Table 1. Features of the CSCN by macro-instrumented indentation</caption><thead><tr><th></th><th>Young Modulus <i>E</i></th><th>Hardness <i>H</i></th><th>Creep ratio</th></tr></thead><tbody><tr><th>Equatorial zone</th><td>1.45 (±0.179) · 10<sup>3</sup> MPa</td><td>0.70 (±0.12) · 10<sup>2</sup> MPa</td><td>8.31± 1.35</td></tr><tr><th>Polar zone</th><td>3.52 (±0.562) · 10<sup>3</sup> MPa</td><td>1.16 (±0.10) · 10<sup>2</sup>MPa</td><td>16.2± 3.51</td></tr></tbody></table>

Looking at the results shown in Table I, we find that the Young's modulus, hardness and creep coefficient are relatively higher in the polar zone than in the equatorial zone. A possible explanation could be differences in thickness from the equatorial to polar zones. It was observed that the nut is thicker in the polar zone compared to the equator.

These results are significantly different from results obtained by Njeugna et al. [5]. The difference could be attributed to difference in test method. The values of Young's modulus obtained in this study are closer to values for polymers and gypsum (Table 2), an indication that the instrumented macro-indentation test may be more suitable for characterising the mechanical properties of this material.

Applying the same indentation tests (Fig. 6) on wood named Azobé with a moisture content of 12% in the same conditions and protocols, we got Young modulus $E(az) = 12.304$ GPa. This value falls within the values gotten from literature review [21]. We found an indentation hardness $H(az) = 41.5398$ MPa and with this hardness $H(az)$, It is clear the CSCNs can be used for sanding wood.
<table><caption>Table 2. Comparison of specific rigidities</caption><thead><tr><th>Materials</th><th></th><th>Young modulus in GPa</th><th>References</th></tr></thead><tbody><tr><td>Polyethylene</td><td></td><td>0.20</td><td>[5]</td></tr><tr><td>Polycarbonate</td><td></td><td>2.40</td><td>[5]</td></tr><tr><td>Epoxide</td><td></td><td>2.40</td><td>[5]</td></tr><tr><td>Polyester</td><td></td><td>5.00</td><td>[5]</td></tr><tr><td>EDP (Gypsum A)</td><td></td><td>9.90</td><td>[16]</td></tr><tr><td>EDP (Gypsum B)</td><td></td><td>2.58</td><td>[16]</td></tr><tr><td>Azobé</td><td></td><td>12.30</td><td>[21]</td></tr><tr><td rowspan="2">Coconuts shells</td><td>Polar zone</td><td>Instrumented indentation test</td><td>3 points flexural test</td></tr><tr><td>Equatorial zone</td><td>3.52</td><td>19,80 [5]</td></tr><tr><td></td><td></td><td>1.45</td><td>12,70 [5]</td></tr></tbody></table>

<figure><img src="image_7.png" /><figcaption>Fig. 6. Comparison between the indentation of CSCNs and Azobé.</figcaption></figure>

**Summary.** Our work focused on determining the mechanical properties of coconut shell from Coco nucifera by an instrumented macro-indentation test. A series of tests were conducted on prismatic samples extracted from 10 mm x 10 mm x 3 mm elongated CSCNs. Sampling was done in two areas: the equatorial zone and polar zone and 50 samples were tested. The method of Oliver and Pharr was applied to analyse the behaviour of the material at the upper limit of loading and the Hertz contact theory was used to calculate fundamental mechanical properties of CSCN. Thus, the Young's modulus, hardness and the creep ratio were determined. The Young's modulus determined is of the order of $3.52 \times 10^3 \pm 562.81$ MPa in the polar zone and $1.45 \times 10^3 \pm 179.29$ MPa in the equatorial zone. The hardness is $1.16 \times 10^2 \pm 10.69$ MPa in the polar zone and $0.70 \times 10^2 \pm 12.01$ MPa in the equatorial zone. The creep ratio is $16.2 \pm 3.51$ in the polar region and $8.31 \pm 1.35$ in the equatorial zone. Currently, we are using these shells after grinding in the manufacturing of abrasives and composites. Therefore, it is imperative to know its mechanical properties for more valorisation.
References

[1] J.A. Banzon, J.R. Velasco: Coconut: production and utilization, Philippine Coconut Res. and Dev., pp. 417, 1982.

[2] B. Warner, Q. Derek, C. Longmore: A review of the future prospects for the world coconut industry and past research in coconut, production and product, Australian Centre for International Agricultural Research, 2007.

[3] U. Tomas, J.R. Ganiron: Sustainable Management of Waste Coconut Shells as Aggregates in Concrete Mixture, Journal of Engineering Science and Technology Review 6 (5), 7-14, 2013.

[4] N. Lesage, M. Sperandio, C. Cabassud: Study of a hybrid process: Adsorption on activated carbon/membrane bioreactor for the treatment of an industrial wastewater, Chemical Engineering and Processing: Process Intensification 10th French Congress on Chemical Engineering, Volume 47, pp. 303-307, 2005, doi:10.1016/j.cep.2007.01.021

[5] E. Njeugna, D. Ndapeu, S. Bistac, J.Y. Drean N.J. Foba, M. Fogue: Contribution to the Characterisation of the Coconut Shells (Coco Nucifera) of Cameroon, International Journal of Mechanics Structural, Volume 4, Number 1, pp.1-22, 2013.

[6] W.C. Olivier, G.M. Pharr: An improved technique for determining hardness and elastic-modulus using load and displacement sensing indentation experiments, J. Mater. Res. 7, 1564-1583, 1992.

[7] D. Tabor: Review of Physics in Technology, 1970.

[8] H. Hertz: Über die Berührung fester elastischer Körper, Journal für die reine und angewandte Mathematik 92, 156-171, 1881.

[9] Standard Test for Microhardness of material, ASTM Standard Method E 384, Annual Book of Standards 3.01, American Society for Testing and Materials, p469, 1989.

[10] C. Gauthier, A.L. Durier, C. Fond, R. Schirrer: Scratching of a coated polymer and mechanical analysis of a scratch resistance solution, Tribology International, Volume 39, pp 88-98, 2006, doi:10.1016/j.triboint.2005.04.014

[11] D. Ndapeu : Caractérisation physico-chimique et mécanique des coques de noix de coco nucifera du Cameroun, en vue de leur utilisation dans l’élaboration des matériaux abrasifs, Thèse de doctorat Ph/D de l’Université de Dschang, 2015.

[12] D.J.A. Mendoza : Détermination des propriétés mécaniques et des lois de comportement par indentation instrumentée, Thèse de Doctorat de l'Université de Lille 1, 2009.

[13] J. Breuils, L. Jacomine, H. Pelletier, P. Peyre : Identification par méthode inverse des propriétés mécaniques de surfaces grenaillées par chocs laser, 19ème Congrès Français de Mécanique, 2009.

[14] T. Chatel : Fluage et recouvrance d'empreintes et de sillons sur surface de polymère, Thèse de Doctorat de l'Université de Strasbourg, 2010.

[15] P.E. Mazeran, M. Beyaoui, M. Bigerelle, M. Guigon: Determination of mechanical properties by nanoindentation in case of viscous materials, International Journal of Materials Research, 103, 2012, doi: 10.3139/146.110687

[16] C. Philippe : Détermination des propriétés mécaniques de céramiques poreuses par essais de microindentation instrumentée sphérique, Mémoire de Thèse de l'Institut national des sciences appliquées de Lyon, 2013.

[17] T. Chatel, V. LeHouérou, H. Pelletier, C. Gauthier: Numerical analysis of the creep of the contact and recovery of the imprint on amorphous polymer surfaces. Mechanics of Time-Dependent Materials, 17(4), 581-595, 2013, doi: 10.1007/s11043-012-9205-x
[18] G.M. Pharr, W.C. Oliver, F.R. Brotzen: On the generality of the relationship among contact stiffness, contact area, and elastic modulus during indentation, Journal of Materials Research 7, pp. 613-619, 1992.

[19] W.C. Oliver, G.M. Pharr: Measurement of hardness and elastic modulus by instrumented indentation: Advances in understanding and refinements to methodology, Journal of Materials Research, Volume 19 No. 1, 2004.

[20] E. Barthel, D. Chicot, J.P. Guin, E. LeBourhis, G. Mauvoisin : L'indentation : un outil de caractérisation multi-échelle des matériaux, aSF2M Info, 2014.

[21] J. Menčík: Determination of mechanical properties by instrumented indentation. Meccanica, 42(1), 19-29, 2007.

[22] J.W.G. Van de Kuilen, H.J. Blass: Mechanical properties of azobé (Lophira alata). Holz als Roh- und Werkstoff, 63(1), 1-10, 2005.