Oil palm, cultivated in more than twenty countries around the world, plays an important role in the peasant economy so far as this crop contributes firstly, to satisfy the domestic needs of farmers and secondly, to provide them with monetary income. Through our investigations, we have found that once the palm nut is separated from the shell, the latter is no longer used for anything or is sometimes used as a fuel for cooking, electricity production and decoration of art objects .
The annual world production of palm kernel shell amounts to about 21,359,000 tons, about 270,000 tons for Cameroon  . In Cameroon, about 70% of these shells are dumped in the wild, causing pollution . Because these plant by-products degrade very slowly.
Research work has been conducted on palm kernel shell, including the determination of physical-chemical properties   . The physical properties of PKS are essential parameters in the development of process methods and equipment design . These properties include rheological, thermal, optical, electrical and some mechanical parameters. Palm kernel shells are also used in the production of activated carbon for water filtration and other applications . Given their importance to the world economy in general and to Cameroon’s economy in particular, palm kernel shell deserves more attention in order to optimize their potential. Further work has been or is being done on PKS. As a background application of palm kernel shell, we can mention their utilisation as fillers in the realization of brake pads, safety helmet. Also, in some localities in Cameroon, such as the Haut-Nkam, West region, palm kernel shells are dumped on muddy country roads in order to facilitate the adherence of car tyres , in addition, composite materials have been made from these shell and although their mechanical characteristics are poorly known, they have demonstrated many qualities, including their machinability and toughness .
In order to improve knowledge of the characteristics of PKS, this work aims to contribute to the physico-mechanical characterization of palm kernel shell.
Palm kernel shells used in this study come from a mature cob walnut from the production area of Nkongsamba for the DURA species and BOMONO for the TENERA species, all located in the Littoral Cameroon. The shells are extracted by drying the nuts. These shells are carefully cleaned before use to get rid of oil and fibres residues and are kept at a temperature of 105˚C in the oven for four hours before packaging.
The determination of the density parameters of the materials indirectly provides an approximation of the quality of their constructive properties.
3.1.1. Absolute Density
The absolute density is determined experimentally by the pycnometer method according to NF P 94-054. The procedure used consisted in carrying out the following operations:
· Select a sample and place it in the oven at 105˚C for 24 hours.
· Weigh a pycnometer filled with distilled water to the mark and note its mass M1.
· Weighing a sample of aggregate of mass M2.
· Introduce the sample into a pycnometer after pouring in a quantity of water.
· Gradually fill the pycnometer up to the mark, eliminate air bubbles and note M3 its mass.
· Note the temperature of the water in the pycnometer.
The values of the individual weights are used to determine the absolute density.
where is water density taken conventionally
3.1.2. Apparent Density
The bulk density was determined by the hydrostatic balance method according to the recommendations of NF P 94-053. The principle of the method consists in determining the volume of a sample by means of the Archimede thrust. It is obtained from successive weighing of the sample. Samples are taken by the quartage method and weighed on a 10−3 g precision scale.
· Let methe mass of the sample, then the sample is immersed in previously melted paraffin.
· Let me+p, the mass of the sample plus paraffin (with a density of 0.87796 g/cm3)
The wax sample is then carefully immersed in water.
o The displaced volume of water Vd is given by the expression
o The volume of the paraffin is:
o The volume of the sample is given by the relationship
The bulk density is expressed as the ratio of the mass of the sample to the volume of the sample.
3.2. Porosity (p), Void Index (e) and Compactness (c)
The dimensionless characteristics, given by Equations (3), (4) and (5), provide information on the voids in a body.
3.3. Mechanical Characterization
The mechanical characterization of palm kernel shell consists here of determining the longitudinal Young’s modulus, the fish coefficient and the impact energy. The species of interest are DURA and TENERA. All palm kernel shells are obtained from mature nuts from the same cob for each species.
3.3.1. Typology of Samples
The palm kernel shell has geometry similar to that of the globe, i.e. it has two poles and meridians. To test the isotropy hypothesis, we propose to take samples in the Meridional direction and the Equatorial direction as shown in Figure 1.
We took the PKS DURA and TENERA specimens in the southern and equatorial directions. The aim was to see whether its mechanical characteristics vary according to the orientation in which the specimen was cut, in order to ensure the isotropy of this material. Table 1 represents the orientation M and E adopted; we have chosen a set of 20 specimens.
3.3.2. Resilience Energy
We determined it experimentally using the Charpy’s Sheep method using a pendulum sheep adapted to plant shells, carried out at the LAMMA laboratory of ENSET, Douala as shown in Figure 2.
Figure 1. Image of palm nut.
Table 1. Adopted sample allocation terminology.
Figure 2. Schematic diagram of the pendulum sheep.
with l: length of the pendulum arm (320 mm);
m: mass of the pendulum arm;
θ: angle of ascent after specimen breakage;
θ0: angle of free upward movement;
g: acceleration of gravity at the test site;
S: cross-section of the test specimen.
Influence of temperature on impact energy
This influence has been assessed by impact tests on specimens subjected to temperatures ranging from 26˚C to 90˚C. For each temperature range, seven specimens were tested. The temperatures were measured using a type K thermocouple.
3.3.3. Young’s Modulus
Due to the curved geometry, PKS does not offer the possibility to obtain straight specimens for the classical uniaxial tensile test; we will limit ourselves to three-point bending and elastic contact tests. The deformation energy will allow us to establish an equation whose parameters will be E and ν. We will take the fish coefficient value of 0.4 because palm kernel shells are similar to wood.
1) Principle of the three-point bending test
The bending test shall be carried out on a specimen in the form of a portion of a cylinder. This test piece is comparable to a curved beam whose mean line is in the form of an arc of a circle. We tested 20 specimens in the southern and equatorial directions. Figure 3 below shows the boundary conditions.
We place specimen (4) on the supporting surface integral with the frame (5), the end of the slide (2), bearing a steel ball is positioned at point C of the shell. The masses (1) are placed above a plate attached to the slide. The feeler of the dial gauge (3) is located under the shell and is used to directly measure the deflection corresponding to a given load in the extension of the slide rail. Several pairs of data (load, deflection) can be collected in the elastic range.
2) Mathematical expression of the deflection-load relationship
The application of Castigliano’s theorem will allow us to establish a linear
Figure 3. (a) Schematic diagram of the test (b) Three-point bending test apparatus.
relationship P = α∙Yc with P the load applied at point C, Yc the deflection at point C andα the slope of the straight line which is a function of the static Young’s modulus, the fish coefficient and the geometric dimensions of the specimen.
3) Expression of the deformation energy of a specimen
The total deformation energy U is due to internal forces     is described by Equation (7) and given by Equation (8).
with U1 = Deformation energy due to normal effort N
U2 = Deformation energy due to sharp effort TR is negligible
U3 = Deformation energy due to flexural moment Mfz
L and e are the dimensions of the cross-section S.
E is the static Young’s modulus in longitudinal direction.
P is the applied load.
is the quadratic moment of the straight section.
The application of Castigliano’s theorem to the midpoint of the specimen C allows us to write we thus obtain the relation below:
Substituting Equation (10) into Equation (9), gives Equation (11)
The factor β depends on the geometrical characteristics of the specimens. From expression ; E is determined. The slope P/Yc has been obtained experimentally as the slope of the linear regression.
4. Results and Discussions
4.1. Density of PKS
The density of the palm kernel shell is given in Table 2 below.
This result shows that palm kernel shells of the species TENERA are more porous than those of the species DURA. This is not due to their parameters of cultivation or area of production. But fundamentally, this is attributable to their microstructure with and important porous network.
4.2. Resilience Energy of PKS
The average impact energy of palm kernel shell is 2.066 J/cm2 and 1.894 J/cm2 for DURA and TENERA species respectively at a temperature of 26˚C. It is clear that DURA PKS are more resistant to breakage. This resistance is directly linked to the highest density, compactness and low void index of Dura variety. Moreover, their structure and thickness are considerable. We also note that the resilience energy of the palm kernel shell decreases linearly with a correlation coefficient R2 of 0.914 with increasing temperature. It varies between 1.63J/cm2 at 50˚C and 0.58 J/cm2 at 90˚C as shown in Figure 4 below.
4.3. Young Modulus of PKS
The Young’s modulus of the palm kernel shell of the DURA variety is 19 GPa and that of the TENERA variety is 17.9 GPa at a temperature of 26˚C. Figure 5 below shows the variation in deflection as a function of load with a correlation coefficient R2 of 0.9956.
In Table 3, we find that palm nut shells have a density close to that of coconut
Table 2. Void index, porosity, compactness and density values.
Figure 4. Impact enregy vs. temperature.
Figure 5. Variation of deflection with load.
Table 3. Comparison of absolute density and Young’s modulus of some wood species.
shells. The Young’s modulus of the Tenera variety is close to that of AZOBE and MOABI that of the dura variety is higher than the majority of hardwood species available in Cameroon.
The physico-mechanical characteristics of the palm kernel shell of the DURA and TENERA varieties have been determined. The density of the palm kernel shells, by the Archimede thrust method, is 1428.81 kg/m3 for Dura and 1395.81 kg/m3 for Tenera respectively. Resilience energy was determined using a pendulum sheep at temperatures ranging from 26˚C to 90˚C. It was found that the resilience energy decreases with increasing temperature. The deformation energy through the bending test was used to determine the longitudinal Young’s modulus of the palm kernel shell. This differs from the specimens taken in the southern and equatorial direction of the shells as shown in Figure 1. It can therefore be seen that PKS are anisotropic materials.
The physico-mechanical parameters of PKS are essential for optimal use in structural and non-structural industrial applications, the development of process methods and even in equipment design. According to the results obtained, direct application can be the use as aggregate in concrete, fillers in composite material and others.
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