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If a material is subjected to high-strain deformation, it deforms permanently (plastic deformation) and ultimately fails. For sufficiently low stresses ans strains, the polymeric material behaves as a linear elastic solid. The point where the behaviour starts to be non-linear is called the proportional limit.

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

0 5 10 15 20

% of Water absorption

weight%

CHAp BPF

0 0.2 0.4 0.6 0.8 1 1.2

00:00 2.5:2.5 05:05 7.5:7.5 10:10

% of Water absorption

weight%

CHAp:BPF BPF:AH

148

The local maximum in the stress-strain curve is called the yield point and indicates the permanent deformation. The corresponding stress and elongation are called yield strength and elongation at yield. Beyond the yield point the material stretches out considerably and this region is called the plastic region.

Further elongation leads to strain hardening and the ultimate rupture of the material. At the rupture point the corresponding stress and strain are called the ultimate strength and the elongation at break. The stress-strain behaviour of a polymeric material depends on various parameters such as molecular characteristics, microstructure, strain-rate and temperature.

The results of the force-extension curves obtained for the developed composites are displayed in Figures 4.20-4.23, while that of the tensile modulus and strengthare shown in Figures4.24-4.27 and Table 4.5(a-d)respectively. From figures 4.20-4.23 it was observed that the composites produced with BPF had the largest area under the force versus extension plot. This is as a result of tougher properties of BPF compared to CHAp. In all the results, the samples had a slight increment in the proportional region until the maximum forces were obtained in the materials.Beyond this point there was a sharp decreased to point zero. Thesamples had their highest force of 4450N and 4950N with composites produced with 7.5%CHA:7%.BPF and 7.5%AH:7.5%BPF respectivel y.

149

Figure 4.20: Variations of Force with Extension for composites produced with varying wt% BPF additions.

Figure 4.21: Variations of Force with Extension for composites produced with varying wt% CHAp additions.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 0

500 1000 1500 2000 2500 3000 3500 4000

Force (N)

Extension (mm) 0BPF

10BPF 20BPF 15BPF 5BPF

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 0

500 1000 1500 2000 2500 3000 3500 4000

Force (N)

Extension (mm) 0CHAp

10CHAp 20CHAp 15CHAp 5CHAp

150

Figure 4.22: Variations of Force with Extension for composites produced with varying wt% BPF: AH additions.

Figure 4.23 Variations of force with extension for composites produced with varying wt% BPF: CHAp additions.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 0

500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500

Force (N)

Extension (mm) 0AH:0BPF 2.5AH:2.5BPF 5AH:5BPF 7.5AH:7.5BPF 10AH:10BPF

0 2 4 6 8 10 12 14 16 18

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Force (N)

Extension (mm) 0CHAp:BPF 2.5CHAp:2.5BPF 5CHAp:5BPF 7.5CHAp:7.5BPF 10CHAp:10BPF

151

Tensile strength and modulus of natural reinforced polymer composites depend on several factors such as the force–extension behaviors of reinforcement and matrix phases, the phase weight fractions, fillers concentrations and the distribution of the reinforcement in the composites.

Modulus is the measure of stiffness of a solid material.A rigid material has an infinite modulus because an infinite force is needed to deform such a material.

Figures 4.24-4.25 showed that the tensile modulus of the composite samples increased significantly after reinforcement with BPF and combinations of BPF:

CHAp and BPF: AH. For example tensile modulus of 247.05, 476 and 501MPa were obtained for the polyester and composite at 7.5%BPF: CHAp7.5% and 7.5%BPF: 7.5%AH respectively (Tables 4.5). This showed that the stiffness of polyester was enhanced with the addition of BPF and CHAp.

The presence of polar group in the matrix contributed to the electrostatic adsorption between the polyester material and the BPF reinforcement. This phenomenon is driven by different charges acting on matrix or reinforcement surfaces.

This mechanism strengthened the polymer-reinforcement interface. It will hold them together and increase their resistance to deformation. The uniformity of reinforcement distribution hindered the chain movement during deformation of the composites. This mechanism increased the stiffness of the composites as well as tensile strength (Akash et al., 2017).

152

Figure 4.24: Variations of tensile modulus with varying weight % CHAp and BPF additions.

Figure 4.25: Variations of tensile modulus with varying weight % CHAp: BPF and BPF: AH developed composites additions.

The tensile strength decreased as wt%CHAp addition increased and increased with increased BPF percentage additions. This indicates that addition of BPF to the polyester matrix improved the loadbearing capacity of the composites, while the addition of CHAp lowered their loadbearing capacity.

0 50 100 150 200 250 300 350 400 450

0 5 10 15 20

Tensile modulus(Mpa)

weight%

CHAp BPF

0 100 200 300 400 500 600

00:00 2.5:2.5 05:05 7.5:7.5 10:10

Tensile modulus(Mpa)

weight%

CHAp:BPF BPF:AH

153

Similar observations were reported by Akash et al., (2017) forothernatural fibre reinforcedpolymer composites. In addition, the developed composites with BPF:

CHAp and BPF: AH deforms less until maximum load, which gave higher tensile strength.

The increase inthe tensile strength with BPF was expected since CHAp is hard its hardness reduced its tensile properties. The tensile strength of the composites increased to maximum values 17.87 and 19.9MPa at 7.5%BPF:

7.5%CHAp and 7.5%BPF: 7.7%AH respectively. The tensile strength obtained at these maximum points was due to the stability of the reinforcement to support stresses transferred from the polymer matrix. Similar results were obtained in the works of Akindapo et al.,(2015); Mohammad, Kuncoro, Mujtahid &Djoko, 2018).

Figure 4.26: Variations of tensile strength of composites developed withvarying Weight % CHAp and BPF additions.

0 2 4 6 8 10 12 14 16 18

0 5 10 15 20

Tensile Strength(Mpa)

weight%

CHAp BPF

154

Figure 4.27: Variation of tensile strength with weight % CHAp: BPF and BPF:

AH addition.

The slightly decreased tensile strength beyond this maximium points increasedinterfacial area of the composites as the particles content increased, which resulted in decreased interfacial bonding between the reinforcement (hydrophilic) and matrix polymer (hydrophobic) components of the composites. Also the reduction in tensile strength may be due to agglomeration of the filler particles in the polyester matrix which formed the domain that looked like a foreign body in the matrix and resulted in physical contact between adjacent aggregates in the composites (Mansour et al., 2011).