Fique fiber in random configuration were used as reinforcement for the composite materials that make up the back, seat, and armrests of the chair. They were supplied by the foundation San Lorenzo in Barichara, Santander Colombia. Fig. 1 shows the fiber under an optical microscope at 50X.

Fig. 1.
Fique fibers seen with 50X magnification.

The fibers have a length of 1.76 ± 0.53 mm and a diameter of 0.0253±0.0033 mm, as reported in study by Gómez et al. [11], where the same reinforcement was used in the same random configuration.

In the fabrication of the chair support structure, fique fiber was used in woven configuration (See Fig. 2), which was obtained from a conventional sack used for coffee transportation.

Fig. 2.
Fibers in woven configuration.

The matrix for the different parts of the chair was polyester reference 102, acquired at the ingequimicas company in Bucaramanga, Santander, Colombia, being the one with the lowest viscosity available by the distributor, this in order to facilitate its handling and application. The percentage of catalyst resin by weight was 100: 2 respectively.

Composite material specimens were made with three, four and five layers of fique reinforcement corresponding to 15.36%, 20.09% and 24.9% by weight, in order to determine the most suitable arrangement to use on the back, seat and armrests according to loads to flexural and tensile.

The specimens were manufactured according to the Hand Lay Up technique, placing layer after layer of fiberglass pre-impregnated with polyester resin on an acrylic mold previously anointed with a release agent. Curing was carried out at room temperature for 48 hours.

The geometry of the specimens for the tensile test was established according to the ASTM D3039 / D3039M standard where measurements of 25 cm long by 2.5 cm wide by 2.5 mm thick are defined.

Regarding the flexural test, the dimensions of the specimens were established according to the ASTM D7264 / D7264M standard where the thickness-span ratio was 32: 1 respectively, and the width was 13 mm. In Fig. 3 specimens manufactured for the flexural test are observed.

Fig. 3
Fique fiber specimens.

The tensile test was carried out following the ASTM D3039/D3039M standard on a universal machine brand MTS model C43.104 of 10 kN at a speed of 2 mm/min with a temperature of 24.2 °C ± 3 °C. For each of the materials, five specimens were tested, considering the average of each of the properties.

The flexural test was carried out according to the ASTM D7264/D7264M-15 standard by means of a universal machine MTS model C43.104 of 10 kN at a speed of 1 mm/min and a temperature of 24.3 °C ± 3.2 °C.

The flexural mechanical characterization was carried out with the use of three points as shown in Fig. 4.

Fig. 4.
Bending test.

Optical microscopy was performed using ZEISS optical microscope model AXIO scope A1, at increases of 20X, 50X, 100X. The images were captured at different focal planes for analysis. The analysis was performed on the compound manufactured with five layers.

For the finite element analysis, the commercial software Ansys 2020 academic research was used. A static analysis was carried out applying a perpendicular load to the surface of the seat, armrests, and backrest, equivalent to 100 kg-f with fixed restrictions at the support points where the pieces would be assembled with screws. The material was assumed to be homogeneous and its properties were defined in the software according to the results obtained from the flexural and stress tensile of the five-layer composite material.

As can be seen in Fig. 5, an increase in the tensile mechanical properties of the composite is evidenced with the addition of a quantity of fique fiber.

Fig. 5.
Tensile strength of composite materials reinforced with fique fibers.

A maximum stress of 27.30 MPa is obtained in the 5-layer composite.

In Fig. 6 the variation of the elasticity module is observed where it is evident that it is increased in the composite with the addition of more quantity of fiber in its manufacture.

Fig. 6.
Modulus of elasticity of composite materials reinforced with fique fibers.

A maximum modulus of elasticity is obtained in the five-layer material being 0.725 GPa.

The mechanical properties of the tensile stress and the modulus of elasticity increased as more layers of fique were incorporated into the biocomposite, which resulted in the material suffering from a stiffening phenomenon. This behavior was also obtained by Hidalgo et al. [12] in the study of the stress behavior of a composite material with a polyethylene/aluminum matrix, varying the amount of fique used as reinforcement.

According to Gañan et al. [13] chemical treatments to the fique fiber improve the mechanical properties of the composite since it increases the adhesion between the matrix and the fiber. For that reason, for future works it is recommended to apply this type of treatments to the fique.

The results obtained show that the best composite material for the manufacture of the back, seat and armrests of the student chair was the one reinforced with five layers of fique.

Fig. 7 shows the flexural stress that was obtained when the test was applied to the biocomposite with the different fique layer arrangements.

Fig. 7.
Flexural stress of composite materials reinforced with fique fibers.

An increase in flexural stress is evidenced with a greater amount of fiber used in the composite, reaching the maximum value of 22.65 MPa with five layers.

This behavior is because the fibers improve the flexural strength of the resins and that there is load transfer [14].

However, the value obtained of flexural stress in the composite materials with fique fiber is lower with respect to those of industrial use, this is due to the low compatibility between the fiber and the matrix for its different chemical nature along the interface [15].

The flexural modulus obtained for the different configurations of layers of composite materials is observed in Fig. 8.

Fig. 8.
Flexural module of composite materials reinforced with fique fibers.

An increase in the flexural modulus is observed with the increase in the number of fibers, the highest being 1.22 GPa with the configuration of five layers of fique fibers.

Hidalgo et al. [16] obtained results with the same behavior of increase in the modulus and flexural stress as the percentage of fique fiber increased in the LDPE / Al matrix composite. Likewise, Hidalgo et al. [17] report the same dynamics of increase in another composite with fique fiber and epoxy and LLDPE matrix.

It is important to emphasize that the properties of the composite reinforced with natural fibers do not depend only on the volume fraction of the fibers, but also on other additional aspects like the orientation and treatment of the fibers, the properties of the matrix and the interfacial adhesion between the fiber and the matrix, in general of the physical and chemical properties of the constituents of the composite [18].

The results obtained show that the best behavior of the composite material subjected to flexural loads for the manufacture of the back, seat and armrests of the student chair, is the material reinforced with five layers of fique.

Fig. 9 and 10 show the five-layer composite material at 20X and 50X magnification, respectively.

Fig. 9.
. Micrograph of composite material at 20X.

Fig. 10.
Micrograph of composite material at 50X.

As shown in the optical microscopy images, there is good adhesion between the fiber and the resin since no defects or spaces between the material's constituents are observed.

In Fig. 11, at 100X magnification, it is evident that the fiber is completely covered by the matrix, which shows that the fique was compatible with the polyester resin.

Fig. 11.
Micrograph of composite material at 100X.

However, in order to carry out a more conclusive study of the morphology and the interface of the composite, a study should be carried out at higher magnifications using scanning electron microscopy as shown by Barba et al. [19] where they compare micrographs obtained with optical microscopy and SEM technique in composite material with recycled constituents.

Fig. 12, 13 and 14 [ST1] show the results obtained from the static stress analysis for the seat, armrest and back respectively, carried out in the finite element software Ansys.

Fig. 12.
Seat stress analysis.

Fig. 13.
Armrest stress analysis.

Fig. 14.
. Back stress analysis.

Additionally, in Fig. 15, 16 and 17, the results obtained from the static analysis in deformation for the seat, armrests and back respectively of the student chair are observed.

Fig. 15.
Seat deformation analysis.

Fig. 16.
Armrest deformation analysis.

Fig. 17.
Back deformation analysis.

The results of the maximum stresses and the deformation of the seat, backrest and armrests are shown in Table I.

Table I.
MAXIMUM STRESSES AND DEFORMATIONS

As can be seen in Table I, the maximum stresses obtained through the simulation of finite elements are less than the stresses obtained experimentally for both flexural and tensile, which allows validating that the chair will support the 100 kg-f load.

The fique fiber composite material was laser cut in a random configuration with the geometry of the armrest, seat, and backrest. With this cutting method, the aim was to improve the quality of the final finish of the pieces, avoiding dimensional errors. Fig. 18 shows the laser cut and the different pieces obtained.

Fig. 18.
Laser cutting of parts.

Each of the five layers were impregnated with the polyester resin using a brush, to ensure uniformity on the surface of the fibers. They were then stacked on top of each other using the Hand Lay Up technique. Pressure was applied manually between each of the layers helping to improve the penetration of the matrix into the fiber. Fig. 19 shows the process.

Fig. 19.
Manufacturing process

Curing was done at room temperature (23 °C +/- 5 °C) for 72 hours. The manufactured seat and armrest parts are shown in Fig. 20.

Fig. 20.
Armrest and seat manufactured.

For the structure, high density expanded polystyrene molds were made with the geometry of the different parts that make it up.

The different profiles of the chair structure were designed with a diameter of 3 cm. The fiber fabric was impregnated with polyester resin and rolled up on itself ensuring its compactness to be introduced into the mold. Long strips of the fabric were cut to ensure continuous pieces reinforced with the fiber. Fig. 21 shows the manufacturing process used.

Fig. 21.
Manufacturing process of the structure.

The curing of the different profiles was carried out at a room temperature of 24 °C +/- 8 °C for 72 hours. Fig. 22 shows the manufactured structure.

Fig. 22.
Structure of the chair.

After all the pieces were made in the composite material with fique fiber, the imperfects present on the surface were eliminated, using 400 and 600 grit sandpaper.

The assembly of the different parts of the chair was carried out using a 2” self-drilling screw. Fig. 23 shows the chair manufactured.

Fig. 23.
Manufactured chair.