Flexural and fatigue behaviour of natural fibrous reinforced polymeric composite materials

In this study, the mechanical properties, fatigue life, and endurance limit values were investigated for composite materials containing natural fiber reinforcement (flax, wool), which were arranged by 0°/90° Angle orientation and a combination of 0°/90° and 45°/-45° angle orientation, then the wool woven mixed by flax woven and unidirectional flax interfered with jute woven. Binary blends containing epoxy and polyurethane at v were mixed at various weight percent (0%, 5%, 10%


Introduction
The composite material consists of two or more constituent materials insoluble in each other: the matrix (continuous phase) and the reinforcement [1]. The growth using synthetic materials in industry applications increased environmental problems because these materials are not environmentally friendly, natural fibers offered an alternative to replace synthetic fibers, and these bio-material fibers have great potential as reinforcements, which provide thermal stability, low cost, lower energy requirement, wide availability, excellent biodegradability and environmentally friendly [2,3]. The fatigue phenomenon is the cause of most fractures in engineering parts during service with the presence of cyclic loads, the fracture occurs under withstanding loads, which are still within the elastic zone under the stress-strain curve. At service, fatigue failures are involved in at least 90% of all mechanical failures. For brittle materials, fatigue failures are particularly dangerous because they do not usually show signs of imminent failure, but this happens suddenly and without noticing the overall plastic deformations, which throughout history have caused important accidents with catastrophic consequences [4,5]. Abdullah, A. H., et al. investigated the tensile and fatigue behavior of epoxy reinforced with unidirectional kenaf. The results were compared with unreinforced epoxy, which showed that increasing the volume fraction of kenaf improved the fatigue life [6]. Saeed, A. R was prepared composite materials with a polymer basis by the match and lay-up method posited materials were prepared from unsaturated polyester resin as a base material supported with natural wool fibers, which were distributed randomly, with weight fractions (3%, 4%, and 5%). The results of the research showed that the value of the impact resistance and electrical insulation increased with increasing the weight of the wool fiber [7]. Bensadoun, F., et al. investigated the fatigue behavior of composite material made by epoxy reinforced with five architectures textile fabric of the flax, which are random mat, plain weave, twill, quasi-unidirectional, and unidirectional with a volume fraction of 40%, except 30% for the mat. The result showed that the fatigue behavior was affected by changing the type of architectures flax fabric type, minimum fatigue life for the random mat fabric, the fatigue life was increased for plain weave and twill fabric, and the highest at using the quasi-unidirectional fabric [8]. Hojo, T., et al. fabricated and tested polymeric composite containing several types of a mat natural fiber, which included unidirectional jute, unidirectional kenaf, and unidirectional bamboo, unsaturated polyester was used as a matrix. The test included tensile and low cycle fatigue. The results were compared with a unidirectional glass mat. The result showed that natural fibers show good properties as reinforcement into composites compared with Glass fiber and show a potential replacement with some types of synthetic fiber [9]. Asgarinia, S., et al. investigated the fatigue the behavior of composites material made by epoxy reinforced with flax (F) as natural fiber woven and E-glass (G) as synthetic woven, the vacuum infusion was used to fabricate the samples several cycles-stress to failure (S-N) of all hybrid configurations were obtained. By comparison between flax and glass hybrid, the flax composites have good fatigue behavior, which shows a potential implementation in bearing applications [10].

Matrix material
The epoxies polymerase has good mechanical properties, but they have low break deformation, low crack propagation resistance, and is brittle. In contrast, the polyurethane polymer has low mechanical properties, flexible and elastic. Depending on the mixing weight percentages ratio between epoxy and polyurethane, epoxies' disadvantages can be reduced. The blend is prepared by mixing the epoxy (Ep) and the polyurethane (Pu) (at 0 %, 5 %, 10 %, and 15 % weight percentages), which are blended by the following steps ( Figure 1a): 1-the epoxy and polyurethane resins were mixed by the requiring weight percentage. 2-The epoxy and polyurethane hardeners were mixed by the required weight percentage. 3-All the components of the resins and hardeners were mixed. 4-A vacuum chamber was used to remove the bubbles from the components.

Natural fiber reinforcements
Three types were woven (Figure 1b) used to reinforce the matrix, which: are flax, wool, and unidirectional flax interfered with jute woven (Jf).

Fabrication of composite materials
The vacuum infusion technique was used to prepare the specimens, Figure  2 shows the equipment used in the system, which is; a vacuum pump, vacuum chamber, trap, digital weighing scale, pressure gauge, and woven fibers stacked within a vacuum bag, this bag must be completely hermetic with its respective input and output blend pipelines, and then the vacuum pump was activated to extract the air and to check the leak. The pressure difference leads to infusion, which allows the blend to pass through the Nomenclature: EP epoxy symbols Pu polyurethane Fa flax laminate arranged by 0°/90° angle orientation Pu5 The blend contains 95% epoxy and 5% polyurethane Fb flax laminate arranged by a combination of 0°/90° and 45°/-45° angle orientation Pu10 The blend contains 90% epoxy and 10% polyurethane Wa wool laminate arranged by 0°/90° angle orientation Pu15 the blend contains 85% epoxy and 15% polyurethane Wb wool laminate arranged by a combination of 0°/90° and 45°/-45° angle orientation JF unidirectional flax interfered with jute woven Hwaf hybrid contains wool and flax woven fibers until saturation, then the inlet and outlet of blend tubes were closed, and the pump is turned off. The mold was kept for three days to cure at room temperature [11]. The wool layers are arranged by 0°/90° angle orientation (Wa) and a combination of 0°/90° and 45°/-45° Angle orientation (Wb), also the flax layers are arranged by 0°/90° angle orientation (Fa) and a combination of 0°/90° and 45°/-45° (Fb).

Figure 2. Vacuum infusion technique
To improve the mechanical properties of wool, the woven wool (Wa) was hybridized by two laminates at the top and two laminates at bottom of flax (HWaf), and the woven wool (Wa) was hybridized by one layer of Jf at the top and the other at the bottom (HWajf), as shown in Figure 3. Bending test : Three-point bending (flexural tests) was applied according to ASTM D790 (Figure 4). The mechanical properties of the unreinforced and reinforced blend were measured. The bending tests were carried out using the universal machine Tinius Olsen h50kt with a maximum capacity of 50KN. The separation between supports was determined using a 16:1 spanto-thickness ratio [12]. The dimensions of the specimens were established by 125x10x4 mm ( Figure 5) and span length = 64 mm, the test speed was 2mm/min at room temperature.   : is the deflection of the specimen (mm) : is the specimen's effective length (mm)

Numerical analysis
ANSYS.19 Workbench was used as numerical software to investigate the fatigue results of unreinforced and reinforced composite. In this work, hex dominant was used to carry out element type, the number of elements was 2400, and the number of nodes was12389, as shown in Figure 8. The boundary conditions of the model applied are fixed support on the first side and the alternative load effect on the other side. The material properties were imported from the experimental results [14].  Figure 10 shows the Stress-Strain curves of the blend matrix which is mixed into the blend by (0%, 5%, 10%, and 15% Pu). The samples were tested without natural fiber reinforcement and the results are shown in Figure 11, which include the yield stress, ultimate stress, and maximum strain.  Comparison results between hybrids (HWaf and HWajf) and Wa showed that the yield stress was increased by 101.219 % and 46.951 %, respectively, while, the maximum strain increased by 8.81 % and 3.953 %.

Fatigue Results
The stress fluctuated between maximum stress (σ max ) and minimum stress (σ min ) The stress ratio R = σ min σ max For fully reverse stress: σ max = −σ min and for static stress: σ max = σ min [15]. The specimens of the fatigue tests were examined at fully reverse stress (R = -1), which was applied to the samples to investigate the number of cycles to fatigue failure.
The S-N curve was obtained by testing a number of the specimens, which were subjected to a specific value of stress. Starting with a high-stress value (less than the yield stress), the number of cycles (N) until rupture was counted. This procedure was repeated gradually by decreasing the value of the stress and counting the number of cycles for each test. Figures (12, 13 The S-N curve was modeled using the parametric relationship known as Basquin's law [64], which represents the relation between the stress and the number of cycles (N) until rupture on an S-N curve. There is no stress limit in the S-N curve for non-ferrous material, such as composite material. In these cases, the S-N curve is reached in a certain number of cycles (10 6 cycles), which is called the endurance limit ( ) [16,17].
Where: is the stress, is the number of cycles until rupture, A 1 and are the adjustment coefficients characteristic of the material and the test conditions.

Figure 19. S-N curve of the hybrids
The fatigue tests results of the blends and the composites are shown in Table 3 and Table 4 respectively, including the experimental S-N curve equations, the regression ( 2 ), ANSYS S-N curve equations, the endurance limit ( ) value at cycles for the experimental and ANSYS results and the Errors between them. Comparing the experimental value of endurance limit ( ) between the pure epoxy with 5%, 10% and 15% of polyurethane blends, the endurance limit increased by 6.452 %, 12.061 %, and 12.421 % respectively.
For the composites, Wa has a higher endurance limit value by 15.174 % than Wb and Fa has a higher endurance limit value than Fb by 11.632 %.
HWaf and HWajf have higher endurance limit values than Wa by 132.895 % and 66.549 % respectively

Conclusions
The main conclusions were withdrawn as follows: 1. Mixing the polyurethane with epoxy decreased the yield stress and maximum stress while increasing the maximum strain. 2. The composites consisting of fiber arranged by 0°/90° have mechanical properties (yield stress, ultimate stress, and maximum strain) higher than composites consisting of fiber arranged by a combination including 0°/90° and 45°/-45°. 3. The mechanical properties of Wa were improved at hybridization by Fa and jute (HWaf and HWajf). 4. For blends, the endurance limit value and fatigue life improved by increasing the polyurethane percentages in the blends to a until10% percentage. 5. The composites consisting of fiber arranged by 0°/90° have endurance limit value and fatigue life higher than composites consisting of fiber arranged by a combination including 0°/90° and 45°/-45°. 6. The endurance limit value and fatigue life of Wa were increased at the hybridization with Fa and jute.