Analysis of Mechanical Properties of Unidirectional Flax Roving and Sateen Weave Woven Fabric-Reinforced Composites

2.1 Composite preparation methods

Flax roving-reinforced composites were prepared by vacuum bagging (Figure 2a) and resin infusion (Figure 2b) [19] methods of composite formation. In both methods, the mold was sealed using a vacuum bag, and atmospheric pressure was applied to hold laminate plies together. The pressure between the outside and inside of the sealed envelope must be equal to 0 to fabricate composite material with better quality. The pressure difference was measured using the gauge bar mounted on a vacuum pump motor. The main difference between the vacuum bagging method (Figure 2a) and the resin infusion method (Figure 2b) is the way of applying the resin to the reinforcement. In case of vacuum bagging method (Figure 2a), each ply of reinforcement has to be wetted manually using epoxy resin before the mold is sealed; in contrast to this, in the resin infusion method (Figure 2b), first, the dry plies of reinforcement have to be placed on the mold and sealed, as shown in Figure 2b, and then epoxy resin is slowly drawn to the envelope through a resin inlet valve, and by using a resin distribution medium, the resins are equally distributed over the surface of reinforcement plies. In both methods, a vacuum outlet valve is used to suck the air in the envelope and excess resin residues.

Figure 2

Methods of composite formation: (a) vacuum bagging method and (b) resin infusion method.

The vacuum bagging method of composite formation was used to produce woven fabric-reinforced composites, and the resin infusion method was used to produce UD composites. Also, the formation of UD composites was tried using a vacuum bagging method, but due to the difficulties of removing air from the gaps between parallelly arranged rovings, the produced composite was not good enough to be used for further experimental tests. In both methods, two layers of reinforcements were stacked together to produce the composite.

2.2 Experiments

The tensile strength and flexural rigidity experimental tests were performed at the Lodz University of Technology in the Faculty of Mechanical Engineering by using the Shimadzu AG-X plus testing machine (Figure 4a), which has a maximum testing load of 50 KN. For the tensile strength experimental test, 10 specimens have been tested for each sample type both in perpendicular and parallel direction to fiber orientation in the composite structure. These specimens’ dimensions were prepared based on the American Standard of Test Method (ASTM) D3039 (Figure 3a) with a testing condition of 2 mm/min speed.

Figure 3

Specimen dimension: (a) tensile strength test and (b) flexural rigidity test.

Figure 4

Shimadzu AG-X plus testing machine: (a) tensile strength test and (b) flexural rigidity test.

Flexural rigidity test was performed to know the force required to bend the flax fiber-reinforced composites and determine the stiffness of the material. To do so, 10 specimens were prepared from each sample, both in perpendicular and parallel direction to fiber orientation in the composite structure with the dimension of the specimens(fig.3b) were prepared based on the ASTM D7264/D 7264-M standard. The specimens were tested using a three-point loading system(fig.4b) method of flexural rigidity test. The samples were tested with a speed of 5 mm/min.

3.1 Analysis of the influence of fiber orientation and reinforcement types on the flexural rigidity of composites

The composite sample reinforced with sateen weave fabric has 28.47% and 13.07% lower ability to withstand flexural stress in comparison to UD composite reinforced by 14 rovings/cm and 10 rovings/cm respectively in the direction of fiber orientation (Figure 5), this illustrates UD reinforced flax composites have higher bendable property than woven-reinforced composites. Also, in a perpendicular direction to fiber orientation, 14 rovings/cm reinforced UD composite has 28.81% and 39.21% higher flexural stress than woven fabric-reinforced composite and 10 rovings/cm reinforced UD composites, respectively (Figure 5). During the flexural rigidity experimental test, when the load applied to specimens in the direction of fiber orientation (0°), the specimens were bent to their maximum deflection point in all types of composites made; after the removal of applied load, each specimen is almost returned to their original position. Also, when experimental tests were carried out on specimens in perpendicular to the fiber orientation, both unidirectionally reinforced composites were cracked. In contrast to this, woven fabric-reinforced composites were almost returned to their original position after the removal of applied force.

Figure 5

The average flexural stress of flax-reinforced composite samples.

3.2 Analysis of the influence of fiber orientation and reinforcement types on the tensile strength of composites

The influence of reinforcement type and density of roving on the tensile strength of composites are shown in Figure 6. The study of tensile test result reveals that when a tensile force is applied to the specimen on a parallel direction to fiber orientation (0°), UD composite reinforced with 10 roving/cm has 18.73% and 34.90% higher tensile strength than woven fabric-reinforced composite and UD flax composite reinforced with 14 rovings/cm, respectively. As a result of the weaving process, the tensile strength of yarns is degraded, and this affects the composite material reinforced either by using woven fabric or UD composite made from yarns/rovings removed from woven fabric, this discloses the influence of single yarn/roving tensile properties on the composite material tensile strength.

Figure 6

The average tensile strength of flax fiber-reinforced composite samples.

The tensile experimental test carried out on the specimens in a perpendicular direction to the fiber orientation in the composite structure shows that woven fabric-reinforced composite has 44.64% and 30.62% higher tensile strength than composite reinforced by 14 rovings/cm and 10 rovings/cm, respectively. In this case, the woven fabric-reinforced composite has exhibited high tensile strength because of the presence of cotton yarn in the warp direction of the fabric.

Summing up the analysis of flexural rigidity and tensile strength of the composites shows that mechanical properties of flax fiber-reinforced composites are highly dependent on the direction of fiber orientation in the composite structure with significantly showing higher flexural rigidity and tensile strength in the direction of fiber orientation, signifying that textile material-reinforced composites have anisotropic properties. Besides, unidirectionally reinforced flax fiber exhibits higher tensile strength and flexural rigidity than woven fabric-reinforced composites. In contrast, woven fabric-reinforced composites have higher tensile strength than unidirectionally reinforced composites when the test was carried out in a perpendicular direction. Woven fabric-reinforced composite has exhibited excellent elongation in comparison with UD flax roving-reinforced composites during the tensile strength test because of the crimpage of yarns in the structure of the woven fabric.