Tensile strength decreases in fiber 1.
This is due to may not be producing the strong interfacial bonding between fibre and matrix. However increasing fibre content there is no remarkable improvement in tensile strength. From the Fig. 5c shows the voids due to fibre pull out. The strong interface region can transfer the maximum load from the matrix to fibre surface.
Fig. 5c 2. When the stress concentrations at the fibre ends leads to matrix cracking. Shorter fiber lengths will create more fiber ends, which eventually act as stress concentration points where failure often occurs at these sites. This possibly clarifies the reduction of tensile strength. 3. The failure mechanisms have shows that under tensile loading the failure start at ends of the fibre and propagate along the fibre matrix interface. 4. In discontinuous fibre polymer composites the stress along the fibre is not uniform. A definite fibre length is required for the effective transfer of stress between fibre and matrix. At lower fibre content this is not enough to impart high strength. 5. Load is not uniformly distributed to more fibres, which are not well bonded with resin and fibre resulting decreases in tensile properties. 6. The reason is that shorter fibre may not be compatible composites because of the improper bonding between the fibres and matrix. Tensile strength was decreased with higher percentage of fibre. 7. In general poor uniform distribution of fibres can be observed for less percentage of fibre. Fiber breakage and fiber pullout and fiber fracture mechanisms can be seen easily. From the SEM photograph, fibre and matrix debonding, fibre fracture, voids due to fibre pull-out are observed as failure modes.
[1] Amuthakkannan, P., et al. "Effect of fibre length and fibre content on mechanical properties of short basalt fibre reinforced polymer matrix composites." Materials Physics and Mechanics 16 (2013): 107-117.
Effect of particulates 1. Occasionally the damaged regions appear on the particle surface in the form of isolated islands treated as voids. During loading some interfacial voids coalesce into larger patches of the fractured material, which is addressed to as interface debonding. Other ones give rise to the volume crack propagation inside the particles. 2. In some cases it is difficult to differentiate exactly the debonding, void nucleation and volume cracking, since these modes can transform into each other during loading. 3. Two basic fracture modes: interfacial fracture that implies failure of the regions belonging to the matrix/particle interface, and volume cracking that occurs inside the particles. 4. For all particles in tension and compression, first cracks appear near the interface due to stress concentration in the near-boundary regions.
[2] Romanova, Varvara Alexandrovna, Ruslan Revovich Balokhonov, and S. Schmauder. "The influence of the reinforcing particle shape and interface strength on the fracture behavior of a metal matrix composite." Acta Materialia 57.1 (2009): 97-107.
1. The composite strength increases with decreasing particle size. Smaller particles have a higher total surface area for a given particle loading. This indicates that the strength increases with increasing surface area of the filled particles through a more efficient stress transfer mechanism. 2. The adhesion strength at the interface determines the load transfer between the components. 3. Effective stress transfer is the most important factor which contributes to the strength of two-phase composite materials. For poorly bonded particles, the stress transfer at the particle/polymer interface is inefficient. 4. Discontinuity in the form of debonding exists because of non-adherence of particle to polymer. Thus, the particle cannot carry any load and the composite strength decreases with increasing particle loading. 5. For composites containing well-bonded particles, addition of particles to a polymer will lead to an increase in strength especially for nanoparticles with high surface areas. [3] Fu, Shao-Yun, et al. "Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites." Composites Part B: Engineering 39.6 (2008): 933-961.
Water absorption test: 1. The reason may be due to the flax fibers contain abundant polar hydroxide groups, which result in a high moisture absorption level of natural fiber reinforced polymer matrix composites. 2. Another effect of increasing water absorption rate of composites is the nature of flax fiber, voids inside the composites lead to the formation of micro channels. After a time immersion in water, the presence of these micro voids and cracks in the composite surface results in the movement of water molecules to the material by capillary action. 3. These results are because of water penetration inside polymers decreasing the connection between fiber and polymer material. 4. Composite materials reinforced with flax fibers had many channels and capillary tube which allowed for water molecules to penetrate inside the materials and acting along the interface between epoxy and flax fiber causing swells in the samples. Then the bonds between resin and fibers will break. So the strength of the composite material will decrease. 5. For the 10% flax fiber reinforcing the ultimate tensile stress of wet samples is higher than that for dry samples. This could be due to the fact that high amounts of water causes swelling of the fibres, which could fill the gaps between the fibre and the polymer–matrix, and eventually could lead to an increase in the mechanical properties of the composites.
Effect of friction: 1.
