Flexuration volume

In order to get the best out of fibre reinforcement it is not uncommon to try to control within close limits the fibre content which will provide maximum stiffness for a fixed weight of matrix and fibres. In flexure it has been found that optimum stiffness is achieved when the volume fraction is 0.2 for chopped strand mat (CSM) and 0.37 for continuous fibre reinforcement. 

Tests by Roe et al. [63] with unidirectional jute fiber-reinforced UP resins show a linear relationship (analogous to the linear mixing rule) between the volume content of fiber and Young s modulus and tensile strength of the composite over a range of fiber content of 0-60%. Similar results are attained for the work of fracture and for the interlaminate shear strength (Fig. 20). Chawla et al. [64] found similar results for the flexural properties of jute fiber-UP composites. 

Fig. 6.5. Yield strengths from flexural tests are plotted against strain rates at the surface of the samples. Tests were performed on polymers A, B, and E test temperature 23 °C. The slope of the three lines correspond to similar activation volumes v = 2 0.1 nm3...

Recently, Oldfield Ellis (1991) have examined the reinforcement of glass-ionomer cement with alumina (Safil) and carbon fibres. The introduction of only small amounts of carbon fibres (5% to 7-5% by volume) into cements based on MP4 and G-338 glasses was found to increase considerably both the elastic modulus and flexural strength. There was an increase in work of fracture attributable to fibre pull-out. A modulus as high as 12-5 GPa has been attained with the addition of 12% by voliune of fibre into MP4 glass (Bailey et al, 1991). Results using alumina fibre were less promising as there was no fibre pull-out because of the brittle nature of alumina fibres which fractured under load. 

Developers have used ASTM C-651-91, "Standard Test Method for Flexural Strength of Manufactured Carbon and Graphite Articles Using Four Point Loading at Room Temperature." Based on 2002 dollars and costs projected to high-volume production (500,000 stacks per... 

Both fiber-matrix interphase-sensitive mechanical tests (interlaminar shear strength, 90° flexure) and interphase-insensitive tests (0° flexure) were conducted on high volume composite samples fabricated from the same materials and in the same manner as discussed above to see if the interphase and its properties altered the composite mechanical properties and in what manner. A summary of the data is plotted as a bar graph in Fig. 7. The first set of bars represents the difference in fiber-matrix adhesion measured between the bare fibers and the sized fibers by the ITS. The composite properties plotted on the figure also show increased values for the epoxy-sized material over the bare fiber composite. 

Steel fiber has been used to reinforce concrete. When 10-mil square bar 2 inches long is incorporated into concrete to the extent of 2% by volume, the modulus of rupture (flexural strength) increases by a factor of 2. When impregnated with polymer the fiber reinforced concrete-polymer increases the modulus of rupture by another factor of 2.5. Thus, an over-all increase by a factor of 5 in the flexural strength is achieved... 

Figure 10. Flexural performance of carbon-fiber-reinforced concrete. is the volume fraction of carbon fiber.

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