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Stress-strain curves for fiber-filled polymers

Posted by admin on July 24th, 2012

The stress-strain curves for a pair of thermoplastic compounds are shown in Figure 7. The base resin is the same for both compounds, except one compound is unfilled while the other contains 30 percent glass fiber as a reinforcement. You can see that the glass fibers significantly increase the ultimate strength, yield strength, proportional limit, and the Young’s modulus while causing the filled resin to rupture at a much lower strain. On the other hand, the unfilled resin shows “drawing” at strains beyond the yield point. The stress decreases to a plateau beyond the yield point before failure. Typically, the cross sectional area of the sample decreases during the drawing, according to Poisson’s Ratio for the material.

FIGURE 7. Stress-strain curves for a fiber-filled and an unfilled resin.

Rate- and temperature-dependency of stress-strain curves

The loading rate (or the strain rate) and temperature can significantly affect the stress-strain behavior of plastics. As an illustration, Figure 8 plots the influence of loading rates and temperature on the tensile stress-strain curve for a semi-crystalline resin. In general, at higher loading rates or lower temperatures, plastics materials appear to be more rigid and brittle. On the other hand, at lower loading rates or higher temperatures, materials appear to be more flexible or ductile because of their viscous characteristics. As you can see in Figure 8, an increase in loading rate significantly increases the ultimate and yield strength, whereas an increase in temperature leads to decreases in ultimate and yield strength and in proportional limit. If the material is semi-crystalline and the glass transition temperature is crossed when raising the temperature, these rate- and temperature-dependent effects can be very large, resulting in entirely different behaviors. If the material is amorphous and the softening range is crossed, the material will undergo viscous flow when loaded.

FIGURE 8. Stress-strain curves for a typical polymer at two test temperatures (high and low) and two rates of loading (fast and slow).

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