Stress-strain curves tensile impact

Proportion of Hard Segments. As expected, the modulus of styrenic block copolymers increases with the proportion of the hard polystyrene segments. The tensile behavior of otherwise similar block copolymers with a wide range of polystyrene contents shows a family of stress—strain curves (4,7,8). As the styrene content is increased, the products change from very weak, soft, mbbedike materials to strong elastomers, then to leathery materials, and finally to hard glassy thermoplastics. The latter have been commercialized as clear, high impact polystyrenes under the trade name K-Resin (39) (Phillips Petroleum Co.). Other types of thermoplastic elastomers show similar behavior that is, as the ratio of the hard to soft phase is increased, the product in turn becomes harder. 

Mechanical Properties. The room temperature modulus and tensile strength are similar to those of other amorphous thermoplastics, but the impact strength and ductility are unusually high. Whereas most amorphous polymers arc glass-like and brittle below their glass-transition temperatures, polycarbonate remains ductile to about — 10°C. The stress-strain curve for polycarbonate typical of ductile materials, places it in an ideal position for use as a metal replacement. Weight savings as a metal replacement are substantial, because polycarbonate is only 44% as dense as aluminum and one-sixth as dense as steel. 

Figure 35-12. Tensile stress-strain curves for poly(styrene), PS, and high-impact poly(styrene), HIPS, at 20 C. The arrows indicate the onset of a whitish coloration. (After C. B. Bucknall.)...

Tensile testing The area under the stress-strain curve is often used to quantify toughness (ASTM D638). However, even with different stress-strain curves that can be obtained, the mechanical responses to the impact loading may dissipate the same impact energy. 

The toughness values are obtained from the stress-strain curves and not from the conventional Charpy or Izod impact tests. Hence their units are obtained in MPa as observed. The variation in ultimate tensile strength and toughness of PVC-PEA blends have been shown in Figures 14.18 and 14.19. 

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