Materials In Today's World

Mechanical Behavior of Polymers


Polymers exhibit a wide range of stress-strain behaviors as shown in the figure below. The brittle polymer (red curve) elastically deforms and fractures before deforming plastically. The blue curve is a plastic polymer and is similar to curves for many metals. Its behavior begins in the linear elastic deformation region. As the curve transitions from the elastic to plastic deformation typically there is a peak stress. For polymer materials, this peak stress is identified as the yield stress. As the material is pulled further, fracture occurs. The stress value when fracture occurs is defined as the tensile strength for polymer materials. The tensile strength can be greater than, equal to, or less than the yield strength. The green curve is a class of polymers known as elastomers. These materials exhibit rubber-like elasticity and will return to their original shape and form unless they are extended to the point of fracture.

Polymer stress strain. See text above. Red: linear, steep. Blue: linear, steep, peaks and flattens out lower than red. Green: Slow increase
Mechanical properties of polymers: stress-strain behavior.
Credit: Adapted from Fig. 7.22, Callister & Rethwisch 5e.

While some of the stress-strain curves for polymers might look similar to ones for metals, polymers are mechanically different than metals (or ceramics). A highly elastic polymer may stretch over 10 times the original length before breaking, while a metal might elastically stretch 10% of the original length elastically and may stretch plastically to double the original length before reaching its fracture point. As seen in the figure below, the largest elastic modulus values for polymers are well under the values for ceramics and metals.

E value: Metal Alloys (40-400), Graphite, Cermaics & Semiconductors (10-1200), polymers (0.2-4), Composites/fibers (.6-400)
Room-Temperature Elastic Moduli for Various Materials
Credit: Callister

As shown in the figure below, the tensile strength of some polymers can rival some ceramics but are no match for even the softest of metals.

Tensile Strength (MPa): Metal Alloys (110-2000), Cermaics & Semiconductors (10-1000), polymers (6-100), Composites/fibers (2-5000)
Tensile strengths of metals/alloys, ceramics, polymers, and composites/fibers.
Credit: Callister & Rethwisch 5e

To Read

Now that you have learned a bit about the mechanical behavior of plastics, please go to your e-textbook and read pages 87 to 89 in Chapter 4 of Materials for Today's World, Custom Edition for Penn State University to learn more about this subject. When finished with the reading proceed to the next web page.