Plastics can be divided into:
Rigid plastics are resistant but not tough materials, they can resist well to stress but less to elongation before breaking.
We can see from the graph that the slope of the rigid plastic curve is very steep, thatâ€™s why a lot of force is needed to deform a rigid plastic. By this we can deduce that it has an high modulus.
Rigid plastics are fragile, as they tend to be resistant, but not very tough, as for example the polystyrene and the polymethacrylate.
Flexible plastics donâ€™t resist very well to deformation, but in the meantime they tend not to break. Their initial modulus is high, so that they can resist to the initial strain, but applying an appropriate force they will deform. Polypropylene and Polyethylene belong to this category.
Fibers are more resistant then tough and they will not deform too much after a tensile stress. As an example, are included in this category nylon, Kevlar and carbon fibers.
Elastomers have a mechanical behavior completely different from other materials. The elastomers have a very low modulus, that allow them to deform very easily; even if this ability is not useful if the material, ended the effort, doesnâ€™t return to its initial position.