text: Polymer melts exhibit different deformation mechanisms when subjected to shear or tensile stresses. For example, a polymer melt’s ductility can increase when a stimulus, such as light, causes fragmentation of the polymer chains through bond breaking. This process is known as chain scission. In the low temperature regime of a polymer melt (T < Tg), crazing or shear banding can occur. The former mechanism resembles crack formation, but this deformation mechanism actually involves the formation of fibrils separated by porous domains or voids. The latter mechanism (shear banding) involves the formation of localized regions of plastic deformation, which typically arise near the position of the maximal shear point in a polymer melt. It is important to note that crazing and shear banding are deformation mechanisms observed in glassy polymers.
  For crystalline polymers, the deformation mechanism is best described by a stress-strain curve for a crystalline polymer, such as nylon. The stress-strain behavior exhibits four characteristic regions. The first region is the linear-elastic regime, where the stress-strain behavior is elastic with no plastic deformation. The characteristic deformation mechanism in the second region is yielding, where plastic deformation can occur in the form phenomena such as twinning. The third region shows the formation of a neck, and the fourth region is characterized as a steep increase in stress due to viscous flow. Additionally, region four corresponds to alignment and elongation of the polymer backbone from its coiled or folded state—eventually leading to fracture.