Table of Contents

Materials and Structure




Because of their very large molecular weight (~ 106) polymers behave differently from the low molecular weight metals during plastic deformation leading to a ductile fracture. The top diagram shows a tensile stress-strain curve for a 6,6 Nylon. It should be noted that the strain at fracture is very large (300%) compared to that for metals.

The lower diagram shows the reorganization of the macromolecules during the plastic deformation process. If the material is initially amorphous, the molecules will have a random organization as depicted in the top picture. As a stress is applied to the sample it deforms and the straining process tends to allign the the macromolecules so that their long axis is parallel to the applied stress (the flow direction). As deformation continues, necking occurs in one region and the molecular structure starts to have the symmetry of a crystalline material. The neck region grows through the sample and fracture occurs by breaking the molecular structure.

From: Askland, "The Science of Engineering Materials," PWS (1994)