This simulation demonstrates "Necking" in a polymer specimen being stretched in a idealised material. A stress/strain curve is generated for an ideal material and a molecular two dimensional "snapshot" is shown of a schematic pair of neighbouring molecules. In reality these molecules are more like writhing interlocked snakes, in a state of near constant movement, constantly making and breaking weak bonds with adjacent molecules in a complex three dimensional structure. The nature of the molecular interaction is statistical and the snapshot only applies to two selected molecules, other molecules could be in widely different states at any stage during the simulation.

To start the simulation hold down the "Stretch" Button to apply an increasing load to the ends of the sample.

Initially the stress/strain response is linear, this means that elastic fully recoverable deformation is taking place. If the load was to be removed then the specimen would retrace the stress strain curve back to the origin. The molecular snapshot shows that recoverable rotations of the molecules are taking place. However the stress/strain curve soon becomes non-linear, now plastic deformation is taking place which is non-recoverable on unloading. The molecular snapshot shows unfolding of the idealised molecules, which will not be reversible if the load was removed.

As a peak stress is reached then note how the sample begins to neck and that the molecules start to straighten out.

As the sample gets more and more stretched more and more molecules will unfold and start to align, forming areas of crystallinity. With increasing strain the molecules can slide past each other by breaking and reforming weak bonds. When nearly all the molecules are unfolded then the specimen may break.

When the sample breaks, press "Reset" if you wish to begin again with a new sample.