In Vellum when an object is bent or stretched, it returns to its original rest shape. However, when plasticity effects are turned on, once an object is bent past a certain threshold it will maintain a new rest shape. Plasticity is typically used with Vellum Hair. For example, a character running it’s hand through a character’s fur, which stays matted down in that area.
There is an Enable Plasticity checkbox on the 
Vellum Constraints node, which will display a few Plasticity parameters. The Threshold determines the point at which the material will either return to it’s original shape, or change into the new shape. The rate at which it changes to the new shape is determined by the Rate parameter, which determines how fast the material adopts its new rest lengths.
Plasticity is also available in conjunction with the Shape Match option of the Constraint Type attribute.
Tip
Turning on the visualization for Stretch Plastic Flow or Bend Plastic Flow is a useful way to see if the material is being affected by the plasticity constraints.
Note
It is difficult to make very stiff material in Vellum, such as sheet metal. For very hard materials, use Bullet Soft Constraints.
The following video shows Crag walking through a field of grass, trampling down the grass (hairs) as he walks. The visualization at the top of the video shows the Bend Plastic Flow.