Most materials that display plastic behavior display elastic behavior below the critical stress or yield stress. Such materials are called lastico- plastic (sometimes called a Prandtl modal) and can be characterized with an analog model of a spring attached to a sliding mass. The mass requires a critical stress to begin moving against friction, but a certain amount of elastic strain is sustained by the spring before the critical stress is reached. Once the mass begins to move (strain) the applied stress has exceeded the critical stress and a constant stress keeps it moving (straining). This is an example of a derived response model one that includes a combination of one or more of the simple or fundamental models (elastic, plastic, viscous) in either series or parallel combinations.

A perfectly plastic material that deforms permanently and without limit when the value of yield stress is reached. It is not necessary to increase stress value to cause any further