Deformation is directly related to force, so that as force increases, an object will deform more and more until its molecular integrity is pushed to the limit. At this point, the object will fracture or fail in some other way.
The above diagram is a plot of stress versus strain, with a characteristic elastic and plastic region. In the elastic region, there is a straight line relationship between stress and strain. Within this loading range, if the force is removed, the strucutre will return to its original dimensions. The slope of this linear portion describes a structure's elastic or Young's modulus, and is calculated as stress/strain.
Beyond the elastic region, but before fracture occurs, the relationship between stress and strain is no longer linear, with small forces resulting in ever-larger deformations. This region is called the plastic region because within this range, if the force is removed, the structure will not return to its original dimensions.At the upper limit of the plastic region, the structure will fracture.
