Final answer:
Plastic deformation is indicated on a force extension graph by a permanent change in the shape or size of a material when subjected to stress beyond the elastic limit. The graph transitions from a straight line segment where Hooke's law is followed to a curved line indicative of irreversible deformation, eventually leading to fracture.
Step-by-step explanation:
Plastic deformation on a force extension graph occurs when the material is subjected to stress beyond the elastic limit, also known as the yield point. In this region, even after the load is removed, the material will not return to its original shape or size, indicating that the deformation is permanent. The force extension graph initially displays a straight line segment following Hooke's law; however, as the load increases, the graph curves, signalling a transition from elastic to plastic deformation. The material continues to deform plastically until it reaches the fracture point, where it breaks.
During plastic deformation, materials like rubber become harder to stretch as stress increases, while ductile materials like metals may actually become easier to deform. The green line on the stress-strain graph indicates how, after the load is removed, the material settles into a new, permanently altered shape and size, reflective of the permanent deformation.
It is important to note that the linear, elastic, and plastic limits represent ranges of values rather than precise points, and the transition between these regions can be gradual. Stress-strain behavior in this graph also varies with how the force is applied and other factors.