Final answer:
Stress that extends the molecules of a material farther apart leads to plastic deformation, where the material cannot return to its original form. This can be due to pulling or stretching, as well as thermal expansion or contraction.
Step-by-step explanation:
When stress extends the molecules of a material farther apart, it causes the material to deform. After a material passes its elastic limit, it displays plastic behavior, meaning it won't return to its original shape or size when the force is removed. This permanent change is known as plastic deformation. Materials such as rubber become increasingly difficult to stretch before reaching a breaking point, while ductile materials like metals decrease in stress with strain, becoming easier to deform as they near the fracture point.
Thermal stress is another form of stress that occurs due to temperature changes causing expansion or contraction; this can have both destructive and beneficial effects, such as rupturing tanks or aiding manufacturing processes.