Final answer:
The unit stress at which a steel bar subjected to a tensile load can return to its original length after the load is removed is called the elastic limit. This is within the elastic region of the material's stress-strain curve where no permanent deformation occurs.
Step-by-step explanation:
When a steel bar is subject to a tensile load, it increases in length up to a certain point without undergoing permanent deformation; when the load is removed, it will return to its original length. The unit stress up to which the material returns to its original shape and size upon removal of the load is known as the elastic limit. Beyond this point, permanent deformation occurs. For steel, as long as the stress is within the elastic region of the material's stress-strain curve, it will fully recover its shape.
The stress within this elastic region is defined as the ratio of the deforming force (F1) to the cross-sectional area (A) of the object being deformed, calculated using tensile stress = F1/A. When the stress exceeds the elastic limit of the material, it enters the region of plastic deformation where it will not return to its original length once the load is removed.
Tensile strain is the ratio of the change in length to the original length, and it provides a measure of deformation under tensile stress. Different materials have different elastic limits and breaking stresses, which are the maximum stresses they can withstand before fracture.