Physics · Mechanical Properties Of Solids · NEET
No. Point A is where the line stops being straight, so Hooke's law (stress = k x strain) stops there. Between A and B the graph is a small curve, but the wire still returns to its old length when you remove the load, so it is still elastic. Point B is the yield point (elastic limit) where elastic behaviour ends. So A comes first, then B. In NCERT they are drawn close together but they are not the same point.
This is the plastic region. Once you cross the yield point B, the strain grows fast even for a small rise in stress. If you now remove the load at any point C here, the wire does NOT return to its old length. There is a leftover strain called the permanent set. So beyond B the wire is permanently deformed.
Point D is the ultimate tensile strength, the largest stress the wire can take. After D the wire starts to thin down at one spot (called necking). Because the area at that neck shrinks, the graph plotted using the ORIGINAL area shows stress falling, even though the wire is close to breaking. At E the wire finally snaps. E is the fracture point.
The wire breaks at E, the fracture point, not at D. D is only the highest stress value (ultimate strength). The material carries on stretching past D, gets a thin neck, and then breaks at E. In a brittle material D and E are very close; in a ductile material they are far apart.
In the experiment you gradually increase the applied force (which sets the stress) and then measure the strain it produces. Since stress is the cause you control and strain is the effect you read, stress is put on the y-axis and strain on the x-axis. This is a NEET convention; if a graph shows the axes swapped, read it carefully because slope meaning changes.
The stress-strain curves are drawn for two different materials X and Y. It is observed that the ultimate strength point and the fracture point are close to each other for material X, but are far apart for material Y. We can say that materials X and Y are likely to be (respectively):
Try the real previous-year questions from this chapter — each with the answer and a full solution.
O (origin), A (proportional limit, end of the straight line where Hooke's law stops), B (yield point or elastic limit), the plastic region B to D, D (ultimate tensile strength, the peak), and E (fracture point where the wire breaks). Memorise them in order: O, A, B, D, E.
Yield strength is the stress value at the yield point B. It marks the end of elastic behaviour. Below this stress the wire returns to its old shape; above it the wire is permanently deformed. It is written as sigma_y and its unit is N per metre squared (Pascal).
Only in the straight-line region O to A. Here stress is directly proportional to strain, so stress = k x strain, where k is the modulus of elasticity (the slope of this line). Beyond A the graph curves and Hooke's law no longer holds.
The proportional limit (point A) is where stress stops being proportional to strain, so the graph stops being straight. The elastic limit (point B) is where elastic behaviour ends and permanent set begins. Between A and B the wire is still elastic but no longer follows a straight line.
Rubber can be stretched to several times its length and still return, so its elastic region is very large. But it does not follow Hooke's law over most of that region, and it has no clear plastic region. Materials like rubber and aorta tissue are called elastomers and their curves are not the standard metal shape.
