Stress-strain curve explanation

The below figure shows the stress-strain graph of a ductile metallic wire has been shown. The different parts of this graph are described below:

i. Straight line OA: In this section, the stress on the wire is proportional to the strain, which means that the metal follows Hooke’s law. The wire behaves like a completely elastic body up to point A.

ii. Point A: This point indicates the proportionality limit.

iii. Line segment AB: In this section, the ratio of stress and strain is comparatively less, which means that the metal doesn’t follow Hooke’s law. However, after reaching this section of strain, if the stress is removed, then the wire will regain its original length, which means that the strain will again be zero.

iv. Point B: This point indicates the elastic limit. In the case of most metals, the two points A and B are found to be quite close to each other. In the case of glass, A and B are identical points and in the case of rubber, the distance between A and B is quite high.

v. Line segment BC: In this section, stress divided by strain becomes even less and the metal gradually loses its elastic property and becomes plastic. After reaching this section of strain, if the stress is removed, then the wire is unable to regain its original length. So, the wire undergoes permanent deformation.

vi. Point C: This point is called the yield point or the upper yield point. At this point, the stress level is known as yield stress. The yield point for most substances can’t be determined accurately.

vii. Line segment CD: In this section, stress divided by strain is negative. It implies that even if stress is decreased, the strain will increase.

viii. Point D: This point is called the lower yield point. If the stress is gradually decreased after the strain reaches this point, the graph DCBAO does not return along DO, but along DO’. In this case, OO’ indicates the permanent deformation. For bodies with nearly perfect elasticity, the points A, B, C and D are situated so close to one another that practically the four-point can be assumed to be identical.

ix. Line segment DE: In this section, stress divided by strain is the least and the metal becomes plastic. The cross-section of certain parts of the wire becomes comparatively lower than that of the remaining parts.

x. Point E: This point indicates the breaking point. The stress developed at this point in the wire is known as breaking stress or ultimate stress.

xi. Line segment EF: In this section, this area of the cross-section at different parts of the wire starts decreasing fast.

xii. Point F: At this point, the wire snaps from its weakest part.