1. The specimen must be of constant cross-section.
2. The material must be homogeneous.
3. The load must produce uniform stress i.e., load is axial.
Fig. 2.2 shows the stress-strain diagram for semi ductile material like mild steel. The different points of the curve are described below.
1. Proportional Limit (OA) : The point A is the point on the stress-strain diagram up to which stress is proportional to strain.
2. Elastic Limit (B) : It is the point on the stress strain diagram up to which the material remains elastic i.e., if unloaded the specimen regains its original length without any permanent set.
3. Yield Point (C or C’) : It is the point on stress-strain diagram at which there is appreciable deformation without appreciable increase in stress. There are two yield points ; upper and lower yield points.
4. Ultimate Stress (D) : It is the highest ordinate attained by the stress-strain diagram.
5. Rupture Stress (E) : It is the stress at which the material breaks. A stress-strain diagram will be nominal when the original cross sectional area of the specimen is always taken into account for calculating the stress. Hence in nominal stress-strain diagram the value of the rupture stress point E will be lower than ultimate stress point D.
6. Actual Rupture Stress (F) : In actual stress-strain diagram the stresses are calculated on the basis of actual area taking in consideration the necking (narrowing down) phenomena (Fig. 2.3).