Stress and Strain Curve for Mild Steel

 Stress and Strain Curve for Mild Steel

Pic-1 shows a typical tensile test specimen of a mild steel on a universal testing machine. Its ends are gripped into universal testing machine. The length between two gripped portion is L₁   

 

The load is applied gradually and at regular interval of loads extension is measured. After a certain Load, extension increases at faster rate. the extension is measured from scale on the universal testing machine. load is increased gradually till the specimen breaks.

Pic 3 shows stress vs strain diagram for the typical mild steel specimen. The following salient points are observed on stress strain curve.

1. Limit of Proportionality (A): it is the limiting value of the stress up to which stress is proportional to strain.

2.Elastic Limit: This is the limiting value of stress up to which if the material is stressed and then released(unloaded) strain disappears completely and the original length is regained. This point is slightly beyond the limit of proportionality.

3. Upper Yield Point (B): this is the stress at which, the load starts reducing and the extension increases. This phenomenon is called yielding of material. At this stage strain is about 0.125 percent and stress is about 250 N/mm² .

4.  Lower Yield Point (C): At this stage the stress remains same but strain increases for same time.

5. Ultimate stress (D): This is the maximum stress the material can resist. This stress is about 370-400 N/mm². At this stage cross-sectional area at a particular section starts reducing very fast. This is called neck formation. After this stage load resisted and hence the stress developed starts reducing.

6. Breaking Point (E): The Stress at which finally the specimen fails is called breaking point. At this strain is 20 to 25 percent.

If the load is removed within the elastic limit the original length is regained i.e. the stress strain curve follows down the loading curve shown in Pic-3. If the load is removed after loading the specimen beyond elastic limit, it follows a straight line parallel to the original straight portion as shown in Pic-3 by line FF’. Thus, if it is loaded beyond elastic limit and then the load is removed a permanent strain (OF) is left in the specimen. This is called permanent set. 

Stress-strain curve in aluminum and high strength steel: In these elastic materials there is no clear-cut yield point. The necking takes place at ultimate stress and eventually the breaking point is lower than the ultimate point. The typical stress-strain diagram is shown in Pic-4 . The stress p at which if unloading is made there will be 0.2 percent proof stress and this point is treated as yield point for all practical purposes.