Explain Stress‐strain Curve for a ductile material.
Answer: Stress‐strain Curve
Stress‐strain curve : Stress‐strain curve gives the relation between the stress and strain induced in a material due to externally applied force. This curve is different for ductile material (steel, rubber copper etc.) and brittle material (cast iron, glass etc.).
The important points on the stress‐strain curve are as follows :
1. Proportional limit : It is also called proportional stress. It is the maximum value of the stress up to which stress is directly proportional to strain. In figure (a) point O to A is a straight line and beyond point A the curve slightly deviates from the straight line.
2. Elastic limit : It is the maximum value of stress up to which the deformation are elastic or temporary (after unloading, metal regains its original shape and size). Beyond elastic limit i.e. point B the deformation of metal is permanent.
3. Yield point : It is the stress at which material yields i.e. deformed plastically without any strain hardening. In this curve the stress corresponding to point C is called upper yield stress and for point D it is called lower yield stress.
4. Ultimate stress : It is the maximum value of stress on this curve without fracture. After this value i.e. point E, the failure of metal begins.
5. Breaking point stress : It is also called failure stress or fracture stress. Breaking stress point F is the fracture or failure point.
6. Proof stress : From figure (b) most of ductile materials such as high strength deformed steel, brass, duralumin etc., does not have position of yield point, so that the curve passes smoothly from elastic deformation to plastic deformation. For such materials a proof stress at a specified strain is calculated. It is generally calculated after test by an offset method.