Explain the 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.).
Stress‐strain curve for ductile material :
The important points on the stress‐strain curve are:
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 is 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 ductile materials such as high strength deformed steel, brass, duralumin, etc., do not have a position of yield point, so that the curve passes smoothly from elastic deformation to plastic deformation. For such materials, proof stress at a specified strain is calculated. It is generally calculated after the test by an offset method.
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