Question

In the stress-strain curve of mild steel, plastic deformation starts at the ________.

• A. proportional limit
• B. elastic limit
• C. upper yield point
• D. lower yield point

Hint:

Elastic limit is the stress point beyond which permanent (plastic) deformation begins—even if it's not immediately visible as at the yield point.

Answer 1

The Correct Option is B

Step 1: Analyze the stress-strain curve of mild steel.
In the stress-strain curve of mild steel:
The proportional limit marks the end of linear elasticity (Hooke's Law).
The elastic limit is the maximum stress up to which the material returns to its original shape upon unloading.
Beyond the elastic limit, the material undergoes plastic deformation.
The yield point (upper and lower) indicates the start of more obvious plastic behavior, especially in mild steel. 
Step 2: Define plastic deformation.
Plastic deformation starts when the material can no longer return to its original shape, which technically begins just after the elastic limit, although yield points make it more evident in mild steel. 
Correct Answer: (B) elastic limit.