Question

In the stress-strain curve of a ductile material, the point at which the material begins to deform plastically is known as

• A. Elastic Limit
• B. Yield Point
• C. Ultimate Tensile Strength
• D. Modulus of Elasticity

Hint:

In the stress-strain curve for ductile materials, the Yield Point (or yield strength) is the critical point marking the transition from elastic deformation (where the material returns to its original shape) to plastic deformation (where the material undergoes permanent, irreversible deformation).

Answer 1

The Correct Option is B

Step 1: Understand the Stress-Strain Curve for Ductile Materials.
A stress-strain curve graphically represents the relationship between stress (force per unit area) and strain (deformation per unit length) in a material subjected to tensile or compressive load. For ductile materials, this curve typically shows an elastic region, a plastic region, and eventually fracture.
Step 2: Define the Elastic and Plastic Regions.
Elastic Region: In this region, the material deforms elastically, meaning it returns to its original shape once the load is removed. Stress is proportional to strain in this region (Hooke's Law), up to the proportional limit.
Plastic Region: In this region, the material undergoes permanent (plastic) deformation. Even if the load is removed, the material will not fully return to its original shape.
Step 3: Identify the point where plastic deformation begins.
The point on the stress-strain curve where the material begins to deform plastically is known as the Yield Point. Beyond this point, any further deformation will be permanent. For some materials, there might be a distinct upper and lower yield point. In engineering practice, the yield strength is often used, which is defined as the stress at which a specified amount of permanent deformation occurs (e.g., 0.2% offset yield strength).
Step 4: Evaluate the given options.
Option 1: Elastic Limit. The elastic limit is the maximum stress a material can withstand without undergoing any permanent deformation upon removal of the load. While close to the yield point, for many ductile materials, the yield point is typically considered the point where plastic deformation begins. In cases where a distinct yield point exists, it effectively marks the onset of plastic deformation.
Option 2: Yield Point. This is the precise term for the point at which a ductile material begins to deform plastically.
Option 3: Ultimate Tensile Strength. This is the maximum stress that a material can withstand while being stretched or pulled before necking begins. It occurs after the yield point.
Option 4: Modulus of Elasticity. Also known as Young's Modulus, it is a measure of the stiffness of an elastic material, defined as the ratio of stress to strain in the elastic region of the stress-strain curve. It is a property, not a point on the curve that signifies plastic deformation. The final answer is $\boxed{\text{2}}. $