Yield strength is the stress at which a material begins to deform plastically.

Before reaching yield strength:

• Deformation is elastic

• Material returns to original shape

After yield strength:

• Deformation becomes permanent

Definition

Yield Strength

The stress at which a material begins permanent plastic deformation.

Core Concept Explanation

During tensile testing:

• Stress increases gradually.

• Up to the elastic limit, deformation is temporary.

• When stress reaches the yield point, the material begins plastic deformation.

From this stage:

• Large strain occurs

• Stress changes very little

Types of Yield Points

Upper Yield Point

The first maximum stress at which yielding begins.

Lower Yield Point

The minimum stress required to maintain plastic deformation after yielding begins.

In mild steel:

Upper yield point > Lower yield point.

Offset Yield Strength

Some materials (like aluminum) do not show a clear yield point.

In such cases, yield strength is determined using the 0.2% offset method.

Properties Related to Yield Strength

Yield strength indicates:

• Maximum stress before permanent deformation

• Safe working limit of materials

• Structural design safety

Applications in Engineering

Yield strength is used in:

• Structural design

• Machine component design

• Pressure vessel design

• Bridge construction

• Automotive components

Engineers design structures so that working stress remains below yield strength.

Exam-Focused Points

• Yield strength indicates start of plastic deformation.

• Mild steel shows upper and lower yield points.

• Materials like aluminum use 0.2% offset yield strength.

• Yield strength is important for safe design of components.

Common Exam Traps

Confusion

Students confuse:

• Yield strength

• Ultimate tensile strength

Correct difference:

• Yield strength → start of plastic deformation

• UTS → maximum stress material can withstand

Example Competitive Exam Questions

Question: What does yield strength represent in a material?
Answer: Stress at which permanent plastic deformation begins.

Question: Which material shows distinct upper and lower yield points?
Answer: Mild steel.

Question: What method is used to determine yield strength in materials without a clear yield point?
Answer: 0.2% offset method.

Quick Revision Summary

• Yield strength = start of plastic deformation.

• Two yield points in mild steel:

  • Upper yield point

  • Lower yield point

• Used in design of machine components.

• Some materials use 0.2% offset method.