Fatigue and Creep Testing

These tests are performed to understand material behavior under long-term loading conditions.
Unlike tensile or hardness tests, these tests study failure occurring after long time or repeated loading.

Fatigue testing studies failure due to repeated stress, while creep testing studies slow deformation under constant stress at high temperature.

Unit 12.1: Fatigue Testing

Fatigue testing is used to determine how materials behave when subjected to repeated or cyclic stresses.

Many engineering components fail due to fatigue even when the applied stress is below the yield strength.

Examples include aircraft wings, rotating shafts, springs, gears, and bridges.

Definition

Fatigue

Fatigue is the failure of a material due to repeated or fluctuating stresses over a long period of time.

Fatigue Testing

A test performed to determine the number of cycles a material can withstand before failure under cyclic loading.

Core Concept Explanation

When a material is subjected to repeated loading:

1. Small microscopic cracks develop.

2. Cracks propagate slowly with each cycle.

3. Eventually, the material fractures suddenly.

Fatigue failure usually occurs without noticeable plastic deformation.

Fatigue Testing Machine

Common fatigue testing machines include:

• Rotating beam fatigue testing machine

• Axial fatigue testing machine

• Torsional fatigue testing machine

The most common type used in laboratories is the rotating beam fatigue testing machine.

Rotating Beam Fatigue Test

In this test:

1. A specimen is mounted in the machine.

2. The specimen is rotated at high speed.

3. A bending load is applied.

4. The material experiences alternating tensile and compressive stresses.

5. The specimen eventually breaks after a number of cycles.

Stress–Cycle Relationship (S–N Curve)

Fatigue behaviour is represented using an S–N curve.

Where:

• S = Stress amplitude

• N = Number of cycles to failure

As the number of cycles increases, the stress required to cause failure decreases.

Fatigue Life

Fatigue life refers to the number of stress cycles a material can endure before failure.

Endurance Limit

Some materials (especially steels) show an endurance limit.

Endurance Limit

The maximum stress that a material can withstand for an infinite number of cycles without failure.

Factors Affecting Fatigue

Important factors influencing fatigue strength:

• Stress concentration

• Surface finish

• Material defects

• Temperature

• Corrosion

• Load type

Applications of Fatigue Testing

Fatigue testing is essential in:

• Aircraft structures

• Automobile components

• Turbine blades

• Railway axles

• Springs and shafts

Exam-Focused Points

Important for JE/AE mechanical exams:

• Fatigue failure occurs due to repeated stress.

• Failure may occur below yield strength.

• Represented by S–N curve.

• Endurance limit mainly seen in ferrous metals.

Example Competitive Exam Questions

Question: What causes fatigue failure?
Answer: Repeated or cyclic stresses.

Question: What does the S–N curve represent?
Answer: Relationship between stress and number of cycles to failure.