Creep testing is used to study slow and permanent deformation of materials under constant stress at high temperature.

Creep becomes important in components operating at high temperatures for long periods.

Examples include boilers, turbines, jet engines, and nuclear reactors.

Definition

Creep

Creep is the slow and continuous deformation of a material under constant stress at high temperature over a long period of time.

Creep Testing

A test performed to determine the creep deformation and creep life of materials under constant load and temperature.

Conditions for Creep

Creep occurs significantly when:

• Temperature is high

• Stress is constant

• Time is long

Creep is especially significant when temperature exceeds about 0.4 times the melting temperature (in Kelvin).

Creep Testing Procedure

In creep testing:

1. A specimen is placed in a creep testing machine.

2. A constant load is applied.

3. The specimen is maintained at constant high temperature.

4. The elongation with time is measured.

The deformation is recorded continuously.

Creep Curve

Creep behavior is represented by a creep curve (strain vs time).

It has three stages:

1. Primary Creep

• Creep rate decreases with time

• Material strain hardening occurs

2. Secondary Creep

• Constant creep rate

• Longest stage

• Stable deformation

3. Tertiary Creep

• Creep rate increases rapidly

• Necking occurs

• Final fracture takes place

Creep Rate

Creep rate is defined as the rate of change of strain with time.

\dot{\epsilon} = \frac{d\epsilon}{dt}

Where
ε = strain
t = time

Factors Affecting Creep

Creep behaviour depends on:

• Temperature

• Applied stress

• Material structure

• Grain size

• Time of loading

Applications of Creep Testing

Creep testing is very important in:

• Gas turbines

• Steam boilers

• Jet engines

• Nuclear power plants

• High-temperature piping

Exam-Focused Points

Key points for competitive exams:

• Creep occurs at high temperature and constant stress.

• Represented by creep curve (strain vs time).

• Has three stages: Primary, Secondary, Tertiary.

• Secondary stage has constant creep rate.

Example Competitive Exam Questions

Question: Under what conditions does creep occur?
Answer: High temperature and constant stress.

Question: Which stage of creep has a constant creep rate?
Answer: Secondary creep.

Quick Revision Summary

Fatigue Testing

• Failure due to repeated stresses

• Represented by S–N curve

• Occurs even below yield strength

• Important for rotating components

Creep Testing

• Slow deformation under constant stress at high temperature

• Represented by creep curve

• Has three stages of creep

• Important for high-temperature components