Material Creep
Material creep is the time-dependent, permanent deformation of a material under sustained mechanical stress, particularly at elevated temperatures (typically above 0.4 times the absolute melting temperature). Unlike elastic deformation, creep is irreversible and progresses continuously as long as the load and temperature persist. It is a critical design consideration in gas turbine blades, nuclear reactor components, steam pipes, and geological structures.
Key Formula
epsilon_dot = A * sigma^n * exp(-Q / (R*T))
LaTeX: \dot{\varepsilon} = A \sigma^n \exp\left(-\frac{Q}{RT}\right)
| Symbol | Meaning | Unit |
|---|---|---|
| \dot{\varepsilon} | Steady-state creep rate | s⁻¹ |
| A | Material constant | dimensionless |
| \sigma | Applied stress | Pa |
| n | Stress exponent (typically 3–8) | dimensionless |
| Q | Activation energy for creep | J/mol |
| R | Universal gas constant (8.314) | J/(mol·K) |
| T | Absolute temperature | K |
Worked Example
Problem
A nickel superalloy turbine blade experiences a steady-state creep rate of 1 × 10⁻⁸ s⁻¹ at 900°C under 200 MPa. Estimate the total creep strain after 10,000 hours of operation.
Solution
Step 1: Convert time to seconds. 10,000 hours × 3600 s/hour = 3.6 × 10⁷ s Step 2: Use steady-state creep assumption. Total creep strain ε = ε̇ × t ε = 1 × 10⁻⁸ s⁻¹ × 3.6 × 10⁷ s Step 3: Calculate. ε = 0.36 = 36%
Answer
Total creep strain ≈ 0.36 (36%) — indicates significant dimensional change requiring replacement
Three Stages of Creep and Their Characteristics
| Stage | Name | Strain Rate Trend | Mechanism | Design Relevance |
|---|---|---|---|---|
| I | Primary (Transient) Creep | Decreasing | Dislocation pile-up, strain hardening | Short initial service |
| II | Secondary (Steady-State) Creep | Constant | Balance of hardening and recovery | Long-term design basis |
| III | Tertiary Creep | Increasing | Void coalescence, necking, cracking | Imminent failure warning |
| — | Creep Rupture | Final fracture | Microcrack propagation | End-of-life criterion |
Interactive Tools
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Related Terms
From Old English "creopan" (to crawl), aptly describing the slow, progressive nature of the deformation. The scientific study of creep was advanced by English engineer Andrade in 1910, who proposed the empirical "Andrade creep law" for metals.