Creep

Definition

Creep is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. It occurs as a result of long-term exposure to high levels of stress that are still below the yield strength of the material.

Main Content

1. The Creep Curve

• The deformation of a material over time at a constant load and temperature is represented by a "Creep Curve."
• It displays the relationship between strain (deformation) and time, identifying the three distinct stages of creep.

Strain (ε)
|          /
|        /  (Tertiary)
|      /
|     / (Secondary)
|   _/
| _/ (Primary)
|/________________ Time (t)

2. Factors Influencing Creep

• Temperature: Creep is highly temperature-dependent; it generally becomes significant when a material is at or above 40% of its absolute melting temperature.
• Stress Levels: Higher constant loads will increase the rate of creep and reduce the time until failure.

3. Creep Mechanisms

• Dislocation Creep: Occurs at high stress where dislocations move through the crystal lattice by climbing over obstacles.
• Diffusion Creep: Occurs at high temperatures where atoms migrate through the crystal structure or along grain boundaries to accommodate stress.

Working / Process

1. Primary Creep (Transient)

• The initial stage where the material exhibits a high rate of strain that gradually decreases over time.
• The material undergoes strain hardening, where dislocation density increases and resists further deformation.

2. Secondary Creep (Steady-State)

• The strain rate becomes constant, resulting in a linear slope on the creep curve.
• This is the most critical stage for engineering design, as the balance between strain hardening and recovery reaches an equilibrium.

3. Tertiary Creep

• The strain rate increases exponentially as the material approaches failure.
• Micro-cracks and voids form within the internal structure, leading to structural necking and eventual fracture (rupture).

Advantages / Applications

• Understanding creep is vital for the design of jet engine turbine blades that must withstand high temperatures and centrifugal forces.
• It is a critical consideration for power plant piping and pressure vessels that operate under continuous high-pressure steam.
• Knowledge of creep properties allows engineers to select specialized superalloys (like nickel-based alloys) that minimize deformation in extreme environments.

Summary

Creep is the time-dependent, permanent deformation of materials under constant stress, particularly at elevated temperatures. It progresses through primary, secondary, and tertiary stages until structural failure occurs. Key concepts to remember include "steady-state creep rate," "homologous temperature," and "rupture life."