Question

Creep occurs under

• A. low temperature and high stress
• B. high temperature and constant stress
• C. room temperature and cyclic stress
• D. low stress and low temperature

Hint:

Creep is a triple-variable phenomenon involving Stress, Temperature, and Time.
The classic creep curve has three stages: Primary (transient), Secondary (steady-state, constant slope), and Tertiary (acceleration leading to rupture).
Always associate creep with high temperatures (\( \gt 0.4 T_m \)) and constant loading/stress.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
This question asks for the environmental and loading conditions required for creep deformation to take place in engineering materials.
Creep is a time-dependent deformation mechanism that poses a major design challenge for high-temperature components.

Step 2: Key Formula or Approach:
Creep is characterized by steady-state strain rates under elevated temperatures and constant stress.
The steady-state creep rate \( \dot{\epsilon}_s \) is mathematically modeled by the power-law creep equation:
\[ \dot{\epsilon}_s = A \sigma^n \exp\left(-\frac{Q_c}{R T}\right) \]
where:
\( A \) is a material constant.
\( \sigma \) is the applied constant stress.
\( n \) is the stress exponent.
\( Q_c \) is the activation energy for creep.
\( R \) is the universal gas gas constant.
\( T \) is the absolute temperature.

Step 3: Detailed Explanation:

• Temperature Requirement: Creep generally becomes significant at temperatures above the homologous temperature threshold, typically \( T_h \gt 0.4 T_m \), where \( T_m \) is the absolute melting point of the metal.
At these elevated temperatures, thermal energy is sufficient to assist dislocations in overcoming barriers via climb and to facilitate atomic diffusion.

• Stress Requirement: Unlike typical plastic deformation which occurs rapidly once the yield strength is exceeded, creep occurs under a constant applied stress that is often much lower than the room-temperature yield strength of the material.
- Low temperature combined with high stress results in instantaneous plastic deformation or cold working.
- Cyclic stress at room temperature leads to fatigue failure, not creep.
- Low stress and low temperature generally result in only elastic deformation with no progressive deformation over time.


Step 4: Final Answer:
Creep is defined as slow, progressive, time-dependent plastic deformation occurring under constant stress at elevated temperatures.
Therefore, the correct choice is option (B).