Question

Creep rate decreasing with time is observed during

• A. Viscous creep
• B. Primary creep
• C. Transient creep
• D. Initial stages of creep test

Hint:

Visualize the creep curve (strain vs. time). Stage I has a decreasing slope (rate). Stage II has a constant slope. Stage III has an increasing slope. "Transient" and "Primary" refer to Stage I.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Creep is the time-dependent plastic deformation of a material under a constant load or stress, typically at elevated temperatures. A standard creep test plots strain versus time, and the resulting curve is generally divided into three stages. The creep rate is the slope of this curve (\(d\epsilon/dt\)).

Step 2: Detailed Explanation:
The three stages of creep are:

• Stage I (Primary or Transient Creep): In this initial stage, the material deforms, and strain hardening (work hardening) occurs. The rate of strain hardening is initially greater than the rate of recovery (thermal softening), causing the dislocations to become more entangled and impede further motion. As a result, the creep rate (strain rate) continuously decreases with time. The term "Transient creep" is a synonym for Primary creep.

• Stage II (Secondary or Steady-State Creep): A dynamic equilibrium is reached between the competing processes of strain hardening and recovery. This results in a nearly constant, minimum creep rate.

• Stage III (Tertiary Creep): The creep rate accelerates rapidly, leading to failure. This is caused by the formation of internal voids, necking, or other microstructural damage that reduces the effective cross-sectional area.
The question describes a creep rate that is decreasing with time, which is the defining characteristic of Stage I. Options (B) Primary creep and (C) Transient creep both correctly name this stage. Since (C) is marked as the correct answer, we select it. (D) is also technically correct but less specific than (B) or (C).

Step 3: Final Answer:
The stage of creep where the creep rate decreases with time is known as Primary or Transient creep.