_________ is the phenomenon of slow extension of the material at a constant load during tensile test.

Explanation:

Creep in Materials

• Creep is the phenomenon of slow and progressive deformation of a material under a constant load or stress, typically occurring over an extended period of time. This process happens when the material is subjected to a constant tensile, compressive, or shear stress, often at elevated temperatures, but it can also occur at room temperature for certain materials.
• During a tensile test or other loading conditions, a material subjected to a constant load may experience deformation that increases gradually over time. This deformation is not recoverable and occurs due to the movement of dislocations, grain boundary sliding, and diffusion processes within the material structure. The creep behavior is typically characterized by three stages:
  • Primary Creep: This is the initial stage where the creep rate decreases over time due to strain hardening.
  • Secondary Creep: In this stage, a constant creep rate is observed, which is the result of a balance between strain hardening and recovery processes.
  • Tertiary Creep: This stage involves an accelerated creep rate leading to material failure due to necking or microstructural damage.

Factors Affecting Creep:

• Stress Level: Higher stress levels lead to increased creep rates.
• Temperature: Elevated temperatures accelerate the creep process, as thermal activation facilitates dislocation movement and grain boundary sliding.
• Material Type: Certain materials, such as metals, polymers, and ceramics, exhibit creep behavior. Metals like aluminum, lead, and nickel-based alloys are more prone to creep at high temperatures.
• Time Duration: Creep deformation increases with the duration of applied stress.

Applications: Understanding creep behavior is essential in industries where materials are exposed to high temperatures and sustained loads, such as:

• Aerospace industry for turbine blades and jet engines.
• Power plants for boiler tubes and heat exchangers.
• Construction industry for bridges and high-rise buildings.
• Automotive industry for engine components.