Slow plastic deformation of metals under a constant stress is known as:
🔬 Understanding Time-Dependent Deformation
Materials don't just deform instantly under a load. Some types of deformation happen slowly, over a long period of time. The question asks for the specific name of this slow, continuous deformation that occurs when a material is held under a constant load, especially at high temperatures.
Creep: A time-dependent and temperature-dependent plastic deformation of a material subjected to a constant stress or load. It is the tendency of a solid material to move slowly or deform permanently under the influence of persistent mechanical stresses that are below the yield strength of the material.
⚖️ Detailed Analysis of the Options
Let's distinguish between the different modes of material failure and deformation.
(a) Creep
Conditions: Constant stress + Time (+ often high temperature).
Description: The material slowly and continuously deforms over time. A classic example is a lead pipe sagging under its own weight over many years, or a turbine blade in a jet engine slowly elongating due to high stress and temperature.
Conclusion: This perfectly matches the description in the question.
(b) Fatigue
Conditions: Fluctuating or cyclic stress + Time.
Description: This is failure under repeated loading and unloading, even if the maximum stress is below the material's ultimate strength. A tiny crack initiates and grows with each cycle until the material suddenly fractures. Think of bending a paperclip back and forth until it breaks.
Conclusion: Incorrect. Fatigue is caused by *cyclic* stress, not *constant* stress.
(c) Endurance
Conditions: Related to fatigue.
Description: The endurance limit (or fatigue limit) is the maximum stress level below which a material can theoretically withstand an infinite number of load cycles without failing due to fatigue. It is a property related to fatigue, not a type of deformation itself.
Conclusion: Incorrect. Endurance is a property, not a process.
(d) Plastic deformation
Conditions: General term.
Description: This is any permanent change in the shape of a material when a load is applied. While creep *is* a form of plastic deformation, it is a very specific type. "Plastic deformation" is a broad category that also includes the instantaneous yielding that happens when you exceed a material's yield strength. "Creep" is the specific name for the slow, time-dependent version.
Conclusion: Incorrect because it is too general; "Creep" is the specific term.
💡 Study Tips for Material Failure
- Creep is Constant and Creepy: Associate the word "creep" with something that happens slowly and continuously under a *constant* load. It "creeps" along over time.
- Fatigue is from Flexing: Think of getting "fatigued" from doing a repetitive exercise. Fatigue failure is from repetitive, *cyclic* loading.
- Endurance is the Limit: The endurance limit is the stress a material can *endure* forever without fatigue failure.