Creep Failure and Design Against Creep in Mechanical Engineering
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Questions and Answers

What is creep in the context of material science?

  • Deformation caused by rapid heating and cooling of a material
  • Temperature-dependent deformation of a material subjected to variable stress
  • Time-dependent deformation of a material under constant stress (correct)
  • Stress-dependent deformation of a material under constant temperature

    • When does creep become significant in a material?

  • When the temperature is less than 0.4 Tm
  • When the stress is greater than the yield stress
  • When the temperature is greater than 0.4 Tm (melting temperature on an absolute scale) (correct)
  • When the stress is less than 0.4 Tm

    • Under what conditions does a material experience creep failure?

  • When the temperature is close to the melting point
  • When the stress exceeds the ultimate tensile strength
  • When the dimensional change renders the material useless in performing its intended function (correct)
  • When the material undergoes rapid heating and cooling cycles

    • What is the result of sufficient strain or creep in a material?

    <p>Fracture, known as stress rupture or creep fracture</p> Signup and view all the answers

    Which process governs creep mechanisms?

    <p>Diffusion-controlled process</p> Signup and view all the answers

    How do vacancies in a material's microstructure contribute to creep?

    <p>They migrate toward grain boundaries to fill the voids, resulting in elongation of the grain</p> Signup and view all the answers

    What are the mechanisms of creep mentioned in the text?

    <p>Vacancies migrating along grain boundaries and dislocations moving to accommodate the applied stress</p> Signup and view all the answers

    What can make materials susceptible to creep at different temperatures?

    <p>The quality of material fabrication and processing</p> Signup and view all the answers

    What is a typical creep test measure?

    <p>Strain as a function of time at a constant stress and temperature</p> Signup and view all the answers

    Which materials are mentioned as susceptible to creep at relatively high temperatures?

    <p>Nickel-based superalloys</p> Signup and view all the answers

    Where do components that experience creep phenomenon exist?

    <p>Boilers, gas turbine engines, and ovens</p> Signup and view all the answers

    What can cause creep failures to appear in a brittle manner?

    <p>High pressure steam lines</p> Signup and view all the answers

    Why are materials with high melting temperatures often selected for turbine engine blades?

    <p>To withstand sustained stress over time at a relatively high temperature</p> Signup and view all the answers

    What is the slope of the strain vs. time curve in a creep test?

    <p>The strain rate or creep rate of the material</p> Signup and view all the answers

    What is an example application whereby components experience creep phenomenon?

    <p>Steam turbine blades</p> Signup and view all the answers

    What can ceramics exhibit at extremely high temperatures?

    <p>A considerable amount of creep</p> Signup and view all the answers

    Ceramics have a very low resistance to deformation by creep due to their characteristically low melting temperatures.

    <p>False</p> Signup and view all the answers

    The rupture lifetime (tr) in a creep test is the time taken for the material to reach its maximum strain.

    <p>True</p> Signup and view all the answers

    Thermal activation of dislocations refers to the reduction in energy required to move pinned dislocations at increasing temperatures.

    <p>False</p> Signup and view all the answers

    Boilers, gas turbine engines, and ovens are examples of components that do not experience creep phenomenon.

    <p>False</p> Signup and view all the answers

    Materials with high melting temperatures are often selected for turbine engine blades due to their resistance to creep at relatively high temperatures.

    <p>True</p> Signup and view all the answers

    Creep failures always appear in a ductile manner.

    <p>False</p> Signup and view all the answers

    In a creep test, strain is measured as a function of stress at a constant temperature.

    <p>False</p> Signup and view all the answers

    Nickel-based superalloys are not susceptible to creep at relatively high temperatures.

    <p>False</p> Signup and view all the answers

    Creep becomes significant in a material only at extremely high temperatures.

    <p>False</p> Signup and view all the answers

    The slope of the strain vs. time curve in a creep test represents the strain rate of the material.

    <p>True</p> Signup and view all the answers

    Creep is a time-independent deformation of a material subjected to a constant stress.

    <p>False</p> Signup and view all the answers

    Creep is not influenced by temperature.

    <p>False</p> Signup and view all the answers

    Creep failure occurs when the dimensional change of a material makes it unsuitable for its intended function.

    <p>True</p> Signup and view all the answers

    Creep fracture is also known as elastic deformation.

    <p>False</p> Signup and view all the answers

    Creep occurs when atoms move in response to the applied stress and temperature, filling the voids in the grains.

    <p>True</p> Signup and view all the answers

    Materials with low melting temperatures are often selected for turbine engine blades.

    <p>False</p> Signup and view all the answers

    Study Notes

    Creep in Material Science

    • Creep is a time-dependent deformation of materials subject to constant stress, particularly at high temperatures.
    • It becomes significant at extremely high temperatures where strain accumulates over time.

    Conditions for Creep Failure

    • Creep failure occurs when the dimensional changes in a material render it unsuitable for its intended function.
    • Creep fractures typically manifest in a ductile manner, contrary to brittle failure modes.

    Mechanisms and Influences on Creep

    • Creep mechanisms are governed by thermal activation of dislocations, where increased temperatures reduce the energy needed to move dislocations.
    • Vacancies in a material's microstructure facilitate creep by allowing atoms to move under stress, filling voids within the grains.

    Creep Conditions

    • Materials with high melting temperatures are often chosen for turbine engine blades due to their enhanced resistance to creep under high temperature conditions.
    • Conversely, materials with low melting temperatures are more susceptible to creep failures.

    Creep Testing

    • A typical creep test measures strain as a function of stress at a constant temperature, indicating how a material behaves under prolonged loading.
    • The slope of the strain vs. time curve during a creep test reveals the strain rate of the material.

    Applications and Examples

    • Common applications where components experience creep include turbines, boilers, and ovens.
    • Ceramics can exhibit unique properties at extremely high temperatures, although they are generally characterized by low resistance to deformation from creep.

    Overview of Creep Performance

    • The rupture lifetime (tr) in a creep test signifies the duration required for a material to reach maximum strain.
    • Nickel-based superalloys are noted for their resistance to creep at elevated temperatures, making them suitable for critical applications.

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    Description

    Explore the concepts of creep failure, creep test, stress and temperature effects on creep behavior, and design against creep in mechanical engineering. This quiz covers Chapter 5 of the Fourth Year First Term curriculum, taught by Dr. Reham Reda Abbas.

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