Creep

Definition

Creep is the gradual and time-dependent deformation of a material under constant load or stress, particularly at elevated temperatures. This phenomenon is critical in understanding how materials behave over time when subjected to prolonged exposure to stress, especially in high-temperature environments. Creep can lead to significant changes in material properties and can ultimately contribute to failure modes in structural applications.

5 Must Know Facts For Your Next Test

1. Creep is most pronounced in materials that are exposed to high temperatures and constant loads over extended periods, such as metals in turbine engines.
2. The creep behavior of materials can be divided into three stages: primary (decelerating), secondary (steady-state), and tertiary (accelerating) creep.
3. Materials that exhibit significant creep can have reduced lifespan and performance under operating conditions, making it crucial for engineers to account for this in design.
4. Creep testing involves subjecting a material specimen to a constant load at elevated temperatures and monitoring deformation over time.
5. The Arrhenius equation is often used to model the temperature dependence of creep rates, indicating that creep becomes more significant as temperature increases.

Review Questions

• How does the phenomenon of creep influence the design considerations for high-temperature applications?
  • Creep influences design considerations for high-temperature applications by necessitating materials that can withstand prolonged stress without significant deformation. Engineers must select materials with favorable creep resistance properties to ensure structural integrity over time. Additionally, they often factor in safety margins and consider potential changes in material performance due to creep when calculating load limits and service life.
• Discuss how the stages of creep (primary, secondary, tertiary) can impact the failure modes of materials under stress.
  • The stages of creep—primary, secondary, and tertiary—each have different implications for material failure. In primary creep, the rate of deformation decreases as the material adjusts to stress. In secondary creep, a steady state is reached where deformation occurs at a constant rate. Tertiary creep sees an accelerating rate of deformation leading to eventual rupture. Understanding these stages helps engineers predict when and how failure may occur, allowing them to implement preventative measures or choose appropriate materials.
• Evaluate the role of temperature on the creep behavior of ductile versus brittle materials, considering their respective failure mechanisms.
  • Temperature plays a crucial role in the creep behavior of both ductile and brittle materials, but their responses differ significantly. Ductile materials may exhibit considerable plastic deformation before failure due to their ability to absorb energy through dislocation movements at elevated temperatures. In contrast, brittle materials tend to fracture with little deformation when subjected to similar conditions. This difference highlights the importance of selecting appropriate materials for applications where high temperatures are expected, as ductile materials may better accommodate time-dependent deformations while brittle materials could fail suddenly under similar stress conditions.

Related terms

viscoelasticity: The property of materials that exhibit both viscous and elastic characteristics when undergoing deformation.

fatigue: The weakening of a material caused by repeatedly applied loads, leading to the development of cracks and eventual failure.

plastic deformation: Permanent deformation of a material that occurs after the yield point has been exceeded, resulting in a change in shape.