Effect of Stress and Temperature on Creep

Summary

The transcript explains the impact of stress and temperature on the creep process in materials. It highlights that an increase in stress or temperature leads to higher initial strain, faster steady-state creep rates, and shorter creep life. The relationship between stress and creep follows a power law, with the stress exponent (n) varying between 1 and 8. Temperature dependence is described using an exponential equation involving activation energy and temperature in Kelvin. The transcript emphasizes that even small changes in stress or temperature can significantly accelerate the creep rate, making creep highly sensitive to these factors.

Takeaways

• 🔥 The effect of stress on creep is similar to the effect of temperature.
• ⬆️ Increasing stress (from sigma 1 to sigma 2) results in greater initial strain, a higher steady-state creep rate, and a shorter creep life.
• ⏳ Creep life at higher stress (t_f2) is shorter than at lower stress (t_f1).
• 📉 The slope of the steady-state strain rate increases with higher stress.
• ⚙️ Steady-state strain rate at sigma 2 is greater than at sigma 1.
• 📈 Stress dependence follows a power law, with stress raised to the power of n, known as the stress exponent.
• 💥 The stress exponent (n) typically ranges from 1 to 8, depending on the creep mechanism.
• ❄️ The temperature dependence of steady-state strain rate follows an exponential law: exp(-Q/RT), where Q is the activation energy and T is temperature in Kelvin.
• ⚡ Small increases in stress or temperature can significantly enhance the steady-state creep rate.
• 🔥 Creep is highly sensitive to both stress and temperature, with even slight changes dramatically reducing creep life.

Q & A

What is the effect of increasing stress on initial strain during creep?

Increasing the stress results in a higher initial strain at the same time point during the creep process.

How does stress affect the steady-state creep rate?

Higher stress increases the steady-state creep rate, meaning the material deforms more rapidly in the steady-state regime.

What happens to the creep life of a material when stress is increased?

The creep life of the material is reduced as stress increases, meaning it will fail more quickly under higher stress.

How is the steady-state strain rate represented graphically?

The steady-state strain rate is represented as the slope in the steady-state regime of a creep curve, and the slope increases with higher stress.

What is the relationship between the steady-state strain rate and stress for different stress levels?

At a higher stress level (sigma 2), the steady-state strain rate is greater compared to a lower stress level (sigma 1).

What is the form of the temperature dependence on the steady-state strain rate in the creep equation?

The temperature dependence follows an exponential form, represented as exponential minus Q over RT, where Q is the activation energy and T is the test temperature in Kelvin.

What is the stress dependence in the creep rate equation?

The stress dependence follows a power law, where the stress is raised to the power of n, and n is called the stress exponent.

What is the typical range for the stress exponent n in creep mechanisms?

The stress exponent n generally varies from 1 to 8, depending on the different creep mechanisms.

How do small changes in stress and temperature affect the steady-state creep rate?

Even slight increases in stress and temperature can significantly enhance the steady-state creep rate, leading to a dramatic reduction in creep life.

Why is creep considered a stress and temperature-sensitive process?

Creep is highly sensitive to both stress and temperature because increases in either parameter can substantially accelerate deformation and reduce the material’s life.