Creep Behavior of Materials

Abstract

Gas-turbine engines’ components, nuclear reactors, oven components, and boilers’ superheater tubes are some examples of machine elements that fail by creep. This chapter first makes a distinction between creep failure and stress rupture; and then explains the dislocation climb mechanism of creep. Creep testing and creep curve has been discussed with the aid of sketches. It has been emphasized that stress, temperature, and grain size are important factors in controlling creep rate. In particular the effects of stress and temperature on the creep rate has been mathematically modeled and analyzed to compute the activation energy for creep. The Larson-Miller parameter has been explained with aid of formula and graphical plots for various titanium alloys. The techniques to design an alloy against creep have been discussed, including superalloy turbine blades. This chapter contains 11 worked examples, 6 diagrams, 6 exercise problems, and 4 MCQs with answers.

Questions and Problems

• 14.1. What is the difference between creep failure and stress rupture?

• 14.2. Give four examples of components that frequently fail by creep.

• 14.3. What is the effect of grain size of a material on the creep rate?

• 14.4. Draw a diagram showing the creep mechanism by dislocation climb.

• 14.5. Explain the three stages of creep with the aid of sketch of creep curve.

• 14.6. A stainless steel specimen was creep tested at 470 °C at a stress of 380 MPa for 360 hours; thereby producing a strain of 0.12. The same specimen was again creep tested at the same temperature at a stress of 270 MPa for 600 hours that produced a strain of 0.03. Calculate the creep rate in the material at a stress of 140 MPa.

• 14.7. The constants for creep behavior of a material are: n = 4.13, and C₁ = 3.95 × 10⁻¹⁸ at a temperature of 500 °C. Calculate the time to produce 0.28% strain in a link bar of the same material when stressed to 120 MPa at the same temperature (500 °C).

• 14.8. A creep test on a material at a stress of 200 MPa results in the steady-state creep rate of 2.0 × 10⁻³s⁻¹ at 850 K; and the creep rate is 5.4 × 10⁻⁶s⁻¹ at a temperature of 730 K. Calculate the activation energy for creep for the temperature range for the material.

• 14.9. By reference to the Larson-Miller parameter data in Fig. 14.7, calculate the time to rupture the Ti₃Al alloy component that is subjected to a stress of 200 MPa at 450 °C.

• 14.10. By using the data in Problem 14.8, calculate the maximum working temperature for the material.if the limiting creep rate is 4.5 × 10⁻⁵s⁻¹.

• 14.11. By reference to Fig. 14.6, what design stress do you recommend for Ti-8-3-4 alloy exposed to a temperature of 550 °C, if the component’s life is desired to be 3 years?

• 14.12. (MCQs). Encircle the correct answers for the following questions.

  • (a) Which stage of creep enables us to calculate the creep rate?

    • (i) Primary creep, (ii) secondary creep, (iii) tertiary creep.

  • (b) Which term corresponds to fracture of a component?

    • (i) stress rupture, (ii) creep, (iii) creep failure, (iv) deformation.

  • (c) Which alloy has the best creep resistance?

    • (i) brass, (ii) bronze, (iii) aluminum alloy, (iv) superalloy.

  • (d) Which scale of temperature is used in calculating the Larson-Miller parameter?

    • (i) ^oC, (ii) ^oF, (iii) ^oR, (iv) Kelvin.