Monkman Grant equation

Two important conclusions are reached from the Monkman-Grant equation. First, the most important engineering requirement to improve the lifetime of materials that fail by creep rupture is to improve creep behavior. Over the years, this has been the goal of much of the research on high temperature alloys. Second, theoretical treatments of creep rupture must be consistent with the Monkman-Grant equation, as has been shown by Ashby and Dyson.111... 

Fig. 4.20 Monkman-Grant curves for two commercial grades of silicon nitride. Some grades give curves that are temperature-independent (a) AY6, SiC -reinforced others give a series of curves depending on temperature (b) NT154. The temperature independent curves have creep rate exponents, m, for the Monkman-Grant equation, tf = ce L, that are approximately 1, whereas the creep rate exponent for the temperature-dependent curves are greater than 1 e.g., 1.7 for NT154.

The starting point for predicting lifetimes during creep is the Monkman-Grant equation, which states that the product of the time to failure tf and the strain rate is a constant, or... 

Figure 12.14 Summary of a large number of tensile rupture data collected from various sources for Si3N4 plotted to verify the Monkman-Grant equation.

Creep rupture data for siliconized silicon carbide were fitted to the Monkman-Grant equation to give p= 1.45. For example, for a strain rate of 2x10 /s the failure time is 100 hours. Determine the steady-state strain rate that would give a failure time of 10000 hours. Discuss the problems that might be associated with such an extrapolation. Describe briefly the creep failure observed in this material (need to check references for this chapter). 

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