Monkman—Grant curve

In Fig. 4.21, creep rupture data from a number of different grades of silicon nitride are plotted in a Monkman-Grant format.30,31,34,115 116 For purposes of comparison with metallic alloys, the temperature dependence of the Monkman-Grant curves has been ignored. As with the metallic alloys, the curves for all of the grades of material tend to plot within a relatively narrow band. These results imply that lifetime can be improved merely by improving creep rate the lower the creep rate, the longer the lifetime. 

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.

Both materials also follow a Monkman-Grant curve ... 

Fig. 4.21 Monkman-Grant curves for experimental and commercial grades of silicon nitride. The results from seven different studies are plotted on the same graph. With the exception of the data by Kossowsky et al., 116 data tend to plot within a relatively narrow band. Within a factor of three in lifetime prediction, all of these data can be represented by the dashed line. It is this line that is plotted in Fig. 4.17 with data from high temperature alloys.

Figure 6.19 (a) Nominal applied stress and time to fracture (steel grade 91, T — 500—700°C) (b) Monkman—Grant curve minimum strain rate and time to fracture. A master curve appears over the entire temperature range [49]. The red star corresponds to a sequential test including pure fatigue test —> pure creep test cyclic loading for 3.3 x 10° cycles under total cyclic strain of 0.15% (Fig. 6.2(b)), followed by a creep under 185 MPa at 550°C until fracture (Fig. 6.5(b)). 

What this relationship, in effect, says is that every material will fail during creep when the strain in that material reaches a certain value independent of how slow or how fast that strain was reached. That the Monkman-Grant expression is valid for Si3N4 is shown in Fig. 12.14, where the range of data obtained for the vast majority of tensile stress rupture tests lies in the hatched area. On such a curve, Eq. (12.42) would appear as a straight line with a slope of 1, which appears to be the case. 

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