Rupture lifetime

Figure 9.3 Illustration of the optimistic prediction of the rupture lifetime of a polyaramid from short-term data, if the presence of a transition is not taken into account (curve shape based on data from [7])...

Fig. 4.18 Summary of data used to establish the validity of the Monkman-Grant equation114 for high temperature metallic alloys. The range of rupture lifetimes corresponds to a total strain to failure of =3 to =30%. The line for Si3N4 was obtained from Fig. 4.20.

Carburisation Carbon deposition can induce the precipitation of coarse internal carbides that embrittle the Ni-Cr-Mo alloys at low temperature [10], Moreover, the creep strain is strongly affected by carburisation [11, 12]. However, it is worth noting that the creep rupture lifetime of Hastelloy X under carburising helium is not far below that in air [13]. 

Given a creep plot for some material, determine (a) the steady-state creep rate and (b) the rupture lifetime. 

Both temperature and the level of the applied stress influence the creep characteristics (Figure 8.30). At a temperature substantially below 0.4r , and after the initial deformation, the strain is virtually independent of time. With either increasing stress or temperature, the following will be noted (1) the instantaneous strain at the time of stress application increases, (2) the steady-state creep rate increases, and (3) the rupture lifetime decreases. 

Figure 8.29 Typical creep curve of strain versus time at constant load and constant elevated temperature. The minimum creep rate Ae/Af is the slope of the linear segment in the secondary region. Rupture lifetime is the total time to rupture.

The results of creep rupture tests are most commonly presented as the logarithm of stress versus the logarithm of rupture lifetime. Figure 8.31 is one such plot for an S-590 alloy in which a set of linear relationships can be seen to exist at each temperature. For some alloys and over relatively large stress ranges, nonlinearity in these curves is observed. 

How do I Solve Problems Using the Stress vs. Rupture Lifetime Graph ... 

The Larson-MiUer parameter—in terms of temperature and rupture lifetime... 

A typical creep curve (strain versus time) normally exhibits three distinct regions (Fignre 8.29) transient (or primary), steady-state (or secondary), and tertiary. Important design parameters available from such a plot include the steady-state creep rate (slope of the linear region) and rupture lifetime (Figure 8.29). 

An increase in the instantaneons initial deformation An increase in the steady-state creep rate A decrease in the rupture lifetime... 

A cylindrical component constructed from an S-590 alloy (Figure 8.31) is to be exposed to a tensile load of 20,000 N. What minimum diameter is required for it to have a rupture lifetime of at least 100 h at 925°C ... 

O have a creep rupture lifetime of at least 20 days at 650°C (923 K). Compute the maximum allowable stress level. 

O that is subjected to a stress of 55 MPa (8000 psi). At what temperature will the rupture lifetime be 200 h ... 

Hua] Tensile, creep rupture tests Yield strength, creep properties (creep rupture lifetime, grain size, ductility)... 

---