Temperature creep rate

Stress Distribution and High Temperature Creep Rate of Discontinuous Fiber Reinforced Metals, Acta Metallurgies et Materialia, 38, 1941-1953 (1990). 26. A. G. Evans, J. W. Hutchinson, and R. M. McMeeking, Stress-Strain Behavior of Metal Matrix Composites with Discontinuous Reinforcements, Scripta Metallurgica et Materialia, 25, 3-8 (1991). 

High-temperature creep rate (during irradiation) ... 

Code Section Notes Minimum Specified Sttength at Temperature Minimum Specified Strength at Temperature Creep Rate of 1000 hrs Average Stress to Rupture in 100,000 hr Average Minimum ... 

Boltzmann s constant, and T is tempeiatuie in kelvin. In general, the creep resistance of metal is improved by the incorporation of ceramic reinforcements. The steady-state creep rate as a function of appHed stress for silver matrix and tungsten fiber—silver matrix composites at 600°C is an example (Fig. 18) (52). The modeling of creep behavior of MMCs is compHcated because in the temperature regime where the metal matrix may be creeping, the ceramic reinforcement is likely to be deforming elastically. 

Creep Resistsince. Studies on creep resistance of particulate reinforced composites seem to indicate that such composites are less creep resistant than are monolithic matrices. Silicon nitride reinforced with 40 vol % TiN has been found to have a higher creep rate and a reduced creep strength compared to that of unreinforced silicon nitride. Further reduction in properties have been observed with an increase in the volume fraction of particles and a decrease in the particle size (20). Similar results have been found for SiC particulate reinforced silicon nitride (64). Poor creep behavior has been attributed to the presence of glassy phases in the composite, and removal of these from the microstmcture may improve the high temperature mechanical properties (64). 

Vessels for high-temperature serviee may be beyond the temperature hmits of the stress tables in the ASME Codes. Sec tion TII, Division 1, makes provision for construction of pressure vessels up to 650°C (1200°F) for carbon and low-alloy steel and up to 815°C (1500°F) for stainless steels (300 series). If a vessel is required for temperatures above these values and above 103 kPa (15 Ibf/in"), it would be necessaiy, in a code state, to get permission from the state authorities to build it as a special project. Above 815°C (1500°F), even the 300 series stainless steels are weak, and creep rates increase rapidly. If the metal which resists the pressure operates at these temperatures, the vessel pressure and size will be limited. The vessel must also be expendable because its life will be short. Long exposure to high temperature may cause the metal to deteriorate and become brittle. Sometimes, however, economics favor this type of operation. 

Here R is the Universal Gas Constant (8.31 Jmol K ) and Q is called the Activation Energy for Creep - it has units of Jmol . Note that the creep rate increases exponentially with temperature (Fig. 17.6, inset). An increase in temperature of 20 C can double the creep rate. 

A well-known example of this time-temperature equivalence is the steady-state creep of a crystalline metal or ceramic, where it follows immediately from the kinetics of thermal activation (Chapter 6). At a constant stress o the creep rate varies with temperature as... 

Viscoelastic creep data are usually presented in one of two ways. In the first, the total strain experienced by the material under the applied stress is plotted as a function of time. Families of such curves may be presented at each temperature of interest, each curve representing the creep behavior of the material at a different level of applied stress. Below a critical stress, viscoelastic materials may exhibit linear viscoelasticity that is, the total strain at a given time is proportional to the applied stress. Above this critical stress, the creep rate becomes disproportionately faster. In the second, the apparent creep modulus is plotted as a function of time. 

The rate of creep and stress relaxation of TPs increases considerably with temperature those of the TSs (thermoset plastics) remain relatively unaffected up to fairly high temperatures. The rate of viscoelastic creep and stress relaxation at a given temperature may also vary significantly from one TP to an-... 

Recently, a method for predicting the remanent life of a reinforcing geotextile was proposed [1] in which the strain to failure of a sacrificial sample was divided by the current creep rate. This requires verification. However, very few methods have so far been proposed or used for monitoring plastics in service and at the same time providing a numerical prediction of their remaining life. The reason for this is not just that the methods are likely to be expensive and complicated, but that there are few applications of plastics which can compete in risk and replacement cost with a high temperature boiler or aircraft structure. 

Figure 12 Ratio of flux creep rate S(T) to the magnetization at 1 second MQ(T) versus temperature. S and MQ are obtained by fitting data such as those in Figure 11 using Eq. (13). The slope of the dashed line corresponds to an effective flux pinning potential U = 83 meV according to a simple thermally activated flux creep model which yields Eq. (14).

A typical creep experiment involves measuring the extent of deformation, called the creep strain, e, over extended periods of time, on the order of thousands of hours, under constant tensile loads and temperature. The resulting plot of creep strain versus time (Figure 5.43) shows the resulting creep rate, e = dejdt, which is the slope of the... 

In Cooperative Learning Exercise 5.9, you studied the effect of temperature on the creep rate of an engine turbine blade. Calculate the viscosity in poise that can be assigned to this material at 500°C under a stress of 20,000 psi. 

---