Creep rates

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. 

Fig. 18. Steady-state creep rate as a function of appHed stress for silver matrix (0) and tungsten fiber—silver matrix composites (A) at 600°C. To convert...

Poly(ethyl methacrylate) (PEMA) yields truly compatible blends with poly(vinyl acetate) up to 20% PEMA concentration (133). Synergistic improvement in material properties was observed. Poly(ethylene oxide) forms compatible homogeneous blends with poly(vinyl acetate) (134). The T of the blends and the crystaUizabiUty of the PEO depend on the composition. The miscibility window of poly(vinyl acetate) and its copolymers with alkyl acrylates can be broadened through the incorporation of acryUc acid as a third component (135). A description of compatible and incompatible blends of poly(vinyl acetate) and other copolymers has been compiled (136). Blends of poly(vinyl acetate) copolymers with urethanes can provide improved heat resistance to the product providing reduced creep rates in adhesives used for vinyl laminating (137). 

Density,g/mL T emperature, K StressJVIPa a Creep rate,s Deformation,p.m Test duration,10 s... 

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. 

The dependence of creep rate on applied stress a is due to the climb force the higher CT, the higher the climb force jb tan 0, the more dislocations become unlocked per second, the more dislocations glide per second, and the higher is the strain rate. 

Steady-state creep rate (s ), for ai applied tensile stress cr of 200iVINm- ... 

Like metals, ceramics creep when they are hot. The creep curve (Fig. 17.4) is just like that for a metal (see Book 1, Chapter 17). During primary creep, the strain-rate decreases with time, tending towards the steady state creep rate... 

Here o is the stress, A and n are creep constants and Q is the activation energy for creep. Most engineering design against creep is based on this equation. Finally, the creep rate accelerates again into tertiary creep and fracture. 

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... 

Polymers are a little more complicated. The drop in modulus (like the increase in creep rate) is caused by the increased ease with which molecules can slip past each other. In metals, which have a crystal structure, this reflects the increasing number of vacancies and the increased rate at which atoms jump into them. In polymers, which are amorphous, it reflects the increase in free volume which gives an increase in the rate of reptation. Then the shift factor is given, not by eqn. (23.11) but by... 

However, world production is only about 55 000 tonnes per annum and this is a reflection of the high volume cost, the rather specialised techniques involving lengthy processing times and to a smaller extent the high creep rate under load. 

From the creep curves for a particular plastic the following values of creep rate at various stress levels were recorded for times between 10 and 10 seconds ... 

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