Increasing the creep resistance

Both damage mechanisms cause a decrease of the effective cross section of the specimen and stress concentrations by notch effects. This is the reason for the rapid increase in the strain rate observed in tertiary creep. 

Materials heavily loaded under creep conditions must meet particular requirements. 

We can conclude from the discussion of the previous sections on mechanisms that a large activation energy of vacancy diffusion is advantageous because vacancy diffusion is important in almost all of the mechanisms discussed. Vacancy diffusion is weak if the formation of a vacancy is difficult and if the diffusion of any formed vacancy is impeded. The enthalpy of vacancy formation is correlated with the binding forces in the material and thus with the melting temperature. Therefore, the homologous temperature T/Tm can be used as parameter to characterise the creep properties. 

Vacancy diffusion occurs by the exchange of a vacancy with its neighbouring atom. The atom has to overcome an energy barrier formed by the surrounding atoms. This barrier is the higher, the closer packed the atoms are close-packed structures are thus more creep resistant. For example, the diffusion coefficient for self-diffusion of iron is 

Similar to time-independent plastic deformation, creep deformation in metals is dominated by dislocation movement, especially at higher stresses. Mechanisms that impede dislocation movement are thus also important in producing creep -resistant materials. However, these mechanisms have to be temperature resistant. 

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In this section we review experimental observations on the creep of ceramic matrix composites. Observations that apply to all ceramic matrix composites are discussed. Creep curves obtained on ceramic matrix composites are compared with curves obtained on metals and metallic alloys. The role of a second phase in increasing the creep resistance of composites is emphasized. Finally, a discussion of creep asymmetry is presented, wherein creep occurs more easily in tension than in compression. 

Adding ceramic whiskers in volume fractions above the percolation threshold has been found to improve creep resistance, often increasing the creep resistance by two orders of magnitude. One would expect a similar effect with fibers but, in some cases, the fibers have such a small grain size (for high strength) that they can show very poor creep resistance. Other important factors that can affect the creep rate of a material are composition, stoichiometry, defect density and environment, often through their dependence on diffusivity. 

General. Polymer resins generally exhibit creep and relaxation behaviour. The addition of fibres increases the creep resistance of the resins. Consequently, creep and relaxation behaviour are more pronounced when a load is applied transversely to fibres or when the composite has a low fibre volume fraction (Karbhari et aL 2003). 

To increase the creep resistance alumina fibres, intergranular silicate phases have to be reduced drastically. This imposes processes other than the addition of silica to control ot-alumina growth. A pure ci-alumina fibre was first produced by Du Pont in 1979 (Dhingra, 1980). Fiber FP was obtained by the addition, to an alumina precursor. 

A flexible a-alumina-zirconia fibre, Nextel 650 fibre, has been recently developed by 3M, with the aim of increasing the creep resistance with respect to that of the a-alumina Nextel 610 fibre. Its smaller diameter and grain sizes, when compared to PRD 166 fibre, give rise to a flexible fibre and 3M reports high strength (2.5 GPa at 25 mm). As... 

Low calcium alloys have been introduced during recent years. Their main advantage is reduced corrosion attack, and thereby reduced grid growth. Silver addition is claimed to increase the creep resistance and improve the corrosion behavior (42). 

The integration of various functional fillers is a traditional route to achieve highly wear-resistant polymer composites. In order to reduce the adhesion of polymers to metallic counterparts, internal lubricants, such as PXra powders and graphite flakes, are frequently incorporated. Short aramid (AF), glass (GF) or carbon (CF) fibers are used to increase the creep resistance and the compressive strength of the polymer matrix. Not so many efforts have been undertaken so far in the development of such composites by integrating inorganic particles with conventional fillers. 

The crosslinking, irradiation, increases the creep resistance of UHMWPE. This was observed by Bhateja and Andrews [148], as shown in Figure 7.13. 

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