Moderate creep

Butyl rubber - This material generally had the least endurance in fatigue tests, but it may be adequate for some cardiovascular applications. Advantages include less sensitivity to stress concentrators than Pellethane, a very low permeability to fluids, a moderate creep resistance and widespread availability at low cost. Disadvantages include a relatively low fatigue resistance compared to the elastomers specifically designed for these applications. The rubber tested was not designed for medical applications and had standard rubber additives and modifiers that were cytotoxic unless the material was extracted after manufacture. 

The outstanding characteristics of this material are its UL 94 V-0 and 5V flammability rating, heat-deflection temperature of 420 F (216 C) at 66 psi (0.45 MPa), high flexural strength and modulus, and excellent chemical and solvent resistance. PBT resins are especially suitable for applications requiring a combination of high heat endurance, sti ess, chemical resistance, and moderate creep (7) (9). 

This is a nickel-chromium-iron alloy with the addition of silicon. Refer to Table 11.1 for its chemical composition. Type 330 stainless has good strength at elevated temperatures, good thermal stability, and excellent resistance to carburizing and oxidizing atmospheres. It is weldable and machinable. This alloy has been used in low-stress applications to temperatures as high as 2250°F (1230°C) and has moderate creep to 1600°F (870°C). 

Dislocations are known to be responsible for die short-term plastic (nonelastic) properties of substances, which represents departure from die elastic behaviour described by Hooke s law. Their concentration determines, in part, not only dris immediate transport of planes of atoms drrough die solid at moderate temperatures, but also plays a decisive role in die behaviour of metals under long-term stress. In processes which occur slowly over a long period of time such as secondaiy creep, die dislocation distribution cannot be considered geometrically fixed widrin a solid because of die applied suess. 

Graphite will creep imder neutron irradiation and stress at temperatures where thermal creep is normally negligible. The phenomenon of irradiation creep has been widely studied because of its significance to the operation of graphite moderated fission reactors. Indeed, if irradiation induced stresses in graphite moderators could not relax via radiation creep, rapid core disintegration would result. The observed creep strain has traditionally been separated into a primary reversible component ( ,) and a secondary irreversible component (Ej), both proportional to stress and to the appropriate unirradiated elastic compliance (inverse modulus) [69]. The total irradiation-induced creep strain (ej is thus ... 

Material behavior have many classifications. Examples are (1) creep, and relaxation behavior with a primary load environment of high or moderate temperatures (2) fatigue, viscoelastic, and elastic range vibration or impact (3) fluidlike flow, as a solid to a gas, which is a very high velocity or hypervelocity impact and (4) crack propagation and environmental embrittlement, as well as ductile and brittle fractures. 

Creep is the gradual extension of a material under a steady tensile stress, over a prolonged period of time. It is usually only important at high temperatures for instance, with steam and gas turbine blades. For a few materials, notably lead, the rate of creep is significant at moderate temperatures. Lead will creep under its own weight at room temperature and lead linings must be supported at frequent intervals. 

Neat thermoplastic polyethylenes have low moduli that involve high strains for moderate loading. Consequently, creep moduli are also low, the more so as the temperature rises, as we can see in Figures 4.3 ((a) and (b)) where creep moduli are displayed as a function of time, load and temperature. 

Figure 4.3(b) displays the fast decrease of creep moduli when the temperature rises moderately. Designers must be vigilant when computing the wall thickness of a part to be used at ambient temperatures (40°C) during warm weather. 

Thermoplastic neat polybutenes have low moduli involving high strains for moderate loading. Consequently, loading must be limited and creep moduli used for designing must be low, for example ... 

Thermoplastic neat COCs have fair moduli that involve moderate strains for moderate loading. Consequently, creep moduli (see Figure 4.23(a)) are fair at room temperature. When the temperature rises (Figure 4.23(b)) creep moduli decrease. 

Rigid PVCs have average moduli, which limits strains and leads to average creep moduli at room temperature. After plasticization, the very low moduli involve high strains for moderate stresses and low creep moduli even at room temperature. 

Amorphous non-alloyed polystyrenes have fair moduli slowly decreasing with a moderate increase in temperature involving fair creep moduli, for example ... 

In Figure 4.41(b) we can observe the rather moderate difference between engineering moduli, roughly 2 to 3 GPa, and creep modulus, roughly 1.5 to 1.8, after 1000 hr at ambient temperature. 

From Figure 4.47(b) we can observe the difference in creep moduli at room temperature (uppermost curve) and at moderate temperatures of 38°C and above. 

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