Properties creep

The creep test is a standard test to determine whether a polymer is a solid. If the polymer exhibits solid behavior, the creep strain reaches an asymptote. If the polymer 

FIGURE 2.7 Standard creep test, (a) Stress, (b) creep, solid behavior, (c) creep, fluid behavior, and (d) creep as a function of stress. 

FIGURE 2.8 Master curve of creep compliance J(t) for a linear viscoelastic polymer. 

The configuration of the creep test specimen is usually made the same as the standard tensile test specimen for static testing. Creep data are generally determined in tension, but creep strains can be measured in compression, shear, or bending also. 

---

In the last chapter we saw how a basic knowledge of the mechanisms of creep was an important aid to the development of materials with good creep properties. An impressive example is in the development of materials for the high-pressure stage of a modern aircraft gas turbine. Here we examine the properties such materials must have, the way in which the present generation of materials has evolved, and the likely direction of their future development. 

Let us first examine the development of turbine-blade materials to meet the challenge of increasing engine temperatures. Although so far we have been stressing the need for excellent creep properties, a turbine-blade alloy must satisfy other criteria too. They are listed in Table 20.1. 

Figure 20.6 shows how this evolutionary process has resulted in a continual improvement of creep properties of nickel alloys over the last 30 years, and shows how... 

One must consider, too, the cost of the materials themselves. Some of the metals used in conventional nickel alloys - such as hafnium - are hideously expensive (at UK 100,000 tonne or US 150,000 tonne ) and extremely scarce and the use of greater and greater quantities of exotic materials in an attempt to improve the creep properties will drive the cost of blades up. But expensive though it is, the cost of the... 

You may be wondering why we did not mention the pure refractory metals Nb, Ta, Mo, W in our chapter on turbine-blade materials (although we did show one of them on Fig. 20.7). These metals have very high melting temperatures, as shown, and should therefore have very good creep properties. 

Austenitic steels have a number of advantages over their ferritic cousins. They are tougher and more ductile. They can be formed more easily by stretching or deep drawing. Because diffusion is slower in f.c.c. iron than in b.c.c. iron, they have better creep properties. And they are non-magnetic, which makes them ideal for instruments like electron microscopes and mass spectrometers. But one drawback is that austenitic steels work harden very rapidly, which makes them rather difficult to machine. 

The trailing edge of the strut develops the highest creep strain. This strain occurs despite the sharp stress relaxation at the trailing edge projection. The creep strain in the strut is well balanced. Transpiration cooling requires a material of porous mesh resistant to oxidation at a temperature of 1600°F (871.1 °C) or more. Otherwise, the superior creep properties of this design... 

Table 3.10. Alloying Effects that Improve Creep Properties [14]...

Alloying effects that improve creep properties.67... 

Throne has reported that the relationship between foam modulus and density can be generalised to other properties such as tensile strength, fatigue strength, creep properties as well as shear and compression modulus. Thus if X is the general material property then... 

Not only do the creep properties of crystalline polymers change rapidly with temperature, but in some cases at a given temperature a crystalline type will creep more with time than will the rigid amorphous or cross-linked (TS) types. However, a crystalline type above its Tg creeps very little, compared to the others. Thus, crystalline types tend to... 

Creep guideline. Here is a summation of the factors to consider when reviewing creep properties ... 

H. Leaderman. Elastic and Creep Properties of Filamentous Materials and Other High Polymers, Textile Foundation, Washington, D.C., 1943. 

The use of soluble zinc soap activators such as zinc 2-ethyl hexanoate instead of conventional stearic acid gives efficiency of vulcanisation and ensures that stress relaxation and creep properties are optimised. Zinc soaps, including the new high efficiency activating types, do not bloom from the compound, either during processing or subsequently during service. 

Time of flight (TOF), 75 660-661 Time-of-flight (ToF) mass analyzers, 24 109 Time of flight diffraction (TOFD), 79 486 Time-of-flight instrumentation, in particle counting, 78 150—151 Time-of-flight-SIMS technique, 24 109 Time-resolved fluorimetry, 74 148-149 Time-resolved spectra, analysis of, 74 613 Time standards, 75 749—750 Time-temperature parameters (TTP), 73 471, 478, 479 creep properties and, 73 480 Time-temperature superposition, 27 746-747... 

Norimoto, M., Gril, J. and Rowell, R.M. (1992). Rheological creep properties of chemically modified wood. Relationship between dimensional and creep stabihty. Wood and Fiber Science, 24( 10), 25-35. 

Yasuda, R., Minato, K. and Norimoto, M. (1994). Chemical modification of wood by nonformaldehyde cross-linking agents. Part 2. Moisture sorption and creep properties. Wood Science and Techrwlogy, 28(3), 209-218. 

Denning s three papers in the late 1960s (a.2-a.4) reviewed the development of closed-cell polyolefin foams, and their mechanical properties. Some of his predictions on materials development turned out to be true. In Part I he explains that non-crosslinked polyethylene (PE) foams have inferior creep properties to crosslinked foams this appeared to be the tensile creep of the melt, rather than compressive creep of the... 

The creep properties of ceramics are of particular importance, especially at high temperatures. In general, the principles of Section 5.2.2 still apply, but the ceramic... 

A power-law expression of the form of Eq. (5.115) adequately describes the creep properties of SMC composites. The creep response of the SMC-R50 composite at various temperatures shown in Figure 5.105 is representative of this behavior. Among the material variables, fiber content once again has the greatest influence on the creep strain. At a given temperature and stress level, creep strain is higher if the fiber content is reduced. 

Elastic recovery or resilience is the recoveiy of length upon release of stress after e xtension or compression. A fiber, fabric, or carpet must possess this property in order to spring back to its original shape after being crushed or wrinkled, Polyolefin fibers have poorer resilience than nylon this is thought to be partially related to the creep properties of the polyolefins. 

Creep Properties. The creep properties, determined at 100 °C under a constant load of 50 kg/cm2, of some CPVC types A, B, and D and of blends (designated E) obtained by mixing 5, 10, and 15% by weight PVC with CPVC type D, are shown in Figure 11 (see Table V). Similar results were obtained for the measurements carried out at 80 °C under a constant load of 100 kg/cm2 (Figure 12). 

All the chlorination processes in which the swelling agent is present give products with similar physical properties, whereas the materials obtained by process without a swelling agent show a lower thermal stability, lower Vicat, poorer creep properties, and little improvement in impact behavior. 

---