Creep types

Polymer processing flows are always laminar and generally creeping type flows. A creeping flow is one in which viscous forces predominate over forces of inertia and acceleration. Classic examples of such flows include those treated by the hydrodynamic theory of lubrication. For these types of flows, the second term on the left-hand side of Eq. 2.5-18 vanishes, and the Equation of motion reduces to ... 

Figure 2.8 Schematics of a creep-type test for a yield stress material...

This section will examine some of the characteristic features of IPN s from a physical and mechanical point of view. Emphasis will be on relating the glass transition behavior to corresponding aspects of morphology. The principal instrumentation employed in the studies discussed here includes a torsional tester for creep-type studies (Section 8.3.1) and a fixed-frequency vibrating unit for dynamic mechanical spectroscopy (see Section 8.3.2). In addition, stress-strain, tensile, and Charpy impact strength values will be briefly discussed. 

Example 5 In the case of a heavy-walled cylinder under internal pressure (see Fig. 3-61). Using methods similar to those used for the thin-walled cylinder, the creep type of radial expansion for a heavy-walled cylinder can be shown to be... 

Since bonded joints can often undergo large displacements, especially when subjected to creep-type loading, the geometrically nonlinear formulation described in References 37 and 38 is used to implement the nonlinear viscoelastic model. The principle of virtual work, in the updated Lagrangian incremental formulation, can be stated as... 

Some spirochaetes exhibit a creeping motihty. If the organism approaches a soUd surface the external viscous shear will be largest when the cell and the surface are closest. If the protoplasmic cylinder is long and irregular, it may not be fi-ee to rotate, and the roll of the exterior layers cause the cell to slide in a direction nearly parallel to the local helical axis, i.e. producing a creeping-type motility. 

Reduction of wind velocity at the fill surface and protection of facilities by strategic placement of wind screens. Given the fact that the majority of the soil movement as a result of wind erosion occurs within one metre of the ground, artificial screens, rows of natural willow twigs and/or other natural materials of limited height may be particularly useful to mitigate the saltation and creep types of soil transport. 

The purpose of these comparisons is simply to point out how complete the parallel is between the Rouse molecular model and the mechanical models we discussed earlier. While the summations in the stress relaxation and creep expressions were included to give better agreement with experiment, the summations in the Rouse theory arise naturally from a consideration of different modes of vibration. It should be noted that all of these modes are overtones of the same fundamental and do not arise from considering different relaxation processes. As we have noted before, different types of encumbrance have different effects on the displacement of the molecules. The mechanical models correct for this in a way the simple Rouse model does not. Allowing for more than one value of f, along the lines of Example 3.7, is one of the ways the Rouse theory has been modified to generate two sets of Tp values. The results of this development are comparable to summing multiple effects in the mechanical models. In all cases the more elaborate expressions describe experimental results better. 

Creep. The creep characteristic of plastic foams must be considered when they are used in stmctural appHcations. Creep is the change in dimensions of a material when it is maintained under a constant stress. Data on the deformation of polystyrene foam under various static loads have been compiled (158). There are two types of creep in this material short-term and long-term. Short-term creep exists in foams at all stress levels however, a threshold stress level exists below which there is no detectable long-term creep. The minimum load required to cause long-term creep in molded polystyrene foam varies with density ranging from 50 kPa (7.3 psi) for foam density 16 kg/m (1 lb /ft ) to 455 kPa (66 psi) at foam density 160 kg/m (10... 

Depending on the type of waving product used, there may be several variations to the procedure outlined above. Thus, instead of wrapping with lotion, the hair is wound wet and the lotion appHed to curled hair. Some instmctions also suggest a creep stage for better tightness and durabiUty. This is simply a 30 min wait between rinsing off the lotion and appHcation of the neutralizer. 

In concrete, triethanolamine accelerates set time and increases early set strength (41—43). These ate often formulated as admixtures (44), for later addition to the concrete mixtures. Compared to calcium chloride, another common set accelerator, triethanolamine is less corrosive to steel-reinforcing materials, and gives a concrete that is more resistant to creep under stress (45). Triethanolamine can also neutralize any acid in the concrete and forms a salt with chlorides. Improvement of mechanical properties, whiteness, and more even distribution of iron impurities in the mixture of portland cements, can be effected by addition of 2% triethanolamine (46). Triethanolamine bottoms and alkanolamine soaps can also be used in these type appUcations. Waterproofing or sealing concrete can be accompUshed by using formulations containing triethanolamine (47,48). 

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. 

In the constant-strain method, the specimen is stretched or bent to a fixed position at the start of the test. The most common shape of the specimens used for constant-strain testing is the U-beud, hairpin, or horseshoe type. A bolt is placed through holes in the legs of the specimen, and it is loaded by tightening a nut on the bolt. In some cases, the stress may be reduced during the test as a result of creep. In the constant-load test the specimen is supported horizontally at each end... 

For outdoor installations the recommended minimum creep distances for all types of voltage or current transformers are given in Table 15.1, according to lEC 60044-1 or lEC 60044-2. 

In design against creep, we seek the material and the shape which will carry the design loads, without failure, for the design life at the design temperature. The meaning of failure depends on the application. We distinguish four types of failure, illustrated in Fig. 17.3. 

Furnace tubes, piping, and exchanger tubing with metal temperatures above 800°F now tend to be an austenitic stainless steel, e.g., Type 304, 321, and 347, although the chromium-molybdenum steels are still used extensively. The stainless steels are favored beeause not only are their creep and stress-rupture properties superior at temperatures over 900°F, but more importantly because of their vastly superior resistance to high-temperature sulfide corrosion and oxidation. Where corrosion is not a significant factor, e.g., steam generation, the low alloys, and in some applications, carbon steel may be used. 

Type of stress. A uniaxial tensile creep test would not be expected to give the required data if the designer was concerned with torsional or compressive creep. 

Polymers of this type have exceptional good values of strength, stiffness and creep resistance (see Table 18.13). After 100 h at 23°C and a tensile load of 70 MPa the creep modulus drops only from 4200 to 3(K)0 MPa whilst at a tensile load of 105 MPa the corresponding figures are 3500 and 2500 MPa respectively. If the test temperature is raised to 150°C the creep modulus for a tensile load of 70 MPa drops from 2400 to 1700 MPa in 100 h. 

The differences between the main types of polysulphone are quite small. The polyethersulphones (Type III in Table 21.3) have markedly better creep resistance at elevated temperatures, e.g. 150°C, significantly higher heat distortion temperatures and marginally superior room temperature meehanical properties than the Type II materials. They also exhibit higher water absotption, dielectric constant and specific gravity. 

The demands on the PSA performance for these types of applications can be very high, especially in the areas of creep, shear resistance and long-term durability. For these reasons, crosslinked acrylics and stabilized block copolymer adhesives have become the main PSA material choices. 

Laminated beams (glulam), parallam (or LSL) and fingerjoints a flat pressed multilayer wood beam, thiek wood planks constituting the layers, used for structural exterior applications and bonded with PRF (phenol-resorcinol-formaldehyde) cold-setting resins, or MUF cold-setting resins, or even with certain types of polurethanes (although the use of these latter ones is only established in one country and can show creep and temperature-induced creep problems). The indi-... 

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