Creep, tensile

Creep experiments are often carried out by hanging a weight on a strip of material of rectangular or cylindrical cross section (ref. 4, page 132 and 162). In this case, the tensile creep compliance D t) is calculated as 

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Creep Rupture. The results from creep mpture tests on tubes under internal pressure at elevated temperatures (71,72) may be correlated by equation 16, in which is replaced by the tensile creep mpture stress after time t at temperature T. 

ISO 899 Determination of Tensile Creep of Plastic, ISO, Geneva, Swit2erland, 1981. 

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. 

Figure 9.11. Recovery from tensile creep of an acetal copolymer at 20°C and 65% relative humidity. (From TCI Technical Service Note G117, reproduced by permission of ICI Plastics Division )...

If only one type of data is available (e.g. tensile creep curves) then it is possible to make conversions to the other test modes. It should always be remembered, however, that these may not always be absolutely accurate for plastics under all situations. 

It should also be noted that in this case the material was loaded in compre-sion whereas the tensile creep curves were used. The vast majority of creep data which is available is for tensile loading mainly because this is the simplest and most convenient test method. However, it should not be forgotten that the material will behave differently under other modes of deformation. In compression the material deforms less than in tension although the efrect is small for strains up to 0.5%. If no compression data is available then the use of tensile data is permissible because the lower modulus in the latter case will provide a conservative design. 

An underground polypropylene storage tank is a sphere of diameter 1.4 m. If it is to be designed to resist an external pressure of 20 kN/m for at least 3 years, estimate a suitable value for the wall thickness. Tensile creep data may be used and the density of the polypropylene is 904 kg/m. ... 

A rod of polypropylene, 10 mm in diameter, is clamped between two rigid fixed supports so that there is no stress in the rod at 20°C. If the assembly is then heated quickly to 60°C estimate the initial force on the supports and the force after 1 year. The tensile creep curves should be used and the effect of temperature may be allowed for by making a 56% shift in the creep curves at short times and a 40% shift at long times. The coefficient of thermal expansion for polypropylene is 1.35 x 10 °C in this temperature range. 

When a pipe fitting is tightened up to a 12 mm diameter polypropylene pipe at 20°C the diameter of the pipe is reduced by 0.05 mm. Calculate the stress in the wall of the pipe after 1 year and if the inside diameter of the pipe is 9 mm, comment on whether or not you would expect the pipe to leak after this time. State the minimum temperature at which the fitting could be used. Use the tensile creep curves and take the coefficient of thermal expansion of the polypropylene to be 9.0 X 10- °C . 

Note < 0.5% so no correction to tensile creep data is needed). (2.10) Critical Stress,... 

Although the creep behavior of a material could be measured in any mode, such experiments are most often run in tension or flexure. In the first, a test specimen is subjected to a constant tensile load and its elongation is measured as a function of time. After a sufficiently long period of time, the specimen will fracture that is a phenomenon called tensile creep failure. In general, the higher the applied tensile stress, the shorter the time and the greater the total strain to specimen failure. Furthermore, as the stress level decreases, the fracture mode changes from ductile to brittle. With flexural, a test specimen... 

Fig. 2-34 Tensile creep and recovery during the intermittent loading.

Fig. 2-39 Tensile-creep behavior of PP top on semilog scale and bottom on log-log scale.

Fig. 7-3 Example of long-term tensile creep curves.

For tensile creep, TJ would be the tensile viscosity. When the viscosity is high (e.g., when working at relatively low temperatures or with very high-molecular-weight polymers) it can be difficult to determine tl-x accurately, so creep recovery measurements are made. Here the load is released after a given creep time and the strain is followed as the specimen shrinks back toward its new equilibrium dimensions. 

For elastomers, factorizability holds out to large strains (57,58). For glassy and crystalline polymers the data confirm what would be expected from stress relaxation—beyond the linear range the creep depends on the stress level. In some cases, factorizability holds over only limited ranges of stress or time scale. One way of describing this nonlinear behavior in uniaxial tensile creep, especially for high modulus/low creep polymers, is by a power... 

ISO 899-1, Plastics - Determination of creep behaviour - Part 1 Tensile creep, 1993. 

ISO 899-1 2003 Plastics - Determination of creep behaviour - Part 1 Tensile creep ISO 899-2 2003 Plastics - Determination of creep behaviour - Part 2 Flexural creep by three-point loading... 

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... 

Capillary pipette Falling sphere Parallel plate Falling coaxial cylinder Stress relaxation Rotating cylinder Tensile creep... 

Nemoto,N. Viscoelastic properties of narrow-distribution polymers. II. Tensile creep studies of polystyrene. Polymer J. (Japan) 1,485-492 (1970). 

Kaufman, J.G. Properties of Aluminum Alloys Tensile, Creep, and Fatigue Data at High and Low Temperatures, ASM International, Materials Park, OH, 1999. Kaufman, J.G. Introduction to Aluminum Alloys and Tempers, ASM International, Materials Park, OH. 2000. 

Such tests, while they constitute an adequate basis for routine evaluation of plasticizers, furnish only a rudimentary picture of the elastic properties of the plasticized resin. More complete studies supply valuable information. For example, tensile creep tests have shown that polyvinyl chlonde resin plasticized with uioctyl phosphate will deform more in response to stresses of short duration than will resin plasticized with tncresyl phosphate the reverse is true for stresses of long duration. 

The next two tests falling into this category are tensile creep and stress relaxation. In tensile creep a load is applied instantaneously to the specimen at zero time and the extension monitored as a function of time. In stress relaxation an extension is imposed and the load monitored as a function of time. 

All these tests are in common use to measure the tensile stiffness of polymers. For example, tests at constant extension rate are often carried out on an Instron tensile testing machine. Tensile creep is used in many cases while stress relaxation is not so common. Dynamic testing is commonly performed using the Rheovibron or other commercial equipment32 or home made equipment33,... 

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