Tensile creep curve

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. 

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 . 

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

FIG. 13.47 Small strain tensile creep curves of rigid PVC quenched from 90 °C (i.e. about 10 °C above Tg) to 40 °C and further kept at 40 0.1 °C for a period of 4 years. The different curves were measured for various values of time te elapsed after the quench. The master curve gives the result of a superposition by shifts that were almost horizontal the arrow indicates the shifting direction. The crosses refer to another sample quenched in the same way, but only measured for creep at a te of 1 day. From Struik (1977,1978). Courtesy of the author and of Elsevier Science Publishers. 

A typical tensile creep curve for a particulate reinforced ceramic matrix composite, siliconized silicon carbide (Si/SiC),28 is shown in Fig. 4.1. In comparison to the behavior of metals and metallic alloys, tertiary creep is suppressed in this material. There is only a slight upward curvature of the creep curve prior to failure. In many other ceramic matrix composites, tertiary... 

Fig. 4.1 Tensile creep curves for siliconized silicon carbide (Carborundum KX01). Over most of the data range, these data can be represented by a constant creep rate there is a short primary creep stage, and almost no tertiary creep. The rupture strain decreases with increasing creep rate. The strain to failure, =1.5%, indicates brittle behavior even at low rates of creep detormation. Figure from Ref. 28.

Figure 12.23 Small strain tensile creep curves for poly(vinyl chloride) quenched from 90°C (i.e., above Tg) to 20°C and further aged for different lengths of time. The master curve ( ) was obtained by horizontal displacement of the curves with only slight vertical shifts. (From Ref. 26.)...

For a single lot of material, just the cost of generating single-point data alone is reported to exceed 2600 (Table 11.24), while the cost of generating a multipoint data package comprising tensile stress-strain curves at five temperatures (23°C and four additional temperatures), dynamic modulus versus temperature at 1-Hz frequency, and tensile creep curves at four stress levels at each of the three temperatures (23° C and two elevated temperatures) amount to nearly 9000, as shown in Table 11.25. 

Fig. 9.32 Tensile creep curves of the monolith and nanocomposite at 1200 °C and 50 MPa. Slight accelerated creep and steady-state creep were present in the monolith, while they were little observed in the nanocomposite [23]. With kind permission of John Wiley and Sons...

The curve of the monolith consists of primary, steady-state and very small tertiary creep. The specimen lifetime was 150 h and 4 % of creep strain was obtained at fracture. A large number of microcracks were also identified by optical microscopy. Compared to the monolith, the nanocomposite exhibited excellent creep resistance. At 1200 °C and 50 MPa, its creep hfe was 1120 h, which is 10 times longer than that of the monolith. The creep strain at fracture was 0.5 %, which is eight times smaller than that of the monohth. Furthermore, the superior creep resistance of the nanocomposite was also obtained by flexure creep tests. Similar to tensile-creep curves, the strain of the nanocomposite tended to decrease over time, while the monolith exhibited steady-state creep and sometimes accelerated creep. 

Fig. 10. Small-strain tensile creep curves for ricrirl x, , ...

Figure 3-9. An example of long-term tensile creep curves at 20 C (49 F) for polypropylene (PP) and nylon (N). The numbers in parentheses refer to the stress level, in MPa.

Figure 3-41. Tensile creep curves for three thermoplastics.

Figure 3-54. An example of tensile creep curves in the direction of maximum fiber orientation, a) A TS polyester RP having 56 percent E-glass, by weight b) glass-fabric/TS polyester RP in 48 percent glass by weight.

Figure 2-23. Tensile creep curve. (Courtesy of Ticona.)...

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