Rupture data

Certain environments reduce rupture performance with particular materials (e.g., polyethylene in contact with detergents), and allowance must be made for this, where necessary, by the incorporation of an additional safety factor to the rupture stress. In general, the resistance to rupture will be greater if the stress is applied in compression than in tension. 

FIGURE 2.7 Isochronous stress-strain curves of various grades of LDPE and HDPE at 23°C (1 h data). 

Differing stress levels have been used for high-density polyethylene and low-density polyethylene for the reasons previously discussed, so an easier comparison between grades can be made by the use of isochronous stress-strain curves. 

For polyethylene, the principal factor controlling creep is density, but the principal factor influencing rupture behavior is the melt index. Thus, for a given density, for long-term applications, the higher the melt index, the lower will be the maximum permissible strain. This is particularly important at elevated temperatures. 

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Stress-rupture data are often presented in a Larson-Miller eurve, whieh indieates the performanee of an alloy in a eomplete and eompaet graphieal style. While widely used to deseribe an alloy s stress-rupture eharaeteristies over a wide temperature, life, and stress range, it is also useful in eomparing the elevated temperature eapabilities of many alloys. The Larson-Miller parameter is... 

Isometric data from the creep curves may also be superimposed on the creep rupture data in order to give an indication of the magnitudes of the strains involved. Most plastics behave in a ductile manner under the action of a steady load. The most notable exceptions are polystyrene, injection moulding grade acrylic and glass-filled nylon. However, even those materials which are ductile at short times tend to become embrittled at long times. This can cause... 

This represents the locus of all the combinations of Ca and Om which cause fatigue failure in a particular number of cycles, N. For plastics the picture is slightly different from that observed in metals. Over the region WX the behaviour is similar in that as the mean stress increases, the stress amplitude must be decreased to cause failure in the same number of cycles. Over the region YZ, however, the mean stress is so large that creep rupture failures are dominant. Point Z may be obtained from creep rupture data at a time equal to that necessary to give (V cycles at the test frequency. It should be realised that, depending on the level of mean stress, different phenomena may be the cause of failure. 

Creep rupture. Creep-rupture data are obtained in the same way as creep data except that higher stresses are used and the time is measured to failure (Figs. 2-28 and 29). The strains are sometimes recorded, but this is not necessary for creep rupture. The results are generally plotted as the log stress versus log time to failure (110). In creep-rupture tests it is the material s behavior just prior to the rupture that is of primary interest. In these tests a number of samples are subjected to different levels of constant stress, with the time to failure being determined for each stress level. General technical literature and product data sheets seldom provide a complete description of a material s behavior prior to rupture. It should include the development of any crazing and stress whitening, its strain-time... 

Fig. 2-36 Stress-rupture data for rigid 2 in. diameter PVC pipe as a function of temperature.

There is generally a less-pronounced curvature when creep and relaxation data are plotted log-log. Tliis facilitates extrapolation and is commonly practiced, particularly with creep modulus and creep-rupture data. 

Manson, S. S., et al., A linear Time-Temperature Relation of Creep Stress Rupture Data, NACA, TN 2890,1953. 

ISO/DIS 9080, Thermoplastics pipes for the transport of fluids - Methods of extrapolation of hydrostatic stress rupture data to determine the long-term hydrostatic strength of thermoplastics pipe materials, 1992. 

In Fig. 4.21, creep rupture data from a number of different grades of silicon nitride are plotted in a Monkman-Grant format.30,31,34,115 116 For purposes of comparison with metallic alloys, the temperature dependence of the Monkman-Grant curves has been ignored. As with the metallic alloys, the curves for all of the grades of material tend to plot within a relatively narrow band. These results imply that lifetime can be improved merely by improving creep rate the lower the creep rate, the longer the lifetime. 

The creep rupture data are derived from laboratory tests on material samples having a standardized geometrical form. The convenient way to express these data is to plot the stress versus the Larson-Miller parameter P ... 

Figure 12.14 Summary of a large number of tensile rupture data collected from various sources for Si3N4 plotted to verify the Monkman-Grant equation.

To construct such a map (see Worked Example 12.4), SCG and creep rupture data must be known for various temperatures. The temperature dependence of the stress levels required to result in a given lifetime are then calculated from Eqs. (12.45) and (12.46). The mechanism that results in the lowest failure stress at a given temperature thus defines the threshold stress or highest applicable stress for the survival of a part for a given time. In other words, the lifetime of the part is determined by the fastest possible path. Such maps are best understood by actually plotting them. 

HDPE has an enormous range of ESC resistance. Figure 10.21 shows creep rupture data for blow moulding and pipe grades under extreme conditions, of 80 °C in 2% Arkopal 110 surfactant solution, which allow the reasonably rapid selection of promising candidate materials. The samples have a 10 X 10 mm cross section, with a circumferential, 1.6 mm deep, razor blade notch. The samples with the longest lives have more tie links between the crystal lamellae. 

Creep rupture data, for a range of circumferentially notched HDPEs. The top three graphs are for 5%, and the rest for 2% Arcopal I 10 surfactant (Fleissner, M., Polym. Eng. ScL, 38, 330, 1998) John Wiley and Sons Inc. reprinted with permission. 

Creep rupture data for polyethylene gas pipes, made from Solvay Eltex TVB 121 at a range of temperatures (Bocker, H. et o/., Kunststoffe, 82, 739, 1992). 

Fig. 14.2 shows creep rupture data for a particular MDPE, for ductile failure at a range of temperatures. An Arrhenius plot, of the logarithm of the creep rupture time versus the reciprocal of the absolute test temperature, is usually a straight line graph. This can be used to estimate the creep rupture times at lower temperatures. If there is ductile failure in the higher temperature tests, it is unlikely that brittle failure will occur at long times at low temperatures. 

Creep rupture data for siliconized silicon carbide were fitted to the Monkman-Grant equation to give p= 1.45. For example, for a strain rate of 2x10 /s the failure time is 100 hours. Determine the steady-state strain rate that would give a failure time of 10000 hours. Discuss the problems that might be associated with such an extrapolation. Describe briefly the creep failure observed in this material (need to check references for this chapter). 

Fig. 18. Creep rupture data for specimens cut at various angles, B. to the fibre axis of an oriented PMMA sheet (birefringence 0-0006). ...

Where to, A17, t) are constants for a given fibre, applied stress and Tis the absolute temperature. Zhurkov achieved an excellent correlation between AU calculated from such stress-rupture data and Al/ , the activation energy for thermal degradation for a wide range of fibres. It was also possible to identify the free radicals and degradation products formed... 

TABLE 4. Summary of Creep and Creep Rupture Data for Ptepreg HiPerComp ... 

Creep rupture data for glass fibre composites in acid are less reliable than creen data as the si7e and shane of the snecimen nhviniislv influence time... 

Fig. 1. Schematic stress rupture data, illustrating the effect of temperature on time to failure...

Tables 13.3-13.5 provide a summary of regression analyses of P7-20FG-0600, P7-30FG-0600, and P7-40FG-0600, respectively. Table 13.6 summarizes the values of the three coefficients Al, A2, and A3 for the three composite materials. The regression analysis statistics indicate a very good fit of observed creep rupture data to Eq. (13.6).

Lyons [142] obtained creep rupture data and tensile behaviour of glass filled PA and polyphthalamides at temperatures between 23-150 °C. A polyphthalamide with... 

There are several design methods utilized in the industry for analyzing creep problems. Three of the most prevalent are the Larson-MiUer relation, the Nor-ton-Bailey power law, and the MFC Omega method. The Larson-MiUer relation attempts to extrapolate creep rupture data from experimental results. The Norton-... 

Fig. 6.94 Stress rupture data of Fig. 6.79 replotted including the high stress flexure data for ARCO alumina [74]. With kind permission of John Wiley and Sons...

Creep and creep-rupture data may be presented as curves in different ways. One way is in terms of stress versus mpture time. Very often, instead of using the rupture time, the time until reaching a steady-state or minimum creep is preferred, because a much shorter period of time is required to collect creep-test data. Figure 6.100 is such a curve. Stress-rupture tests are used to determine the failure time, as mentioned above. The data are plotted as log—log curves. A straight line is... 

The resulting stress rupture data will be used to examir ie the applicability of a generalized fatigue-life (slow crack growth) model. If necessary, model refinements will be implemented to account for both crack blur tlng and creep damage effects. Insights obtained from the characterization studies will be crucial for this modification process. 

Creep rupture data minimum jreep rate as a function of the time to failure. Although the data set is limited, time-to-failure seems to be determined uniquely bye the rate of creep in this material, suggesting that improvement of creep rate is of paramount importance to improving lifetime. 

To determine the susceptibility of the NT154 to stress rupture, six specimens were permitted to creep to failure. Both the time to failure and the creei ate were measured during these experiments. As can be seen in figure all of the creep rupture data fall on a single curve when the creep rate is plotted as a function of the time to failure. This curve, in combination with creep curves can be used to establish stre.ss allowables for high temperature applications. During the coming period, creep rupture data will be obtained on material that has been annealed for extended periods. 

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