Plateau creep

Pre-exponential factor Plateau creep rate Lennard-Jones force constant Draw ratio... 

Fig. 38. Curves fitted to plateau creep data on the basis of the two-process model with no constraint on parameters ( ) Rigidex 50, X = 20 (X) 006.60, X = 20 (A) y-irradiated Rigidex 50, X = 20 (O) H020, >. = 20 (o) Hostalen GUR, solution-spun fibre...

Recent work has shown that the creep and recovery behaviour of ultra high modulus polypropylenes is very similar to that of LPE. Again the Sherby-Dorn plots form a good entry to the detailed examination of the creep response. Plateau creep behaviour similar to that of LPE has been observed, and the high stress process correlates well with the a-relaxation process in terms of its activation energy. 

Fig. 38 shows a set of curves fitted to plateau creep data on the basis of the two-process model, and the corresponding activation parameters are shown in Table 1. The activation volume for ther 2 dashpot is in the range of 100 A3, and, as remarked by Wilding and Ward, this comparatively small activation volume could be consistent with a slip process in the crystalline regions. Support for this hypothesis comes from examination of the temperature dependence of this process. Results for selected samples show that there is a good linear relationship between log ep and 1/T. At high stress levels the behaviour corresponds to that of an apparent single activated process, with... 

The properties of the Dyneema fiber get better when the temperature is lower. This holds especially with strength and creep. The tenacity rises fast and a brittle point has not yet been measured. Creep is also sensitive to temperature (Arrhenius type of dependency), reaching lower figures at lower temperatures. The absolute figure for creep also strongly depends on the process as mentioned before and on the grade of UHMWPE that is used to produce the fibers giving rise to differences in the plateau creep rate of more than one decade. 

Fig. 15. Steady plateau creep strain rate e vs stress a for drawn LPE. A R50 = 10 ) 0 R50 y-...

Part of the polyolefin family are PBs. They are similar to PPs and HDPEs but exhibit a more crystalline structure. This crystallinity produces unusual high strength and extreme resistance to deformation over a temperature range of-10 to 190F. Its structure results in a rubberlike, elastomeric material with low molded-in stress. Tensile stress that does not plateau after reaching its yield point makes possible films that look like PE but act more like polyester (TP) films. Compared to other polyolefins, they have superior resistance to creep... 

The critical stress can now be seen to be essentially an experimental limitation. The smallest strain reading on our present creep apparatus is 5x 10 . The anticipated plateau strain rate of 10" s for a typical sample with a critical stress of 0.2 GPa would therefore only produce a measurable creep response after 500 days, which is on the limit of the time scale of present creep tests at Leeds University. 

The small changes taking place in the compliance function with time at the plateau also obey Andrade s equation for creep, that is (26-28),... 

In the glass-like zone, the values of the creep compliance function seem to be independent of the concentration however, the changes that take place in the values of J t) in the transition zone are larger the lower the concentration. The length of the plateau increases with the concentration, and the plateau and terminal zones merge into a single region at low concentrations. The location of the isotherms on the time scale is shifted to shorter chains as the concentration decreases. 

Figure 8.26 Double logarithmic plots showing the concentration dependence of both the creep compliance and the relaxation modulus at the plateau. (From Ref. 8.)...

Figure 5 Flow curve for a 6 wt% solution of diblock Ej/sB/j (E = oxyethylene, B = oxybu-tylene, subscripts = number of repeats) in water at 25 C. Every point was measured from an individual creep curve on fresh samples, to eliminate orientation effects. The plateau at around a-50 Pa indicates inhomogeneous flow, probably shear bandingf ...

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