Polyethylene creep

Problem 4 An empty pipe in a trench is covered with a depth of 1 m of soil of density 2000 kg m. The soil load can be considered as a concentrated force as in Fig. 14.7a. What is the maximum allowable pipe SDR if the vertical deflection after 1 h is not to exceed 10% of the pipe diameter Use polyethylene creep data from Fig. 7.6. 

The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

Other polyolefins A variety of other crystalline polyolefins are available such as polybutene-1 (improved creep resistance over polyethylene), poly-4-methyl pentene-1 (excellent temperature deformation resistance) and ethylene-vinyl acetate (greater flexibility). 

Polyolefins (Polyethylene, Polypropylene) Powder, pellets Tough and chemical resistant. Weak in creep and thermal resistance. Polyethylene maximum use temperature 210 F, polypropylene 260 F. May be injection and extrusion molded, vacuum formed. Low cost. Antistatic sheet and tiles, heat-shrinkable tubing, deicer boots. 

Figure 11 Shear creep of polyethylene (density = 0.950) at different loads after 10 min, and as a function of applied stress. Deviation firm the value of 1.0 indicates a dependence of creep compliance on load.

Many other data in the literature show a strong dependence of creep compliance on the applied load, although in some cases the authors did not discuss this aspect of creep. Stress dependence is found with all kinds of plastics. For example, the creep of polyethylene has been studied by... 

Annealing can reduce the creep of crystalline polymers in the same manner as for glassy polymers (89,94,102). For example, the properties of a quenched specimen of low-density polyethylene will still be changing a month after it is made. The creep decreases with time, while the density and modulus increase with time of aging at room temperature. However, for crystalline polymers such as polyethylene and polypropylene, both the annealing temperature and the test temperatures are generally between... 

These equations predict a continuously diminishing rate of creep. Many empirical and semi-empirical models of creep-strain have been made and are described by Ward [24], One of these has been used successfully to describe the later stages of creep in polymers such as oriented polyethylene. The Arrhenius equation was modified by Eyring to apply to the rate of creep (deJdt) in the following way ... 

Figure 12.1 Creep-rupture (time to failure plotted horizontally against applied hoop stress) of polyethylene pipes under pressure at different temperatures...

Figures 3.7 (a)-(d) show some aspects of creep for polypropylene and polyethylene ...

Figure 3.7.(Contd) (d) Creep strength versus time example of polyethylene at20°C, 50°C and 80°C...

Neat thermoplastic polyethylenes have low moduli that involve high strains for moderate loading. Consequently, creep moduli are also low, the more so as the temperature rises, as we can see in Figures 4.3 ((a) and (b)) where creep moduli are displayed as a function of time, load and temperature. 

ISO 16770 2004 Plastics - Determination of environmental stress cracking (ESC) of polyethylene - Full-notch creep test (FNCT)... 

General drawbacks are the innate relative sensitivity to heat and creep, low rigidity, significant shrinkage and poorer low-temperature behaviour than polyethylene. 

Figure 4.35(a) displays examples of creep under relatively high stresses (10 to 30 MPa) for rigid PVC at room temperature. We can note that the creep moduli are much higher than those of polyethylene or polypropylene (less than 1 GPa). 

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... 

---