Polyethylene tensional stress

Fig. 7.5. (a) Thedependenceof x, the elongational viscosity, on tensile stress for branched and linear polyethylene. At stresses below 10 Pa, both polymers approach Newtonian behaviour (A independent of a). Non-Newtonian behaviour occurs above 10 Pa branched PE tension-stiffens linear PE tension-thins. 

The data presented in Figure 19.7 were obtained on a Sonntag-Universal machine which flexes a beam in tension and compression. Whereas the acetal resin was subjected to stresses at 1800 cycles per minute at 75°F and at 100% RH, the nylons were cycled at only 1200 cycles per minute and had a moisture content of 2.5%. The polyethylene sample was also flexed at 1200 cycles per minute. Whilst the moisture content has not been found to be a significant factor it has been observed that the geometry of the test piece and, in particular, the presence of notches has a profound effect on the fatigue endurance limit. 

Fig. 1. Apparent secant modulus (stress/strain) in tension at 0.1% strain and at various sample aspect ratios measured at a strain rate of 10 4s 1. Isotropic polyethylene sheet at 22 °C (sample cross section 1 mm X 3 mm)...

Mohandas and co-workers (18), confirming previous findings of Weiss and Blumenson (19), have also shown that cells in an environment free of adsorbable proteins (which rapidly modify the surface properties of polymeric or inorganic substrates) will exhibit a similar direct relationship between their adhesion and the critical surface tension of the surface they contacted. DiflFerential adhesion of red blood cells was measured by determining the fraction of cells retained on a surface after the application of well-calibrated shear stresses (IS). In protein-free experiments, the red cells (themselves dominated in adhesive interactions by their protein membranes) had greatest adhesion to glass, intermediate adhesion to polyethylene and siliconized glass, and least adhesion to Teflon. 

Creep. One of the most remarkable aspects of the deformation of polydiacetylenes is that it is not possible to measure any time-dependent deformation or creep when crystals are deformed in tension parallel to the chain direction (14,24). This behviour is demonstrated in Figure 3 for a polyDCHD crystal held at constant stress at room temperature and the indications are that creep does not take place at temperatures of up to at least 100 C (24). Creep and time-dependent deformation are normally a serious draw-back in the use of conventional high-modulus polymer fibres such as polyethylenes (28). Defects such as loops and chain-ends allow the translation of molecules parallel to the chain direction in polyethylene fibres. In contrast since polydiacetylene single crystal fibres contain perfectly-aligned long polymer molecules (cf Figure lb) there is no mechanism whereby creep can take place even at high temperatures. 

If the materials are anisotropic, they will present different properties in the different directions. Examples of these polymeric materials are polymer fibers, such as polyethylene terephthalate, PET, nylon fibers, injection-molded polymers, fiber-reinforced composites with a polymeric matrix, and crystalline polymers where the crystalline phase is not randomly oriented. A typical method for measuring the modulus in tension is the stress-strain test, in which the modulus corresponds to the initial slope of the stress-strain curve. Figure 21.4 shows typical stress-strain curves for different types of polymeric materials. 

To probe the models for nucleation-controlled plastic flow we compare the predicted temperature dependence of the tensile plastic resistance with the tensile-yield-stress experimental results of Brooks and Mukhtar (2000). For comparison the polyethylene PE3 of average molecular weight = 131000 with a crystallinity of only 0.673 and lamella thickness of 34.3 nm is chosen. For the predictions of the temperature dependence, eqs. (9.26)-(9.28) of Section 9.4.3 are used, where we take in the denominator of eq. (9.25) the factor (1 + K), since the experiments were performed in tension. Noting that the lamella thickness of this polymer type is 1 = 34.3 nm, which is thicker than that for mode A of monolithic-screw-dislocation nucleation, we consider only modes B and C involving nucleation of screw-dislocation half loops and edge-dislocation half loops, and, together with the results of Fig. 9.21, we state the expected tensile yield stress Oy to be... 

Equation (24.30) provides the desired connection between Ki and dh/dt. In the derivation both the stress level a and the original crack length ho were used but both canceled out, with the unexpected result that the crack propagation rate is independent of both The experimental results support Eq. (24.30) as shown for instance in Fig. 24.3 for Hoechst PEs studied under uniaxial tension in water medium at 60 °C. Each symbol pertains to a different stress level and a different original notch length. It is clear that all polyethylenes with the molecular mass Ma form a common curve, and the same is true for the other molecular masses. Moreover, we see that a higher M results in a lower crack propagation rate this result is related to the constituents of CRC listed at the end of Section 24.1.3, particularly the first two of them. 

There is a substantial increase in the shear yield stress up to a hydrostatic pressure of about 300 MPa. After this pressure brittle failure occurs, unless prevented by protecting the specimens from the hydraulic fluid [24] (e.g. by coating with a layer of solidified rubber solution). A study of polyethylene under conditions of combined pressure and tension has shown that the yield stress of... 

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. 

A sample of linear polyethylene tested at 23 "C and 10" s" yielded at 30.0 MPa in uniaxial tension, and at 31.5 MPa in uniaxial compression. Assuming that the yield stress is a linear function of hydrostatic pressure, calculate under superimposed hydrostatic pressure of 500 MPa. 

We have reported [66] a limited study of spread polymethyl methacrylates and polyethylene oxide. Figure 12.19 shows the variation in surface tension, shear viscosity and dilational modulus obtained from SQELS data as a function of surface concentration. The viscoelastic moduli both show maximum values at finite values of the surface concentration. As the capillary waves generate oscillatory stress and strain, these are related via the complex dynamic modulus of the surface... 

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