Yield strain mechanism

It was found that increasing Ca caused the yield stress and yield strain to increase, along with cell deformation at the yield point. At sufficiently high values of Ca, cell distortion is so severe that film thinning and rupture can occur, resulting in mechanical failure of the foam (Fig. 6). This implies the presence of a shear strength for foams and HIPEs. The initial orientation of the cells was also found to affect the stress/strain behaviour of the system in the presence of viscous forces [63]. For some particular orientations, periodic flow was not observed for any value of Ca. 

Notwithstanding this great variety of mechanical properties the deformation curves of fibres of linear polymers in the glassy state show a great similarity. Typical stress-strain curves of poly(ethylene terephthalate) (PET), cellulose II and poly(p-phenylene terephtha-lamide (PpPTA) are shown in Fig. 13.89. All curves consist of a nearly straight section up to the yield strain between 0.5 and 2.5%, a short yield range characterised by a decrease of the slope, followed by a more or less concave section almost up to fracture. Also the sonic modulus versus strain curves of these fibres are very similar (see Fig. 13.90). Apart from a small shoulder below the yield point for the medium- or low-oriented fibres, the sonic modulus is an increasing, almost linear function of the strain. 

The macroscopic stress cr is this force times the number of interparticle bonds that cross a unit area of the sample this latter factor should scale as 0 /a (Russel et al. 1989). As long as the local applied force increases with increased strain, cr increases with increasing strain, and the gel maintains its mechanical stability. But once the strain reaches the point -that the slope W of the potential is a maximum (see Fig. 7-23), any further strain produces a decreasing force, and the interparticle structure breaks apart. This corresponds to the point of yield. Thus, the yield strain yy is given by the condition that the second derivative W of W D) is zero that is, W" Dy) = 0, where Dy = lyyU + (yy + 1)Dq is the value of D for which W" = 0. Very roughly, we might expect that W is a maximum (W — 0) when separation D = Dy % on the order of twice Dq, the value of D at static equilibrium. This would imply that the yield strain yy is roughly hence, for particles 100 nm in... 

Equation (13-12) and (13-13) plus a small noise term—which introduces small imperfections into the macrolattice—complete the description of the the model. Because of the periodic nature of the elastic stress [Eq. (13-9)] in large-strain shearing, instabilities occur that allow layers to slip with respect to each other, and the strain y, then varies from layer to layer. Thus flow occurs by yielding a mechanism similar to that discussed in Section 1.5.3. 

Blend composition has a strong influence on the mechanical properties of NR/EPDM blends (66). Figure 15.8 shows that tensile strength stress at 100% and yield strain is increasing with the increase in the concentration of EPDM in the blend. This is attributed to the reinforcing effect of EPDM domains in the NR matrix. The... 

Figure 6.14 shows a stress-strain curve from which the mechanical properties are obtained. The maximum slope gives the compressive Young s modulus, the maximum stress is the yield stress (ciy) and the corresponding strain is the yield strain (Sy). A high yield stress indicates a strong material. 

The experimental values of yield strain, Ey, are shown in Fig. 3. The experimentaly measured strains are higher than the theoretically predicted ones and increase in the following order SIC, TTS, and no treatment. This effect should be attributed to some non-homogeneous deformation mechanism. The yielding phenomenon in ductile filled polymers is often attributed to a crazing effect or to a de-wetting... 

COD analysis, based on general yielding fracture mechanics as outlined by Wells [ ] and Burdekin and Dawes [% has been used for more than ten years. The COD test is a slow, three-point bend test where the distance through which the notch faces move is used as a measure of prefracture ductility, rather than the total strain to fracture. This distance is called the crack opening displacement, or COD. 

The mechanical properties of the scaffolds are usually measmed by performing compression tests. For scaffolds with 85-95% porosities, stiffness ranged between 100 and 150kPa, yield stresses ranged between 25 and 35kPa, and yield strains ranged between 15 and 60%. Similar values for stiffness are reported in the literature for these porosity levels [54,55]. Yield properties, however, are often not reported or are poorly defined, and are thus difficult to compare. 

Let us consider now the results of the concept [44, 45] application to polymers yielding process description. The yielding can be considered as polymer structure loss of its stability in the mechanical stresses field and the yield strain is measure of this process resistance. In Ref. [44], it is indicated that specific lifetime of suprasegmental structures t is coimected with AG as follows ... 

As clearly discussed, the mechanical behaviour of polymers changes rapidly as the temperature is reduced or the strain rate increased. Brooks et al. have shown that for polyethylene there is a sudden transition in the yield strain at temperatures below ambient, the exact temperature depending on the sample morphology. Figure 12.31 shows results for linear PE. It was also found that this temperature marks the change from classical elastic-plastic behaviour to time-dependent viscoelastic behaviour where the yielded samples show... 

Radiation experiments with a Co source at room temperature detected favorable behavior in SBS block copolymers at small doses, Table 5.32. Slight yellowing is observed which, however, can be reversed by storage in daylight. Figure 5.135. Mechanical properties, such as tensile strength and yield strain hardly change under small radiation doses. 

Figure 5.336 Change in mechanical properties of polyamide 11 caused by exposure to a glycol-water mixture at 40 and 70 °C Left fracture and tensile strength Right yield strain...

The improvement in toughness (impact strength) when EPR is added to PP matrix normally results in a reduction of its stiffness, which is usually related to a decrease in flexural modulus, yield stress, and an increase in the yield strain. A balance between toughness and stiffness is always required for optimum performance of the rubber-toughened polymer. The mechanical performance, shrinkage, and processing behavior of heterophasic polypropylene is influenced by rubber particle size, rubber composition, and rubber content. 

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