Geometrical softening

Contrary to this latter explanation of the load drop in terms of geometric softening, results reported by Whitney and Andrews [20] showed a yield drop in compression for polystyrene and poly(methyl methacrylate) (PMMA) where there are no geometrical complications. Brown and Ward [18] then made a detailed investigation of yield drops in PET, studying isotropic and oriented specimens in tension, shear and compression. They concluded that in most cases there is clear evidence for the existence of an intrinsic yield drop, i.e. that a fall in true stress can occur in polymers, as in metals. This is reflected in the work of Amoedo and Lee [7], shown above in Figure 11.5(a). 

We have chemically analyzed the glass spheres and from this estimated the viscosity at 800 and 850 C by means of the correlation equations of Lyon ( ) The range of viscosity thus obtained over the above temperature range is 6.3 X 10 to 2.9 X 10 poises. This is near the geometric mean of the viscosities at the softening and working temperatures of soda-lime glass ( ). 

Another geometric factor affecting Tg is cis-trms configuration. Double bonds in the cis form reduce the energy barrier for rotation of adjacent bonds, soften the chain, and hence reduce Tg (Table 4.5). 

Like the extrusion of metals, the extrusion of plastics involves the continuous forming of a shape by forcing softened plastic material through a die orifice that has approximately the geometric profile of the cross-section of the work. The extruded form is subsequently hardened by cooling. With the continuous extrusion process, such products as rods, tubes, and shapes of uniform cross-section can be economically produced. Extrusion to obtain a sleeve of the correct proportion almost always precedes the basic process of blow molding. 

There remains the question of whether the drop in load observed at yielding arises from the purely geometrical strain softening associated with a true-stress-strain curve of the form shown in fig. 8.4(c), where there is no drop in the true stress but merely a reduction in slope of the stress train curve, or whether there is actually a maximum in the true-stress strain curve as shown in fig. 8.4(d). Experiments on polystyrene and PMMA in compression, under which the geometrical effect cannot take place, show that a drop in load is still observed. Results from extensive studies of PET under a variety of loading conditions also support the idea that a maximum in the true-stress train curve may occur in a number of polymers. 

A geometric parameter of the softened clay capsules is also shown which is the dimension plan of the collapses (L) This parameter is the total minimum gap/space or free span, being bridged by the upper layers. 

Also, in the softened clay bags the geometrical parameter (H) is important, as it represents the minimum height of the horizontal clay layers which bridge the weakened area. 

By using this method, different geometric situations have been modelised in which the uniform charge is situated on a clay layer which is at the same time supported by a non-deformable layer. A softened area could exist in the core of the clays. 

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