Principal Deformation Mechanism

These results are summarized in a simple sketch (Fig. 4.15) that describes the principal deformation mechanism of the studied TPU materials —still neglecting domain formation, transformation or destruction. The virgin injection-molded material (Fig. 4.15a, = 0) exhibits a nanostructure of hard and soft domains with poor inter-domain correlation. Thus the SAXS only sees uncorrelated sandwiches , e.g. two hard domains with a soft domain in between (or two soft domains with a hard domain in between). In the sketch only one type of sandwich is drawn. Thus the main scattering effects in the CDF on the meridian are the domain peak (at very small 3), and the long period peak (L-peak) that has been discussed in the previous 

4 Thermoplastic Polyurethane Elastomers Under Uniaxial Deformation 

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Shear yielding is well established as the principal deformation mechanism and source of energy dissipation in both uiunodified and rubbo -toughened epoxy resins [2,3,27,83,121]. As molecular mobility in the epoxy resin network chains decreases, the ability of the matrix to deform by shear yielding is reduced. This is the reason why epoxy resins become both more brittle and more difficult to toughen as the epoxy resin crosslink density increases and/or as the network chains increase in rigidity, e.g. by use of highly aromatic epoxy resin monomers (see Section 19.7.1.1). 

In an excellent review Bucknall [124] explains that rubber toughening involves three principal deformation mechanisms shear yielding, crazing and rubber particle cavitation. The rubber particles, with a much lower stiffness than the matrix polymer, give rise to stress concentrations for the initiation of shear yielding and crazing. 

As one might intuitively expect, the incorporation of rubber particles within the matrix of brittle plastics enormously improves their impact resistance. Indeed, the impact resistance imparted by the rubber is the principal reason for its incorporation (Rosen, 1967) in rubber-plastic blends and grafts. Toughening in such polymers is also observed under other loading conditions, such as simple low-rate stress-strain deformation and fatigue. It is believed that several deformation mechanisms are important in all such cases, though their relative importance may depend on the polymer and on the nature of the loading. 

Both principal fracture mechanisms, shear yielding and crazing, are influenced by the particle size. In PPBC matrix, where spherical elastomeric particles are chemically bonded, the energy absorption takes place mainly by deformation of the matrix. In such systems, a large amount of shear yielding is to be expected. The shear yielding becomes more prominent upon increasing the concentration of EPDM as well as reduction of their particle size. The micro-shear bands in the fracture surface (Pig. 10.23e) clearly support these expectations. 

It may be shown that the Raman measurements are capable of yielding information on both < cos 0 > and < cos d >. The availability of < cos 0 > data can be valuable is distinguishing between the differing types of stress deformation mechanisms that have been proposed. However, an interpretation of the band intensities in terms of and is possible only when the principal components of the derived polarisability tensor are known. This information is often not available and assumptions must then be made these then render the method non-absolute. Examples of this approach will be considered briefly below. 

III The Laws of Thermostatics for Large Principal Deformation A combined statement of the first and second laws of thermo-statics, when only mechanical work is involved, may be written in the form ... 

Figures 14 and 15 show the orientation distribution functions, w(, 0, rj) and qjiCp 0) calculated as above for the extension ratios of the spherulite from 1.1 up to 1.4 by choosing the parameters so that the calculated results give the best fit to the observed orientation distribution functions in Figures 9 and 11. As can be seen by a comparison of Figure 14 with Figure 11, the development of the two populations in the orientation distribution function is well reproduced by theory. The orientation distribution functions, qjiCp 0), calculated for the principal crystallographic axes shown in Figure 15 also agree fairly well with the observed functions shown in Figure 9. The uniaxial deformation mechanism of spherulites can now be discussed in terms of the changes of the model parameters. ...

Among the basic mechanical properties of fibers are their deformability and tenacity. When an axial stretching force is applied to the fiber, the principal quantitative indices of deformability are the axial elastic modulus (E)... 

These use crystalline materials in which the electrical properties of the material are changed when it undergoes slight deformation by, for example, the application of mechanical pressure. The principal effect is to cause a change in the frequency at which the material resonates. This change in resonant frequency can be detected and measured, so giving an indication of the change in pressure. 

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