most 1%, the linearity extends to, for instance, e¼ 10 ^4 ,3? 10 ^4 and
14? 10 ^4 , for hydrogenated palm oil crystals in oil, margarine fat, and
butter, respectively. Presumably, the differences are largely due to
differences in the anisometry of the crystals, which tends to increase in
the same order; in other words, the crystals in butterfat can be bent more
readily than those of the other fats. For strains larger than about 10^3 , the
deformation tends to become partly irreversible, and the extent of
irreversibility increases in the reverse order. For increasingly larger
deformations, the changes become increasingly irreversible.
Deformation Regimes. Figure 17.24 illustrates what will happen
when a typical margarine fat is subjected to increasing strain. The following
regimes can be distinguished:
- Linear. The strain is proportional to the stress and the loss tangent
is small (<0.1). The deformation is fullyelasticand hence reversible, which
implies that no bonds between crystals are broken. - Nonlinear. Note that this region extends over a strain range about
200 times that of the linear region. With increasing strain, the deformation
becomes more and moreviscoelastic, and the apparent loss tangent increases
up to a value of about unity. This also means that the shape of the curve will
depend on strain rate (cf. Figure 17.6). Bonds are broken, the more so for
greater strains. Presumably, van der Waals bonds can reform on removal of
FIGURE17.24 Schematic diagram of an example of the relation between stress (s)
and strain (e) during deformation of a typical margarine fat.syis the yield stress.