affected. Just plotting force against linear deformation can readily lead to
wrong conclusions. For very small strains, as applied when measuring
moduli, the discrepancy is usually negligible.
Figure 17.4a shows a wide variety in deformation behavior; such, or
even larger, differences are readily observed when comparing different
systems. Curves 1 and 2 show the same modulus, and even the fracture stress
is almost the same, but the fracture strains differ widely; also the toughness
differs greatly (by a factor of about 15). Material 1 is stiff and short, like
chocolate, whereas 2 is much ‘‘longer,’’ like a semihard cheese. Curve 5
shows a much smaller modulus, while fracture stress and strain are like
those of curve 2; however, the material is less tough, and the shape of the
curve is as observed for some polymer gels. Curves 3–6 have the same
modulus, but they are otherwise quite different. Curve 3 is of a weak and
brittle material, e.g., a rusk. Curve 6 shows no fracture; instead, it shows
yielding; see below.
FIGURE17.3 Apparent and true deformation parameters of a test piece (of
semihard cheese) under compression (as in Figure 17.1d).sis stress,eis strain.
Apparent stress is force over initial cross-sectional area; apparent strain is change in
height over initial height (i.e., Cauchy strain).