well with ‘‘firmness.’’ Another variable is the inhomogeneity of a material,
which gives rise to local stress concentration. This hardly affects the
modulus, but may strongly affect the large deformation behavior.
Fracture Mechanics. In a purely elastic material, the overall stress
needed to cause fracture is greatly lowered (by up to a factor 100) by local
stress concentration: near a tiny crack or other defect, the local stress can
readily surpass the fracture stress. However, surpassing the fracture stress is
insufficient, since energy is needed for fracture to proceed, i.e., for crack
propagation. The energy is derived from the elastic energy stored upon
deformation. Each system has a critical crack length above which fracture is
propagated.
Soft solids show more complex behavior, and fracture is often
‘‘elastic–plastic,’’ implying that near the tip of the crack, where the stress is
highest, yielding occurs; the pieces remaining after fracture do not precisely
fit to each other. The local yielding increases the energy needed, i.e., the
specific work of fracture.
For viscoelastic soft solids, the whole test piece shows lasting
deformation before and during the event of fracture, making the relations
even more complicated. The fracture parameters now are markedly time-
scale dependent. The specific work of fracture is increased, since much
energy is dissipated during deformation.
Materials may break in shear or in tension; the former occurs in
‘‘short’’ (e.g., brittle), the latter in ‘‘long’’ (e.g., rubbery) materials. Another
material property is notch sensitivity. In a test piece that is put under
tension, notches can be applied, and in several materials the ensuing stress
concentration greatly lowers the overall stress needed for fracture
propagation. The notch sensitivity is smaller if the material contains defects
of the size of a small notch, or if the bonds between structural elements are
much stronger in the direction of the applied stress than in a perpendicular
direction, as in many fibrous systems.
Elongational flow, due to uniaxial or biaxial tensional stress (e.g.,
spinning of a thread, or blowing of a film, respectively), tends to lead to
fracture: the stress will generally be largest where the test piece is thinnest, so
that the thinnest part will experience an ever increasing stress. This need not
occur if the material is strongly strain hardening. More precisely, the stress
should increase with increasing strain to a certain extent. Some concentrated
protein solutions show this behavior, so that they can be spun, and wheat
flour doughs do the same, preventing rupture of thin dough films between
gas bubbles upon rising of the dough in the oven.
Texture perceptionis briefly discussed, and pitfalls in relating sensory
perceived texture to instrumentally measured properties are pointed out.