Inhomogeneity. For large deformation, most rheological relations,
including time scale dependence, can be quite different from what has been
discussed so far if the material is inhomogeneous. What is meant is
inhomogeneity at length scales larger than those of the smallest building
blocks that affect the rheological properties. The deformation of a test piece
will then always result in an inhomogeneous stress distribution throughout
the sample. In other words, localstress concentrationwill occur, which will
result ininhomogeneous deformation. It is generally impossible to predict the
distribution of stress or strain throughout the test piece. Interpretation of
the measured values in rheological or structural terms then is quite difficult.
The problem can be aggravated by the testing itself. Because the
material is inhomogeneous, structure breakdown can occur in specific local
regions, thereby increasing the inhomogeneity. In several cases, a kind of
yielding zones or planes are formed throughout the test piece. A well-known
phenomenon isslip, i.e., formation of a yielding plane close to the surface of
the measuring body holding the test piece. The measured stresses then are
much smaller than would have been obtained in the absence of slip. In other
words, the test results may be useless (although the observation that slip
occurs may in itself be significant). A comparable problem is that handling
of a soft solid prior to testing, e.g., by forcing a specimen into the measuring
body of the rheometer, may cause considerable structure breakdown, by
which the rheological properties of the material are greatly altered.
Note In an experiment as illustrated in Figure 17.1c, the material
must slip over the platens if we are to measure its elongational
viscosity. In such a case, lubrication by a thin liquid that does not
diffuse into the sample is often applied.Question
An important property of a product like margarine is its consistency or, more
precisely, its ‘‘spreadability’’, that is, can it readily and evenly be spread onto a slice
of bread? Presumably, the yield stress of the materialsyis essential in this respect: if
it is too low, the product may flow under its own weight during storage, if too high,
spreading takes too much effort and the bread will crumble. An empirical test that is
often done to assess the consistency of spreads is cone penetration. A sharp cone of
given mass and cone angle is clamped in a vertical position above the test piece, the
cone tip just touching the surface, and released. The penetration depthLpof the cone
in the test piece then is measured.
How would the relation be betweensyandLp? Do you think that it is a good
test for the yield stress? What criticisms can be made? How would the magnitude of