stress in the material can relax before a considerable strain is attained; this
means thatscan not attain a high value. If deformation is fast, there is little
time for relaxation and a relatively high stress results. At smallCvalues, the
material thus behaves predominantly liquidlike (high tandvalue), at highC
values, more solidlike (lower tandvalue).
Another consequence of stress relaxation is that, for a higher stress
applied to a material, it takes a shorter time before a high strain is attained
and a shorter time before failure occurs (i.e., yielding or fracture, depending
on the material). This is illustrated in Figure 17.6b.
Figure 17.6c relates to a material that exhibits stress overshoot
(indicated in the figure for the upper curve) before yielding is complete. The
magnitude of the overshoot increases with increasingCvalue. Again, a
higher stress can then be reached before much structure breakdown has
occurred. The relaxation will occur over roughly the same time span, i.e., the
relaxation time, for various strain rates. At quite low C values, stress
overshoot tends to be negligible.
It should be emphasized that various soft solids behave much
differently in a quantitative sense, although the time dependence is
qualitatively the same for most materials. It primarily depends on the
Deborah number [Eq. (5.14)] how strong the time effects will be.
Furthermore, stiffness, strength, and shortness of the material affect the
result.
FIGURE17.6 Time effects in deformation. The arrows through the curves indicate
increasing values, generally by two or three orders of magnitude, of the parameter
indicated.sis stress,eis strain,Cis velocity gradient (strain rate), andtis time after
starting deformation. In (a) and (c), the strain rate is kept constant during the test,
implying thateis proportional totfor each value ofC;sis measured. In (b), the
stress is kept constant andeis measured as a function oft.