  An experimental system has been found recently, a coagulated CaCO3 suspension
system, which shows very variable yield behaviour depending upon how it is
tested and, specifically, at what rate it is sheared. At P\'eclet numbers Pe >
1 it behaves as a simple Herschel Bulkley liquid, whereas at Pe < 1 highly
non-monotonic flow curves are seen. In controlled stress testing it shows
hysteresis and shear banding and in the usual type of stress scan, used to
measure flow curves in controlled stress mode routinely, it can show very
erratic and irreproducible behaviour. All of these features will be attributed
here to a dependence of the solid phase, or, yield stress, on the prevailing
rate of shear at the yield point. Stress growth curves obtained from step
strain-rate testing showed that this rate-dependence was a consequence of
P\'eclet number dependent strain softening. At very low Pe, yield was
cooperative and the yield strain was order-one, whereas as Pe approached unity,
the yield strain reduced to that needed to break interparticle bonds, causing
the yield stress to be greatly reduced. It is suspected that rate-dependent
yield could well be the rule rather than the exception for cohesive suspensions
more generally. If so, then the Herschel-Bulkley equation can usefully be
generalized to read (in simple shear). The proposition that rate-dependent
yield might be general for cohesive suspensions is amenable to critical
experimental testing by a range of means and along lines suggested.
