Slump test for yield stress

A slump test for yield stress probes a material by allowing a molded sample to deform under its own weight. Fill a known mold on a level support, remove the mold, and measure the decrease in height or the spread of the final deposit. Gravity supplies the driving stress; resistance associated with the yield stress can arrest the motion before the sample becomes a thin layer.

The measured distance is not itself a stress. Inferring yield stress requires a calibrated relation or a mechanical model using the initial geometry, final shape and density. The final slump alone generally cannot identify both yield stress and dynamic viscosity: a time-resolved measurement provides additional information. A modified concrete test, for example, records the time required for a specified fall as well as the final slump.^[ref] The procedure is useful as a simple field measurement, with interpretation dependent on sample history and geometry.