In Geotechnical engineering the term asperity is mostly used for unevenness (roughness) of the surface of a discontinuity, grain, or particle with heights in the range from approximately 0.1 mm to many decimetre. Smaller unevenness is normally considered to be a material property (often denoted by material friction or basic material friction).
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Dilation
An often used definition for asperities in geotechnical engineering:
Unevenness of a surface are asperities if these cause dilation if two blocks with in between a discontinuity with matching asperities on the two opposing surfaces (i.e. a fitting discontinuity) move relative to each other, under low stress levels that do not cause breaking of the asperities.
Contrast with asperity in materials science
Materials science recognizes asperities ranging from the sub-visual (normally less than 0.1 mm) to the atomic scale.