A simple technique for the measurement of the shear stress field on a flat surface is presented. The technique is based on a correlation established between the erosion patterns of fine grains spread on a horizontal smooth plate and the known shear stress values obtained by conventional, well established, means for the flow produced by a round jet perpendicular to a flat plate. The critical shear stress values are found to be practically independent of the overall flow configuration and properties, and depend on the particle distribution and characteristics. Various granular materials of different sizes, shapes and densities were used at varying surface concentration and flow conditions. The experimental scatter of the results is small enough to make the erosion technique useful for quantitative assessment of wall shear stresses with an accuracy compatible with most practical applications. Furthermore it combines simplicity with promptness, given that in each run a curve of constant shear stress is obtained instead of a single local value.

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