Sdr, the Developed Interfacial Area Ratio, is expressed as the percentage of additional surface area contributed by the texture as compared to an ideal plane the size of the measurement region.
Christopher A. Brown, William A. Johnsen, Kevin M. Hult, Scale-sensitivity, Fractal Analysis and Simulations, Int. J. Mach. Tools Manufact. Vol 38, Nos 5-6, pp. 633-637, 1998.
Sa= 0.52 µm, Sdr=0.0023%.
Sa= 0.33 µm, Sdr=0.0623%.
Sdr may further differentiate surfaces of similar amplitudes and average roughness. Typically Sdr will increase with the spatial intricacy of the texture whether or not Sa changes. Sdr is useful in applications involving surface coatings and adhesion. Sdr and may find relevance when considering surfaces used with lubricants and other fluids. Sdr is affected both by texture amplitude and spacing. Thus higher Sa, wider spaced texture may have actually a lower Sdr value than a lower Sa but finer spaced texture, as displayed above.