The Areal Material Ratio, Smr(c) is the ratio (expressed as a percentage) of the cross sectional area of the surface at a height (c) relative to the evaluation cross sectional area. The height (c) may be measured from the best fitting least squares mean plane or as a depth down from the maximum point of the Areal Material Ratio Curve.
Smr(c) may used to determine the amount of bearing area remaining after a certain depth of material is removed from the surface. A typical application may be in the specification of engine cylinder bore surfaces prior to running-in. Typically a cylinder bore may be honed to produce a pattern consisting of a plateau-like surface upon which are superimposed fine peaked structures. The fine peaked structure is provided to augment final running-in/seating of the sliding piston rings. Thus a specification for a surface may be Smr(0.5µm) > 40%, measured from the Max Value Areal Material Ratio curve with 1% peak and 1% valley offsets. This specification would be developed based on experiments that determine, for example, that running-in typically removes the top 0.5µm of the surface heights before surface stabilization.