asperities

Asperity, the singular form of asperities, refers to a small protruding point on a rough surface that bears contact in a pair of interacting bodies. In engineering and tribology, asperities are the microscopic peaks that come into contact under load, so the real area of contact is the sum of many discrete contacts rather than the apparent macroscopic area.

In geology and seismology, fault asperities are patches on a fault surface that are locally stronger or locked, resisting shear motion. They concentrate stress and influence the initiation and propagation of earthquakes. The rupture of asperities can release seismic energy and affect the timing and extent of slip on adjacent fault zones. The distribution and properties of asperities help explain rupture complexity, seismic cycles, and fault segmentation.

On engineered surfaces, asperities determine friction, wear, and lubrication behavior. When loaded, asperities deform elastically or plastically, leading to energy dissipation and heat. The real contact area, which grows with load, depends on roughness, material properties, and surface treatments. In lubrication, films may separate asperities, reducing friction, while in dry contact, asperity interactions govern wear and aging.

Modeling approaches for asperity contacts include statistical methods that treat the surface as a distribution of asperities with individual contact forces, such as the Greenwood–Williamson model. Measurement techniques like profilometry, atomic force microscopy, and scanning electron microscopy are used to characterize asperity height distributions and geometry.