Shear

Shear is a type of deformation or internal force that acts parallel to a material's plane, causing adjacent layers to slide past one another. In solids, a shear force produces shear stress, defined as tau = F/A, where F is the force parallel to the plane and A is the area over which it acts. The resulting change in shape is quantified by shear strain, gamma, defined as the displacement parallel to the plane divided by the thickness. The linear relation for small deformations is tau = G gamma, where G is the shear modulus. Materials respond to shear through yielding, plastic flow, or fracture, depending on their properties and loading rate. Shear testing, including torsion and simple shear, characterizes G and shear strength.

In fluids, shear is described by the velocity gradient perpendicular to the flow direction, du/dy, called the shear rate. The ratio of shear stress to shear rate defines viscosity; Newtonian fluids have constant viscosity, while non-Newtonian fluids change viscosity with the rate of strain. Shear thinning and shear thickening are common phenomena.

In geology and geophysics, shear stresses act along faults and may produce slip and earthquakes. In engineering, design against shear involves calculating shear forces in beams and joints, and ensuring that shear stress remains within material limits.

Common units include pascals for stress and a dimensionless measure for strain. The concept of shear appears across disciplines, from microscopic crystal lattices to planetary crusts.