# Shear strength

Shear strength is the strength of a material or component against the type of yield or structural failure when the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force.

When a paper is cut with scissors, the paper fails in shear. In structural and mechanical engineering, the shear strength of a component is essential for designing the dimensions and materials to be used for the manufacture or construction of the component (e.g., beams, plates, or bolts).

In Soil Mechanics, shear strength is referred to because soils, being particulate in nature, deformations (and failure) occur primarily by relative sliding between grains. The “shear strength” of a soil in one direction means the maximum tangential stress, that can be applied in a given direction before “failure” occurs.

The shear resistance of soil is a result of friction and interlocking of particles, and possibly cementation or bonding at particle contacts. Due to interlocking, particulate material may expand or contract in volume as it is subject to shear strains. If soil expands its volume, the density of particles will decrease and the strength will decrease; in this case, the peak strength would be followed by a reduction of shear stress. The stress-strain relationship levels off when the material stops expanding or contracting, and when interparticle bonds are broken. The theoretical state at which the shear stress and density remain constant while the shear strain increases may be called the critical state, steady state, or residual strength.

The shear strength of granular (i.e., non-cohesive) soils is commonly defined by the Mohr-Coulomb (Terzaghi) equation expressed in terms of effective stresses, which expresses the dependence of the available shear strength at failure on the plane of failure as a function of the effective normal stress at the plane of failure.

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