What is Shear Strength?

A measure of resistance to wear and tear

Shear is defined as a strain in the structure of a substance produced by a lateral shifting of its layers in relation to one another.  Broadly, many branches of science concerned with measuring physical stress on an object have developed various techniques and measurements to quantify the amount of lateral stress a structure can take.  Some materials break with the slightest application of force; others twist or compensate, maintaining structural integrity.

Engineering

In engineering, scientists require very specific tests of a material's response to tension, compression, shear, and repeated combined actions of these three.  Testing machines have been developed to provide data for engineers.  In order to build structures capable of surviving in the real world, and not just on paper, engineers and the builders who execute their plans fastidiously scrutinize a material's physical and chemical properties, predicting its behavior down to the tiniest variance.  Some engineering concepts are required for understanding these properties, as the tests and measurements involved tend to use complicated notations and terminology.

In structural engineering shear strength is a term used to describe the resistance against the type of structural failure where a component splits into two parts that slide past each other.  The shear strength of a component is most important for beams but also relevant for other members.  In a reinforced concrete beam, the main purpose of stirrups is to increase shear strength.

Geology

In geology, shear strength is a term used to describe the compressive strength of materials, especially soil. Under a downward load, soil resists the force of the great weight above it by two internal mechanisms.  In other words, the weight of the soil produces one force (pressure on top of the soil at the bottom) and the soil itself produces other kinds of forces depending on its condition.

In fine-grained soil, the particles of the soil itself produce cohesion.  The condition of pores in the soil may also effect these measurements.  Contact between the particles in soil also produces friction, especially in clay soil.  A series of complex equations measure these competing forces.  Clays and sands have different shear strengths.  Cohesive forces determine the shear strength of sand; friction determines the sheer strength of clay.
Constructions in soil can fail due to shear; if miscalculated, buildings placed on top of different types of soil might adversely effect the complicated interactions of the weight of the soil on top of itself and cause a collapse or sink hole.

Gardening

The thickness of the material a pair of metal shears can cut is determined by the design of the tool and its material composition.  Garden shears are numbered for size; larger numbers tend to indicate larger models, although this varies by manufacturer.  A long-handled shear, such as a tree shear, tends to be able to cut greater diameters because of leverage.  Shorter-handled shears, such as pruning shears, make smaller cuts for detailed work.

More information on shears