Shearing (physics)
Encyclopedia
Shearing in continuum mechanics
refers to the occurrence of a shear strain
, which is a deformation of a material substance in which parallel internal surfaces slide past one another. It is induced by a shear stress
in the material. Shear strain is distinguished from volumetric strain, the change in a material's volume in response to stress.
Often, the verb shearing refers more specifically to a mechanical process that causes a plastic
shear strain in a material, rather than causing a merely elastic
one. A plastic shear strain is a continuous (non-fracturing) deformation that is irreversible, such that the material does not recover its original shape. It occurs when the material is yielding. The process of shearing a material may induce a volumetric strain along with the shear strain. In soil mechanics
, the volumetric strain associated with shearing is known as Reynolds' dilation
if it increases the volume, or compaction
if it decreases the volume.
The shear center (also known as the elastic axis or torsional axis) is an imaginary point on a section, where a shear force can be applied without inducing any torsion. In general, the shear center is not the centroid. For cross-sectional areas having one axis of symmetry, the shear center is located on the axis of symmetry. For those having two axes of symmetry, the shear center lies on the centroid of the cross-section.
In some materials such as metal
s, plastics, or granular material
s like sand
or soil
s, the shearing motion rapidly localizes into a narrow band, known as a shear band
. In that case, all the sliding occurs within the band while the blocks of material on either side of the band simply slide past one another without internal deformation. A special case of shear localization occurs in brittle
materials when they fracture
along a narrow band. Then, all subsequent shearing occurs within the fracture. Plate tectonics
, where the plates of the Earth's crust slide along fracture zones, is an example of this.
Shearing in soil mechanics is measured with a triaxial shear test
or a direct shear test
.
Continuum mechanics
Continuum mechanics is a branch of mechanics that deals with the analysis of the kinematics and the mechanical behavior of materials modelled as a continuous mass rather than as discrete particles...
refers to the occurrence of a shear strain
Strain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
, which is a deformation of a material substance in which parallel internal surfaces slide past one another. It is induced by a shear stress
Shear stress
A shear stress, denoted \tau\, , is defined as the component of stress coplanar with a material cross section. Shear stress arises from the force vector component parallel to the cross section...
in the material. Shear strain is distinguished from volumetric strain, the change in a material's volume in response to stress.
Often, the verb shearing refers more specifically to a mechanical process that causes a plastic
Plasticity (physics)
In physics and materials science, plasticity describes the deformation of a material undergoing non-reversible changes of shape in response to applied forces. For example, a solid piece of metal being bent or pounded into a new shape displays plasticity as permanent changes occur within the...
shear strain in a material, rather than causing a merely elastic
Elasticity (physics)
In physics, elasticity is the physical property of a material that returns to its original shape after the stress that made it deform or distort is removed. The relative amount of deformation is called the strain....
one. A plastic shear strain is a continuous (non-fracturing) deformation that is irreversible, such that the material does not recover its original shape. It occurs when the material is yielding. The process of shearing a material may induce a volumetric strain along with the shear strain. In soil mechanics
Soil mechanics
Soil mechanics is a branch of engineering mechanics that describes the behavior of soils. It differs from fluid mechanics and solid mechanics in the sense that soils consist of a heterogeneous mixture of fluids and particles but soil may also contain organic solids, liquids, and gasses and other...
, the volumetric strain associated with shearing is known as Reynolds' dilation
Reynolds' dilatancy
Reynolds' dilatancy is the observed tendency of a compacted granular material to dilate as it is sheared. This occurs because the grains in a compacted state are interlocking and therefore do not have the freedom to move around one another. When stressed, a lever motion occurs between...
if it increases the volume, or compaction
Soil compaction
In Geotechnical engineering, soil compaction is the process in which a stress applied to a soil causes densification as air is displaced from the pores between the soil grains. When stress is applied that causes densification due to water being displaced from between the soil grains then...
if it decreases the volume.
The shear center (also known as the elastic axis or torsional axis) is an imaginary point on a section, where a shear force can be applied without inducing any torsion. In general, the shear center is not the centroid. For cross-sectional areas having one axis of symmetry, the shear center is located on the axis of symmetry. For those having two axes of symmetry, the shear center lies on the centroid of the cross-section.
In some materials such as metal
Metal
A metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s, plastics, or granular material
Granular material
A granular material is a conglomeration of discrete solid, macroscopic particles characterized by a loss of energy whenever the particles interact . The constituents that compose granular material must be large enough such that they are not subject to thermal motion fluctuations...
s like sand
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
or soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
s, the shearing motion rapidly localizes into a narrow band, known as a shear band
Shear band
A shear band is a narrow zone of intense shearing strain, usually of plastic nature, developing during severe deformation of ductile materials....
. In that case, all the sliding occurs within the band while the blocks of material on either side of the band simply slide past one another without internal deformation. A special case of shear localization occurs in brittle
Brittle
A material is brittle if, when subjected to stress, it breaks without significant deformation . Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Breaking is often accompanied by a snapping sound. Brittle materials include most ceramics and glasses ...
materials when they fracture
Fracture
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
along a narrow band. Then, all subsequent shearing occurs within the fracture. Plate tectonics
Plate tectonics
Plate tectonics is a scientific theory that describes the large scale motions of Earth's lithosphere...
, where the plates of the Earth's crust slide along fracture zones, is an example of this.
Shearing in soil mechanics is measured with a triaxial shear test
Triaxial shear tests
A triaxial shear test is a common method to measure the mechanical properties of many deformable solids, especially soil and rock, and other granular materials or powders...
or a direct shear test
Direct Shear Test
A direct shear test also known as shearbox test is a laboratory or field test used by geotechnical engineers to measure the shear strength properties of soil or rock material, or of discontinuties in soil or rock masses.- Soil :...
.