Stress (mechanics)

In <a href="/facts/Continuum_mechanics/vI7LpFDk">continuum mechanics</a>, stress is a <a href="/facts/Physical_quantity/S8pWBEpK">physical quantity</a> that describes <a href="/facts/Force/533Q1uzq">forces</a> present during <a href="/facts/Deformation_(physics)/xK7R59oI">deformation</a>. For example, an object being pulled apart, such as a stretched elastic band, is subject to <a href="/facts/Tension_(physics)/waNGYp3a">tensile</a> stress and may undergo <a href="/facts/Elongation_(materials_science)/xK7R59oI">elongation</a>. An object being pushed together, such as a crumpled sponge, is subject to <a href="/facts/Compression_(physics)/2wwcVmM0">compressive</a> stress and may undergo shortening. The greater the force and the smaller the cross-sectional area of the body on which it acts, the greater the stress. Stress has <a href="/facts/Dimension_(physics)/t6UskoCD">dimension</a> of force per area, with <a href="/facts/SI_Units/S7YLglNz">SI units</a> of newtons per square meter (N/m2) or <a href="/facts/Pascal_(unit)/Am1To2Qm">pascal</a> (Pa).
Stress expresses the internal forces that neighbouring <a href="/facts/Particle/JHKr2w6h">particles</a> of a continuous material exert on each other, while <a href="/facts/Strain_(mechanics)/xK7R59oI">strain</a> is the measure of the relative <a href="/facts/Deformation_(mechanics)/xK7R59oI">deformation</a> of the material. For example, when a <a href="/facts/Solid/hj5UsXno">solid</a> vertical bar is supporting an overhead <a href="/facts/Weight/9NMHCsQd">weight</a>, each particle in the bar pushes on the particles immediately below it. When a <a href="/facts/Liquid/t9GkLWzt">liquid</a> is in a closed container under <a href="/facts/Pressure/eyc37GRG">pressure</a>, each particle gets pushed against by all the surrounding particles. The container walls and the <a href="/facts/Pressure/eyc37GRG">pressure</a>-inducing surface (such as a piston) push against them in (Newtonian) <a href="/facts/Reaction_force/g2rCCqwq">reaction</a>. These macroscopic forces are actually the net result of a very large number of <a href="/facts/Intermolecular_force/xIcFMvL7">intermolecular forces</a> and <a href="/facts/Statistical_mechanics/6zMsNYYG">collisions</a> between the particles in those <a href="/facts/Molecule/N9ljSfFq">molecules</a>. Stress is frequently represented by a lowercase Greek letter sigma (σ).
Strain inside a material may arise by various mechanisms, such as stress as applied by external forces to the bulk material (like <a href="/facts/Gravity/bqAN7DdQ">gravity</a>) or to its surface (like <a href="/facts/Contact_force/l997TYVu">contact forces</a>, external pressure, or <a href="/facts/Friction/d0108Rpu">friction</a>). Any <a href="/facts/Deformation_(mechanics)/xK7R59oI">strain (deformation)</a> of a solid material generates an internal elastic stress, analogous to the reaction force of a <a href="/facts/Spring_(device)/jknGXzTq">spring</a>, that tends to restore the material to its original non-deformed state. In liquids and <a href="/facts/Gas/hVVAoodV">gases</a>, only deformations that change the volume generate persistent elastic stress. If the deformation changes gradually with time, even in fluids there will usually be some viscous stress, opposing that change. Elastic and viscous stresses are usually combined under the name mechanical stress.

Significant stress may exist even when deformation is negligible or non-existent (a common assumption when modeling the flow of water). Stress may exist in the absence of external forces; such built-in stress is important, for example, in <a href="/facts/Prestressed_concrete/0fFlgCju">prestressed concrete</a> and <a href="/facts/Tempered_glass/I2Q7fX7n">tempered glass</a>. Stress may also be imposed on a material without the application of <a href="/facts/Newton%27s_laws_of_motion/XKzGTXK1">net forces</a>, for example by <a href="/facts/Thermal_expansion/BP2vjX0N">changes in temperature</a> or <a href="/facts/Chemistry/SrNqkGVm">chemical</a> composition, or by external <a href="/facts/Electromagnetic_field/MacraS8y">electromagnetic fields</a> (as in <a href="/facts/Piezoelectricity/65k6BDqx">piezoelectric</a> and <a href="/facts/Magnetostriction/hkma4mTU">magnetostrictive</a> materials).
The relation between mechanical stress, strain, and the <a href="/facts/Strain_rate/QptTh1KW">strain rate</a> can be quite complicated, although a <a href="/facts/Linear_elasticity/pPLIvs7a">linear approximation</a> may be adequate in practice if the quantities are sufficiently small. Stress that exceeds certain <a href="/facts/Strength_of_materials/CE77cUcB">strength limits</a> of the material will result in permanent deformation (such as <a href="/facts/Plasticity_(physics)/2VKo6keA">plastic flow</a>, <a href="/facts/Fracture/nxDNAj0d">fracture</a>, <a href="/facts/Cavitation/RFSxKS41">cavitation</a>) or even change its <a href="/facts/Crystal_structure/bJ4bCeHd">crystal structure</a> and <a href="/facts/Chemistry/SrNqkGVm">chemical composition</a>.

Stress (mechanics) open-in-new

Stress (mechanics)