In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces arise as a reaction to external forces applied to the body. Because the loaded deformable body is assumed to behave as a continuum, these internal forces are distributed continuously within the volume of the material body, and result in deformation of the body's shape. Beyond certain limits of material strength, this can lead to a permanent shape change or structural failure.
The stresses considered in continuum mechanics are only those produced during the application of external forces and the consequent deformation of the body; that is to say, relative changes in deformation are considered rather than absolute values. A body is considered stress-free if the only forces present are those inter-atomic forces (ionic, metallic, and van der Waals forces) required to hold the body together and to keep its shape in the absence of all external influences, including gravitational attraction. Stresses generated during manufacture of the body to a specific configuration are also excluded.
The dimension of stress is that of pressure, and therefore the SI unit for stress is the pascal (symbol Pa), which is equivalent to one newton (force) per square meter (unit area), that is N/m2. In Imperial units, stress is measured in pound-force per square inch, which is abbreviated as psi.
Read more about Stress (mechanics): Introduction, Theoretical Background, Euler–Cauchy Stress Principle, Equilibrium Equations and Symmetry of The Stress Tensor, Principal Stresses and Stress Invariants, Maximum and Minimum Shear Stresses, Stress Deviator Tensor, Octahedral Stresses, Alternative Measures of Stress
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