In mechanics and materials science, stress (represented by a lowercase Greek letter sigma – σ) is a physical quantity that expresses the internal forces that neighboring particles of a continuous material exert on each other. At the same time, strain is the measure of the deformation of the material, which is not a physical quantity.
Although it is impossible to measure the intensity of this stress, the external load and the area to which it is applied can be measured. Stress (σ) can be equated to the load per unit area or the force (F) applied per cross-sectional area (A) perpendicular to the force as:
When a metal is subjected to a load (force), it is distorted or deformed, no matter how strong the metal or light the load. If the load is small, the distortion will probably disappear when the load is removed. The intensity, or degree, of distortion, is known as strain. A deformation is called elastic deformation if the stress is a linear function of strain. In other words, stress and strain follow Hooke’s law. Beyond the linear region, stress and strain show nonlinear behavior. This inelastic behavior is called plastic deformation.
Stress is the internal resistance, or counterforce, of a material to the distorting effects of an external force or load. These counterforces tend to return the atoms to their normal positions. The total resistance developed is equal to the external load.
From an internal point of view, stress intensity within the body of a component is expressed as one of three basic internal load types: tension, compression, and shear. In engineering practice, many loads are torsional rather than pure shear. Mathematically, there are only two types of an internal load because tensile and compressive stress may be regarded as the positive and negative versions of the same type of normal loading.
Shear stress exists when two parts of material tend to slide across each other in any typical plane of shear upon application of force parallel to that plane. Torsion is a variation of pure shear in which a structural member is twisted. Torsional forces produce a rotational motion about the longitudinal axis of one end of the member relative to the other. Shear stress is also of great importance in nature, intimately related to the downslope movement of earth materials (as in the case of avalanches).