Shear stress
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In physics, shear stress is a stress state in which the shape of a material tends to change (usually by "sliding" forces -- torque by transversely-acting forces) without particular volume change. The shape change is evaluated by measuring the relative change in the angle between initially perpendicular sides of a differential element of material (shear strain). A simple definition of shear stress is the components of stress at a point that act parallel to the plane in which they lie.
In laboratory testing, shear stress is achieved by torsion of a specimen. Direct shear of a specimen by a moment induces shear stress, as well as tensile and compressive stress.
Structural members in pure shear stress are the torsion bars and the driveshafts in automobiles. Riveted and bolted joints may also be mainly subjected to shear stress. Cantilevers, beams, consoles, and column heads are subject to composite loading, consisting of shear, tensile, and compressive stress.
Image:SubsidedRoad.jpg Also constructions in soil can fail due to shear; e.g., the weight of an earth-filled dam or dike may cause the subsoil to collapse, like a small landslide.
Shear stress vectors are relevant to the motion of fluids upon surfaces, which result in the generation of shear stress. Particularly, the laminar flux on the surface has a 0 m/s velocity, and the shear stress appears.
The biological importance of shear stress relies on blood flux. The endothelial cells recognise shear stresses and transduce signals to vascular muscular cells and others in order to modify the vessel structure. It is necessary, because high shear stress vessel regions must have larger vessel walls.