Shear stress
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Shear stress is a stress state where 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 change of the angle's magnitude (shear strain).
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.
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.
A simple definition of shear stress is, 'The components of stress at a point that act parallel to the plane in which they lie'.
Shear stress vectors have a important relevance on surfaces due to movement of fluids on it. When a fluid moves uppon a surface, there is generation of shear stress. Particularly, the laminar flux on the surface have a 0 m/s velocity, and the shear stress appears.
The biological importance of shear stress relies on blood flux. The endothelial cells recognise them and tranduces signals to vascular muscular cells and others in order to modify the vessels structure. It is necessary, because high shear stress vessels regions must have larger vessels walls.