Shear stress

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.

Shear stresses within a semi-monocoque structure may be calculated by idealizing the cross-section of the structure into a set of stringers (carrying only axial loads) and webs (carrying only shear flows). Dividing the shear flow by the thickness of a given portion of the semi-monocoque structure yields the shear stress. Thus, the maximum shear stress will occur either in the web of maximum shear flow or minimum thickness.

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 is relevant to the motion of fluids upon surfaces, which result in the generation of shear stress. Particularly, the laminar flux on a surface has a zero velocity, and shear stress occurs between the zero-velocity surface and the higher-velocity flow away from the surface.

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.

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