Shear load causes shear stress in structural members, occurring in the plane perpendicular to normal stress. Engineers calculate shear load to prevent mechanical failures, as high loads can cause permanent sagging or deformation. Shear load is important in calculating stresses within a beam, and the maximum allowable load depends on the material and geometry of the beam. Failure distorts the geometry of a material and makes it more susceptible to fracture. Steel can tolerate more internal stresses than aluminum, and some theories emphasize shear stress as key to explaining when materials will fail.
A shear load is a force that causes shear stress when applied to a structural member. Shear stress, which is a force per unit area, occurs in the plane perpendicular to the normal stress; is created when two planes of the same object try to slide past each other. Engineers must calculate the shear load on structures to ensure that no mechanical failures occur. Too high a load can cause permanent sagging or deformation of materials.
Normal stresses occur when a material is put in tension or compression. In this case, both applied forces are along the same axis. If forces are applied along different axes, there will be shear stresses in addition to any normal stresses. A square element of material will experience forces that tend to distort it into a parallelogram. The average shear stress in a material is equal to the shear load divided by the cross-sectional area in question.
While shear stress is force per unit area, shear load generally refers only to the force itself. Therefore, the appropriate units are the units of force, most commonly Newtons or pound-forces. When a shear load is applied to a restrained material, a reaction force is responsible for holding the material stationary. This reaction force constitutes the “second” applied force; when combined with a reaction force, a single force can give rise to shear stresses.
The shear load is important in calculating the stresses within a beam. The Euler-Bernoulli beam equation relates the shear load to the bending motion through a beam. A bending moment is the torque that deflects a beam. The maximum allowable load on a beam is related to both the material and geometry of the beam: thicker beams made of stronger materials can withstand higher shear loads.
When the forces cause the internal stresses to become too high, the material fails. Settlement permanently changes the relaxed shape and dimensions of a material, such as occurs when the material is free from external forces. A paper clip can easily be carried by hand to the point of surrender. Failure not only distorts the geometry of a material, it can make materials more susceptible to fracture. Managing this risk is of crucial importance for civil and mechanical engineers.
Deciding which materials are the strongest or have the highest yield points is easier to do through experiment than through theoretical analysis. It is known, for example, that steel can tolerate more internal stresses than aluminum. The explanation of why this is the case is the subject of several competing theories. Some of these theories emphasize shear stress as key to explaining when materials will fail.
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