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A shear load is a force that causes shear stress when applied to a structural element. Shear stress, which is a force per unit area, occurs in the plane perpendicular to normal stress; it is created when two planes of the same object are trying to slide past one another. Engineers need to calculate the shear load on structures to make sure they don’t experience mechanical failure. Too high a shear load can cause materials to yield or permanently deform.
Normal stresses occur when a material is put into tension or compression. In this case, both applied forces are along the same axis. If the forces are applied along different axes, there will be shear stresses in addition to any normal stresses. A square element of the material will experience forces that tend to skew 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 unites are the units force, most commonly Newtons or pounds-force. When a shear load is applied to a constrained material, a reaction force is responsible for keeping the material stationary. This reaction force constitutes the “second” force applied; 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 movement throughout a beam. A bending moment is the twisting torque than causes a beam to deflect. The maximum allowable shear load on a beam is related to both the material and geometry of the beam—thicker beams made of stronger materials can endure higher shear loads.
When forces cause internal stresses to become too high, a material will yield. Yielding permanently changes the relaxed shape and size of a material, as occurs when the material is free from external forces. A paperclip can easily be brought to the yield point by hand. Yielding not only distorts the geometry of a material, but it can make materials more susceptible to fracture. Managing this risk is of crucial importance to civil and mechanical engineers.
Deciding which materials are the strongest, or have the highest yielding points, is easier to do through experiment than through theoretical analysis. It is common knowledge, for example, that steel can tolerate more internal stresses than can aluminum. The explanation of why this is the case is the subject of several competing theories. Some of these theories emphasize shear stress as fundamental to explaining when materials will yield.