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Static loads are slowly applied mechanical forces, while dynamic loads are rapidly applied. Static load tests determine maximum allowable loads on structures and materials’ mechanical properties. Elevators experience static loads, while dynamic loads occur when loading conditions change over time. Tensile tests determine a material’s tensile strength by subjecting it to tension until it breaks.
A static load is a mechanical force slowly applied to an assembly or object. This can be countered with a dynamic load, which is a force that is rapidly applied. Static load tests are useful for determining maximum allowable loads on engineering structures, such as bridges, and can also be useful for finding out the mechanical properties of materials.
This force is often applied to engineering structures upon which human safety depends because engineers need to know the maximum force a structure can support before it collapses. Any force that is consistently applied without moving an object is considered a static load, and knowing how much load a structure can handle is helpful in setting safety margins for the structure. Limiting the load to half of a structure’s maximum will give a safety factor of two.
An elevator is an example where a static load occurs. When ten people stand in an elevator waiting for the doors to close, they are putting a load on it that is static because the people and the elevator aren’t moving relative to each other. The stresses within the lift have time to reach equilibrium under such conditions. A lift must be tested to establish a maximum weight limit with an acceptable safety margin.
Dynamic loading, on the other hand, occurs when loading conditions change over time. As people move in an elevator, they create a dynamic load and the stresses at one point in the elevator can vary considerably.
The materials themselves can be tested to discover their fundamental properties. All materials have an inherent limit to the amount of tension or compression they can tolerate before permanently failing or deforming. Stress is a measure of the force per unit area in the cross section of a material, and when the force per unit area becomes too great, microscopic fractures develop. If the force continues to increase, the material may break altogether.
A tensile test can be used to determine the tensile strength of a material. Objects come into tension when outward forces are applied along the same axis. If forces are applied vertically, objects will tend to get slightly taller but thinner. This deformation is temporary and will disappear once the forces subside. When stresses exceed the yield point, however, a material will have its dimensions permanently changed.
A specimen subjected to a tensile test can typically withstand stresses in excess of its yield strength without breaking. At some point, however, the specimen will break into two pieces because the microscopic cracks resulting from the yielding will grow. The stress at the point of complete failure is called the tensile strength of a material.
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