A static load is a mechanical force applied slowly to an assembly or object. This can be contrasted with a dynamic load, which is a force that is applied rapidly. Tests of static load are useful in determining the maximum allowable loads on engineering structures, such as bridges, and they can also be useful in discovering the mechanical properties of materials.
This force is often applied to engineering structures that peoples' safety depends on because engineers need to know the maximum force a structure can support before it will collapse. Any force applied steadily without moving an object is considered a static load, and the knowledge of how much loading a structure can handle is useful for setting safety margins for the structure. Limiting the loading to one half of a structure’s maximum will give a factor of safety of two.
An elevator is an example where static loading occurs. When ten people stand in an elevator waiting for the doors to close, they are exerting a load on it that is static because the people and the elevator are not moving relative to each other. The stresses within the elevator have time to reach equilibrium under such conditions. An elevator must be tested to establish a maximum weight limit with an acceptable margin of safety.
A dynamic load, on the other hand, results when loading conditions change with time. When people move around in an elevator, they are creating a dynamic load, and the stresses at a point on the elevator may vary considerably.
Materials themselves can be subjected to a tests to discover their fundamental properties. All materials have an intrinsic limit on how much tension or compression stress they can tolerate before yielding or permanently deforming. Stress is a measure of force per unit area in a material’s cross section, and when the force per unit area becomes too great, microscopic fractures develop. If the force continues to rise, the material can break altogether.
A tensile test can be used to determine a material’s tensile strength. Objects go into tension when outward forces are applied along the same axis. If forces are applied vertically, objects will tend to become slightly taller but thinner. This deformation is temporary and will disappear once forces have subsided. When stresses surpass the yield point, however, a material will have its dimensions permanently modified.
A sample subjected to a tensile test can typically withstand stresses higher than its yield stress without breaking. At a certain point, however, the sample will break into two pieces because the microscopic cracks that resulted from yielding will grow. The stress at the point of complete breakage is called a material’s ultimate tensile strength.