the cables of a suspension bridge as well as the elements of many man-made structures are designed to behave elastically. whey it is absolutely necessary that such elements function elastically?
When loaded past their elastic limit, and the loads are repetitive, there is frictional heating and weakening of the element (hysteresis) and a permanent deformation that reduces the load-bearing ability, shifting the load to other elements until a failure occurs.
If there is only a steady load, there are situations in which loading beyond the elastic limit is permissible, but it is not good engineering practice. It is called "yielding" or plastic flow.
The reason that elements in structures, such as cables in a suspension bridge, are typically designed to behave elastically is to ensure their safety and longevity. Elastic behavior refers to the ability of a material or component to deform under stress and return to its original shape once the load is removed.
When a structure or its components are loaded within their elastic limit, they will deform under the applied load or stress but return to their original shape when the load is removed. This elastic behavior allows the structure to withstand the external forces without any permanent damage or deformation.
However, if the elements are loaded beyond their elastic limit, they enter a realm called plastic deformation. In this situation, the material or component permanently deforms and does not fully return to its original shape once the load is removed. This can lead to significant changes in the structural integrity and load-bearing capacity.
When a material or component undergoes plastic deformation, it experiences hysteresis, which is the energy loss due to frictional heating and weakening. Hysteresis can cause the element to weaken over time and reduce its load-bearing ability. This process ultimately leads to failure, as the load is shifted to other elements of the structure.
While there may be situations in which loading beyond the elastic limit is permissible under steady loads, such as in certain engineered designs like yielding or plastic flow, it is generally not preferred. Designing structures to function elastically ensures that they can withstand repetitive loads, maintain their integrity, and avoid permanent damage or failure over time.