Short Answer:
A gradually applied load is a type of load that increases slowly from zero to its full value over a period of time. In this case, the material or structure gets enough time to adjust to the increasing load without causing sudden stress or shock. The stress produced by a gradually applied load is usually less than that caused by a sudden or impact load.
When a load is applied gradually, the deformation of the body also increases slowly, allowing the internal stresses to distribute uniformly. This type of loading condition is very common in real-life structures such as bridges, machines, and buildings where the load acts progressively rather than instantaneously.
Detailed Explanation :
Gradually Applied Load
A gradually applied load refers to a force that acts on a material or structure slowly and steadily until it reaches its full magnitude. Unlike sudden or impact loads, a gradually applied load does not cause shock or vibration because the structure has sufficient time to resist and balance the increasing force. It is one of the most common types of loading conditions considered in mechanical and structural engineering.
When a load is applied gradually, the stress and strain increase proportionally according to Hooke’s law until the material reaches its elastic limit. This means the deformation produced in the body is directly proportional to the load applied, and if the load is removed, the material returns to its original shape.
In mechanical design and analysis, engineers assume most working loads are gradually applied because machinery and structures are usually subjected to forces that rise steadily over time. This assumption helps in designing safe and durable components.
Behavior Under Gradually Applied Load
When a load acts gradually, the internal resistance of the material develops simultaneously as the load increases. Therefore, the work done by the load is equal to the strain energy stored in the material. The relation between stress, strain, and energy helps in understanding the strength and deformation behavior of materials under such conditions.
For a gradually applied load, the work done by the load is given by:
where,
- = Final load applied
- = Final deformation of the body
This equation shows that the energy stored in a material due to a gradually applied load is half of that stored when the same load is suddenly applied. Hence, gradually applied loads are safer for structures and machine parts.
Examples of Gradually Applied Load
- Bridge loading – When vehicles move on a bridge, the load increases gradually as the vehicle comes onto the span.
- Machine operation – Machines like presses or cranes often apply loads gradually during their operation.
- Construction structures – When floors, beams, or columns are loaded by furniture or human occupancy, the load increases progressively.
- Hydraulic systems – The pressure is increased slowly, which causes a gradual increase in the load on the piston or cylinder.
In all these cases, since the load increases slowly, the materials experience less dynamic stress and have a longer service life.
Comparison with Sudden Load
A sudden load is applied instantly, while a gradually applied load takes time to reach its maximum value. The stress produced by a sudden load is twice that produced by a gradually applied load of the same magnitude. This is because the kinetic energy associated with the sudden application of force is absorbed by the material as strain energy. Hence, gradually applied loads are more desirable and safe in most engineering designs.
Applications in Mechanical Engineering
Gradually applied loads are commonly found in:
- Machine frames and foundations.
- Structural members such as beams and columns.
- Lifting machines and cranes.
- Hydraulic and pneumatic systems.
- Vehicle suspensions where loads increase smoothly due to road contact.
Designing components for gradually applied loads helps reduce fatigue, deformation, and failure, ensuring reliability and long-term performance.
Conclusion
A gradually applied load increases slowly from zero to its final value, allowing the structure to adjust without any sudden impact. The stress and deformation caused by this type of load are smaller and more predictable than those produced by sudden loads. In engineering, it is often considered the most practical and realistic form of loading, ensuring safety, durability, and efficiency of machines and structures.