Stress

Stress is defined as the load (force) put on a piece of material or a structure.

Types of stresses

1. Tensile stresses: Is the stress state leading to expansion.

   

2. Compressive stresses: Is the stress on materials that leads to a smaller volume.

   

3. Shear stresses: Is a stress state where the stress is parallel to the surface of the material.

   

Forces acting on a ship

The modern ship is made up of steel plating, sections, and built-up girders so connected as to provide adequate strength in all parts to withstand the forces acting on the ship under all conditions of service.

The forces acting on a ship may be static or dynamic.

• Static forces: Due to the difference in the weight and the buoyancy, which occur throughout the ship.

• Dynamic forces: Due to the motion of the ship at sea and the action of the wind and waves.

  

These forces create:

1. Longitudinal stress

2. Transverse stress

3. Local stress

Longitudinal Stress

• The forces are two in number: the weight of the ship and all that it carries acting downwards, and the vertical component of the hydrostatic pressure.

• Depending upon the direction in which the bending moment acts, the ship will Hog or Sag.

Hogging

• If the buoyancy amidships exceeds the weight due to loading or when the wave crest is amidships, the ship will Hog (as a beam supported at mid-length and loaded at the ends).

  

  

Sagging

• If the weight amidships exceeds the buoyancy or when the wave trough is amidships, the ship will Sag (as a beam supported at its ends and loaded at mid-length).

  

  

Transverse Stress

• A transverse section of amidships is subjected to static pressure due to the surrounding water as well as internal loading due to the weight of the structure, cargo, etc.

  

The modern ship is made up of steel plating, sections, and built-up girders so connected as to provide adequate strength in all parts to withstand the forces acting on the ship under all conditions of service.