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Bulk Stress

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Quick
Bulk Stress may also be known as volume stress, volumetric stress, or hydraulic stress.


Equations

(Eq1)      p = B ΔV V

Bulk stress

Nomenclature

B	bulk modulus
ΔV	absolute value of the change in volume of object
V	original volume of object

Details

Fig1. A solid sphere subject to uniform bulk stress from a fluid.

In Fig1, the stress is the fluid pressure p on the object. The quantity ΔV/V is the bulk strain. The object is said to be under hydraulic compression, and the pressure can be called the hydraulic stress. The bulk stress is then:

(Eq1)

p = B ΔV V

Generally solids have rigid atomic lattices whereas the atoms or molecules of liquids are less tightly coupled to their neighbors. This means that usually, solids are less compressible than liquids.

When a submersible plunges deep into the ocean, the water exerts nearly uniform pressure everywhere on its surface and squeezes the submersible to a slightly smaller volume. This is a different situation from the tensile and compressive stresses and strains. The stress is now a uniform pressure on all sides, and the resulting deformation is a volume change. The terms bulk stress (or volume stress) and bulk strain (or volume strain) are used to describe these quantities. Another familiar example is the compression of a gas under pressure, such as the air in a car's tire.

If an arbitrary cross section within a fluid at rest is chosen, the force acting on each side of the section is always perpendicular to it. If a force was exerted parallel to a section, the fluid would slip sideways to counteract the effort. When a solid is immersed in a fluid and both are at rest, the forces that the fluid exerts on the surface of the solid are always perpendicular to the surface at each point. The force F per unit area A on such a surface is called the pressure p in the fluid:

(Eq1)

p = F A

When a pressure is applied to the surface of a fluid in a container, such as in a cylinder, the pressure is transmitted through the fluid and also acts on the surface of any body immersed in the fluid. This principle is called Pascal's law. If pressure differences due to differences in depth within the fluid can be neglected, the pressure is the same at every point in the fluid and at every point on the surface of any submerged body.

Materials with small bulk modulus and large compressibility are easy to compress; those with larger bulk modulus and smaller compressibility compress less with the same pressure increase.