When a solid (say sphere) is placed in a fluid under high pressure and compressed uniformly on all sides, then the force applied by the fluid acts in a perpendicular direction at each point of the surface of the solid.

This is calledHydraulic Compression.This leads to a decrease in the volume of the solid without any change in its geometrical shape.

As a result, the body develops internal restoring forces that are equal and opposite to the forces applied by the fluid. The body restores its original shape and size when taken out from the fluid.

The internal restoring force per unit area in this case is known as **hydraulic stress** and in magnitude is equal to the **hydraulic pressure** (applied force per unit area).

**Definition of hydraulic stress**

A solid body develops internal restoring forces when it faces hydraulic compression. This internal restoring force per unit area of the solid body is called Hydraulic Stress.

**Formula of hydraulic stress**

Hydraulic stress = F/A = hydraulic pressure.

F = restoring force developed internally in a solid body under hydraulic compression

A = surface area of that solid body

**Volume strain**

The strain produced by hydraulic pressure is called **volume strain** and is defined as the ratio of change in volume (∆V) to the original volume (V).

Volume strain=∆V/V

Read More: Mechanical Properties of Solids