Difference Between Pressure and Stress
Pressure, stress measured in terms of force per unit area. A given force may generate high or low pressure, depending on the area over which it is applied. For example, if a total force of 100 pounds were to be distributed evenly over an area of 100 square inches, the resultant pressure (compressive stress) would be 1 psi (pound per square inch). If the same force were applied to an area of 1 square inch, the resultant pressure would be 100 pounds per square inch. This is why needles and nails are pointed. The application of a relatively low force at the head of the nail or needle generates a high pressure at the point because of the point’s tiny area, enabling it to break through tough materials.
The term pressure is applied particularly to stresses exerted uniformly in all directions, as distinguished from unidirectional compressive stresses. If a liquid or gas, for example, is compressed in a cylinder by a piston, the stress created is exerted against the entire inner surface of the cylinder, not merely against the surface opposite the piston. The weight of the earth’s atmosphere generates a pressure of approximately 14.7 pounds psi (1 atmosphere) at sea level. It is this pressure that supports the column of mercury in a barometer and drives air into our lungs when we expand our chests.
Stress in a body can be defined as resistance to external forces. It is measured in terms of the force exerted per unit of area. Generally, the force is given in pounds and the resisting area in square inches. Consequently, stress usually is expressed in pounds per square inch (psi). Sometimes it is expressed in kilopounds per square inch, which is commonly abbreviated as kips.
Stress is produced in all bodies that are subjected to external forces. The three main kinds of stress are tensile stress, compressive stress, and shearing stress. Other stresses are similar to these basic stresses or are a combination of them. For instance, a bending stress in a beam actually is a combination of tensile, compressive, and shearing stresses.