Stresses are measured in pounds per square inch.
For example, if we have a bar in tension there is a stress distributed equally all over its cross-section. In other words, if the bar is 1 in. square, each particle of that square inch will bear the same stress or load. If a bar is 2 in. square then its area is 4 sq. in. and each inch of this area has an equal load or stress acting upon it. The pounds of stress per square inch on a piece in tension or compression is called the unit stress.
If a bar of 4 sq. in. cross-section is under a total pull of 36,000 lbs., the unit stress is then one-fourth of 36,000 lbs., or 9000 lbs. per square inch.
When a piece is stressed beyond the elastic limit and consequently breaks, we say that it has been stressed to its ultimate strength. The ultimate strength or breaking stress is a unit stress and is always given in pounds per square inch. The ultimate strength is that unit stress which is found just before rupture and is the greatest unit stress the piece will bear.
Suppose we find, by testing, that a bar of wrought iron 2 in. square breaks under a tension of 240,000 lbs. What is its ultimate strength? Since the sectional area is 4 sq. in. and the total stress which it took to break the bar was 240,000 lbs., the stress per unit of area will be 240,000 ÷ 4, or 6000 lbs., the ultimate strength.