26-Sep-06 Yield Strength vs. Tensile Strength

Found a great post on the Gripboard, describing the difference between Yield Strength and Tensile Strength, regarding nail bending.

First off, tensile strength and yield strengths are completely different. The tensile strength is indeed the theoretical strength that it would take to pull a piece of the steel apart if you pulled on it like a rope. It is not a force but is given as a pressure (psi-pounds per square inch) So, if you wanted to know the force, you would take the tensile strength (in psi) and multiply that number by the cross-sectional area. So a larger piece of metal (5/16 vs. 1/4) is harder to bend because it has a larger cross-sectional area. Its also why square stock bends easier one way than the other (diamond vs. square)

When you are bending, 2 things happen to the piece at the same time. The outside radius of the bend is put into tension and the inside is put into compression. So the tensile strength does have some bearing as to how difficult a piece is to bend.

Yield strength is usually lower than tensile strength for the same piece of metal. It is the pressure at which a piece moves out of the plastic range and into the yield range. So when you bend, you are exceeding both the yield and tensile strength. If you can only flex the piece of metal and it springs back straight, you have not exceeded the yield strength.