The Importance and Use of Torque Testing
What is proof load? How is proof load distinguished from ultimate and yield strength?
All of the following terms refer to the mechanical properties that aid to define the anticipated tensile strength performance of a precise fastener and can be calculated in units of force. The force is typically reported in Newtons (N) and/or pound-force (lbf). Sometimes these forces are reported in kilonewton (kN) and kilopound-force (kip). The difference is that one is used for standard measurements (inches) and the other is mostly used for metric measurements (mm).
Applying sufficient tensile force to a fastener where the outcome will not show plastic deformation is known as proof load. Proof load is considered to be around 85% – 95% of the yield strength. That is to say, the fastener must stay in its elastic range when force is applied up to its proof load. We can define Yield Strength as the tensile force that will produce permanent distortion (most commonly at 0.2% plastic strain) on a specified fastener. The maximum force a fastener must tolerate before fracture is defined as Ultimate Tensile Strength.