Glossary

Proof load is the maximum load that a fastener (such as a bolt, screw, or nut) can withstand without experiencing any permanent deformation. It represents the highest level of stress that can be applied to the fastener while ensuring that it remains within its elastic range—meaning it will return to its original dimensions once the load is removed. Beyond the proof load point, the material begins to deform plastically, leading to a permanent stretch or distortion that can compromise the fastener’s performance and integrity.

In practical terms, proof load serves as a safety benchmark used to verify that a fastener meets its specified strength requirements. During testing, the fastener is tightened or loaded up to its proof load (usually expressed as a specific percentage of its tensile strength, often around 85% to 95%) and then released. If it returns to its original shape and dimensions without yielding, it passes the proof test.

Proof load is usually specified in terms of force (N or lbf) or stress (MPa or ksi), depending on whether it’s defined for the entire fastener or for a particular cross-sectional area. The corresponding test ensures that the fastener can handle expected service loads without stretching, loosening, or losing clamping force.

For example, in high-strength steel bolts such as property class 8.8, the proof load is approximately 80% of the minimum tensile strength. This ensures that in normal service conditions, the bolt remains in the elastic range even when fully tightened, providing consistent and reliable clamping without risk of permanent deformation.

In summary, proof load defines the upper limit of safe, elastic loading for a fastener, ensuring it can perform reliably under tension without suffering permanent damage. It is a critical parameter in fastener design, testing, and quality assurance—helping engineers select the right fastener grade and tightening torque for each application.