The load at which a fastener experiences a specified amount of permanent deformation is its:

The load at which a fastener experiences a specified amount of permanent deformation is referred to as its proof load. Proof load is defined as a predetermined load that is applied to a fastener to ensure that it can withstand the specified service conditions without suffering permanent deformation. This is critical in applications where maintaining the integrity of the fastener is essential for the safety and reliability of the overall assembly.

In contrast to proof load, tensile load refers to the maximum load a fastener can withstand without breaking. Ultimate load is the maximum load that a fastener can endure prior to failure, which often means it is beyond the elastic limit of the material, leading to catastrophic failure. Working load is typically the load that a fastener is expected to carry during regular operation and is often a fraction of the proof load to provide a safety margin.

Understanding proof load is essential in design and engineering, as it helps in selecting the right fastener for specific applications, ensuring that it will perform reliably under expected conditions without experiencing deformation that could lead to failure.