A test load applied to a complete bolt, screw or nut, which must survive it without permanent set. For bolts, length is measured before and after loading and any elongation beyond a tiny tolerance fails the part; a nut must neither strip nor seize on the test mandrel. Proof stress sits around 90 percent of minimum yield, for example 85 ksi for SAE Grade 5 and 120 ksi for Grade 8, and preload targets are commonly set as a percentage of proof load.
All five pieces passed proof load with no stretch on the length check, so the lot can ship.
Also heard as
- proof stress
- proof test
Do not mix it up with
- Tensile strength: Tensile strength is the load at which the fastener breaks; proof load is the lower load it must survive with no permanent stretch.
Related terms
Preload
The tension created in a bolt by tightening it, which clamps the joint and keeps it from slipping or opening.
Tensile strength
The maximum pulling load a fastener can take before breaking, stated as stress over its tensile stress area.
SAE grade
The inch bolt strength system, chiefly Grades 2, 5 and 8, identified by radial lines on the head.
Property class
The metric strength grade of a bolt or nut, stamped on the head as a number such as 8.8, 10.9 or 12.9.