What is proof strength of a bolt and how it is different from yield strength?

Proof strength, or proof load, is the full size equivalent of a yield test. Yield testing is typically only performed on machined specimens. In most fastener standards, full size testing is the preferred method, and the referee method if there is a discrepancy between the two test results.

Because of the difficulty of getting reliable test results for yield when testing full size fasteners, the proof load test was developed as an alternative. In a proof load test, a headed bolt is placed in a testing machine with a nut on the threaded end, and a wedge under the head. The wedge varies from 4-10 degrees depending on the size and configuration of fastener, and serves to evaluate the ductility of the bolt. In a machine specimen test, you test ductility by measuring elongation and reduction of area, but those are impossible during a full size test, so head deflection is used in its place. Proof load testing is typically performed at 90-93% of the expected minimum yield strength and is a simple pass/fail test. The bolt length is measured, and after being subjected to the published proof load value for 10 seconds, if it has not elongated more than 0.0005″, it is deemed to have passed.

Portland Bolt can test many grades and sizes in our in house test laboratory, including full size bolts up to 200,000lbs, and machine specimens for bolts that exceed 200,000lbs.

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7 comments

    @Hao- The proof strength on your 2-56 18-8 screw should be about 111lbf. We are unable to help with a torque value for these. Apologies.

    @Halnettige – A194 gr.16 heavy hex nuts are proofloaded to 175,000psi. Other than hardness testing, there are no other mechanical or ductility tests required.

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