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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    hot dip galvanizing bolts are acceptable for API 20E Standard?, API 20E says Under sizing of external threads are not acceptable.

    @Shivaraju- In the US, we do not undersize our galvanized threads, but rather oversize the nut. That said, we are not very familiar with the specifics of API 20E, so we cannot comment about what the API says about galvanizing.

    Hi Dane,
    Why was the proof load given the preference in the US instead of yield strength (which is adopted in Europe as a critical bolt parameter)? Is there any historical standpoint behind these?

    For applications where elongation is a crucial factor, yield strength needs to be studied.


    @Jashim- I looked through ISO 898, which I am admittedly not very familiar with, and I did see proof load and full size testing required for some grades. It appears to vary by grade, some grades require full size and proof load, others require tensile/yield/elongation/RA. That is the case in many of the US standards as well – some require full size and proof load testing whereas others require tensile/yield/elongation.
    As for historical information, most of these were written decades ago, so I cannot be certain. I know for the structural grades (like A325/A490), the full size performance is a preferred unit of measure to a machined specimen.

    Hi Dane , Is it possible the yield strength of grade 10.9 bolt is O.K but its proof load test result to be greater than 12.5 micron (according to ISO 898-1 ).

    @Bijan- Are you asking if the bolt can pass the yield test but fail the proof load length measurement? Yes it can. However, depending on the language in the testing standard, it may or may not be determined to have failed. We are not familiar with the wording of ISO 898-1, but in most ASTM standards the proof load length test is the referee test method.

    @Noel- We can perform full size and machine sample axial testing on many grades of fastener. We are not familiar with the specifics of API 20E, so we are unable to comment specifically on that standard.

    Dane..heres where I am confused..the .2 elongation in newton meters squared vs full yield..for 304 threaded rod, one mtr offers a yield of 30 ksi in 0.2 N/mm^2 and the other offers yield in ksi without the .2 elongation mentioned..the two are significantly different values..65ksi for “yield” and 30 ksi for 0.2 N/mm^2..both are 304 threaded rod..are they they the same?
    Tensile Strength Rm N/mm^2 : 550
    Yield Strength Rp 0.2 N/mm^2 : 205
    HardnessHB : 125
    Yield Strength (N/mm’2) /Ksi 65min actual 119-122

    see that the .2 is missing on the bottom yield calc..help?

    @Yvon- Proof load is listed in ksi for brevity. Some ASTM bolting standards also list out a more detailed chart sorted by diameter with proof load values in lbf, and we would be happy to provide lbf values upon request.

    Are proof load test performed on F 1554 GR 55 or GR 105 Threaded Rods. I am proposing to raise a drilled shaft with couplers and threaded rods and the EOR is requiring the rod to meet a 100 ksi proof load stress.

    @Bruce- F1554 rods are not typically proofloaded as it is not a requirement of the standard. We can however calculate the theoretical proof stress by using the tensile stress area of the bolt and the yield strength of the material.

    @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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