A fastener tensile test pulls a bolt, stud or screw until it breaks and records the highest load it carried. A proof-load test pulls it to a stated load, releases it and checks that it has not stretched permanently. Together they show whether a fastener really meets the grade marked on its head.
Ask your supplier for both, on samples from your own lot, and ask for the report before the lot ships.
What does each test show?
The tensile test gives ultimate tensile strength, which is the breaking load divided by the stress area of the thread. Where required, it also gives yield or elongation. It tells you the fastener will not fail under a given load.
The proof-load test is gentler. The bolt is loaded to a specified proof load, held, then unloaded and measured. If the length is unchanged within tolerance, the fastener passes. It shows that normal service loads will not leave a permanent set.
Neither test replaces a hardness test, which is quicker and checks for heat-treatment problems across a batch. Our note on hardness testing for castings and forgings explains how hardness readings are used the same way.
Which standard sets the numbers?
The grade on the head, such as 8.8 or 10.9 for metric bolts, points to a standard that sets minimum strength, hardness and test methods. The test method itself is also standardised. Ask the supplier which standard each test follows, and make sure your purchase order names the same one.
For the common grades and what they mean, see fastener grades and standards. Do not accept a report that only says 'tested OK'. It should show the standard, the grade, the sample size and the measured results.
How should samples be taken?
Tensile and proof tests destroy or stress the part, so they are done on a sample, not the whole lot. Agree the sample size and the sampling plan in advance, and choose samples at random from the finished, plated lot. Samples picked by the factory from a favourite carton tell you little.
Test finished parts, not raw bar. Heat treatment, plating and thread rolling all change the result. Plated high-strength bolts also need hydrogen-embrittlement control, so ask how the supplier handled baking after plating.
How do you read the report?
Check these points on every test report:
- The lot or batch number matches your order and the markings on the product.
- The standard and grade are named, and the measured values meet the minimum for that grade.
- Each test line shows the individual result, not just a pass.
- The laboratory is named, with its accreditation or the date the machine was calibrated.
- Failure location is noted for tensile tests, since a break in the thread is normal and a break under the head is a concern.
Should you witness the test?
For critical applications, send a third-party inspector to witness sampling and testing, or have samples sent to an independent laboratory. Our guide on reading a lab certificate shows the checks that apply to any laboratory report.
Next step
We can specify the tests with the manufacturer, pick the samples and have them tested. To set up a fastener order with proper checks, book a free sourcing review.