Knowledge base
Test bar
Why a companion specimen from the same heat accompanies every lot
Term explained
Why a companion specimen from the same heat accompanies every lot
A test bar is a section of the same bar stock as the parts of a lot, which passes through all production steps with them and is used at the end for destructive materials testing. Because it comes from the same heat and has received the same heat treatment, its values verify the strength, toughness and hardness of the entire lot without a finished bolt having to be destroyed. Testing is carried out to ISO 6892-1 and ISO 148-1 or to ASTM A370, E8 and E23, depending on the standard or customer specification.
How the test bar travels through production at RS Randack
During cutting, at least one test bar is sawn for each lot, and always for parts subject to third-party inspection. The last digits of the order number are stamped on its end face, with an additional character where the same diameter comes from different batches. The bar travels visibly with the parts through forging and heat treatment. After quenching and tempering, quality assurance removes it and tests it in our own materials laboratory: tensile test with tensile strength, yield strength, elongation and reduction of area, hardness, on request impact energy at test temperature, and microstructure in the micrograph. If a test fails, the entire order is blocked until the deviation has been resolved.
How long does a test bar need to be?
That depends on the tests: the Charpy impact test requires at least 55 mm, the tensile specimen considerably more for large diameters, and elevated-temperature tensile tests and bend tests even longer bars. Production planning sets the length for each order and writes it on the works order.
Test bar or full-size bolt?
Both. The test bar provides the material properties, while the tensile test on the full-size bolt also shows the behaviour of the thread and the head-to-shank transition. For third-party inspections, both tests are often required; for bolts above M64 in 10.9, the test bar remains the evidence, because the full-size bolt exceeds the capacity of the tensile testing machine.
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