Threads and the fillet under a head are notches. ASTM E292 treats notch-rupture life as a measure of a material's ability to deform locally without cracking, so that stress can spread out around a notch. A notch-sensitive material can break at a notch sooner than a smooth bar of the same section.
The combination bar checks both in one test. The smooth section and the notch root have the same diameter, so they carry the same nominal stress; the notch adds the stress concentration. ASTM A453, A1014 and the B16 supplementary requirement of A193 all require the break to be in the smooth section.
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model dataCombination bar drawn to ASTM A453 Fig. 1 and Table 7 (type 4, after ASTM E292); the threaded ends and the shoulder length are not specified there and are drawn to look right only. Temperature, stress, minimum life, elongation and the 5,000 psi step-up from ASTM A453 Table 6 and section 8.2.4; constant load at temperature per ASTM E139; retests per ASTM A962. The lever and dead weights are one common way of applying a constant load, and the machine and furnace are generic. The stretch is exaggerated; the hours to rupture and the 8% elongation are for illustration.
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