Internal-pressure tests on hot-rolled- and cold-rolled-steel cylinders yield pressure-expansion curves of the same shape as those determined mathematically. The value of the flow stress can be determined by finding the theoretical pressure-expansion curve which most nearly fits the data. Values of flow stress determined in this way agree closely with the yield strength found from a tensile test. Die-steel cylinders, heat-treated to hardnesses up to 62 Rockwell C, exhibit plastic flow before failure, and the stress-strain curve permits the calculation of the flow stress by this method. The flow stress increases with the hardness, reaching a value as high as 440,000 psi for a Rockwell C hardness of 62. It can be shown mathematically that a partly plastic cylinder may fail due to a circumferential stress which is well below the flow stress. Values of fracture stress determined from cylinder tests bear out the foregoing conclusion, although the fracture stress is slightly higher than that determined by tensile tests.

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