The Freely Expanding Ring Test—A Test to Determine Material Strength at High Strain Rates

The freely expanding ring test (ERT) is a conceptually simple test for determining the stress-strain behavior of materials at large strains and at high strain rates. This test is conducted by placing a thin ring of test material in a state of uniform radial expansion and then measuring its subsequent velocity-time history. The ring is usually propelled by a high explosive driving system. The test has not become popular in the materials property community, however, because there has been some concern about how the launching of the ring sample with an explosively generated shock wave might affect the properties to be measured. To determine the suitability of the ERT for these fundamental investigations, a series of experiments was performed on a carefully controlled material—oxygen-free electronic fully annealed copper. Recovered ring samples were analyzed and the change in hardness determined. Comparisons of the ERT data with that from Hopkinson bar tests at strain rates of about 5 × 10³ s⁻¹ indicate that the shock-induced hardness is approximately equivalent to a strain hardening of 5 percent. ERT data on this material at strain rates up to 2.3 × 10⁴ s⁻¹ are presented.