NIST Room Temperature Low-Energy and High Energy Verification Specimens: Feasibility Study
Enrico Lucon, Christopher N. McCowan, Raymond L. Santoyo
The feasibility of certifying Charpy reference specimens (SRMs) for testing at room temperature (RT, 21 °C ± 1 °C) instead of -40 °C was demonstrated at NIST by performing 130 RT tests from nine low-energy and high-energy lots on the three master machines located in Boulder. The statistical analyses performed show that in most cases the variability of results (i.e., the experimental scatter) is reduced when testing at room temperature. For eight out of nine lots considered, both the coefficient of variation and the sample size were lower at 21 °C than at 40 °C. The results of this study will allow NIST to satisfy a request for room temperature SRMs that was received from customers for several years: testing at 21 °C removes from the verification process the operators skill in transferring the specimen from the cooling bath, and puts the focus back on the machine performance. For NIST, it also reduces the time and cost for certifying new verification lots. For one of the low-energy lots tested with a C-shaped hammer, we experienced a couple of specimens jamming and yielding unusually high values of absorbed energy. For both specimens, signs of jamming were clearly visible. Jamming is slightly more likely to happen at 21 °C than at 40 °C, since at room temperature low-energy samples tend to hover in the vicinity of the test area after impact. In the evaluation of a verification set, any jammed specimen should anyway be removed from the analyses.