A Comparative Analysis of NIST Charpy Machines and Internal Reference Materials
Enrico Lucon, Raymond L. Santoyo
This report documents the results of a series of comparative analyses which were conducted at on the Charpy machines located at NIST in Boulder, Colorado. The analyses were performed both on historical data, collected over a period of more than 20 years, and newly obtained test results from almost 300 instrumented and non-instrumented impact tests on verification specimens of low, high, and super-high energy. Test results, obtained both in the past and within the current study, show that one of the so- called master machines (used to establish reference values for ASTM verification specimens) delivers consistently lower energy values in the 15 J - 20 J range. This appears to be due to its higher stiffness and hammer design (C-type), which increases its natural frequency of vibration and causes early fracture in a quasi-brittle (low-energy) test. The extremely large number of instrumented Charpy tests performed within this investigation (more than 200) allowed us to assess the reliability of the conventional static calibration procedure for our instrumented strikers, as compared to different approaches for the adjustment/correction of measured force values. Even though these approaches were found beneficial in improving the between-machine consistency, statistically significant differences remained nonetheless. The only truly reliable way to calibrate an instrumented striker appears to be by applying forces at loading rates that are equivalent to those encountered during actual tests (dynamic calibration of instrumented strikers). This should become a clear objective of the NIST Charpy program going forward.