The low cost and simple configuration of the test have made it a common requirement in codes for critical structures. However, the test is not useful if the variation in test results between laboratories is not tightly controlled. The control is accomplished by requiring the indirect verification of impact machines. NIST administers a program to verify the performance of Charpy impact machines by selling specimens with certified breaking strength as a standard reference material (SRM). The program is conducted in accordance with ASTM Standard E 23-02a, "Standard Test Methods for Notched Bar Impact Testing of Metallic Materials".
The verification program works as follows. NIST obtains a pilot lot of 75 Charpy specimens from a supplier and then measures the breaking strength of the specimens using three master machines. If the measurements meet certain criteria, then the rest of the specimens are machined and sent to NIST. An additional 75 specimens are selected at random from the lot and broken. If the breaking strength of the additional specimens is in agreement with the pilot lot, then the lot is certified as a standard reference material by NIST. Customers test sets of five specimens (one SRM unit) and then return the broken specimens and observed values to NIST for analysis. The data are stored in a database for future reference. Charpy SRMs are the biggest selling SRMs at NIST, accounting for 22.4 % of all SRM revenue in FY09.