NIST Impact Verification Program

The NIST impact verification program has provided 22 years of service to manufacturers and consumers and currently certifies eight SRMs that underpin quality control testing of impact toughness for structural steels. We provide SRMs used to verify the measurement of absorbed energy at three energy levels: low energy (14-20 J); high energy (88-136 J); and super high energy (176-244 J). We also provide two SRMs that are used to verify the measurement of maximum force in a Charpy impact test. SRMs are available for both Charpy and Izod impact testing (Figure 1).

The Charpy program supplies 2,000 units per year to customers worldwide. Typically, we evaluate and report on over 1,000 verification tests per year. We are available to our customers by email, fax or phone (1,500 contacts per year) and continually update our customer website and database to improve customer interactions. Most recently, the program has gone paperless, improving the already short turnaround time for our post-test evaluation to same-day service.

We recently added a means for our customers to obtain a proficiency test result for their impact test (Figure 2). When a customer purchases a set of SRMs, the data from all NIST verification tests for that lot are made available. Customers can compare their test results to this compiled data, giving them an alternate means (aside from meeting ASTM or ISO requirements) to evaluate the performance of their impact machines, and the relative performance of multiple impact machines within their organizations. This information supports efforts by industry to achieve or maintain quality system certification through ISO or other standards bodies.

NIST maintains leadership positions on ASTM and ISO standards committees and has long been active in improving impact testing standards both in the U.S. and around the world. We maintain an extensive database of customer data in order to further improve measurement accuracy and support our ISO and ASTM activities. For example, we publish a guide on uncertainty analysis for Charpy tests that offers users a full explanation of the uncertainty associated with the NIST reference specimens and the customer's verification test.

We are currently working with several national metrological institutes (NMIs) to develop an approach to SRM certification that reduces the bias among the NMIs. Our focus is on standardizing the design of instrumented strikers to provide comparable load-displacement data across all types of Charpy machines. If successful, absorbed energy measured under the instrumented impact curve will be traceable to force and time, moving the underpinnings of the measurement to more fundamental quantities (Figure 3). This approach, like the current approach of measuring the energy absorbed from a pendulum swing (in Joules), has many practical issues that will need to be addressed. However, we see it as the future in impact testing and the best hope for establishing a scale for measuring absorbed energy that can help to reduce bias among NMIs.

Figure 3. Force/time data acquired from instrumented Charpy test.