The National Institute of Standards and Technology (NIST), Boulder, Colo., reports that its engineers are working to improve striker design and associated instrumented strain gauges for more accurate Charpy tests. The goal is to provide SI-traceable, consistent force measurements that enable comparable force-time data across all types of Charpy machines and achieve a true dynamic calibration procedure for instrumented Charpy strikers. The design of the striker is important because it impacts the ability to accomplish this.
Instrumented strikers offer the possibility of greatly enhanced accuracy. At present, however, “there is no international agreement on the shape and configuration of the striker, where the strain gauges are placed, how many gauges are used, how close to the striking edge they are located, and more,” says Enrico Lucon, a veteran engineer in NIST’s Charpy lab in Boulder. “We’ve been working for a couple of years now, and we’re about halfway to the point of proposing an optimized design for instrumented Charpy strikers.”
NIST researchers are also working on an important related problem: accuracy concerns about the present widely used method of calibrating the strain gauges. It is a static process (static calibration) in which an exactly known force is applied to the striker and the resulting voltage is recorded.
“But impact is a highly dynamic process,” says NIST physicist Akobuije Chijioke. “The ratio of force and output voltage from the striker can change greatly during an impact that typically lasts from less than 1 millisecond to 5 milliseconds.”
NIST’s instruments can record millions of strain and voltage readings per second. “To determine the proportion of ductile to brittle fracture in various steel types, you need that kind of resolution,” Dr. Lucon says.
