Dynamic calibration of instrumented Charpy strikers

Charpy V-notch testing of metallic materials involves the application of kilonewton-range force pulses on millisecond time scales, generated by impacting the notched test specimen with a striker mounted on a swinging hammer. In the instrumented version of the test, strain gauges are affixed to the striker, and the voltage output from a strain gauge bridge amplifier is recorded in real time during the impact event. Calibration of the striker allows the voltage output to be converted to an applied force. The striker is typically calibrated statically, by dismounting it from the hammer and loading it with a universal testing machine. The highly dynamic impact conditions of a test can lead to errors in the force measurement based on such static calibration, and furthermore the dynamic response of the instrumented striker may be dependent on the hammer and the mounting of the striker to the hammer. Therefore we seek to perform a true dynamic calibration of the instrumented Charpy striker, and to perform this calibration on the striker as mounted in the Charpy machine. We have developed a dynamic calibration method for instrumented Charpy strikers based on a short (relative to natural frequencies of the striker) force pulse, which approximates a delta function in time. By impacting the striker against small and very brittle ceramic specimens, we were able to generate force pulses as short as approximately 10 microseconds. Such short pulses have a nearly uniform force spectral amplitude up to a high frequency, allowing determination of the striker's frequency response function and impulse response function shapes. Furthermore, the calibration is carried out in conditions closely simulating those that occur in the use of the calibrated striker during Charpy tests. We present the results of such dynamic calibrations for instrumented Charpy strikers on two different Charpy machines, the strikers for the different machines having different geometries.