On the accuracy of measurement and calibration of load signal in the instrumented Charpy impact test

Significant variation in measured load of an instrumented Charpy impact test is observed if specimen materials or sizes are changed from the standard specimen. Load calibration factors for different materials and sizes of specimens are evaluated by means of both experiments and finite element analysis. Elastic finite element analysis shows that the strain fields in the instrumented striker in contact with different materials and sizes of specimens are not exactly the same as those for the standard specimen. As a result of experiment and analysis, the decrease in stiffness or size of specimens results in a decrease in the calibration factor. Those are attributable to strain localization near the region in which the strain gage is attached. Therefore, the measured load for low-stiffness materials or miniaturized specimens is overestimated when the decrease of calibration factor is disregarded. Those results strongly suggest that the system must be calibrated for the different materials and sizes of specimens to obtain accurate toughness data.