Design Optimization of Instrumented Strikers for Charpy V-Notch Pendulum Impact Testing

What is it about?

The Charpy impact test was originally devised to measure the amount of energy absorbed by a material during fracture. Instrumentation of the striker extended the scope of this inexpensive dynamic test of a notched specimen significantly. By not only recording the absorbed energy but also the force applied to the specimen as a function of time, additional information about the material’s properties and a better understanding of the ductile-to-brittle-transition response is obtainable. At present day however, no internationally accepted procedure to calibrate or verify the measured dynamic force exists. The static force calibration of an instrumented striker is no guarantee for a good dynamic performance. Differences in distribution and location of the applied force between calibration and actual test can result in large deviations between the angle-based energy and the energy computed from the force-time curve. And in such cases the reliability of the obtained forces is questionable. From an engineering viewpoint, an instrumented striker for which a static force calibration is sufficient to accurately measure the force applied to the specimen during a Charpy impact test would be ideal. To investigate if such an instrumented striker can be designed, the influence of the striker geometry, the location of the strain gauges and the static force calibration procedure on the force measured by the striker is assessed using finite element analysis (FEA). Due to the large amount of scenarios only a striker with a 2 mm radius (ISO) is treated in this study. The effect of the various parameters on the measured force is determined as follows. The voltage response of the strain gauge bridge on the striker is calculated as a function of the force applied to the back of the striker. With the striker in contact with a special support block or imprinted specimen, the static force calibration procedure is simulated. The obtained voltage-force relationship (static calibration curve) is used to convert the voltage response of a simulated Charpy impact test (quasi-static simulation) to force values. The force measured by the striker and derived energy is compared with the applied force and corresponding energy. The results of this elaborate study, including design recommendations to improve the accuracy of the force measured by instrumented Charpy impact strikers, are summarized in this paper.

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Why is it important?

In this paper it has been demonstrated that • A static force calibration is sufficient to accurately measure the force during an impact test when the instrumented striker is designed to be insensitive to the load distribution at the striking edge. • Finite element analysis shows that an ISO striker with a height of 12 mm and strain gauges at a distance of 17 mm from the striking edge, located left/right or top/bottom, is the best design for testing standard Charpy V-notch specimens. • The conversion of the output voltage of the strain gauge bridge to force values using the static calibration curve introduces errors in the force when the striker is sensitive to the load distribution at the striking edge. This distortion is non-linear and adjustment of the force values to equalize the computed and angle-based energy is not allowed. • The accuracy of the dynamic measured maximum force using standard reference materials can be greatly improved when instrumented strikers are designed to be insensitive to the load distribution at the contact between striker and specimen.