Influence of striker design on the absorbed energy of low-energy reference specimens used for the indirect verification of charpy impact testers

Attempts to optimize the design of instrumented strikers for accurate dynamic force measurement were successful for a Toni MFL 300J Charpy impact tester (C-type hammer). However, the instrumented strikers for a Tinius Olsen IT406 machine (U-type hammer) failed to meet the indirect verification requirements at the low-energy level specified in ASTM E23-18, Standard Test Method for Notched Bar Impact Testing of Metallic Materials. To investigate whether the design of the striker contributes to the absorbed energy of low-energy reference specimens, the Charpy impact test was simulated for different striker designs by using finite element analysis. This exercise confirmed a contribution to the absorbed energy, attributed to the elastic energy stored in the striker at the point of brittle fracture (striker energy). The striker energy is proportional to the striker compliance, which is much larger for strikers for U-type machines than for C-type machines. Consequently, the absorbed energy of low-energy reference materials tested on C-hammer machines will be lower than when tested on U-hammer machines. The possibility of correcting the absorbed energy by subtracting the striker energy was investigated. For this purpose, historical records of instrumented tests on low-energy reference materials performed on pendulum impact testers with different strikers were analyzed. Correcting the absorbed energy shifts the striker bias. With no correction, more than half of the indirect verifications at the low-energy level failed, whereas nearly all passed after correction. Although a bias remained for individual strikers, the averaged corrected bias of all strikers was close to zero, indicating that the proposed method of correcting the absorbed energy of low-energy reference materials for the striker energy can potentially cancel out the striker influence, thereby yielding a striker/machine-independent value for low-energy reference materials.