Design and Dynamic Force Verification of Instrumented Charpy Strikers for the Tinius Olsen Pendulum Impact Tester

What is it about?

The reliable measurement of characteristic force values (yield, maximum, brittle fracture and crack arrest) from instrumented Charpy impact tests is considered of high importance at SCK•CEN in view of the Enhanced Surveillance Approach of nuclear reactor pressure vessels. International standards related to instrumented Charpy impact testing, ASTM E2298 and ISO 14556, try to provide some guarantee on the accuracy of the measured dynamic force by specifying a permissible error for the static force calibration of the striker and limiting the difference between the absorbed energy, given by the dial and/or optical encoder, and the energy computed from the force-time curve. The ISO 14556 standard considers a maximum difference of ±5 J acceptable. In contrast, the ASTM E2298 standard allows a much larger difference of up to 15% or 1 J, whichever is larger. If this requirement is not met but the difference does not exceed 25 % or 2 J, whichever is larger, force values shall be adjusted to equalize absorbed and computed energy. From practical experience it is known that the dynamic performance of poorly designed instrumented strikers is often characterized by large differences between absorbed and computed energy and non-linear distortions of the force-time curve, and the requirements of the ASTM standard may not guarantee an accurate dynamic force measurement or even introduce large errors. The use of standard reference materials for the verification of the dynamically measured maximum force is under investigation. Interlaboratory studies however showed that the reproducibility limit on the maximum force can be as large as ±33 %. The large scatter is mainly attributed to differences in pendulum impact tester, possible influence of the striker radius but also the striker design. In the absence of a better understanding to what extent the dynamically measured maximum force is influenced by external factors, the dynamic force verification using standard reference material is considered unreliable. To gain insight in the dynamic performance of instrumented strikers and to identify ways to improve the accuracy of the dynamic force measurement, instrumented ASTM (8 mm radius) and ISO (2 mm radius) strikers were developed for the Toni MFL 300 Joule pendulum impact tester at SCK•CEN. This research showed that the design of the striker, in particular the location of the strain gauges, significantly affects the performance of instrumented strikers used for Charpy impact testing. Placing the strain gauges further away from the striking edge improves the linearity of the static force calibration and reduces the difference between the absorbed and computed energy. The general yield and maximum force are a function of the position of the strain gauges and striker radius. And the characteristic ‘tail’ in the final portion of the force-time curve for an ASTM striker disappears when the strain gauges are placed further from the striking edge. Comparison of the yield stress obtained from dynamic tensile tests and the general yield stress calculated from the instrumented impact tests of the same material revealed that by placing the strain gauges further from the striking edge, the general yield stress converges till in good agreement with the yield stress from dynamic tensile tests. Because this procedure holds for a large temperature range, is (nearly) independent of striker radius and is based on the comparison of a material property, this method has been adopted as the recommended in-house procedure for verifying instrumented strikers. In continuation of this research and to further improve our design know-how, instrumented Charpy impact strikers are developed for the Tinius Olsen model 74 pendulum impact tester at SCK•CEN. New insights to improve striker performance are incorporated in the design. The strikers are extensively characterized. Performance conform the requirements set by the standards, is verified by static force calibration of the striker and evaluation of the difference between absorbed and computed energy over a wide energy range. The accuracy of the dynamic force measurement is assessed by testing NIST (National Institute of Standards and Technology) reference specimens with a certified maximum force value and the comparison of the general yield stress calculated from the instrumented impact test with the yield stress obtained from dynamic tensile testing of the same material. This paper summarizes the design, validation and dynamic force verification of the instrumented ASTM (8 mm radius) and ISO (2 mm radius) strikers for a Tinius Olsen pendulum impact tester.

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Why is it important?

• The developed strikers meet the accuracy requirement for the static force calibration and the permissible difference between absorbed and computed energy as set by the current ASTM E2298 and ISO 14556 standard. • The ASTM strikers also meet the more stringent requirement on the permissible energy difference stipulated in the ISO 14556 standard of ±5 J. This indicates that when instrumented strikers are properly designed there is no need for different requirements on the absorbed energy between the ISO 14556 and ASTM E2298 standard. • The comparison of the general yield stress inferred from instrumented Charpy impact tests, using Server’s equation, with the yield stress obtained from dynamic tensile testing of the same material at an equivalent strain rate, proved to be successful for the verification of the dynamically measured force of the instrumented impact strikers.