Influence of Strain Gage Position on the Static and Dynamic Performance of Instrumented Impact Strikers

What is it about?

The Charpy test is more than 100 years old and is still widely used in laboratories all over the world to obtain information on the toughness of materials. Instrumented impact testing started as a laboratory curiosity in the early 1920’s. The volume of work on instrumented impact testing increased drastically in the late 1960’s. At SCK·CEN, the reliability of force measurements obtained from instrumented Charpy tests is considered of primary importance in view of the so-called “Enhanced Surveillance Strategy” of nuclear reactor pressure vessel steels. We have demonstrated that the effects of neutron exposure on fracture toughness can be more reliably assessed by using alternative Charpy index temperatures obtained using the load diagram approach, in which characteristic force values (yield, maximum, brittle fracture, and crack arrest) are represented and analyzed as a function of test temperature. Characteristic temperatures obtained from the load diagram can more reliably assess the effect of service exposure on the ductile-to-brittle transition temperature (DBTT) and cleavage fracture toughness than index temperatures corresponding to fixed amounts of Charpy absorbed energy (41 J). Starting in 2002, the development of in-house instrumented strikers was initiated at SCK·CEN. The motivation was that most of the commercially available instrumented strikers, besides being expensive, had several shortcomings and some did not perform satisfactorily. From a mechanical viewpoint, it was observed that often the wiring was cumbersome and the strain gages had insufficient protection from manipulation (especially in a hot cell environment) or flying broken specimens. For every failure, a long delivery time for a new striker has to be taken into account and replacement requires an expensive intervention. Evidence was also found that the position of the strain gages with respect to the striking edge could significantly affect striker performance, in terms of nonlinearity errors and signal drift during static calibrations. These observations prompted further research aimed at optimizing the strain gage location with respect to: • static performance during striker calibration, i.e., linearity and hysteresis; • dynamic performance, i.e., relationships between absorbed energies (measured by the machine encoder and calculated from the instrumented force/displacement curves) and between dynamic yield stresses (from Charpy yield forces and from tensile tests).

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