Volume 37, Issue 6 (November 2009)
Influence of Loading Rate on the Calibration of Instrumented Charpy Strikers
One of the key factors for obtaining reliable instrumented Charpy results is the calibration of the instrumented striker. The conventional approach for establishing an analytical relationship between strain gage output and force applied to the transducer is the static calibration, which is preferably performed with the striker installed in the pendulum assembly. However, the response of an instrumented striker under static force application may sometimes differ significantly from its dynamic performance during an actual Charpy test. This is typically reflected in a large difference between absorbed energy returned by the pendulum encoder (KV) and calculated under the instrumented force/displacement test record (Wt). Such difference can be either minimized by optimizing the striker design or analytically removed by adjusting forces and displacements until KV=Wt (the so-called “dynamic force adjustment”). This study investigates the influence of increasing force application rates on the force/voltage characteristics of two instrumented strikers, one at NIST in Boulder, Colorado and one at SCK•CEN in Mol, Belgium. Force was applied to the strikers using servohydraulic testing machines up to maximum loading rates of around 3,000 kN/s, approximately 60 times lower than the typical loading rate during the elastic portion of an instrumented Charpy test (∼180,000 kN/s). At SCK•CEN, force was applied to the striker by cyclic loading at increasing frequencies and using a single ramp function having the desired slope (“single-shot” loading). It is observed that, under cyclic loading, the sensitivity of the strain gages drops significantly above a threshold loading rate, which may depend on the striker design (∼100 kN/s for the NIST tup, ∼1,000 kN/s for the SCK•CEN tup). Conversely, in the case of single-shot loading, the force/voltage relationship does not vary significantly up to ∼3,000 kN/s and remains close to the one obtained from the static calibration.