Mechanism underlying irreversible effects after flexure excursions

Determine the precise physical mechanism responsible for the irreversible shift observed in the required holding force (i.e., the zero position/force offset) after large excursions of the flexures in the NIST QEMMS Kibble balance mechanism, which employs precipitation-hardened Copper Beryllium alloy C17200 flexures, in order to explain the persistent offset that remains after returning the beam to the null position despite mitigation of anelastic relaxation.

Background

The paper investigates anelastic relaxation in flexures used for Kibble balances, combining analytical modeling with experiments on a simplified beam-balance mechanism employing one main and two end flexures made from precipitation-hardened Copper Beryllium alloy C17200. Beyond the expected time-dependent anelastic aftereffect, the authors observed a residual shift in the equilibrium force/position after large deflections that does not relax away, which they refer to as an irreversible effect.

This irreversible offset scales approximately as a power law with excursion amplitude (with an experimentally observed exponent near 1.73) and, while small for the controlled mass-exchange motions planned for QEMMS, could in principle bias precision measurements if not understood and mitigated. The authors note potential mechanisms discussed in the literature (e.g., dislocation pinning, work hardening, clamping effects, lattice stick–slip) but emphasize that the specific cause in their system remains unidentified.