Dynamic calibration of uni-axial material testing machines

A calibration method of wide applicability for uni-axial material testing machines is proposed and experimentally verified. The result of the calibration is an estimate of the linear deviation between the forces indicated by the load cell of the machine, and that experienced by the tested specimen. The analysis relies upon parameter estimation of a measured frequency response function, with a dynamic model derived from a low-frequency vibration analysis of the mechanical machine structure. Details of higher order, possibly created by a flexible machine base, are also investigated. The accuracy is then considerably enhanced whenever the spectrum of the exciting force has significant amplitude at a weakly damped resonance of the base. Since the suggested method indicates the achieved model accuracy, it is easy to detect the cases when the approach is not applicable. The dynamic model can also be used for optimizing machine performance. Optimal machine damping is shown to equal 1/2, similar to the maximum flat design of second order Butterworth filters. The measured machine was found to have a damping of only 3%.