Logo PTB

Investigations of impact hammer calibrations

13.09.2017

Theoretical studies and experimental tests were conducted to calibrate impact hammers in dynamic measurement set-ups with primary traceability methods. The first comparison measurements were carried out in the frame of a scientific cooperation with the Mexican National Metrology Institute CENAM.

Impact hammers are a widely used tool for the dynamic excitation and modal testing of mechanical structures. The hammers feature an integrated force transducer to measure the pulse-shaped force stimulus. The pulse width can be varied by using hammer tips of different hardness to meet the requirements. While performing modal testing, the hammer signal is compared with the structure response which is, for instance, picked up by acceleration sensors at specific points of interest.

Motivated by the increasing demand for the calibration of impact hammers, calibration set-ups for dynamic calibrations with primary traceability methods have been developed and theoretically and experimentally tested at PTB. The impact hammer hits a mass body and the transferred force pulse, the reference force, is given by the product of mass (known by weighing) and acceleration (measured during the impact). The measurement set-ups use cylindrical mass bodies of different sizes, guided with low friction by a string suspension (pendulum) or an air bearing. The acceleration in the axial direction of impact is measured by a laser vibrometer or an acceleration sensor at the body’s opposite end face. This signal and the hammer signal are simultaneously sampled with high temporal resolution.

The shock sensitivity defined as the ratio of the hammer signal to the reference force is determined as the calibration result. The signal analysis can be performed in the time or frequency domain. Pulse peak values are typically measured while performing time domain analyses. Shock-excited signal components of high frequency can be suppressed by an appropriate filtering.

In the frame of a scientific collaboration with the Mexican National Metrology Institute CENAM, comparison measurements could be conducted at the various measurement set-ups of both institutes for the first time. Figure 1 shows the measured pulse peak values of an impact hammer equipped with a steel tip. The gradient of the regression line represents the shock sensitivity to be determined. The values of PTB and CENAM show very good agreement (deviation of about 1 %) and meet the specification given in the datasheet.

 

Figure 1: Calibration of an impact hammer (Kistler 9726A20000) with a steel tip.

 Further information is given in [1].

 

Literature:

 [1] M. Kobusch, L. Klaus, L. Muñiz Mendoza, “Investigations of impact hammer calibrations”, Proc. of IMEKO 23rd TC3, 13th TC5 and 4th TC22 International Conference, Helsinki, Finland, 2017. Opens external link in new window http://www.imeko.org/publications/tc3-2017/IMEKO-TC3-2017-029.pdf

Contact person:

Michael Kobusch, FB 1. 7, AG 1. 73, E-Mail:Opens window for sending email michael.kobusch(at)ptb.de