Dear All,

It is well known that above 10 Hz in 4PB tests mass inertia forces due moving masses between the two inner clamps and the load cell play a role. Some producers of 4PB devices corrected for this before presenting the output of bending deflection Vb and phase lag between force F and deflection Vb. If not the output data consist out of the measured total deflection Vt and phase lag between Vt and F and the user has to correct this. Because the two differential equations for Vb and the shear deflection Vs are “coupled” by the mass inertia forces this is not easy. At the 4PB platform there is an Excel program “Bending and Shear” in which this problem is solved by an iteration process. This isn’t a desirable option for automatically processing the measured data. For the frequency range of 10 to 30 Hz (often used in frequency sweeps) I have developed an approximation that can be used in the processing of data. It is based on an adopted constant ratio of |Vb|/|Vt| = 0.96 and assuming that the phase lags of Vb and Vt don’t differ much. The note describing the used formulas is available on the same 4PB platform on the tab “Excel programs”.

Best regards and stay safe

Ad