A Study on the Measurement Instrumentation for a Custom-Made Bipendulum Impact Testing Machine

Experimental investigations of structures under impact loads are normally carried out by means of drop-weight testing rigs, which are often tailor made, and require sensors for fast transient measurement of mechanical quantities (namely, force, displacement, velocity, and absorbed energy). Since these tests are quite expensive, time consuming, and, often, not repeatable, the sensors must provide reliable and interpretable results in the first trial. In this paper, a thorough study on the measurement instrumentation suitable to carry out reliable low-velocity impact tests by means of a custom-made bipendulum impact testing machine is presented. Attention has been focused on the choice of the quantities to be measured, on the measurement problem, on the calibration of the instrumentation, and on the mechanical characterization of the impact rig. Original transducers for the measurement of either the initial impact velocity or the impact force have been developed, and their uncertainty has been estimated. The use of polyvinylidene difluoride (PVDF) sensing films for the realization of both single- and multiple-channel tailored dynamic load cells proved to be effective, gaining some significant advantages when compared to the use of more expensive commercial load cells or accelerometers fixed to the impacting mass. The detailed mechanical characterization of the bipendulum impact testing machine accomplished by exploiting the new developed transducers allowed the equivalent mass of the impactor, the alignment between the swinging mass and the sample holder, and the friction loss model to be experimentally assessed. Finally, an example of the application of the instrumented impact testing machine is presented.