Considerations in the design and manufacturing of a load cell for measuring dynamic compressive loads

Quasi-static loads are transmitted to the sensing element of the load cell by the particles of metallic parts and recorded without any problems or complications. In dynamic or impact loading where the duration period of the impact pulse is ranging from microsecond to millisecond the situation is quite different as the stress caused by the impact load is transmitted to the sensing element by stress waves either longitudinal for the tensile and compressive stresses or shear (torsional waves) for the shear stresses. The latter speeds are less than the first longitudinal waves, being (E/p)¹/², where E is the modulus of elasticity of the metallic part material and p is its density. These two types of waves require consideration in the design of the metallic part due to the wave transmission, reflection of these waves: when they meet a stress discontinuity defined as a change in cross sectional area or change in the acoustic impedance or both. In this paper, considerations in the design of a load cell for measuring the dynamic loads are presented and discussed under three main headings: the metallic part to which the sensing element is going to be fixed, the strain sensing element which is an electric resistance wire, adhesive, the measuring device to measure the output of the sensing element. Finally, the manufacturing and the calibration of the load cell are supported by the Hopkinson pressure and the split Hopkinson torsional bar for measuring longitudinal stresses and shear stresses respectively.