A strain gauge is very commonly used for this purpose. The basic principle of operation of strain gauge is the variation of resistance of a metallic conductor when the load is applied onto it. The applied load is limited to elastic deformation regime and the change in resistance of the strain gauge is very small in the range of 0.1% of the resistance of the strain gauge. Therefore, direct measurements of the resistance change by current and voltage measurements are not expected to give an accurate result and strain resolution of such a method is also not expected to be very good. Therefore, the change in resistance is generally measured by means of imbalance in Wheatstone bridge and the imbalanced voltage signal is magnified as well by using an amplifier. In these experiments, the three electrical techniques i.e. direct measurement of current and voltage across the strain gauge resistor, direct measurement of the imbalance in Wheatstone bridge arrangement and Wheatstone bridge and amplifier techniques have been explored. The objectives of this experiment are:
The experimental rig, shown in Figures 1 and 2, has a cantilever beam with a pair of 120 strain gauges, attached one on the upper and one on the lower beam surfaces. The strain-setting device is a bolt with a metric thread.
The strain in the cantilever beam gets transmitted to the strain gauges mounted on it and causes a corresponding change in the resistance of these gauges. The strain gauge mounted on the upper side is under tensile strain and its resistance increases while the one mounted on the lower side gets compressed and its resistance decreases. An analysis to calculate strain theoretically from the measured voltage signal is presented below:
When strain is applied on the cantilever beam, the strain is transmitted to the strain gauge and the strain gauge resistance changes.