Strain Gauge

The purpose of this project is to develop an understanding of the fundamentals regarding strain gages, Wheatstone bridges, and analyzing data appropriately using equations related to thin walled pressure vessels. Strain gases are very versatile, from determining the material property to being able to constantly observe the pressure inside a container. During the experiment, a strain gauge was attached to a soda can in the radial and axial direction. The attached strain gauge allowed for the strain to be measured as the can was opened. Upon completion, the results for the experiment yielded 1.01e-3 2.60e-5 for the radial direction while the strain in the axial direction was 4.39e-5 2.20e-5.
The purpose of this lab was not only to find the pressure inside a soda can, but to develop a better understanding of strain measurements and strain gages. The pressure vessel used in this lab was a can of room temperature mountain dew. A strain gauge is used in a variety of experiments that include deformation, sheering, torque and pressure sensing. Strain gauges are a series of wire or foil arranged in a grid like pattern with a thin plastic housing. They measure the amount of deformation on an object by passing an electric current through the foil. When the object starts to deform, the foil deforms as well causing a charge in resistance. This change of resistance can be used to calculate strain. Figure 2 shows an example of a typical strain gauge.
For a strain gauge to function properly, a wheat stone bridge must be used. A quarter bridge circuit was used for the lab, which is composed of three identical resisters, which is then connected to the strain gauge, which is similar to the schematic depicted in figure 3.
Experimental Approach
The strain gage and pressure vessel lab consisted of testing the strain and pressures that occur within a soda can. Measuring the strain within the soda can an Omega prewired strain gage type KFG-2-120-D16-11L1M2S was used. The gage factor for the strain gage is 2.06 1%. The power supply used to power the strain gage is an Instek PSS-2005, bridged with an Omega BCM-1 module. Also two Digital multimeters were used to obtain the change in volts within the strain gage. Microsoft Excel was used to compliment this study to obtain mathematical calculations and statistical analysis of the results.
Beginning with the lab setup, one sealed soda can at room temperature. Some preliminary preparation of the soda can must be taken to place the strain gage. With a fine grit sandpaper remove a large enough portion of paint to place the strain gage, make sure the soda can is will cleaned with no oils or debris from the paint. Glue the strain gage with super glue onto the can in a direction which both axial and radial directions are being measured.
Perform the proper connections on the Omega BCM-1 module is set, calibrate the strain gage to read zero by twisting the dial on the wheat stone bridge seen in Figure 5. Apply is small load onto the can by squeezing its sides and make sure the two multimeters are receiving a readout. The final step is to open the can and record the data shown on the multimeters. Obtaining the proper dimension of the can is important for the analytics of this lab. Measuring the thickness of the aluminum a Westward Micrometer .0001 inch is used.
Upon completing the setup and zeroing of the strain gages, the can was squeezed and the voltage change was observed to ensure proper connection of the gages. When the soda can was squeezed the voltages changed as expected. This change is resultant of the increase in internal pressure due to the increased exterior pressure applied from squeezing the can. The change in voltage through the strain gage is caused by the small wires of the strain gage being stretched and thus changing electric properties of the wire. The change in voltage signifies the change in the cross-sectional area of the wires in the strain gage.
After releasing the soda can the voltages returned to zero, and the soda can was then opened, via the pull tab on the top. The release of pressure from the interior of the soda can creates a small contraction