Strain Gauge - Lab Report Example

LAB REPORT: STRAIN GAUGE By Location Table of Contents .....................................................................................................................................3 Introduction................................................................................................................................3 Aims and Objectives..................................................................................................................4 Equipment..................................................................................................................................4 Experimental Procedure.............................................................................................................6 Experimental Data......................................................................................................................6 Statistical Properties of the Data...............................................................................................8 Effects of Temperature on Results and Errors in Experiment...................................................9 Conclusion................................................................................................................................10 References................................................................................................................................11 Abstract 3 Introduction 3 The strain gauge, an instrument that was invented in 1938, is used to measure the strain on an object. Strain is the resultant deformation or fractional change that occurs on an object as a result of the effect of a certain force. The instrument usually has a metallic foil attached to it and as much as the object is deformed, the foil is also deformed. As a result, the electrical resistance of the foil changes and this change is determined using the Wheatstone bridge, a value which correlates with the strain of the object through the gauge factor.The figure below from Wikipedia explains what strain is. 3 Aims and Objectives 4 Equipment 5 Experimental Procedure 6 Experimental Data 7 Relation of Load and strain 7 Calculation of the Mass of the Connecting Rod 9 Statistical Properties of Data 9 Lab Report: Strain Gauge / Bridge Circuit Abstract An experiment was performed to study variation of strain in the beam with changing load. For this purpose, the load-strain data was plotted and the equation was derived to obtain a mathematical relationship. Later, the mathematical expression was used to find the load of connecting rod. The statistical properties of the data were studies in light of material properties. Introduction The strain gauge, an instrument that was invented in 1938, is used to measure the strain on an object. Strain is the resultant deformation or fractional change that occurs on an object as a result of the effect of a certain force. The instrument usually has a metallic foil attached to it and as much as the object is deformed, the foil is also deformed. As a result, the electrical resistance of the foil changes and this change is determined using the Wheatstone bridge, a value which correlates with the strain of the object through the gauge factor.The figure below from Wikipedia explains what strain is. Aims and Objectives The main aims and objectives of this particular experiment are to achieve the following: 1. To obtain Load and strain data 2. To plot stains with varying loads. 3. To work out mathematical relation between strain and load. 4. To calculate weight and mass of given connecting rod using strain value. 5. To discuss statistical properties Equipment The equipment used for this experiment is shown in Figure 1 below. Figure 1: Equipment used for Experiment Some of the uses of the equipment shown in Figure 1 above are as follows: 1. Bridge Circuit / Strain gauge meter 2. Strain gauge 3. Bending beam 4. Weights Figure No 2: Connections on Signal processing box Figure No 3: Connecting rod Experimental Procedure Following procedure was used for this experiment: 1. Bridge circuit connections were made as shown in Figure No 2. 2. The weight of 5N was added on the hanger on the beam. 3. The strain value was noted. 4. The weights were added with an increment of 5N until 20N. Strain values were noted for each load. 5. The process was repeated twice to get two sets of readings. The data is tabulated in Table No 1. 6. The connecting rod, shown in Figure No 3 was then hanged and its strain value was noted. . Experimental Data Load Strain Gauge N Reading-1 Reading-2 0 0 0 5 0.9 1 10 1.8 2 15 2.9 3 20 3.8 4 Table No 1: Experimental Data Relation of Load and strain The plot shown in Figure No 4indicates variation of average strain on the strain gauge reading with varying loads on hanger. It can be observed that the strain varies linearly throughout the range of experiment. Using this data, the equation of strain is calculated using equation of line: Now, using this expression, the load can be worked out for any measured value of strain. Figure N 4: Variation of strain with Load Calculation of the Mass of the Connecting Rod Statistical Properties of Data In order to examine the statistical properties of the data, the mean and standard deviation of the two set of data is calculated and is shown in Table No 2. From the data, it can be observed that strain values in reading-1 are lower than the values in reading-2. The reason why initial strain readings were low is that the structure had an initial ‘slackness’ due to which the resulting strains were lower. However, once the structure was deformed, the subsequent deformation values were higher. The higher strain values are also due to the fact that the beam was not given enough time to come back to its original position and therefore, reading-2 was slightly higher. Therefore, mean value was chosen for calculation of connecting rod. The standard deviation varies between 0.071 – 0.1414 (Buivolov 2000). Load Strain Gauge statistical Properties N Reading-1 Reading-2 Mean Value Standard Deviation 0 0 0 0 0 5 0.9 1 0.95 0.070711 10 1.8 2 1.9 0.141421 15 2.9 3 2.95 0.070711 20 3.8 4 3.9 0.141421 Table No 2: Statistical Properties of data Effect of Temperature Variations on the Results An increase in temperature would cause the object to undergo thermal expansion hence increase in size. Resultantly, the change in size will be detected by the gauge as strain with its resistance and that of the connecting wires changing. However, the constantan alloys that make up the gauges are made in such a way that they counter resistance changes that are recorded due to thermal expansion. It is upon the person carrying out the experiment to select the best alloy for the object being experimented on because thermal expansion varies among various materials. Errors The zero Offset – after connecting the strain gauge to the source of force, a zero offset would occur if the impedance of the four arms of the gauge are different. However, this can be rectified when one uses a resistor that is parallel to one of the arms. Temperature Coefficient of Gauge Factor is an error that occurs due to effects of temperature on the recorded strain. As discussed earlier, changes in temperature would cause changes in the size of the object hence this change would in turn be recorded as strain. To correct this, a fixed resistance should be introduced at the input pole with voltage minimizing temperature sensitivity. Linearity error occurs when the sensitivity of the value of strain is affected by pressure in a function of thickness and quality of bonding. Overloading error may also occur if the strain gauge is loaded with weights it is not designated to accommodate (Yan et al. 2005). Conclusion A linear relation between load and strain values was observed. This indicates that the applied load corresponded to the stress level which is within linear-elastic range. However, the two data sets had a difference causing some standard deviation. This variation is not due to any precision of the instrument. Instead, this variation is caused by material slackness and some residual deformation in the beam from previous loading. References Buivolov, A.L. 2000, "Analytical method for determining the inverse functions of a system of nonlinear equations describing the static characteristics of a wind-tunnel strain-gauge balance", Measurement Techniques, vol. 43, no. 1, pp. 8-11. Yan, T., Jones, B.E., Rakowski, R.T., Tudor, M.J. & al, e. 2005, "Metallic resonant strain gauges with high overload capability", Sensor Review, vol. 25, no. 2, pp. 144-147. Read More