Digital Image Correlation Compared to Strain Gauge

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1 Digital Image Correlation Compared to Strain Gauge Abstract: This report presents the test results from a stereo digital image correlation system, in which two samples were pulled on a tensile test machine. The samples had strain gauges attached and then a speckle pattern was applied. Then the samples were pulled in the tensile machine and the strains were then compared to each other. Introduction: An experimental test was conducted in order to measure the strain data developed by using Vic-3D and compare the data as recorded by a strain gauge in the same region of the sample. The samples that used in the experiment were 1018 steel and aluminum alloy. The test are being used to compare the accuracy and repeatability of digital image correlated in relation to strain gauges. Procedures: The first step was to apply the strain gauges to the sample making sure that the gauge was in line with the loading direction. The next step was to apply a thin coat of white spray paint to the surface over the strain gauge and sample, then a speckle pattern was applied using black spray paint. Using Vic-Snap to view the sample as shown in Figure 1, the cameras were then positioned, focused, and the apertures were adjusted to optimize the digital image correlation. After the cameras were in place and adjusted, the system was calibrated using a calibration grid with circular markers located at a know spacing. To calibrate the stereo camera system, images of the grid were recorded at various locations and angles within the field of view of both cameras. Once the system was calibrated, the apparatus was then positioned and a reference image was captured.

2 The test consisted of pulling the two different samples in the elastic region of the material. Strain Gauge and Area of Interest for DIC Figure 1: Test sample with strain gauge and area of interest for digital image correlation Results and Discussion: After acquiring the images using Vic-Snap, the paired images were imported into Vic-3D along with the data from the strain gauge. The analysis parameters for the test were run with a subset size of 59 and a step of 10. With this setup, ~5600 points were analyzed in the area of interest. Then the strains in the loading direction, εyy, were extracted over the strain gauge and averaged. The strains of the digital image correlation and the strain gauge were plotted in relation to the loading from the tensile test. Graphs 1 & 2 display the data acquired from the steel sample and Graphs 3 & 4 are from the aluminum sample.

3 Graph 1: Strain gauge verses digital image correlation using 1018 Steel Graph 1 displays the load verses strain data in the elastic region of the material. By analyzing the differences of the data collected from the strain gauge and the DIC measurements the standard deviation of the difference between the two was 16.07, and the data has a correlation coefficient of which means that there is a very strong linear relationship between the two data sets.

4 Graph 2: Strain gauge verses digital image correlation using 1018 Steel The standard deviation of the difference between the two was 20.46, and the data has a correlation coefficient of which means that there is a very strong linear relationship between the two data sets during Test 2 of the 1018 steel.

5 The standard deviation of the difference between the two was 9.24, and the data has a correlation coefficient of which means that there is a very strong linear relationship between the two data sets during Test 1 of the aluminum alloy.

6 The standard deviation of the difference between the two was 19.26, and the data has a correlation coefficient of which means that there is a very strong linear relationship between the two data sets during Test 2 of the aluminum alloy.