Digital Image Correlation systems are the next wave of advanced deformation and strain measurement tools. Most test labs or stress labs have a few primary measurement tools such as strain gauges for measuring strain, LVDTs for measuring displacement, and accelerometers for measuring acceleration. While these are all accurate tools for collecting a specific type of data, this data is provided in one direction. Alternatively, Digital Image Correlation (DIC) systems offer a dynamic process for collecting strain and deformation data at a few points vs. thousands of points.
The traditional process of collecting strain and displacement data at different points takes careful consideration. The placement of each sensor must be considered carefully; each sensor must be calibrated and adhered to the surface of the part, referenced or zeroed, and connected to its specific data acquisition system. This process can take days for more complex measurements. Often, institutions have an expert for each of these sensors.
Overall, the test program could take months just to collect data on a few points of the object!
Traditional Set Up:
In spite of expert technicians applying these sensors, some of these sensors inevitably fail during the test. This could be due to heating, sample geometry, excessive forces, or issues with the adhesive that applies them to the surface. When you have 5 strain gauges and one fails, you’ve just decreased your amount of strain data by 20%!
Of course, you don’t want to run one test. In order to get statistically significant information, your team would likely run a few tests. This procedure that was just described will now be repeated for every test that you run. The quality of the data that you collect will depend on the number of sensors, placement, and accuracy.
Hopefully, the sensors were also placed in a meaningful area. Otherwise, you will be left scratching your head and squinting at all of these graphs and tables you’ve just created, wondering what it all means.
If this time consuming and frustrating process sounds familiar, then a digital image correlation system could be perfect for your test lab.
DIC systems are optical measurement systems that measure strain and displacement in 3D at thousands of points on the surface of the subject. Every point on the surface of the part within the optical systems field of view becomes a 3D measurement point.
Instead of calibrating and applying each sensor one by one, the DIC system is calibrated and then directed at the test article. This process takes 15 minutes, and then you are ready to collect data at thousands of points.
During the test, the DIC system samples data at a user-defined interval and saves it for post-processing later. Once a test is finished, you can bring another test article into the frame and test that one with no additional calibration needed by the DIC system.
The testing process that lasted days, weeks, or even months can now be completed in a week.
To make the interpretation even easier, the data is presented in 3D contour plots similar to finite element analysis (FEA). With this 3D contour plot, any point can be analyzed, section cuts can be made, and this data can be fed directly back into you FEA model for validation.
These graphics demonstrate the ease of interpretation of this 3D data collected using DIC compared to traditional methods.
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