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Editor’s note: This article has been adapted from an episode of ASNT’s podcast, Chat NDT with ASNT featuring Tsuchin Philip Chu, School of Mechanical, Aerospace, and Materials Engineering, Southern Illinois University, tchu@siu.edu; and Anish Poudel, MxV Rail (formerly Transportation Technology Center Inc.), anish_poudel@aar.com. Listen to the full conversation below or search for Chat NDT with ASNT in your favorite podcast platform.
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by Haley Cowans
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Overview of Digital Image Correlation
Digital image correlation (DIC) is a noncontact optical technique used for measuring full-field displacements and strains on the surface of objects undergoing deformation under loading conditions. DIC allows us to estimate the stress in an object—if there’s an area of stress concentration that could possibly lead to a failure of the object, we can use that finding as the indication of a defect or an anomaly.
DIC works using optical methods: you take pictures of the object before it’s been stress loaded, and then take pictures of the object after being loaded (Figure 2). To use DIC, the surface of the object under test must be covered with a random pattern (Figure 1), which is then used to compare the “before” and “after” images of the object. The pattern we use is usually just a random pattern in the form of speckles or dots on the surfaces.
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After deformation, we try to correlate the images of the object. You can zoom in to one section of the reference images before deformation, and then find the same section in the images of the object after deformation. When the images are correlated, you can determine the displacement of that subset. All the subsets throughout the images are tracked, and you can determine the displacement of every single point on the object.
After that, by using the derivation of the displacement field, you can get a measurement of strain. Once you get a strain, you can estimate stress using theoretical equations. For example, say there’s one area where there’s a crack, and once loaded, the crack is going to create higher stress. You’ll see stress concentration, and that will create the higher strain values in that spot. So, by using image correlation, you can identify that particular spot and find the crack.
What Is “Strain”?
The strain is basically the change of the size or shape of the object that’s been deformed or loaded. Imagine a rubber band that’s originally one inch long: you stretch it, and it becomes 1.2 inches. That’s 20% elongation. So, 20% is the strain, and from the strain for different materials and different material properties, you can estimate stress.
If there’s any area on the structure or the parts you are using that can create higher stress values, and therefore higher strain values, that’s indication of the failure of the object. Again, that can be used to determine if there are cracks or discontinuities somewhere on the surface or subsurface.
It’s important to note that depending on different types of loading modes, there can be different types of strain. If it’s a tensile force, you’re going to see a tensile strain; if it’s a compressive force, you can calculate the compressive strain, and so on.
It’s also important to note that the strain is typically measured as a dimensionless quantity, because it usually expresses the ratio of the change in the length or set of material to its original length and set.
How Is Digital Image Correlation Different from Previous Ways of Measuring Strain?
The traditional way of measuring strain is by placing a strain gauge on the surface of the object. A strain gauge is made of metal wires, and the change of the length of the strain gauge creates some kind of change of the resistance of the electric circuit. So, you can use that to measure strain very accurately. However, you can only put strain gauges in certain locations, so it’s kind of like point measurement.
The digital image correlation technique is a whole field measurement. You can measure strain for the whole area, and you can just zone in to find the values anywhere you want to. It’s similar to ultrasonic testing: you use one transducer and measure the one area, but with a C-scan, then you can create the whole map.
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Additional Information
- Figure 1 is from “A Three-Camera Digital Image Correlation System for Full-Field 3D Shape and Motion Measurement,” November 2022 Volume 80, Issue 11 of Materials Evaluation, DOI: https://doi.org/10.32548/2022.me-04293
- Figure 2 is from “Digital Image Correlation and Its Role in NDE,” November 2022 Volume 80, Issue 11 of Materials Evaluation, DOI: https://doi.org/10.32548/2022.me-04306
- Read “Assessment of Composite Aluminum Adhesive Joints Using Digital Image Correlation” by Anish Poudel and Tsuchin Philip Chu, https://doi.org/10.32548/2022.me-04281
- For more information on DIC, read the November 2022 Technical Focus issue of Materials Evaluation.
- Listen to the full conversation on Chat NDT with ASNT here, or wherever you listen to podcasts.
Note: ASNT members get free access to all papers published in Materials Evaluation.
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Haley Cowans is the Content Strategist at ASNT, hcowans@asnt.org.
Share your ideas for future podcasts with Debbie Segor, CAE, Marketing and Communications Manager at ASNT, dsegor@asnt.org.