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Introduction to Digital Image Correlation (DIC)

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This presentation introduces Digital Image Correlation, the optical technique that compares images of a tested specimen’s surface to generate full-field strain and displacement maps.

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Introduction to Digital Image Correlation (DIC)

  1. 1. Ian McEnteggart, Instron® Composites Marketing Manager An Introduction to Digital Image Correlation (DIC)
  2. 2. 2 What is Digital Image Correlation? Images Displacement Strain Analysis of image surface over time Use of cross correlation to determine displacement Strain calculated from displacement An optical method to measure deformation on an object’s surface
  3. 3. 3 • Non-contact strain measurement • Full-field data (like thousands of tiny extensometers) • Extract “conventional” 1D strain plots • Identify strain hot spots over a large area • Don’t need to know where to place the strain gauge/extensometer before the test starts • Validate correct specimen alignment What are the Benefits of DIC?
  4. 4. 4 Sample Preparation • Apply speckle pattern using: • Airbrush • Standard spray aerosol • Brush—flicking • Transferable stickers • Rubber stamp • Some materials can be left without speckling: • Certain composites • Concrete • Textiles/fabric
  5. 5. 5 How Does It Work?
  6. 6. 6 Calculating Full-Field Displacement • Repeated for each subset over the entire surface • The result is a regular map of displacements over the entire specimen surface Specimen surface image Split into small subsets Pattern recognized for each subset As the specimen deforms, axial (x) and transverse (y) displacements for each subset are calculated
  7. 7. 7 Calculating Strain • Strain at each location is calculated using central differencing • Strain calculated in the x and y directions separately • For the x direction: ∆𝐿 = 𝐿 𝑡 − 𝐿0 𝜀 = ∆𝐿 𝐿0
  8. 8. 8 Analysis of Various Strain and Displacement Data Axial Strain Transverse Strain Shear Strain Poisson’s Ratio Minimum Normal Strain Maximum Normal Strain Axial Displacement Transverse Displacement
  9. 9. 9 Extracting 1D Plots • Use virtual extensometer for calculating strain/displacement between to points. • Use virtual strain gauge for calculating average strain over a defined area.
  10. 10. Advanced Video Extensometer (AVE) 2 & DIC Replay
  11. 11. 11 AVE 2: Versatile and Capable • Doesn’t require operator to attach extensometer, reducing operator influence and increasing consistency • 1 micron accuracy—measure modulus to ISO 527 • Patented LED lighting and fan system eliminates environmental influences • Measures both tensile and compressive strain • Can be used on chambers for cold and hot tensile tests • Can be used for full-field strain measurement using Digital Image Correlation Software
  12. 12. 12 DIC Replay • Streamlined full-field strain package tailored for the materials testing market • Allows users to analyze advanced strain characteristics after the test • No PhD degree required! Simple integration & convenient mounting •Consumes images saved by Instron® AVE camera •No spaghetti cabling •Synchronizes with data collected from the testing system
  13. 13. 13 Why Instron DIC? Integrated and synchronous collection of all data from testing system, e.g. force and camera Only 1 PCIntegrated camera and lighting unit sits on the frame out of the way of testing area. Polarized light is used so ambient light doesn’t matter. Easy to use, where users can focus on analyzing and understanding their results rather than assembling test rigs.
  14. 14. 14 DIC Application Examples • Multiple extensometers for the same sample • Can decide location post test
  15. 15. 15 DIC Application Examples • Component—Identify strain “hot spots”
  16. 16. 16 DIC Application Examples • Foam specimen—Traditional strain measurement was not possible • Speckled with felt pen • Split seen in DIC • Virtual extensometers at 25mm and 50mm GLs
  17. 17. 17 DIC Application Examples • Composite Laminate—Open-Hole Tension • Complex 2D strain distribution • Measure all components of 2D strain tensor (axial, transverse, shear), along with maximum and minimum principle strains Shear Strain Axial Strain
  18. 18. 18 What Can We Do Next? • If you’re interested in further discussions: • Arrange a demonstration on site by our sales engineer • Visit our applications laboratories and bring samples • Send samples for us to test
  19. 19. Thanks for your time!

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