11. Digital Image Correlation

Question 1

What are some general points on the usage of Digital Image Correlation (DIC)

Answer 1

• Allows the measurement of full-field displacements - Easy to use, accurate and fast - Track patterns to determine displacements - Cost effective - Wide range of applications

Question 2

What are the principals of the DIC method

Answer 2

• Image correlation relies upon “fingerprint” of digital grayscale patterns
• Pattern is discretized into small windows/subsets
• Displacements calculated and differentiated to obtain strains

Question 3

What does DIC stand for?

Answer 3

Digital image correlation

Question 4

DIC relies on digital patterns, what must the characteristics of those patterns be?

Answer 4

• High contrast - Random pattern - The scale must be relevant to deformation and the recording device - Sharp grayscale - resolution should be 1 pixel at recording device i.e. tone can change every pixel

Question 5

If a pattern naturally exists on a specamine, does another pattern need to be projected for DIC?

Answer 5

NO!

Question 6

Name 4 typical methods of creating a pattern on a sample for DIC?

Answer 6

• Spray painting - usually with black and white paint, with large deformation it is better to speckle the surface to avoid paint crazing.
• Abrasion of surface - for small displacements
• Chemical Etching
• Use of natural pattern on surface

Lighting plays a a role in the type of speckle pattern used

Question 7

What are the assumptions involved in Digital image correlation?

Answer 7

• Changes are comparably small
• Changes are continuous, like in a strain field
• Two (or more) states of an object exist
• Comparison by correlation of images and/or by feature extraction

Question 8

Draw a sketch of a 2D experimental arrangement for DIC

Answer 8

CCD camera perpendicular to the surface of the object

Random pattern on the surface of the object

White lights provide illumination

Moving the camera further from the specamine minimizes erros due to out of plane displacements

Question 9

What are the basics of 2d image correlation in DIC?

Answer 9

• Small square subsets of an undeformed image are matched to their locations in deformed images
• typical subset sizes are 11x11 to 41x41 pixels, it can go up to 61x61 pixels
• once the first subset is analysed an overlapping subset is then analysed, this continues until the entire displacement field is analysed
• Current programms can run over 1000 subsets per second on a 500 MHz PC

Because it is possible to match undeformed square subsets with deformed non-square subsets, deformations and in-plane rotations are not a problem for this method

Question 10

What are some requirements for the accurate usage of DIC?

Answer 10

• Initial estimate for rigid body motion
• Planar surfaces, with minimal out-of-plane motion in 2d systems
• Appropriate subset size for applied pattern
• Analysis is algorithm dependent, feature dependent - rule of thumb: 1/100 pixel

Question 11

How are the initial estimates for rigid body motion in DIC made?

Answer 11

• The estimate is provided by a simple point and click procedure incorporated in the program
• The estimation of initial rigid body motion and strain allows direct comparison of the initial image to any other image from the test
• The program provides two different types of initial guesses
• If the location of the first point experienced mostly translations, the user need only indicate the same location in the second image.
• Multiple points are needed for image pairs with large strains.

If the images were taken at incremental load steps no initial guess is required

Question 12

How do we minimize the interference of out of plane motion in 2d systems for DIC?

Answer 12

• By using telephoto lenses and keeping the camera as far from the specimen as possible
• Tolerance is W/Z < 0.0001 for region of interest
• W is the out-of-plane displacement of the specimen during loading
• Z is the distance from the camera to the specimen

Question 13

What is the Appropriate subset size for the applied pattern in DIC methods?

Answer 13

• Pattern application methods have been developed for areas of interest from 0.5 mm x 0.5 mm to 200 mm x 200 mm
• There needs to be enough information in each subset to obtain a good match between the images. If the features of the pattern are large, larger subsets are required

Question 14

What are the limits of an algorithm dependant DIC analysis?

Answer 14

each system uses slightly different methods to track the displacements of the patterns

Question 15

What are the limits of a feature dependant DIC analysis?

Answer 15

if the features being tracked cannot be picked up by the camera or they are not random then the method won’t work
However generally speaking can accurately discern movements down to 1/10 pixel