Painting your digital image correlation samples for accurate measurement

The accuracy of your results depends on how well you prepare your sample surface and apply the paint. Adhesive failure causes delamination of the paint from the surface of the substrate. Cohesive failure can change the geometry of your speckle pattern and the local elasticity of the paint.

Digital image correlation (DIC) is becoming a common process to measure material displacement during tensile and dome tests. This visual method uses cameras and software to follow the movement of points defined by a speckle pattern painted on the material surface.

By tracking the vectors of displacement during deformation, you can measure such events as local deformations, cracking, plastic instabilities, and phase transformations. It is generally accepted that the precision of the DIC displacement measurement is limited by the quality of the image and is reliable down to 0.1 pixel. And the accuracy of your test results depends on your awareness and management of the test procedures:

• Paint the surface of the sample with white paint.
• Paint a contrasting speckle pattern (usually black) on top of the white paint.
• Select a sample region or regions of the speckle pattern and record it as a reference.
• Mount the sample and begin your deformation operation.
• Capture images of the sample area during motion/deformation.
• Compare the final image (at the failure event) to your reference image to determine displacement length and vectors.

Your ability to make good decisions about materials and forming processes depends on your test procedures and their effect on DIC imaging. As your geometry and forming operations approach the limits of your material, test accuracy becomes more critical to making good decisions.

DIC test precision and accuracy are greatly affected by how the paint is applied to the material surface and how well it adheres.

While cameras and software can provide highly precise measurements, the accuracy of your test depends on how closely the movement of the paint follows the movement of the substrate. Paint is formulated to withstand the elasticity of the underlying surface during normal operation; it isn’t generally formulated to withstand the high levels of deformation during a tensile or dome test.

Researchers are finding that paint adhesion is a limiting factor in the accurate measure of displacement. Without a proper paint application on the surface of the material, your test risks showing incorrect results. Inadequate adhesion can result in your test equipment measuring the elasticity of the paint rather than the material you are testing.

The accuracy of your results depends on how well you prepare your sample surface and apply the paint. The two important elements of paint performance are cohesion—the strength of the molecular bonds of the paint—and adhesion—the strength of the paint’s bond to the substrate.

Cohesive failure can change the geometry of your speckle pattern and the local elasticity of the paint. Adhesive failure is the delamination of the paint from the surface of the substrate. When adhesion fails, the speckle pattern cannot be relied on to follow the movement of the substrate. Both types of failure can give you inaccurate results.

Paint adhesion remains somewhat poorly understood, but it is generally assumed to have three components:

1. Adsorption—Adsorption happens when the paint molecules flow freely over the surface of the substrate and form interface bonds.
2. Chemical Adhesion—Chemical adhesion results when the chemical components of both the paint and the substrate bond with each other.
3. Mechanical Bond—Mechanical bonds occur when the paint penetrates the roughness of the substrate surface and interlocks with the substrate.

Reliable adhesion requires proper “wetting,” which happens when the paint flows into all the valleys on the rough surface. This provides maximum surface area contact for the three components of paint adhesion.

Finding the right combination of paint and application for each type of material requires much trial and error. The most important variables to consider include the following:

• Cleaning—The cleanliness of your sample surface is the most important consideration in proper paint application. Remove all contaminants and particles from the sample before applying the paint. Throughout the process of cleaning, applying, and curing paint, handle the sample carefully to prevent introducing dirt and body oils to the paint and substrate surface.
• Material Properties—Ferrous metals, aluminum, and stainless materials have different surface properties and chemistry. Your paint selection will influence the strength of adsorption and chemical bonds. Try to choose a paint appropriate for your sample material.
• Application Method—Select an appropriate spray tip to ensure adequate coverage and cohesion across the test surface. Your ambient environment also should be clean and at appropriate temperature and humidity levels. Experiment with curing times to find the best elasticity and viscosity for your paint.

As with any process, you can’t take the variables affecting your results for granted. Your best approach is to take a close look at all the components of your test and question their reliability. A disciplined and informed process will help you to attain the precision and accuracy you need from your test results, ultimately leading to improved productivity.