The VIC Speckle Pattern Application Kit
Digital Image Correlation (DIC) relies on a high-contrast random pattern applied to the specimen’s surface to produce the most accurate results. In practice, if the pattern is too sparse, too dense, or the speckles are too large, spatial resolution of the measurement will suffer. Conversely, if the speckles are too small, aliasing may result causing noisy data. To reduce noise and lower uncertainty, the ideal speckle pattern for the desired field of view (FOV) and camera resolution must be applied. The VIC Speckle Pattern Application Kit enables the user to easily apply the ideal pattern to the surface of the specimen without any precise knowledge of sample area. The kit is easy-to-use, flexible, and available for a wide range of applications suitable with your VIC-2D or VIC-3D system. The kit virtually eliminates the potential bias that would result from poor sample preparation.
The speckle application kit from Correlated Solutions provides an array of tools which can be used by new and experienced DIC users alike to quickly create the ideal speckle pattern. Speckle size suggestion tables for multiple fields-of-view and sensor resolution combinations are also provided. The items included in the kit are shown and listed below.
There is a wide range of speckle pattern application techniques, but many provide inconsistent dot sizes from sample to sample, which also yield inconsistent results. This can be especially frustrating when recurring tests are performed. Producing repeatable dot sizes with the VIC Speckle Pattern Application Kit from Correlated Solutions provides the ability to quickly create predictable and controlled speckle patterns, which allows for the highest level of accurate and consistent results.
Why Is Your Speckle Pattern Important?
The ideal speckle pattern is high contrast, contains random speckles, consistent dot sizes, and equal amounts of black and white on the sample’s surface. If all of these conditions are met, the area of interest will produce low levels of noise and may be tracked with very high levels of certainty.
Below is an example of how various speckle patterns can effect measurement noise. The middle pattern was created using a roller from the VIC Speckle Pattern Application Kit, while the patterns on either side were created using spray paint. As you can see, the roller from the speckle pattern kit produces a high contrast pattern with approximately 50% black and white coverage, while the painted pattern produces a lower contrast pattern with inconsistent dot sizes.
Figure 1: A relatively flat plate with three different speckle patterns analyzed to show the variable Sigma_Z
Figure 1 shows a 2D data overlay of the variable Sigma_Z, which is the confidence margin of the measurements taken in the Z direction (out-of-plane shape measurement). The closer Sigma is to zero, the more statistically likely the value shown is indeed the correct value. The outside patterns are low in contrast and overly fine which result in larger Sigma_Z values, which indicates less confidence. The middle pattern shows lower Sigma_Z values, which indicates high confidence in the measurement. The results speak for themselves.
Figure 2: The 3D plot of Sigma_Z displays how poor patterns create low confidence margins and noisy shape measurements.
Why Is Sigma Important?
High Sigma values in VIC-2D and VIC-3D indicate higher measurement uncertainty, which can result in an increase in measurement noise in the shape, displacement, and strain data. As Figure 2 shows, the outside patterns show a larger range of Z values of a relatively flat sample, while the center pattern shows much smaller bias.
In Figure 3 below data for the variable exx (transverse strain in the X direction) is shown for the same plate after a rigid body translation, where there should be zero strain. The range of strain values shown in the contour plot are much higher on the outside patterns than the center pattern. The result shows that the local strain measurements on the outside patterns are more noisy and will produce data that is less accurate. This is a good illustration of how poor speckle patterns can result in noisy measurements, while favorable patterns (applied with the speckle kit) will produce much better results.
By applying the optimal speckle pattern using the VIC Speckle Pattern Application Kit your results will be less noisy and more accurate.
Figure 3: Transverse strain is shown after a rigid body translation. The outsides of the plate display a much higher level of noise than the middle of the plate due to the poor speckle patterns.
The VIC Speckle Pattern Application Kit Will Improve Your Results:
The VIC Speckle Pattern Application Kit from Correlated Solutions, Inc. can help produce very low noise and very high confidence results for DIC measurements. The speckle kit allows both new and experienced DIC users to create ideal and repeatable patterns for many fields of view for nearly any lens and sensor size combination. Having this capability can streamline the sample preparation process, ensure consistent patterns for recurring tests, and provide a means to obtain a consistently low strain noise floor for all measurements acquired with your VIC-2D or VIC-3D system.
As the world-leader in Digital Image Correlation, Correlated Solutions is continuously developing innovative and new measurement techniques and solutions. The VIC Speckle Pattern Application Kit from Correlated Solutions is one more example of how we are always helping to improve your DIC measurements. The speckle kit is an affordable addition to your VIC-2D or VIC-3D system. Call our sales department today for a quotation at +1 (803) 926 – 7272 x1
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