Comparing 3D DIC Data to Theory/Strain Gauges/Other Techniques
Posted by Elisha Byrne, Last modified by Elisha Byrne on 23 August 2017 12:22 PM
Interpreting 3D DIC Data
Why does my strain or displacement data not match theory or other measuring techniques?
Digital Image Correlation (DIC) data is often used as a validation method, so being able to compare DIC data to theory, FEA analysis, strain gauge measurements, or other techniques is important. When comparing data, it is important to consider the following:
DIC is a surface technique for a continuous surface
Standard DIC techniques only use images of a material's surface. This means the only information available is from the surface, and therefore results are only relevant to the surface behavior of the specimen.
Additionally, because of how we track and match images in order to obtain data, and because of the fundamentals of strain theory, we must treat the area of interest as a continuous surface. Erroneous strains and unreliable data and can sometimes arise due to behavior of the specimen that interrupts the ability of DIC to track a specimen’s deformation as a continuous surface. Some examples of this discontinuous behavior are:
For more on Continuous Surfaces in DIC, please refer to the link below:
Interpreting Strains in Vic-3D
Comparing local and global strains
When comparing strains from Vic with those from strain gauges or other similar techniques, it is important to note that Vic reports a full-field contour plot of strains at individual points, whereas strain gauges report strain over the length of the gauge. To analyze a user defined area of strains, you may use inspector tools, like “Inspect rectangle” and extract the average strain value within that rectangle through “Plot extractions”. When comparing strain gauge/extensomer data, you may use the inspector tools to place a extensometer on either end of the gauge that you are comparing it to and again “Plot extractions,” and then select “extensometers” in the drop down menu that you’ll see on the upper left of the screen.
Also note that the size of each individual virtual strain gauge on the contour plot is controlled by both the step size and filter. The localized virtual strain gauge size is the step size x strain filter. For more explanation on interpreting local/global data and strain filter selections, please refer to the link below:
Also a brief explanation of how strain is calculated in 3D DIC is here:
Strain tensor types
To compare strain values obtained from DIC to results from other methods, it is important to compare the same strain tensor type. By default, Vic-3D reports strain as Lagrange strain, but the strain tensor can be selected at run-time in the postprocessing tab or during the strain calculation dialogue. There are many strain tensors available in Vic-3D. For more information on different strain tensor options, please refer to the link below:
Below are a few things to consider if your DIC data is not as predicted by strain gauges or theory:
Strain Variables and Consistency of Coordinate Systems
Strains reported in Vic are given in terms of exx (strain along the X axis), eyy (strain along the Y axis), exy (shear strain), e1 (major/first principal), e2 (minor/second principal), and gamma (major strain angle, the angle between the positive x-axis and major strain axis). It is important to consider the different strain variables when comparing strain data.
For many setups, the X axis or Y axis will align with the longitudinal or transverse strain axis, based on Vic-3D’s default “best plane fit.” But do note that in order to compare strains along given xy axes the coordinate system in Vic-3D should be consistent with the coordinate system it is being compared to. If alignment is critical, you should specify a coordinate system in Vic-3D that matches what it is being compared to. The coordinate system can be user-defined through the coordinate tools in the data tab. Note that applying a coordinate transform will not transform associated strains - you will be prompted of this when applying transforms; re-calculate strain to get strains in your new coordinate system.
If you are interested in comparing major and minor strains, it may be helpful to visualize the major and minor directions. The major and minor strain directions can be displayed on the contour plot by using the tools in the “Vector” tab of the "Plotting Tools" menu.
Troubleshooting Test Setup
Projection Error: Indication of a Problem in Test Setup
When an analysis is run in Vic-3D, the reported projection error can be a good indicator of potential problems in the test. While test setup issues do not always result in an increased projection error, if the projection error is over 0.1 this does indicate that there is likely problem with the test setup. The following are common causes of high projection error or other problems that can bias results:
All of the above items are discussed in detail here:
Visible strain gauges
When using DIC and strain gauges at the same time, it is likely that the strain gauge will be visible where the cameras are imaging. Even if the gauge is painted over with the speckle pattern, any slipping between the gauge or gauge connections/tape and surface that occurs will result in the DIC results being inaccurate. Remember that DIC must assume a continuous surface. So it is necessary to analyze a location which is not obstructed by the gauge, but still near the gauge so that you can compare similar areas.
Alignment of Test Fixture
For tests that are in pure tension or pure compression, if exx and eyy differ from the principal strains, this could indicate a problem in the test setup. If theory predicts pure tension and these strains do not match (after you investigate the coordinate system to make sure the coordinate axis lines up with the load axis), it is likely that there might be misalignment in the setup, or some torsion introduced in the test.
Slippage of Specimen within Test Fixture
If the specimen slips within the test fixture, it will likely not stain as predicted. Often times, a specimen will slip in tensile tester grips, for example. If this occurs, it can slip in the beginning of the test and then catch within the grips and strain for the rest of the test, or it can strain as predicted, then slip at higher loads.
Viewing through windows
It is sometimes necessary to view specimen through a glass window pane or other medium that causes distortions. There are some considerations to take if your test setup requires a window and/or medium. Refer here for the procedure for this application:
Speckle Pattern Slipping on Surface or Degrading in Test Conditions
It is imperative that the speckle pattern does not slip on the surface of the specimen. It must deform with the surface that is being tested. It is also imperative that the speckle pattern holds up to test conditions (does not melt in heated tests, does not lose speckles due to the event being dynamic, must not lose speckles due to the test being in water or being exposed to convection, does not crack before the surface does, etc.).
Noise and Bias in DIC Data
DIC data will always have some noise, but it is possible that calibration, test set-up, or other conditions can introduce bias and increase noise into the results from DIC.
Bias in DIC measurements can often arise from a number of issues related to the calibration of the stereo camera system. Below are some causes of bias calibration.
All of the above items are further discussed here:
Noise in DIC Data
In some situations, especially when looking at very small strains, high noise levels may make it difficult to get good results. As with all measuring techniques, noise is unavoidable in DIC, but steps can be taken to minimize it.
Preparation of the specimen and setup are the most important factors to reducing noise. The following can increase the noise in DIC data:
Most of the time, the cause of increased noise is due to a non-ideal speckle pattern. For details regarding ideal speckle patterns, please refer here:
With good preparation and setup, strain noise can be reduced down to about 50 microstrain. For more on how to reduce noise in your data, refer to here:
If you wish to quantify the noise in your setup, you can refer here: