Comparing 3D DIC Data to Theory/Strain Gauges/Other Techniques

 INTERPRETING 3D DIC DATA

 Why does my strain or displacement data not match theory or other measuring techniques?

 INTRODUCTION

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 measures surface strains of continuous materials
• Vic-3D provides local and global strain data with various strain variable and tensor options
• Test set-up can affect data
• There are many causes of noise and bias in DIC results (and many ways to eliminate these causes)

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:

• Cracks or holes in the specimen are typically removed from results because once a material breaks apart because it is not strained across the hole/crack any longer. Any attempt to use subset size and thresholding to bring this data back into the contour plot will result in erroneous strain data. 
• Some materials such as composites, textiles, and biological tissues that have micro-structure can behave continuously on a macro level but then have fibers slipping past each other or other discontinuities on a smaller level. This could produce erroneous strains as well.

For more on Continuous Surfaces in DIC, please refer to the link below:

Continuous Surfaces in DIC

 

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, as well as how strain is calculated in 3D DIC please refer to the link below:

Strain Filter Selection

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: 

 Strain Tensors and Criteria in VIC 

 Below are a few things to consider if your DIC data is not as predicted by strain gauges or theory:

• Engineering strain should be selected in the strain calculation dialog in order to compare data from strain gauges/extensometers, etc.
• If you notice that the strains you are comparing match well at low strains and deviate at high strains, you are likely comparing Lagrange strain to Engineering strain. (At large strains, Lagrange strain can become much larger than Engineering strain due to the higher order term.)
• If you notice that your shear strain is off by a factor of 2, you are likely comparing Lagrange strain to Engineering strain. (Lagrange shear strain is equal to half the Engineering shear strain.)

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:

• Cameras moved relative to each other after calibration (i.e. broken calibration, non-rigid stereo-rig)
• Desynchronized images
• Glare and reflections
• Motion blur
• Heatwaves
• Biased calibration

All of the above items are discussed in detail here:

Projection Error Explanations and Causes

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 strain 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:

Viewing Through a Window

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.

Biased calibration

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. 

• Not enough data.  This might result in a low calibration score, but only because not enough data was obtained for a good calibration.  The following can result in a bias calibration due to lack of data collection.
• Calibration grid is too small. The grid should fill a minimum about 70% of the image.
• Not enough images taken.  Aim for at least 15 image pairs.
• The grid is not tilted enough out-of-plane about all x and y axes.  One in-plane rotation image is also required.
• Use “High Magnification” option in calibration dialog if the grids are not tilted enough (this is needed when the depth-of-field is limited).
• Select a higher distortion order in the calibration dialog for short lenses.  As a rule of thumb:
• Select distortion order of 3 for 8mm.
• Select distortion order of 2 for 12mm and 17mm lenses. 
• Longer lenses should remain at the default distortion order of 1.
• Other potential causes of poor calibration:
• Using a grid that is not rigid
• Parts of the grid being obstructed from view (by hand or test fixture)
• Glare/reflections on calibration grid

All of the above items are further discussed here:

Troubleshooting Calibration Problems

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:

• Image processing in third party software (this is never beneficial):
  Processing Images for DIC Analysis
• High signal-to-noise ratio: Color images, or noisy sensors will increase strain noise.
• Blur/Defocus: Can be due to poor focus or the specimen moving beyond the depth-of-field
• Poor Contrast/Lighting
• Glare
• Heatwaves: Can even come from lights being used
• Low F-stop: Images can be diffraction limited
• Stereo-Angle/Lens selection:
• For 8mm lenses, stereo angle minimum is 35 degrees
• For 17mm lenses, stereo angle minimum is 25 degrees
• For 35mm lenses, stereo angle minimum is 15 degrees
• For lenses 50mm or longer, stereo angle minimum is 10 degrees
• Good speckle pattern

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:

Speckle Pattern Fundamentals

With good preparation and setup, strain noise can be reduced down to about 50 micro-strain. For more on how to reduce noise in your data, refer to here:

Minimizing Noise and Bias in DIC

If you wish to quantify the noise in your setup, you can refer here:

Resolution and Accuracy