2D Digital Image Correlation

Along with strain measurement, you obtain valuable additional information on specimen behavior!
ZwickRoell digital image correlation makes local strain visible in 2D across the entire specimen surface.

Description Examples Difference 2D / 3D Specimen preparation Procedure Analysis tools Extensometers

What is 2D digital image correlation?

2D digital image correlation visualizes deformations and strain over the entire visible specimen surface. The non-contact videoXtens extensometer records image series during the test, compares image by image, and calculates the displacement in a pre-defined facet field, where each facet includes a specified number of camera pixels. This data is used to create two-dimensional color strain maps, which allow you to analyze the specimen behavior at a glance.

DIC - Digital Image Correlation

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Where is digital image correlation used?

ZwickRoell 2D digital image correlation (abbr.: 2D DIC) is used for analysis of the specimen behavior under load. The specimen behavior is visualized in color in the software and provides indications of inhomogeneous local strains and other special features. You can evaluate these local strains more precisely using various analysis tools, such as virtual gauge lengths or virtual strain gauges.

Digital image correlation is also used to verify the results from the live strain measurement. In addition, errors in the test arrangement, such as inaccurate specimen alignment, quickly become visible.

ZwickRoell 2D DIC is a software option for the videoXtens. The full-surface evaluation leaves plenty of scope for the specimen shape: Even components, complex specimens with recesses or non-homogeneous materials are analyzed with digital image correlation.

2D digital image correlation applications

The use of cost-efficient virtual strain gauges for shear tests with notched specimens to ASTM D5379 and ASTM D7078
Open-hole tension (OHT) strength test to ASTM D5766 with determination of the stress concentration near the hole
FE (finite element) model validation: Comparison of the displacement and strain field through the FE simulation
Determination of stress-strain curves (true, technical)
Evaluation of specimen failure through assessment of the point of fracture, for example through determination of a local strain maximum at the point of fracture
Verification of the heterogeneity of the material and identification of local failure

Digital image correlation used with fiber-reinforced plastics and composite specimens — Digital image correlation used with composite specimens

Digital image correlation used with composite specimens

Difference between 2D and 3D digital image correlation

Many applications do not require 3D DIC. Two-dimensional digital image correlation is sufficient if the measurement surface is flat and there is no twisting, no tilting of the surface, and no significant lateral specimen movement occurs during the test.

A 3D DIC system is used for three-dimensional measurements, for example of components and round specimens, and requires special hardware and software. Systems for 3D digital image correlation are connected to the ZwickRoell testing machine via a module.

Specimen preparation for 2D DIC

A high-contrast pattern can simply and quickly be sprayed onto the specimen.

Additional markings are not necessary for the live strain measurement. Virtual gauge marks are placed on the existing pattern through the software.

Video extensometer and digital image correlation

Simply see more: the 2D DIC software option from ZwickRoell

The 2D digital image correlation option does not require additional hardware. The software option is simply combined with the videoXtens and expands the functionality of the already installed strain measurement system.

It allows for live strain measurement with a single extensometer, after which the 2D DIC analysis can be carried out.

ZwickRoell array systems offer you high resolution combined with a large field of view. They include several cameras, such as the videoXtens 2-150 HP, which allow you to simply see more in 2D DIC mode.

One software for all of your tasks: testXpert

The 2D digital image correlation option is fully integrated in testXpert. This means that with just one software program, you can perform live measurements and 2D DIC analyses. All measured values, test results and images are stored, managed and evaluated together. The strain values obtained from the 2D DIC analysis can be displayed in the stress-strain curve and evaluated. This makes it a powerful option.

Measured values, test results and images are stored, managed and evaluated together. You can easily and extensively analyze all measured values with each other. The strain values obtained from the 2D DIC analysis can be displayed in the stress-strain curve and evaluated.
You will reach your goal quickly and with just a few clicks: by specifying a workflow, you are guided step by step from setup to analysis and display of the measured values.
Prepare analysis parameters once, then save them in testXpert and use them over and over.
Reliable results: the measured values resulting from digital image correlation are synchronized with the measured values from the testing machine.
A new specimen can be created via test re-run for each different evaluation. This makes the evaluation accessible at any time.

Example of a digital image correlation analysis procedure

1. Defining masks and grids

Easily define the image region to be analyzed by means of a mask. Using a toolbox of mask geometries such as circles or polygons, you can also create irregular masks or define recesses. You also have the option of using multiple masks for which different resolutions can be specified.

Three very helpful default settings are available to define the facets and resolution. Settings can also be individually selected or adjusted. In addition, measurements can be carried out on different planes that have varying distances to the test axis, which is the case with offset specimens for example. Here, the distance from the specimen plane to the test axis can be individually adjusted.

2. Starting the correlation

The correlation is used to calculate the displacements and strains between facets using the parameters defined in the mask. Images such as those recorded after specimen fracture can be selectively deselected for the correlation.

3. Analyze

You have a wide selection of analytical tools and displays at your disposal for the analysis process.

The color map and diagram are clearly displayed in a common analysis layout. Analytical tools, such as gauge lengths, can be dragged on the color map to move them, simultaneously displaying current values in the diagram – without time delay! You can use the timeline to access any point in time within the test and apply the analysis tools precisely to the essential regions.

4. Test re-run

Results of the individual 2D DIC analysis tools are combined with the measured values of the live test via the test re-run option in testXpert. Thus, the strain values of the 2D DIC analysis are displayed in the stress-strain curve.

From these combinations, material characteristic values can also be recalculated retrospectively.

2D Digital Image Correlation

DIC analysis range

2D Digital Image Correlation: A mask defines the image region to be analyzed

Analysis tools

Various analysis tools for 2D digital image correlation

Graphical evaluations

Each analysis tool contains a selection of graphical evaluations of the digital image correlation method

Various analysis tools for 2D digital image correlation (DIC)

Drag & drop

All 2D DIC analysis tools can be moved with the mouse, whereby the graphical evaluation appears simultaneously

Cutting line stack

Selected time steps are displayed in a diagram, making the temporal progression of the cutting line visible.

Virtual strain gauge

The 2D DIC virtual strain gauges can also be placed on top of each other at different angles

2D Digital Image Correlation (DIC):Simply Analyze

Analysis tools

Measuring points: these can be placed in any desired locations within a strain map.
Gauge lengths or “virtual extensometer“: establish two points on the strain map, between which the change in distance is to be determined.
Cutting lines: the strain progression is visualized along the lines. The cutting line deforms along with the specimen. In addition, there is cutting line stack through which selected time steps are displayed in a diagram, making the time-based development of the intersection visible.
Virtual strain gauges: for the virtual strain gauges, you can individually define the position, size and angle. In addition, multiple virtual strain gauges can be placed on top of each other at different angles. For example, two virtual strain gauges can be combined to form a biaxial strain gauge with measuring grids oriented at 90° to each other. The digital image correlation virtual strain gauges can save you a lot of time and cost.

Creation of graphs / diagrams

Analysis tools, such as gauge lengths, can be dragged on the color map to move them, simultaneously displaying current values in the diagram – without time delay! You can use the timeline to call up any test point.

The following measured values can be displayed as strain maps and in diagrams:

Displacements in X direction
Displacements in Y direction
Local longitudinal strains Ɛ_x
Local transverse strains Ɛ_y
Local shear strains Ɛ_xy
Maximum normal strains
Minimum normal strains
Poisson's ratio
Equivalent Von Mises strains

Vector maps can be displayed in all strain maps, showing the main strain directions.

Export options

For simulation purposes or other use of the data outside of testXpert, the digital image correlation option offers export possibilities:

Export of individual data in .csv format
Export of the video in .avi format
Export color map / diagram in .bmp format

What is special about the 2D digital image correlation cutting line?

With the cutting line, strain progression is displayed along or diagonally across the specimen. The cutting line deforms along with the specimen. It is therefore not a fixed part of the image, but rather a line that actually follows the behavior of the specimen throughout the test.

One special function of the cutting line is the cutting line stack: selected time steps can be displayed in a diagram, allowing you to see the development of the cutting line over time.

What makes virtual strain gauges so efficient?

Virtual strain gauges are efficient because they provide a cost-effective alternative to adhesive strain gauges. This eliminates the time required to apply the strain gauges. An extremely effective analysis tool for digital image correlation.

The virtual strain gauges are flexible: position, size and angle are individually determined. They can also be placed on top of each other, whereby two virtual strain gauges create a biaxial strain gauge with measuring grids oriented at 90° to each other.

In addition to local strain information at the strain gauge position, 2D DIC also provides a view of the entire specimen.

What is the benefit of vector maps?

Vector maps display the main strain directions. This makes the strain conditions visible over the entire evaluation range and you quickly gain a good overall understanding of what is going on with the specimen.

The function is an effective extension of the visualization options within digital image correlation.