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.
ZwickRoell 2D digital image correlation (abbr.: DIC) offers you different DIC analysis options, including:
- Color displays of specimen behavior provide indications of inhomogeneous local strain and other special features.
- Local strains can be determined via the cutting line and point measurements, as well as via virtual strain gauges and virtual gauge lengths.
- You can also analyze components, complex specimens with notches or non-homogeneous materials.
- Verify your live strain measurement results.
- Errors in the test arrangement, such as inaccurate specimen alignment quickly become visible.
Specific application examples:
- The use of cost-efficient virtual strain gauges for shear tests with notched specimens to ASTM D5379 and ASTM D7078 (link to video)
- Open-hole tensile (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
You have a wide selection of analytical tools and diagram displays at your disposal for the analysis process.
Many applications do not require 3D DIC. Two-dimensional DIC analysis 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. A system for 3D digital image correlation can be connected to the materials testing machine via a module.
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.
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.
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.
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.
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.