Strain mapping by digital image correlation
While a speckle-patterned flat specimen is loaded in the tensile machine, two synchronised cameras image its surface. Matching image subsets between frames yields the displacement field and its derivative the strain map; the concentration around the notch is compared with the threshold.
For every load step the strain map and the peak strain value are written to the test record; a threshold crossing is sent to the test software as a stop signal and the strain-time curve is shown on the display.
- 01 Clamping
- 02 Reference
- 03 Loading
- 04 Correlation
- 05 Decide
The specimen is clamped; both cameras are aligned to its surface.
- Peak strain
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- Force
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- Displacement
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Scenes are illustrative simulations; they are not customer references or performance guarantees.
Need
A strain gauge reads a single point, while crack initiation happens where there is no gauge, at the notch root. The surface also moves out of plane under load; a single camera mistakes this motion for strain.
Solution approach
A random speckle pattern is applied to the surface; two cameras in an angled stereo set-up are triggered together and an image is taken at every load step. Small subsets are matched between frames by correlation, 3D displacement follows from the stereo geometry and the strain field from its derivative. Out-of-plane motion is thereby separated from strain.
Process steps
- 01
Clamping
The specimen is clamped; both cameras are aligned to its surface.
- 02
Reference
A reference image pair is taken in the unloaded state and the subsets are defined.
- 03
Loading
As the load increases, a synchronised image pair is taken at every step.
- 04
Correlation
Subsets are matched; the displacement and strain fields are computed.
- 05
Decide
Peak strain is compared with the threshold.
Related applications
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