Digital image correlation (DIC) is a technique for full-field displacement and strain measurement. It relies on the specimen being treated (via paint spray, printing, transfers etc.) with a stochastic (random) speckle pattern; this pattern is then photographed with one or more digital cameras before and after the specimen is loaded, and a map of the displacement of the pattern from one image to the next is measured using postprocessing software which correlates the new image with the original one. The displacement field is then differentiated with respect to spatial position to obtain a map of strain. If only a single camera is used, displacements can only be measured in-plane (and out-of-plane displacements are recorded as spurious in-plane strains). If a pair of cameras is used, full 3D displacement data can be captured and compensation made for out-of-plane displacements. Displacements of a small fraction of a pixel can be detected. The strain sensitivity of DIC is dependent on the resolution of the camera but is independent of the overall field of view. There is a trade-off between spatial resolution and strain resolution, so small strains (in the order of 100 microstrain) can be measured with poor spatial resolution, but greater spatial resolution can only be achieved reliably when measuring larger strains (in the order of thousands of microstrain). There is no practical upper limit on the strains which can be measured over several loading steps). DIC is therefore well suited to large strains (in the plastic region, for example) but less well suited to measuring small elastic strains. The field of view goes from around 20mm x 20mm upwards, with no practical limitation (though very large or very small applications would require additional calibration targets).
CAMERA has purchased a Dantec Q400 3D DIC system with HILIS high-intensity light source. It is equipped with laptop, two 5MB high-speed cameras which can capture images at a rate of several frames per second. However, the system is fully compatible with CAMERA’s pair of high speed cameras which can be interfaced to the system laptop via an Ethernet switch. System control and image processing takes place on a laptop computer using the ISTRA4D software. We have and additional licence for ISTRA4D; furthermore, that software can be used without restriction in “viewing mode” only. For 3D work the system includes an automatic calibration procedure involving the use of a “chessboard” target; we have targets from 30mmx30mm up to 200mmx200mm; this procedure can be used both with the 5MB and HS cameras. The system also includes analogue inputs.
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