In this application video, a 2-cam (16 Mpx) MSpeedDIC system is used to measure the strain & displacement
of a wooden specimen with holes in a 3-point bending test.
In the measurement, the real-time speckle pattern quality evaluator indicates the optimal facet/subset size for
the correlation evaluation. Then after, the projection calibration is performed by rotating the stereo rig (whilst
keeping the cameras and lenses fixed) away from the specimen. The 10mm double-sided calibration target is
automatically recognized in the Istra4D V4.11 software via it’s QR-code, which entails information about the grid
spacing, std. tolerances, and front/rear properties (such as thickness, which is used for double-sided biplanar setups).
The projection parameters are computed live with feedback. Users can easily manoeuvre & orientate the target based
on the visual indicators.
A ringbuffer recording is performed at 15 Hz capturing a total of 800 frames/steps. From using the measured series &
projection calibration, the correlation evaluation is then performed. A mask is set from the reference camera over the
central part of the sample whilst excluding the holes. The startpoint search performs a pre-correlation indicating
whether or not the correlation is possible, based on the selected startpoints, facet/subset size, and tolerance limits.
The correlation evaluation is performed with live feedback of the selected visualization.
From the graphical visualization, the Y-displacement (mm) with rigid body motion removed (RBMR) is displayed as a
2D-overlay. As expected, the displacement is symmetrical around the point of contact of the impactor die. The sample
depresses around the contact and the longer sides expand. The maximum compressive strain at the point of contact is
ca. -7 mstrain, that can be seen from the line gauge. Interestingly, the mean principal strain around both circles was
different, indicating that the alignment of the specimen was not exactly symmetrical around the impactor die, as otherwise
though. Additionally, the strain was 1.5x higher around circle 1, then circle 2, indicating the stress concentration factor as
the location for where eventual fracture would occur. This is easily visible in the 2D-overlay of the engineering principle
strain 1, specifically in the upper top left of circle 1.
3-point bending test of a wooden specimen with holes using the MSpeedDIC system in Istra4D V4.11
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