VDE 2626: Pre-Acceptance Test

VDI/VDE 2626 is a standard practice that describes a general procedure for performing an acceptance & interim check for a stereoscopic Digital Image Correlation (DIC) setup. The standard was issued in November 2019 by the Association of German Engineers (Verein Deutsche Ingenieure – VDI) to provide a standardized pre-acceptance and monitoring protocol for the accuracy quantification of a DIC test using three (primary) technical parameters, including: Zero Strain Deviation (ZS), Zero Displacement Deviation (ZD) and Displacement Deviation (D).

Figure 1 – Front page extract of VDI/VDE 2626

The acceptance and monitoring of a DIC system can be carried out by measuring the movement of a rigid test piece captured in different orientations or positions within a projected measurement volume. The ZS and ZD parameters characterize the ability of a DIC measuring system to reproducibly capture the deviation of displacement of, and strain on, an object. The DD parameter characterizes the ability of a DIC measuring system to determine absolute displacements for a defined test procedure.

Video – Measurement example of a test piece moved to twelve locations. The Total Displacement with Rigid Body Motion Removed (RBMR) in µm can be observed in the visualization and the Total Displacement can be seen in the temporal plot below. Three acquisition steps were acquired at each location. The Region-of-Interest (RoI) is 80 x 80 mm.

Figure 2 – Determination equations of parameters ZD & ZS

The determination of the parameters ZS and ZD are found through sub-dividing the measurement points into a grid of squares, q, with a fixed size. For each square, the values of the measuring points are combined to form one value. The determination of the zero displacement (ZD) results from the mean squared error of the averaged displacements of all squares. The determination of the zero strain (ZS) results from the root mean square of strain X, Y and shear (XY) strain over all squares. The determination of the displacement (D) results from the absolute difference between measured output displacement data and input displacement data.

Figure 3 – Zero Displacment Deviation with respect to test step acquisitons

Figure 4 – Zero Strain Deviation with respect to test step acquisitons

The parameters ZD and ZS describe acceptance requirements for the relative deviation behaviour of a DIC measuring system for the rigid body motion of a test piece, without it being deformed. As such, ZD and ZS are calculated with the removal of rigid body motion (RBMR) for which in an ideal case should be zero. For the measurement example shown above, both the ZD and ZS increase with respect to absolute displacement. The mean parameters for ZD and ZS are 1.5 µm and 75 microstrain, respectively.