A 3D Displacement Control by Digital Image Correlation for the Multiaxial Testing of Materials with a Stewart Platform

A complex mixed-mode crack propagation test performed with a 6-axis testing machine and full-field measurements

“…In the present case, 3 targets covered with a black and white speckle and 3 cameras (Leutron Picsight P141M-GigE camera, definition: 1392 × 1040 pixels, It has been shown [45] that the displacement error of the machine in such a configuration is less than 2 µm, and can be reasonably assessed to less than 1 µm because the error is partly due to the validation setup at this scale. (1) The loading path is governed by two main features, namely, the crack is ini-215 tiated in mixed mode loading and the process zone influence is checked by crack reorientation.…”

“…Another solution is to directly measure the position of the endeffector and use an end-effector position control. The latter can be obtained with a set of uniaxial sensors [4,8,6], or with a non-contact optical full-field 140 measurement technique [45]. One advantage of the optical method is that it does not hide the space around the specimen if used from a long distance.…”

“…• The hexapod own DIC system: fast (i.e., 20 Hz), low uncertainty (≤ 0.2 µm for in-plane motion of each camera [45]), but only 3 translations and 3 ro- of the hexapod to be calculated. The measurement areas are on the octogonal reference plate (Figure 8(a-b)), closer to the U-shaped plates than the DIC targets but still a few centimeters away from the specimen upper end.…”

“…Second, as underlined by Nooru-Mohamed himself [22], only the displacements along the two loading axes could be acquired, while 3D displacements would be of interest because of the high sensitivity to boundary conditions. In spite of these drawbacks and the fact that only point-wise kine- 45 matic data were available (in addition to the final crack path), the so-called NM tests have become a benchmark for numerous models and numerical implementations [23,24,25,26,27,28,29,30,31,32,33,34,35] since it is one of the few mixed-mode crack propagation experiments (i.e., Refs. [36,37,38,39] are among the few other mixed-mode crack propagation tests on cement-based 50 material).…”

On the validation of integrated DIC with tapered double cantilever beam tests

“…Any out-of-plane rotation of the sample, in combination with a non-telecentric lens, will be interpreted as in-plane deformation if not considered. Using a pinhole camera model, these apparent displacements can be modeled and a first order approximation returns the following five shape functions [34],…”