Martin Poncelet

Multiaxial Testing of Materials Using a Stewart Platform: Case Study of the Nooru-Mohamed Test

A first application of a Stewart platform used as a multiaxial testing machine is presented. A specific case study is chosen, namely the so-called Nooru-Mohamed experiment, which is a tension-shear test on a notched concrete specimen. The calibration, analysis of performances, and a validation test are investigated. The results confirm that the hexapod can be used to apply multiaxial loadings under small displacements, and allow also the measurement of the six components of forces and torques. Additional digital image correlation procedures are implemented at two scales of observation. Global measurements show the effect of the compliance of the device. Local measurements around the crack tip, necessary due to the small deformation of the specimen, give quantitative information about crack nucleation and propagation and will allow for future accurate numerical model validations.

CARPIUC benchmark overview: crack advance, reorientation, propagation and initiation under complex loadings

A series of experiments has been performed to build a benchmark for the numerical modeling of mixed mode crack propagation in concrete. Two tests are selected so that the propagation is almost always stable although the material is quasi-fragile. Moreover the variation of mode mixity enables the study of the reorientation of the crack, as well as more complex phenomena such as branching or link-up. To control stability and mode mixity, the loading is imposed with a 6-axis testing machine while the crack propagation is assessed by full-field measurement. In this paper, a description of the experimental setup is provided as well as numerical simulation results illustrating the sensitivity to boundary conditions and geometry. Links to data needed to perform numerical simulations of these benchmark experiments are given, together with references to publications giving full descriptions of the protocol and experiment results.

Separation of superimposed images with subpixel shift

The problem of the separation of superimposed images is considered in the particular case of a steady background and a foreground that is composed of different patterns separated in space, each with a compact support. Each pattern of the foreground may move in time independently. A single pair of these superimposed images is assumed to be available, and the displacement amplitude is typically smaller than the pixel size. Further, assuming that the background is smoothly varying in space, an original algorithm is proposed. To illustrate the performance of the method, a real test case of X-ray tomographic radiographs with moving patterns due to dust particles or surface scratches of optical elements along the beam is considered. Finally an automatic and simple treatment is proposed to erase the effects of such features.

Hybrid Stereocorrelation for 3D Thermomechanical Field Measurements

Three dimensional displacement fields are measured thanks to an original stereo system made of InfraRed (IR) and visible light cameras. This configuration provides the thermal fields that drive the thermomechanical loading and the 3D surface displacement fields, i.e. the response of the studied material. The calibration of the stereo rig is performed by using a 3D target of known geometry and the large gray level variations that occur on the IR images are accounted for by applying a low-pass filter. Because of a slight disorientation of the target, a further calibration correction step revealed necessary. After correction of the target orientation, the measured displacement fields display a good qualitative agreement (no quantitative comparison has been performed yet) with the expected fields.