Optimized optical setup for DIC in rock mechanics

Abstract

Abstract. In order to investigate the micromechanical mechanisms of argillaceous rocks under combined mechanical load and varying moisture, and observe very low strain rates of about 10 -10 s -1 , i.e. about 10 -4 per week, a specific optical setup for DigitalImage Correlation (DIC) has been developed at the Solids Mechanics Laboratory, Ecole Polytechnique. This paper mainly presents two procedures to optimize the experimental setup and enhance the full-field strain measurement accuracy. The first one allows us to minimize the systematic DIC errors by choosing an optimal optical setup, including appropriate lens aperture and light source with small wavelength, in combination with a specific grey level interpolation method (bilinear, bicubic, biquintic) in DIC algorithms. The second procedure aims at reducing the errors induced by overall out-of-plane motions of the sample. This procedure makes use of a specifically designed optical microscope, which tracks the few tens of micrometers out-of-plane motions of the sample all along the several months lasting test, to compensate the magnification fluctuations which disturb the macroscopic measurements. The current optimized optical setup gives a better than 10 -5 accuracy at macro scale (3 mm gage length) and 10 -4 accuracy at microscale (about 100 µm gage length).

Localized deformation induced by heterogeneities in porous carbonate analysed by multi-scale digital image correlation

“…Beam size and scanning rate are also adjusted to get a good compromise between the quality of the signal to noise ratio and the problem of electrical charge accumulation. Noise levels are however larger in SEM images than in optical images, as discussed by Yang et al (2010).…”

“…The quantification of the errors due to an overall uniform out-of-plane motion is further discussed in Section 5. Additional general discussions on these aspects can be found in Sutton et al (2008) and an ad-hoc improvement of the present set-up is described by Yang et al (2010).…”

“…The systematic error is then expressed as the difference between the evaluated and theoretical displacements, associated with the global camera motion. Additional information on this procedure of systematic-error estimation is reported in Yang et al, 2010. The error on the displacement field propagates to the calculation of the transformation gradient P P F . However a suitable choice of the grid size and of the number of neighboring points for displacement integration can minimize this effect.…”

Experimental characterization of the intragranular strain field in columnar ice during transient creep

“…In an attempt to evaluate systematic errors associated with subpixel displacements (so-called 'S curve') the method proposed by [38] was used. Successive images of the fixed specimen were taken for small camera translations along the optical axis.…”

Experimental multiscale measurements for the mechanical identification of a cortical bone by digital image correlation

“…The identification procedure of the prior stochastic model of this apparent elasticity field is detailed in Nguyen (2013); Nguyen et al (2015) for which the measurements of the displacement field are required both at the macroscopic scale and at the mesoscopic scale. The implementation of the experimental methodology by optical measurements of mechanical fields, the development of a measuring bench, the specimen preparation, the experimental measurements, and the DIC method, have already been the object of research in the context of biological materials Bertolino et al (2007); Bornert et al (2010); Chauvet et al (2010); Gusachenko et al (2012); Heripre et al (2007); Houssen et al (2011); Kalouche et al (2010); Yang et al (2012Yang et al ( , 2010; Yamaguchi et al (2011). In this paper, new advances are proposed to circumvent the difficulties induced by the complex nature of the cortical bone and the need to observe for the same loading two very different scales.…”