Abstract
In order to investigate the micromechanical mechan isms 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 Digital Image Correlation (DIC) has been developed at the Sol ids 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 sma ll wavelength, in combination with a specific grey level interpolation method (bilinear, bicubic, biquintic) in DIC algorithms. The second procedure aims at reducing the errors in duced 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 micrometer s out-of-plane motions of the sample all along the several months lasting test, t o 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).
Highlights
The first procedure aims at precisely quantifying actual systematic Digital Image Correlation (DIC) errors obtained with the images in use and reducing the amplitude of these errors by an appropriate choice of the lens aperture, which controls the size of diffraction patterns, so as to adapt it to the available natural contrast of the rock used as DIC patterns and the specific DIC algorithm
The experimental setup mainly consists of three parts: macroscopic and microscopic optical setups; mechanical loading device; suction control equipment
The MicroDIC setup is opposite the MacroDIC setup but both are mounted on the same rigid optical table, which is itself mounted on a rigid electromechanical machine used to apply the axial force on the cylindrical sample
Summary
This paper starts with a short presentation of the experimental setup, which will be described in detail elsewhere [3], and focuses on two specific procedures which have been developed to improve the accuracy of optical measurements. The second one consists in correcting the errors induced by the overall micrometric out-of-plane motion of the sample on the macroscopic DIC measurements. The alternative solution consists in tracking the motion of the sample with the optical microscope designed to investigate deformations at microscale. A contrast detection algorithm automatically keeps the microscope in focus and centres the area of interest; it records in addition the distance between the sample and the macroscopic camera with micrometric accuracy
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