Ruben Balcaen

Stereo-DIC Uncertainty Estimation Using the Epipolar Constraint and Optimized Three Camera Triangulation

Even though stereo-DIC is widely used in the field of experimental mechanics, uncertainty quantification techniques lag behind. Camera motion for example is often neglected, even though it is present and it can introduce bias. We propose to estimate the error caused by camera motion, based on the epipolar distance, by using a 3-camera system. This solves the problem of the lack of sensitivity along the epipolar line by placing a third camera perpendicular, providing sensitivity in both directions. The third camera provides modest improvement on the results with an optimized triangulation.

Evaluation of Camera Motion in Stereo-DIC

The full-field nature of stereo-digital image correlation (stereo-DIC) makes it a widespread technique with vast possibilities. The uncertainty quantification of the technique is however not yet fully understood due to the non-linear optical-numerical measurement chain, limiting analytical research of the technique. Camera motion is a perfect example of this; it is hard to measure and the influence of it is not yet investigated. A simulator (presented in [Balcaen et al. Finite element based image generator for stereo-DIC uncertainty quantification applied on stereo-DIC calibration, 2016] and used in [Balcaen et al. Stereo-DIC uncertainty quantification based on simulated images, 2016]) is used to study how camera motion during a test influences the resulting deformation- and strain-field. Since the exact imposed deformation-field is known in a simulator the resulting errors can be clearly identified. Camera motion between the calibration-stage and the actual measurement is not investigated since this comes down to a poorly calibrated rig. We refer to the literature for more information concerning the influence of poor calibration data on measurement results. Numerous tests indicate that small camera-rotations can introduce a linear displacement-offset and thus also a constant strain-offset, mainly depending on the focal length of the lenses and the angle of rotation. Camera translation on the other side seems to be less pernicious.

Model Based Bolt Preload Monitoring Using Digital Image Correlation

Bolt preload is one of the most significant parameters in bolted connections, which affects the static and fatigue performance of bolted connections. Traditional preload measuring methods give an estimation of the preload at the moment of tightening but cannot monitor preloading losses. In this investigation an initial proof of concept of a DIC based approach to relate bolt elongation to bolt preload is presented. Bolt elongation tests were performed to M16 bolts and a calibration method was proposed to obtain the bolt preload-elongation relationship for a given clamping length range.