Stereo-DIC uncertainty estimation using the epipolar constraint and optimized three camera triangulation

© 2017, The Society for Experimental Mechanics, Inc. 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.status: publishe

An FE-based image deformation tool to quantify the metrological performance of stereo-DIC

Stereo-Digital Image correlation (stereo-DIC) is being used increasingly when it comes to mea- suring shape, motion and deformation since the cost of computational power has dropped the last few years and since more commercial solutions are becoming available. The stereo-DIC process chain is how- ever largely non-linear, making the quantication of the metrological performance very cumbersome. As a consequence, usually empirical rules of thumb are being used together with user expertise. Since the uncer- tainty on a measurement is not known, the transition from valuable research tool to industrial measurement tool is being stalled. An FE-based image deformation tool is proposed to generate images as if they were made during a real experiment, giving ground truth simulated data and thus oering the possibility to evaluate and validate existing DIC-codes. It also oers the users an understanding on how the dierent parameters are in uencing the result of a DIC-measurement, giving the possibility to improve their results by introducing new rules of thumb. Since the image generator uses an FE-mesh and a real speckle pattern a best-case set up for each experiment can be determined.status: accepte

Evaluation of Camera Motion in Stereo-DIC

© The Society for Experimental Mechanics, Inc. 2017. 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.status: publishe

Stereo-DIC uncertainty quantification based on simulated images

Stereo digital image correlation (stereo-DIC) is in wide-spread use for full-field shape, motion and deformation-measurements. However there are very few papers investigating the influence of the setup on the measurement uncertainty. This is mainly due to the highly non-linear measurement chain involving both optical and numerical aspects, making it difficult to investigate how error sources are propagated through the stereo-DIC chain. Indeed, it is impossible to separate all the error sources that are present during a physical measurement. This paper tries to investigate a selection of error sources that are present during experiments. This is based on a simulator introduced in a previous article (Balcaen et al., Exp Mech, 1–16 2017) and briefly reviewed here. Based on these simulations we suggest some ”best-practices” guidelines of optimal stereo-DIC setups.status: publishe

Influence of camera rotation on stereo-DIC and compensation methods

© 2018, Society for Experimental Mechanics. Camera rotation during stereo digital image correlation (DIC) measurements is always present in some amount and is caused by environmental vibrations during testing (e.g. blast testing, testing in industrial environments, vibration testing Helffrick et al., Mech Syst Signal Process 25(3):917–927, 2011, etc.). The impact of changing extrinsic parameters on stereo-DIC measurements was investigated in this paper by simulating several DIC experiments with a shaking camera system. To obtain an appropriate order of magnitude of the camera motion, the actual camera rotation during a test was observed and reported. The impact hereof was investigated by analyzing static images and by simulating camera rotation with a stereo-DIC simulator presented in Balcaen et al. (Exp Mech 57(5):703–718, 2017). Insight on this underestimated error source is explained by investigating which rotations are more critical to the resulting displacements and strains. Two possible compensation methods were investigated and their capability of compensating camera motion was evaluated.status: publishe

Stereo-DIC calibration and speckle image generator based on FE formulations

Stereo digital image correlation (stereo-DIC) is being accepted by the industry as a valid full-field measurement technique for measuring shape, motion and deformation, and it is therefore of utmost importance to provide uncertainties on the obtained measurements. However, the influences on a stereo-DIC measurement are not fully understood; indeed, stereo-DIC is a complex optical-numerical process and it is not always clear how errors are propagating throughout the measurement chain. In order to investigate the magnitude of the different error-sources a simulator for stereo-DIC is proposed. This simulator is able to generate realistic synthetic images as if they were made during a real set-up, so the error sources can be investigated separately and an optimal set-up can be chosen before any physical test is performed. We present in this paper the mathematical approach to the DIC simulator including details on how to convert FE displacement field results to stereo-DIC images. The simulator includes the ability to control the lighting and to create synthetic calibration images. The synthetic images are compared to simulations for a bulge test as a validation of the simulator. Synthetic calibration images are compared to experimental calibration studies to verify those. Finally a brief look at how the simulator could be used for looking at calibration quality is conducted.status: publishe

Evaluation of methodologies for compensation of out of plane motions in a 2D digital image correlation setup

© 2015 Wiley Publishing Ltd. In 2D digital image correlation, out of plane motions are an important experimental factor to consider. Because a 2D setup does not provide any depth information, movements towards the camera (out of plane motions) are disadvantageous for the surface measurement. The effects of out of plane motions in a 2D digital image correlation (DIC) setup have been investigated profoundly in previous works. The compensation of these motions however is less investigated. Therefore, this paper will handle on solutions for correcting or minimising out of plane motions in 2D DIC measurements. Three compensation methods are implemented and validated. Firstly, a mechanical camera positioning tool is developed to avoid misalignments of the camera and therefore avoid most of the out of plane motions. Secondly, the camera is aligned numerically using the camera pinhole model and numerically deforming the images. Finally, a method proposed in literature using a region of compensation is used in this validation. All three methods have shown to be able to minimise out of plane motions. Validated in an experimental setup, a great improvement of the identified Poisson ratio is observed during multiple tensile tests.status: publishe

Model Based Bolt Preload Monitoring Using Digital Image Correlation

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Impact of motion blur on stereo-digital image correlation with the focus on a drone-carried stereo rig

Stereo‐digital image correlation (DIC) is a wide‐spread technique in the field of experimental mechanics for measuring shape, motion, and deformation and it is frequently used for material identification by using inverse methods (e.g., virtual fields method and finite element model updating). New applications emerge due to the reached maturity level of the technique, which poses new challenges towards reaching a desired level of accuracy in operating conditions. In this work, the possibility of a drone carrying an in‐house‐made portable DIC setup is explored, and the effect of the drone‐induced vibrations on the accuracy of stereo‐DIC for shape and strain measurement is evaluated. During acquisition, the relative motion between the camera system and the measured item generates motion‐blurred images. The effect of this phenomenon on the precision of stereo‐DIC is further evaluated in this paper.status: publishe

Evaluation of the influence of cyclic loading on a laser sintered transtibial prosthetic socket using Digital Image Correlation (DIC)

People with a transtibial amputation worldwide rely on their prosthetic socket to regain their mobility. Patient comfort is largely affected by the weight and strength of these prosthetic sockets. The use of additive manufacturing could give the prosthetist a range of new design possibilities when designing a prosthetic socket. These new design possibilities can in turn lead to improved socket designs and more comfortable prosthetic sockets. This new way of designing and producing prosthetic sockets radically differs from the manual traditional production process. This makes it difficult for prosthetists to understand how all these new design possibilities influence the mechanical properties of the additive manufactured prosthetic socket. Therefore there is a growing need for a method to evaluate the strength and stiffness of newly developed socket designs.We propose a method to evaluate the strength and stiffness of prosthetic sockets. A robotic gait simulator is used to apply realistic kinetics of amputee gait to the tested socket. A Digital Image Correlation (DIC) system is then used to measure the deformation of a prosthetic socket under different loading conditions. This way it is possible to check if plastic deformation will occur in the designed transtibial socket. Furthermore it is possible to assess the effect of cyclic loading on the 3D printed socket.To illustrate the proposed method, a transtibial prosthetic socket was designed using CAD software and produced with laser sintering PA12. DIC measurements were performed on this transtibial socket both before and after it was subjected to a cyclic load of 1 million cycles (mimicking realistic amputee gait).status: accepte