Removing Quasi-Periodic Noise in Strain Maps by Filtering in the Fourier Domain

Experimental Techniques - Quasi-periodic noise due to various reasons often corrupts strainmaps obtained with full-field measuring systems. The aim of this didactic paper is to show how to remove...     

DIC Challenge 2.0: Developing Images and Guidelines for Evaluating Accuracy and Resolution of 2D Analyses

Experimental Mechanics - The DIC Challenge 2.0 follows on from the work accomplished in the first Digital Image Correlation (DIC) Challenge Reu et al. (Experimental Mechanics 58(7):1067, 1). The...     

A Robust-to-Noise Deconvolution Algorithm to Enhance Displacement and Strain Maps Obtained with Local DIC and LSA

Experimental Mechanics - Digital Image Correlation (DIC) and Localized Spectrum Analysis (LSA) are two techniques available to extract displacement fields from images of deformed surfaces marked...     

On the Propagation of Camera Sensor Noise to Displacement Maps Obtained by DIC - an Experimental Study

This paper focuses on one of the metrological properties of DIC, namely displacement resolution. More specifically, the study aims to validate, in the environment of an experimental mechanics laboratory, a recent generalized theoretical prediction of displacement resolution. Indeed, usual predictive formulas available in the literature neither take into account sub-pixel displacement, nor have been validated in an experimental mechanics laboratory environment, nor are applicable to all types of DIC (Global as well as Local). Here, the formula used to account for sub-pixel displacements is first recalled, and an accurate model of the sensor noise is introduced. The hypotheses required for the elaboration of this prediction are clearly stated. The formula is then validated using experimental data. Since rigid body motion between the specimen and the camera impairs the experimental data, and since sensor noise is signal-dependent, particular tools need to be introduced in order to ensure the consistency between the observed image noise and the model on which prediction hypotheses are based. Pre-processing tools introduced for another full-field measurement approach, namely the Grid Method, are employed to address these issues.     

On the Optimal Pattern for Displacement Field Measurement: Random Speckle and DIC,or Checkerboard and LSA?

Experimental Mechanics - This paper deals with the optimal pattern that can be used to retrieve displacement fields by minimizing the optical residual calculated over small regions of contrasted...     

Extracting Displacement and Strain Fields from Checkerboard Images with the Localized Spectrum Analysis

Experimental Mechanics - The performance of white-light full-field measurement methods strongly depends on the nature of the pattern used to mark the surface on which displacements and strains are...     

Investigation of the Tensile Behavior of Recycled Asphalts in the Small-Strain Domain with the Grid Method

Under mechanical loading, asphalt mixtures exhibit in their bulk heterogeneous strain fields characterized by localized gradients concentrated within the binder. Measuring such fields constitutes a challenge for the full-field measurement methods currently used in the experimental mechanics community. This is particularly true when the objective is to measure strains in the linear viscoelastic domain, which is characterized by low strain levels (about some 200 με). In the present study, the strain distribution is measured on the surface of several recycled asphalt pavements (RAP) subjected to direct tensile tests. Four asphalt mixtures incorporating respectively 0%, 20%, 40% and 100% of RAP are studied. The tests are performed using a servo-controlled machine for rheological tests (MAER) and the local strain fields are measured using the Grid Method (GM). Out-of-Plane Motions (OPM) and camera noise are the main causes of disturbance that significantly affect the strain measurements. A method is proposed here to compensate the OPM. It is specially dedicated to bituminous mixtures. Sensor noise is filtered over time to improve the measurement resolution. Obtained results indicate that such compensations allow the use of GM to obtain quantitative measurements of the asphalt deformation in the small-strain domain. Finally, the behavior of the different samples are compared and the effect of RAP inclusion on the local strain distribution is observed and characterized.     

Towards Criteria Characterizing the Metrological Performance of Full-field Measurement Techniques

Users of full-field measurement methods like Digital Image Correlation (DIC) often aim to perform measurements with the best trade-off between spatial resolution, bias and measurement resolution. Whenever two full-field methods are compared, it is essential that these criteria are taken into consideration. Recently a metrological efficiency indicator for full-field measurements has been proposed and discussed. This indicator combines measurement resolution and spatial resolution. It has been shown to be invariant to the subset size in the case of Local DIC. The goal of this article is to discuss a method, which determines both the spatial and the measurement resolutions for a given bias for two different DIC methods, in order to obtain the metrological efficiency indicator for each of these methods. The benefit of this indicator is that it does not depend on setting parameters such as the subset size, which are chosen by the user. As such, it can be considered as intrinsic to each technique, thus enabling fair comparison. Local DIC and triangular finite element based Global DIC will be the subject of this investigation. With this setting, their respective subset and triangular element sizes will be related to the spatial resolution of both methods for a given acceptable bias. By using the metrological efficiency indicator, the performance of the two methods will be compared and discussed to a new level of detail. Generally speaking, the indicator shows that the metrological performance of both methods is similar, confirming their popularity. However, it will be shown that, depending on the choice of what an acceptable bias is, one of the method may be preferred to another. The results show that for the specific DIC versions used in the study, for cases for which a significant bias is acceptable, Local DIC outperforms Global DIC, while the opposite is true in the case for which the bias requirements are more stringent. Finally, the quadratic versions of both DIC versions are shown to significantly outperform their respective linear versions.