Spherical indentation of granite was investigated using Digital Volume Correlation (DVC) aiming at probing constitutive laws of the studied rock. In situ indentation was performed within an X-ray tomograph. Finite element simulations of the problem, using different constitutive models, were carried out and their trustworthiness was assessed thanks to DVC residuals. Three laws were investigated, namely, pure elasticity, then compressible elastoplasticity, and finally compressible elastoplasticity coupled with damage. Frictional contact effects were studied as well. The results show that compressible elastoplasticity should be accounted for to achieve high accuracy of results, and that frictional effects are of importance in terms of damage extent. If macrocrack initiation is also sought, then damage features should be included in the model.

There is renewed interest in using advanced techniques to characterize ancient Roman concrete due to its exceptional durability and low-carbon footprint. In the present work, samples were drilled from the "Hospitium" in Pompeii and were analyzed by synchrotron microtomography (μCT) and neutron radiography to study how the microstructure, including the presence of induced cracks, affects their water adsorption. The water distribution and absorptivity were quantified by neutron radiography. The 3D crack propagation, pore size distribution and orientation, tortuosity, and connectivity were analyzed from μCT results using advanced imaging methods. Porosity was also measured by mercury intrusion porosimetry (MIP) as a reference. Ductile fracture patterns were observed once cracks were introduced. Compared to Portland cement mortar/concrete, the Pompeii samples had relatively high porosity, low connectivity, and a similar coefficient of capillary penetration. In addition, permeability was predicted from models based on percolation theory and pore structure data to evaluate the fluid transport properties. Understanding the microstructure of ancient Pompeii concrete is important because it could inspire the development of modern concrete with high durability.

In order to characterize the mechanical behavior of a reinforced concrete beam–column joint, an experiment is designed whereby different histories of increasing cyclic loads are applied, and the structure deformation (as well as the morphology of apparent surface cracks) is studied using multi-view correlation. The complex three-dimensional geometry and the slender nature of the beam and column call for numerous digital cameras were positioned around the specimen. Assessing the feasibility of image acquisition and estimating a priori uncertainties on multi-view correlation became a true challenge. It is shown that the recourse to photo-realistic rendering software provides the needed tools to perform this experiment design optimization.

A recently developed Projection-based Digital Image Correlation (P-DVC) method is here extended to 4D (space and time) displacement field measurement and mechanical identification based on a single radiograph per loading step instead of volumes as in standard DVC methods. Two levels of data reductions are exploited, namely, reduction of the data acquisition (and time) by a factor of 1000 and reduction of the solution space by exploiting model reduction techniques. The analysis of a complete tensile elastoplastic test composed of 127 loading steps performed in 6 min is presented. The 4D displacement field as well as the elastoplastic constitutive law are identified.

Digital volume correlation consists in registering series of 3D images of experiments to yield 4D displacement fields. These 4D analyses have been conducted for the last two decades. Some achievements and current challenges are reviewed herein.

A projection-based digital volume correlation method (presented in a companion paper) is extended to an integrated approach for the calibration of an elastoplastic law based on a single radiograph per loading step. Instead of following a two-step sequential procedure (i.e. first, measurement of the displacement field; second, identification), the integrated method aims at identifying few model parameters directly from the gray-level projections. The analysis of an in situ tensile test composed of 127 loading steps performed in 6 min is presented. An isotropic elastoplastic constitutive law with free-form hardening behavior (i.e. controlled by only eight parameters) is identified and shows a ductile behavior (up to 6.3% strain before failure). A large improvement on the residual quality is shown and validates the proposed model and procedure. The obtained displacement fields are similar to those measured with no mechanical integration. A different parameterization of the constitutive law provides a very close result, thereby assessing the robustness of the procedure.

The measurement of 4D (i.e., 3D space and time) displacement fields of in situ tests within X-ray Computed Tomography scanners (i.e., lab-scale X-CT) is considered herein using projection-based Digital Volume Correlation. With one single projection per loading (i.e. time) step, the developed method allows for loading not to be interrupted and to vary continuously during the scan rotation. As a result, huge gains in acquisition time (i.e., more than two orders of magnitudes) to be reached. The kinematic analysis is carried out using predefined space and time bases combined with model reduction techniques (i.e., Proper Generalized Decomposition with space-time decomposition). The accuracy of the measured kinematic basis is assessed via gray level residual fields. An application to an in situ tensile test composed of 127 time steps is performed. Because of the slender geometry of the sample, a specific beam space regularization is used, which is composed of a stack of rigid sections. Large improvements on the residual, whose SNR evolves from 9.9 dB to 26.7 dB, validate the procedure.

The present study deals with a three dimensional analysis of a cyclic uniaxial experiment on a glass fibre reinforced epoxy resin composite. From monotonic and cyclic uniaxial mechanical experiments and by using a 2D digital image correlation optical setup, the macroscopic material behaviour is revealed. Based on the observed response, an ex situ cyclic tomography experiment was defined and conducted. The investigated dog-bone sample was loaded at four different load levels. At each subsequent unloaded stage, the sample was scanned in a laboratory tomograph. The 3D reconstructed volumes were analyzed with FE-based digital volume correlation in order to measure displacement fields in the region of interest. Since very low permanent strains were reported, the gray level correlation residuals were studied to observe the damaged zones in the bulk of the sample. Overlaying the correlation residuals with the corresponding microstructure accurately depicts damage mechanisms occurring in the investigated material.

The inelastic behavior of Bohus granite is investigated based on experimental and numerical results. The yield surface and related dilation angle are determined based on quasi-oedometric tests performed in an earlier work. It is shown how to obtain the yield surface and dilation angle from this test for hydrostatic pressure levels up to 750MPa. In the constitutive modeling, a Drucker-Prager law is employed together with a variable dilation angle. The constitutive model is first applied to simulate the quasi-oedometric test and the stress and strain fields are obtained. Furthermore, the validation of the model is investigated by simulation of the spherical indentation test. The results are compared with corresponding experimental data and a good agreement is found.

Digital volume correlation consists in registering series of 3D images of experiments to yield 4D displacement fields. These 4D analyses have been conducted for the last two decades. Some achievements and current challenges are reviewed herein.

Two dynamic Brazilian tests are analyzed via digital image correlation for acquisition rates equal to 5 and 10 million frames per second. Displacements of the order of 1 mm, velocities of the order of +/- 250 m/s, and accelerations as high as +/- 4 x 10(7) m/s(2) are measured with spacetime DIC. Uncertainty quantifications enable spacetime DIC to be compared with instantaneous analyses. The gains provided by the temporal regularization are very significant for the acceleration fields. The observed levels are consistent with a priori estimates.

In the last decade, the exotic properties of pantographic metamaterials have been investigated and different mathematical models (both discrete or continuous) have been introduced. In a previous publication, a large part of the already existing literature about pantographic metamaterials has been presented. In this paper, we give some details about the next generation of research in this field. We present an organic scheme of the whole process of design, fabrication, experiments, models and image analyses.

The present study aims at demonstrating the feasibility of performing a fracture test in less than 1 min in a laboratory CT scanner despite the severe time constraints of tomography acquisition. After introducing the basic concepts of projection-based digital volume correlation, the specific implementation of this methodology to a wedge splitting test on a refractory material is presented. The kinematics of the test is described over a mesh tailored to the sample geometry, and the elastic behavior of the sample is exploited through finite element computations to provide sensitivity fields of experimental boundary conditions to allow for their “measurements.” Enhancing the simulation to account for crack advance with extended finite element analyses allows the sensitivity of the procedure to the crack position to be assessed. A confidence interval for the refractory toughness is finally obtained.

Digital image correlation (DIC), which consists in registering image pairs to measure displacement fields, can be tailored to analyze image sequences from videos taking advantage of a common reference image. In the present study it is no longer the first image of the sequence but rather a computed image from the entire video. The sought kinematics, which is separated in space and time, offers the opportunity to extract 'modes', each of which is a spatial displacement field multiplied by a scalar function of time only. These modes are not chosen a priori but rather computed from a specific formulation of DIC so that they capture the displacements at best. The exploited mathematical technique to achieve this modal representation is a form of proper generalized decomposition that makes use of the DIC variational formulation, where both spatial and temporal regularizations can be included. Two image videos acquired with an ultra-high speed camera at 5 and 10 million frames per second are analyzed to illustrate the proposed technique. Very large computation time gains are obtained with no noticeable differences in the kinematic measurements. Moreover, it is shown that motions have a lower complexity, i.e. require less modes, than the direct proper orthogonal decomposition (or principal component analysis) performed on the entire video.

An online calibration procedure for x-ray lab-CT is developed using projection-based digital volume correlation. An initial reconstruction of the sample is positioned in the 3D space for every angle so that its projection matches the initial one. This procedure allows a space-time displacement field to be estimated for the scanned sample, which is regularized with (i) rigid body motions in space and (ii) modal time shape functions computed using model reduction techniques (i.e. proper generalized decomposition). The result is an accurate identification of the position of the sample adapted for each angle, which may deviate from the desired perfect rotation required for standard reconstructions. An application of this procedure to a 4D in situ mechanical test is shown. The proposed correction leads to a much improved tomographic reconstruction quality.

The measurement of four-dimensional (i.e. three-dimensional space and time) displacement fields of in situ tests within X-ray computed tomography scanners (i.e. lab-scale X-ray computed tomography) is considered herein using projection-based digital volume correlation. With a single projection per loading (i.e. time) step, the developed method allows the loading not to be interrupted and to vary continuously during the scan rotation. As a result, huge gains in acquisition time (i.e. more than two orders of magnitude) need to be reached. The kinematic analysis is carried out using predefined space and time bases combined with model reduction techniques (i.e. proper generalized decomposition with space–time decomposition). The accuracy of the measured kinematic basis is assessed via gray-level residual fields. An application to an in situ tensile test composed of 127 time steps is performed. Because of the slender geometry of the sample, a specific beam space regularization is used, which is composed of a stack of rigid sections. Large improvements on the residual, the signal-to-noise ratio of which evolves from 9.9 to 26.7 dB, validate the procedure.

3D imaging has become popular for analyzing material microstructures. When time lapse series of 3D pictures are acquired during a single experiment, it is possible to measure displacement fields via digital volume correlation (DVC), thereby leading to 4D results. Such 4D analyses have been performed for almost two decades. The present paper aims at reviewing the achievements of and challenges faced by such measurement technique. Ex-situ and in-situ experiments are discussed. A general and unified DVC framework is introduced. Various sources of measurement bias and uncertainties are analyzed. The current challenges are studied and some propositions are given to address them.

A recently developed Projection-based Digital Image Correlation (P-DVC) method is here extended to 4D (space and time) displacement field measurement and mechanical identification based on a single radiograph per loading step instead of volumes as in standard DVC methods. Two levels of data reductions are exploited, namely, reduction of the data acquisition (and time) by a factor of 1000 and reduction of the solution space by exploiting model reduction techniques. The analysis of a tensile elastoplastic test composed of 127 loading steps performed in 6 minutes is presented. The 4D displacement field as well as the elasto-plastic constitutive law are identified. Keywords-Image-based identification, Model reduction, Fast 4D identification Introduction The identification and validation of increasingly complex mechanical models is a major concern in experimental solid mechanics. The recent development of computed tomography coupled with in-situ mechanical tests and full field measurements offers the opportunity to identify kinematic and/or mechanical quantities on the tested sample. The 4D frameworks [1] based on a series of 3D scans (each composed of approximately 1000 radiographs requiring 1 hour scan time in lab-CT) at different loading steps have proved to be very efficient but extremely time intensive (and in turn fragile with respect to slow drifts). These approaches are thus often limited to few scans and forbid time dependent mechanical characterizations. A recently developed Projection-based Digital Volume Correlation (P-DVC) [2] overcomes these difficulties by extracting the measured quantities directly from few radiographs instead of reconstructed volumes. The technique was originally developed for crack characterization in synchrotron or lab-CT [3] by using only two orthogonal projection angles per step resulting in a gain of 300 in acquisition time compared with the classical full scan method. P-DVC is herein extended to elastoplastic identification from a single projection per loading step. The proposed method allows on-the-fly radiograph acquisitions to be carried out while continuously loading and rotating the sample with an additional improvement of more than 2 in the acquisition time. P-DVC is based on the minimization of the quadratic difference between the projection of the deformed sample acquired at angle θ(t), g(r,t), and the computed projection of the reference 3D image,

An identification framework is introduced herein to calibrate material parameters at the microscale in order to analyze ductile damage. It is applied to study a dog-bone sample, which is made of spheroidal graphite cast iron, loaded in tension and imaged via in situ microtomography. The region of interest is analyzed via Digital Volume Correlation (DVC) to measure kinematic fields. Finite Element (FE) simulations, which account for the studied microstructure that is explicitly meshed thanks its 3D image, are driven by Dirichlet boundary conditions extracted from DVC measurements. The plastic behavior of the ferritic matrix is calibrated via integrated DVC. The three mechanisms of ductile damage are then analyzed in view of the predictions of numerical simulations at the microscopic scale.