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In-situ (tomography) experiments are generally based on scans reconstructed from a large number of projections acquired under constant deformation of samples. Standard digital volume correlation (DVC) methods are based on a limited number of scans due to acquisition duration. They thus prevent analyses of time-dependent phenomena. In this paper, a modal procedure is proposed that allows time-dependent occurrences to be analyzed. It estimates spacetime displacement fields during the whole loading history. The spatial modes are based on standard DVC, which is subsequently enriched using projection-based digital volume correlation (P-DVC) to measure the temporal amplitudes. The method is applied to two cases, namely, a virtual experiment mimicking wedge splitting and an actual shear test on a pantographic metamaterial inducing large motions. With the proposed method, the temporal amplitude in the real test was measured for each projection leading to a temporal resolution of one tenth of a second and the analysis of 16,400 time steps. For the proposed algorithm, the sensitivity to the acquisition angle of the sample was investigated and measurement uncertainties were assessed.

Wood is a material with anisotropic elastic properties at the macroscale. In the present work, a sample made of Beninise teak was subjected to in situ torsion. Digital Volume Correlation (DVC) analyses were run at the mesoscale to measure displacement fields. The corresponding strain fields were obtained at the same scale in addition to the gray level residuals at the voxel scale. The out-of-plane shear modulus could be calibrated at the macroscale and was in good agreement with earlier results of the coauthors (MCH and CAK). The ultimate shear strength was also assessed at the same scale. Last, damage was detected and quantified at the mesoscale thanks to strain fields and at the microscale via gray level residual fields.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A combined computational-experimental framework is introduced herein to validate numerical simulations at the microscopic scale. It is exemplified for a flat specimen with central hole made of cast iron and imaged via in-situ synchrotron laminography at micrometer resolution during a tensile test. The region of interest in the reconstructed volume, which is close to the central hole, is analyzed by digital volume correlation (DVC) to measure kinematic fields. Finite element (FE) simulations, which account for the studied material microstructure, are driven by Dirichlet boundary conditions extracted from DVC measurements. Gray level residuals for DVC measurements and FE simulations are assessed for validation purposes. [ABSTRACT FROM AUTHOR]

In this work 3D strain and damage analyses are performed in the immediate vicinity of the notch root of a flat CT-like specimen made of aluminum alloy. Experimental data, partially exploited by Morgeneyer et al. (Acta Mat 69:78-91, ), were obtained by using synchrotron laminography and the 3D reconstructed volumes are subsequently analyzed via Digital volume correlation. These data enable for in situ assessments of strain fields and ductile damage in the zone where the stress triaxiality evolves from elevated to lower levels, which is accompanied by flat-to-slant crack transition. The measured strain field patterns in this area are analyzed herein in a systematic manner by studying the incremental strain activity during several loading steps. It is shown that from the very beginning of the loading history multiple slant strained bands appear in front of the notch root while the corresponding damage growth sets in at later loading stages and higher strains. The activity of the different strained bands at the notch root is alternating between different locations over the loading history. However, the band leading to final rupture is always active. The region where slant fracture occurs is identified to be in plane strain condition with respect to the crack propagation direction. [ABSTRACT FROM AUTHOR]

Abstract: Fatigue crack propagation is usually analyzed as a one-dimensional problem and the identification of Paris’ type propagation law is often performed using standardized samples. The present paper is devoted to the analysis of three dimensional fatigue crack propagation using advanced experimental, imaging, measurement and numerical simulation techniques. Fatigue experiments are performed in situ in a tomograph. The images are analyzed using digital volume correlation to extract displacement fields and crack geometry. Stress intensity factors are evaluated along three-dimensional crack fronts and compared to numerical simulations using measured boundary conditions and detected crack shape. Last, using a single experiment, local da/dN vs. ΔK laws are estimated within a large range of ΔK values. [Copyright &y& Elsevier]

Abstract: A full three-dimensional (3D) study of a compression test on a sample made of stone wool is presented. The analysis combines different tools, namely X-ray microtomography of an in situ experiment, image acquisition and treatment, and volume correlation. This set of tools allows for the measurement of 3D displacement fields and enables Poisson’s ratio of this type of material to be evaluated. The strain fields are used to analyze localization phenomena. The uncertainty of the analysis is evaluated by actually moving the sample in its undeformed and deformed states, qualifying both image acquisition and correlation steps. [Copyright &y& Elsevier]

Damage and strain changes during shear dominated loading were assessed in situ via synchrotron laminography. In combination with digital volume correlation (DVC), local strain fields inside the material were determined thanks to the contrast provided by intermetallic particles. Testing was performed on a thin sheet sample with an optimized geometry that induced shear stress states in two ligaments opposite to each other with respect to the loading axis. Examining one of the ligaments, the strain fields in the material showed the expected localization in the shear plane with a central peak in shear strain. The Lode parameter fields associated with incremental strain measurements were determined for the analyzed zone and found to be close to zero. The total Hencky von Mises strain close to failure was measured in the material bulk and reached values between 0.6 and 0.7 for a DVC element length of 20 μ m. In the shear zone, damage onset in the form of grain-related elongated cracks and of voids nucleating on intermetallic particles was detected from about half of the maximum value of the equivalent strain to failure onward. Voids between particle clusters grew in shear as the particles were rigid and mostly brittle. Damage was mainly concentrated in the gage section subjected to shear and away from free surfaces, in particular away from the notches. It was characterized and quantified with respect to the maximum equivalent strain found inside the band. • Damage development under simple shear was imaged by 3D in situ laminography • 3D Strain fields were measured by DVC • The Lode parameter was assessed based on strain increments • Damage nucleated and grew in the aluminum alloy • Shear and intermetallic particle cracks were found [ABSTRACT FROM AUTHOR]

Based on the assumption that the time evolution of a sample observed by computed tomography requires many less parameters than the definition of the microstructure itself, it is proposed to reconstruct these changes based on the initial state (using computed tomography) and very few radiographs acquired at fixed intervals of time. This Letter presents a proof of concept that for a fatigue cracked sample its kinematics can be tracked from no more than two radiographs in situations where a complete 3D view would require several hundreds of radiographs. This 2 order of magnitude gain opens the way to a "computed" 4D tomography, which complements the recent progress achieved in fast or ultrafast computed tomography, which is based on beam brightness, detector sensitivity, and signal acquisition technologies. [ABSTRACT FROM AUTHOR]

Abstract: High-resolution in situ synchrotron X-ray laminography combined with digital volume correlation (DVC) is used to measure the damage and plastic strain fields ahead of a notch introduced within a 2198 Al–Cu–Li alloy sheet. Synchrotron laminography is a technique specifically developed for three-dimensional (3-D) imaging of laterally extended sheet specimens with micrometre resolution. DVC is carried out using the 3-D image contrast caused by iron-rich intermetallic particles present in the alloy. The alloy is recrystallized and tested in T8 artificial ageing condition involving relatively low work hardening. Inclined strain localization bands are shown to develop at ∼800μm from the notch prior to the onset of damage. Damage in this region results mainly from the nucleation of voids on micrometric intermetallic particles and occurs at a very late stage of deformation, followed by very limited void growth. The accumulation of strain in the slanted localization band is found to be a steady process, whereas the crack propagation is a sudden process. Standard 3-D FE calculations using either von Mises plasticity or Gurson’s model do not capture the plastic localization process. [Copyright &y& Elsevier]

Abstract: Three-dimensional (3-D) tomographic images of a nodular graphite cast iron obtained using a laboratory X-ray source were used to analyze the opening of a fatigue crack during in situ mechanical loading. Direct image analysis and digital image correlation are utilized to obtain the 3-D morphology and front location of the crack, as well as the displacement fields in the bulk of the specimen. From digital image correlation results it is possible to extract the crack opening displacement (COD) map in the whole sample cross-section and to compute stress intensity factors (SIFs) all along the crack front, even for COD values that are less than the image resolution. The comparison of COD maps with local values of the SIF enabled for an estimation of the opening SIF (Kop ) equal to 6MPam1/2. [Copyright &y& Elsevier]

Crystallographic aspects of heterogeneous plastic flow during ductile tearing are assessed experimentally by combining in situ synchrotron laminography imaging of notched flat samples with a full-field measurement technique and numerical simulations with the crystal plasticity finite element method (CP-FEM). Advantage is taken of the plane strain condition in the region ahead of the notch root (i.e. , in the plane normal to the crack propagation direction) to perform two-dimensional image correlation in the material bulk. Intermetallic particles provide image contrast that is registered during the correlation analyses. They reveal the early onset of slant strained bands along which the final slant crack propagated. Two-dimensional plane strain CP-FEM simulations using the grain shape and crystallographic orientations of the studied material show the same trends as those observed experimentally, namely, the appearance of early slant strained bands that cross tens of grains and that remain stable over the loading history. The ratio of the strains within and outside the bands is also similar to that observed experimentally. • In situ ductile tearing of an AA7075 T651 alloy was performed by 3D laminography. • Image correlation techniques allowed strain to be measured inside the material. • Early slant strained bands were found crossing tens of grains through the thickness. • CPFEM simulations using the real grain shape and orientation were performed. • The simulations captured the experimentally found slant strained bands. [ABSTRACT FROM AUTHOR]