3D-volumetric Digital Image Correlation Research Articles

Localised deformation in a wide-grained sand under triaxial compression revealed by X-ray tomography and digital image correlation

Understanding the mechanisms of deformation and failure of soils is essential in geotechnical engineering. In particular, strain localisation is a key issue for the failure of soils. In laboratory mechanical testing, localised deformation can be investigated efficiently through full-field measurements of displacement and strain in soil specimens. The objective of this study is to investigate the deformation and the failure of a soil under triaxial compression using in situ X-ray (i.e., X-ray scanning during the loading test) computed tomography (CT) and digital image correlation (DIC). This paper is concerned with the characterisation of internal strains and with strain localisation patterning in wide-grained sand undergoing triaxial compression at different confinement pressures using Three Dimensional (3D) volumetric DIC. Complete 3D images of specimens have been recorded at several stages throughout the triaxial tests and analysed using 3D volumetric DIC in order to obtain 3D incremental displacement and strain fields. Based on the results of the combined X-ray tomography and DIC, the deformation process and especially the strain localisation are quantitatively characterised in space and time under different triaxial compression conditions.

Quantitative analysis of deformation in hollow cylinder tests on anisotropic clay formations

A series of triaxial laboratory experiments are performed on thick-walled hollow cylindrical samples of boom clay. The aim of this testing program is to better understand the anisotropic deformation during the excavation. The testing conditions are similar to those to be experienced by host rocks around disposal galleries for radioactive waste. X-ray computed tomography is performed at different steps for each test with the samples remaining inside the loading cell. Initial analysis of the tomography images allows of the observation of the deformation of the central hole. In addition, particles manual tracking and 3D volumetric digital image correlation processing methods are considered being used to analyze the particles displacements and the boundary deformation of the sample quantitatively. An unsymmetrical damaged zone is induced around the hole, with a reverse deformation trend being found at the boundary after unloading, which indicates that the significant anisotropic deformation of boom clay can be induced by mechanical unloading.

Full-field Measurements of Strain Localisation in Sandstone by Neutron Tomography and 3D-Volumetric Digital Image Correlation

Recent studies have demonstrated that the combination of x-ray tomography during triaxial tests (“in-situ” tests) and 3D- volumetric Digital Image Correlation (3D-DIC) can provide important insight into the mechanical behaviour and deformation processes of granular materials such as sand. The application of these tools to investigate the mechanisms of failure in rocks is also of obvious interest. However, the relevant applied confining pressures for triaxial testing on rocks are higher than those on sands and therefore stronger pressure containment vessels, i.e., made of thick metal walls, are required. This makes in-situ x-ray imaging of rock deformation during triaxial tests a challenge. One possible solution to overcome this problem is to use neutrons, which should better penetrate the metal-walls of the pressure vessels. In this perspective, this work assesses the capability of neutron tomography with 3D-DIC to measure deformation fields in rock samples. Results from pre- and post-deformation neutron tomography of a Bentheim sandstone sample deformed ex-situ at 40MPa show that clear images of the internal structure can be achieved and utilised for 3D-DIC analysis to reveal the details of the 3D strain field. From these results the character of the localised deformation in the study sample can thus be described. Furthermore, comparison with analyses based on equivalent x-ray tomography imaging of the same sample confirms the effectiveness of the method in relation to the more established x-ray based approach.

Shear-enhanced compaction band identification at the laboratory scale using acoustic and full-field methods

This paper presents results from the analysis of shear-enhanced compaction bands that developed in a porous sandstone during triaxial compression tests at high confining pressures. The analysis uses non-destructive full-field experimental methods: X-ray tomography, 3D-volumetric digital image correlation (DIC) and acoustic emission (AE) monitoring including source mechanisms analysis. The 3D-volumetric DIC measurements reveal that these bands are zones with: a small component of band-parallel slip; a larger component of vertical shortening; compactant volumetric strains; and high maximum shear strains. Low X-ray tomography gray-scale standard deviation values within the bands indicate regions of grain size reduction and grain fragmentation. AE hypocenters detected during loading were concentrated inside these narrow bands and showed predominantly pure and hybrid collapse mechanisms; the latter implies some shear strain and is consistent with the oblique geometry of these bands. The experimental results in general support the hypothesis that laboratory developed shear-enhanced compaction bands, at least those studied here, share more characteristics with compaction bands than with compactant shear-bands; these latter deformation features differ from shear-enhanced compaction bands not only in the mechanical behavior, but also in the kinematics and the grain-scale deformation mechanisms.

Characterization of shear and compaction bands in a porous sandstone deformed under triaxial compression

The study of localized deformation in porous sandstones at the laboratory scale can yield valuable insights into the internal structures and mechanisms of shear zones and compaction bands that might impact on flow at a reservoir scale. Herein, we report results of a laboratory study of shear and compaction band formation in a porous sandstone using a range of full-field experimental techniques: acoustic emissions, ultrasonic tomography, X-ray tomography, and 3D volumetric digital image correlation, plus thin section and Scanning Electron Microscope observations. The two main mechanisms involved in shear and compaction band formation, grain breakage (damage) and porosity reduction (compaction), are both well captured by the combination of all these laboratory techniques. The combined use of these techniques demonstrated the processes of shear and compaction band generation and the associated strain components that developed in the laboratory, and potentially also increased understanding of the naturally developed equivalents. The physical mechanisms of shear and compaction involved seem to be similar, but at the laboratory scale they show differences in the proportions and the order of occurrence in time.

Imaging sand deformation at the grain scale

Shear banding, the localization of deformation into thin zones of intense shearing, is a phenomenon commonly observed in sand and other granular materials. The most valuable experimental contributions to the understanding of shear banding are those measuring, in one way or another, the full field of deformation in the specimen – which is the only means by which test results can be appropriately interpreted [1]. Full-field analysis of strain localization in sand possibly started in the late 1960s in Cambridge and was continued over the last decades in the work of a number of groups, including Grenoble. Most of these works were performed using specifically designed plane strain devices, and used a range of full-field methods, the more advanced of which allowed observation of the specimen throughout loading by optical methods, thereby permitting measurement of the evolving strain field. In the 1960s, x-ray radiography was first used to measure 2D strain fields in sand. From the early 1980s, x-ray tomography was used by a few groups working in geomechanics, including Desrues and coworkers (see [2] for a review). These studies provided valuable 3D information on localization patterning in sand, and also demonstrated the potential of x-ray tomography as a quantitative tool, e.g., for measuring the evolution of void ratio inside a shear band and its relation to critical state. The recent advent of x-ray micro tomography, originally with synchrotron sources and now with laboratory scanners, has provided much finer spatial resolution, which opens up new possibilities for understanding the mechanics of granular media (in 3D) at the scale of the grain. For example, Oda and coworkers [3] presented micro tomography images of sand grains inside a shear band, showing organized structures that would not have been seen in standard x-ray tomography images (because of insufficient resolution) and that had only previously been observed in 2D thin sections. It should be noted that the images by Oda were obtained post-mortem, i.e., after testing. However, a full understanding of the mechanisms of (localized) deformation can only be achieved if the entire deformation process is followed throughout a test while the specimen deforms. This is possible by using in-situ x-ray tomography (in-situ meaning x-ray scanning at the same time as loading). A number of such in-situ studies for triaxial tests on sand were performed over the last ten years or so, mostly using medical or industrial tomography systems, and in a few cases synchrotron micro tomography, which allows identifying individual sand grains and tracking their displacements throughout a test. The aim of the present study is both to observe the material evolution under loading with grainscale resolution and to image the deformation processes. In recent work [4], we applied 3D Volumetric Digital Image Correlation (V-DIC) to a sequence of x-ray tomography images taken during a triaxial test on a clay-rock specimen. In the present paper, we show results of a similar DICbased analysis of deformation for sand specimens under triaxial compression. Two different granular materials were tested: Hostun sand, a fine-grained, angular siliceous sand with a mean grain size (D50) of about 300 m, and Caicos ooid, a material characterized by spheroidal grains with D50 of about 420 m. In addition, we have developed a grain-scale V-DIC method that permits the characterization of the full kinematics (i.e., 3D displacements and rotations) of all the individual © Owned by the authors, published by EDP Sciences, 2010 DOI:10.1051/epjconf/20100622021 EPJ Web of Conferences 6, 22021 (2010)

Experimental characterisation of shear and compaction band mechanisms in porous sandstone by a combination of AE and 3D-DIC

Shear and compaction bands represent two fundamental failure modes in sandstones. The identification of these features in both the natural outcrops and the laboratory scale yields useful insights into the mechanisms of localised deformation. Several conventional and advanced experimental full-field methods have been developed in order to shed light into the processes involved, as well as qualify and quantify parameters such as the orientation of these localised bands. In this paper, we present results of two triaxial tests, investigating shear and compaction bands in porous sandstone, using analysis of Acoustic Emission recorded during deformation and 3D-volumetric Digital Image Correlation based on pre- and post deformation x-ray tomography images. (Less)