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Solid Mechanics

Faculty of Engineering, LTH

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Experimental Mechanics

Introduction

Since mid-2011 experimental mechanics has become a key area of research in the group, both in terms of method development and in terms of application to investigate the mechanics of a range of different materials. In the following a number of examples are given of the experimental mechanics applications and developments being carried out by members of the group. In addition, experimental applications for biomechanics are described elsewhere.

For more information contact: Steve Hall

General research approach

The focus of the developments in experimental mechanics is in advanced methods and in particular full-field measurements (where fields of properties are measured as opposed to point-wise, boundary measurements generally made using traditional methods). Furthermore, the general approach is to investigate, develop and apply advanced experimental tools that are the most appropriate for probing the properties or phenomena of interest, which may involve the combination of multiple methods simultaneously. Key tools that are used and developed in our research include 2D-surface and 3D-volumetric DIC, 4D Digital Image analysis, x-ray tomography, neutron imaging, x-ray and neutron scattering (diffraction and small angle) and ultrasonic methods (including ultrasonic tomography). A number of the experimental developments involve collaboration with Laboratoire 3SR (Grenoble, France) and other international research groups plus experiments are often performed at large scale facilities including MaxLab (Synchrotron x-ray source, Lund, Sweden) and the European Synchrotron Radiation Faciltity (Grenoble, France). We also have strong links with the ESS (Europe's future neutron source) and MAX IV (Sweden's future synchrotron x-ray source) in Lund. The majority of the applications to date have concerned investigations of the mechanics of geomaterials (rocks and soils), but the focus is broadening to other materials, including polymers, paper and other materials of interest in relation to the theoretical and numerical work of the group.

- ALERT Doctoral School 2012: Advanced experimental techniques in geomechanics, Viggiani, G., Hall, S.A and Romero, E. (eds), ALERT Geomaterials, ISBN 978-3-00-039683-0.

- Viggiani, G., and Hall, S.A., 2012, Full-field measurements in experimental geomechanics: historical perspective, current trends and recent results, in ALERT Doctoral School 2012: Advanced experimental techniques in geomechanics, Viggiani, G., Hall, S.A and Romero, E. (eds), ALERT Geomaterials, p 3-68, ISBN 978-3-00-039683-0.

- Viggiani, G., and Hall, S.A., 2008, Full-field measurements, a new tool for laboratory experimental geomechanics, Fourth Symposium on Deformation Characteristics of Geomaterials, 22-24 September 2008, Atlanta, USA (eds Burns, S.E., Mayne, P.W. & Santamarina, J.C.) Amsterdam, IOS Press, 1, 3-26.

A few examples

Analysis of localised behaviour in geomaterials by 2D-surface and 3D-volume Digital Image Correlation

Surface Digital Image Correlation (DIC) is a useful tool to assess full-field displacements and strains. In particular 2D-DIC is a very powerful tool in conjunction with plane-strain testing, as the surface observations can be taken as being representative of the deformation through the test specimens depth. 2D-DIC has been used in a number of studies by S.Hall in the Solid Mechanics group (mostly whilst he was based in Grenoble). This research has involved development of 2D-DIC codes and approaches for the treatment of discontinuities or the study of 2D granular materials, including adaptations to follow individual grain kinematics (2D displacements and in-plane rotations). 

 

Hall, S.A., 2012, Digital Image Correlation in Experimental Geomechanics, in ALERT Doctoral School 2012: Advanced experimental techniques in geomechanics, Viggiani, G., Hall, S.A and Romero, E. (eds), ALERT Geomaterials, ISBN 978-3-00-039683-0.

- Nguyen, T.L., Hall, S.A., Vacher, P. and Viggiani, G., 2011, Fracture mechanisms in soft rock: identification and quantification of evolving displacement discontinuities by digital image correlation, Tectonophysics

- Hall, S.A., Muir Wood, D., Ibraim, E. and Viggiani, G., 2010, Localised deformation patterning in 2D granular materials revealed by digital image correlation, Granular Matter, 12, 1-14


3D image warping and 3D-Volumetric DIC for 3D deformation measurements from km’s to microns


Another important area of development is in 3D-volumetric DIC that can be applied to volume images, e.g., from x-ray tomography, to have full 3D tensor strain fields throughout the bulk of test specimens. These developments stem from S.Hall's work on time-lapse seismic data analysis, where multiple snapshot images of subsurface geologic hydrocarbon reservoirs are acquired through production. This work led to the development of 3D image warping with the objective of correcting spatially varying 3D offsets between 3D images; Hall et al. (2005; 2006). In carrying out such work it was realised that the vector displacement fields for alignment also revealed the displacements due to subsidence and compaction around the producing reservoirs (Hall et al., 2005). The methodology was thus developed as a tool for 3D deformation analysis in time-lapse data (Hall, 2006). Subsequently, the codes were adapted for use with 3D x-ray tomography images acquired either during, or before and after, loading, to determine the 3D kinematic and strain fields in studies of, for example, strain localisation in geomaterials (e.g., Hall et al., 2007; 2009; 2010; Charalampidou et al., 2011): 

- Charalampidou, E-M. Hall, S.A., Stanchits, S., Lewis, H. and Viggiani, G., 2011, Characterization of shear and compaction bands in a porous sandstone deformed under triaxial compression, Tectonophysics, 503, 8-17

Hall, S.A., Lenoir, N., Viggiani, G., Bésuelle, P., and Desrues, J., 2010, Characterisation of the evolving grain-scale structure in a sand deforming under triaxial compression, in Advances in Computed Tomography for Geomaterials, GeoX 2010. Ed. Khalid .A. Alshibli and Allen H. Reed, Wiley, & Sons, Hoboken, NJ, p. 34-42.

Hall, S.A., Lenoir, N., Viggiani, G., Desrues, J., and Bésuelle P., 2009, Strain localisation in sand under triaxial loading: characterisation by x-ray micro tomography and 3D digital image correlation, Proceedings of the 1st International Symposium on Computational Geomechanics (ComGeo I), Juan-les-Pins, Cote d'Azur, France.

- Hall, S.A., Pannier, Y., Bornert, M., Desrues, J., and Viggiani, G., 2007, Deux approches de la corrélation 3D d’images volumiques comparées sur  des données de tomographie à rayons X, 18ème Congrès Français de Mécanique, Grenoble, 27-31 août 2007.

- Hall, S.A., 2006, A methodology for 7D warping and deformation monitoring using time-lapse seismic data, Geophysics, 71, O21–O311.

-  Hall, S.A., MacBeth, C., Barkved, O.I. and Wild, P., 2005, Cross-matching with interpreted warping of 3D streamer and 3D OBC data at Valhall for time-lapse assessment, Geophysical Prospecting, 53, 283-297.

4D grain-scale analysis of granular mechanics

3D x-ray tomography imaging of triaxial compression tests on sand specimens during the loading (in-situ testing) carried out in Grenoble have been analysed to extract quantified, meaningful information about the 3D mechanisms of deformation. This started with the application of 3D-volumetric DIC (see above) to determine 3D displacement and strain fields in deforming sand specimens to reveal strain localisation phenomena (Hall et al., 2009; 2010). In particular the analysis revealed the localisation of shear strain well before it was visible in the tomography images. This approach also revealed structuring in the shear bands, which might be indicative of columns of grains associated with so-called force chains. 3D image analysis techniques have been developed to investigate the structural evolution of the material, including porosity and grain contact measurements (e.g., Hall et al., 2010b).  In an effort to better understand the mechanisms of deformation at a pertinent level, i.e., the grain-scale, “discrete-DIC” approaches have been developed to track all the individual grains’ kinematics (Hall et al., 2010 - collaboration with M. Bornert, Institut Navier, France - and Andò et al., 2012 - collaboration with Laboratoire 3SR, France). A systematic study of sands with different grain shapes and under different loading conditions has been performed in a PhD project in Grenoble (E.Andò, co-supervised by S. Hall in Lund).

 

- Hall, S.A., Desrues, J., Viggiani, G., Bésuelle, P., and Andò, E., 2012, Experimental characterisation of (localised) deformation phenomena in granular geomaterial from sample down to inter- and intra-grain scales, in Full field measurements and identification in Solid Mechanics, Procedia IUTAM, 4, 54–65

- Hall, S.A., Bornert M., Desrues J., Pannier Y., Lenoir N., Viggiani, G., Bésuelle, P., 2010, Discrete and Continuum analysis of localised deformation in sand using X-ray micro CT and Volumetric Digital Image Correlation, Géotechnique, 60, 315 –322.

Hall, S.A., Lenoir, N., Viggiani, G., Bésuelle, P., and Desrues, J., 2010, Characterisation of the evolving grain-scale structure in a sand deforming under triaxial compression, in Advances in Computed Tomography for Geomaterials, GeoX 2010. Ed. Khalid .A. Alshibli and Allen H. Reed, Wiley, & Sons, Hoboken, NJ, p. 34-42.

3DXRD and spatially-resolved neutron diffraction applied to the assessment of force transfer in granular materials.

One of the holy-grails of experimental analysis of granular mechanics is the detection and characterisation of force transmission and, in particular, “force chains”. The DIC analysis described above for assessing the full grain-kinematics from 3D images of sand samples undergoing triaxial compression provide indications of such structures, but no actual measures that can be related to force transmission. A new experimental method, 3D x-ray diffraction (3DXRD) has been tested as a tool to measure force transmission in granular materials. The first experiments using 3DXRD for granular materials were performed by S. Hall in 2010 with first results published in 2011 (Hall et al., 2011). This approach has significant potential to produce a major step forward in experimental geomechanics. In related work spatially-resolved neutron diffraction has also been used to measure grain strains in sand specimens under compression in a continuum sense (Hall et al., 2011) revealing new insight into the partitioning of strain between pore-space reduction and grain straining; this also has strong potential for future advances in geomechanics.

 

- Hall, S.A., Wright, J., Pirling, T., Andò, E., Hughes, D.J. and Viggiani, G., 2011, Can intergranular force transmission be identified in sand? First results of spatially-resolved neutron and x-ray diffraction, Granular Matter, 13, 251-254

Ultrasonic tomography to assess damage evolution in geomaterials

Whilst techniques such as DIC provide new insight into mechanisms of deformation they do not provide information on the material evolution in terms of the mechanical properties. Ultrasonic methods provide measurements of the elastic properties of materials and thus provide the potential to assess changes in the mechanical properties due to deformation. However, ultrasonic methods do not usually allow mapping of elastic properties, which is essential in the study of heterogeneous phenomena such as strain localisation. To this end ultrasonic tomography has been investigated as a tool for full-field mapping of elastic properties and their evolution due to deformation and damage (e.g., Charalampidou et al. 2010; Charalampidou, 2011, Hall and Tudisco 2012).

      

 

- Hall, S.A. and Tudisco, E. 2012, Full-field ultrasonic measurement (ultrasonic tomography) in experimental geomechanics, in ALERT Doctoral School 2012: Advanced experimental techniques in geomechanics, Viggiani, G., Hall, S.A and Romero, E. (eds), ALERT Geomaterials, ISBN 978-3-00-039683-0

- Charalampidou, E-M. Hall, S.A., Stanchits, S., Lewis, H. and Viggiani, G., 2011, Characterization of shear and compaction bands in a porous sandstone deformed under triaxial compression, Tectonophysics, 503, 8-17

- Charalampidou, E-M., 2011, Experimental Study of Localised Deformation in Porous Sandstones, PhD Thesis, Heriot-Watt University and Université de Grenoble

- Hall, S.A., 2009, When geophysics met geomechanics: Imaging of geomechanical properties and processes using elastic waves, in Mechanics of Natural Solids, Kolymbas, D. and Viggiani, G. (Eds.), VIII, 304 p., Springer, pp 147-175

- Hall, S.A., Viggiani, G., Charalampidou, E., Bésuelle, P., and Rousseau, C., 2007, Caractérisation de l’endommagement localisé dans les géomatériaux à l’aide des ondes ultrasonores en conditions de laboratoire, 18ème Congrès Français de Mécanique, Grenoble, 27-31 août 2007

 

Neutron Imaging of fluid flow in rocks

Understanding the controls on the permeability of porous rocks, such as sandstone, is important in a number resource engineering challenges, in particular hydrocarbon production and CO2 sequestration (in the first case fluids are extracted from subsurface geologic formations and in the latter fluids are injected into the same or similar formations). During such engineering operations deformation of the rock mass can occur that leads to alterations in the flow properties of the rock. However, deformation in these porous materials is, in general, not homogeneous as deformation localises into narrow shear or compaction bands, which might then evolve into fractures. These local deformation features can act as barriers to or conduits for fluid flow, depending on their evolution and resultant properties; their effect on flow is therefore a key factor in determining the storage or production capacity of a reservoir.

Traditionally rock permeability is measured by bulk measurements involving measurement of the in-flow and out-flow of fluids at the ends of test specimens. In the presence of material heterogeneity, e.g., localised deformation features, such measures do not provide a good understanding of the factors that alter the measured permeability values, as the causes are local internal modifications to the flow profile. Therefore, to better understand the controlling factors on permeability evolution due to localised deformation requires determination of what modifications to the permeability field have occurred, where and why by mapping of the full permeability field through the test specimens. Neutron imaging is an ideal tool for the observing fluid flow in rocks as hydrogen (in the flowing water) is highly absorbing to neutrons whereas sandstone, for example, is not. Therefore fluid distributions and movements inside a rock sample can easily be detected. In this work neutron imaging has been used  to observe and map local fluid flow variations in locally deformed sandstone samples using neutron radiography and digital image analysis techniques (Hall et al, 2010).

 

Hall, S.A., Hughes, D. and Rowe, S., 2010, Local characterisation of fluid flow in sandstone with localised deformation features through fast neutron imaging, proceedings of 14th International Conference on Experimental Mechanics (ICEM 14), EPJ Web of Conferences, 6, 22008.

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