Influence of the physico-mechanical properties of clay minerals on stone deterioration

La desquamation en plaque est une forme de dégradation de la pierre monumentale fréquemment observée. Cette morphologie spécifique résulte de l’initiation et la propagation d’une fissure subparallèle à la surface, qui subit diverses sollicitations en conditions naturelles d’exposition. L’objectif de cette recherche est de mieux comprendre ces mécanismes d’altération et d’appréhender les paramètres déterminants dans l’initiation de la fissuration au sein des pierres mises en œuvre. Une approche en mécanique de la rupture a ainsi été proposée. L’influence des minéraux argileux, mis en cause dans ce phénomène, est étudiée et discutée. Trois pierres ont été sélectionnées, une molasse, un grès et une kersantite, présentant de nombreux exemples de desquamation lorsqu’elles sont exposées. Leur comportement hydromécanique est caractérisé en tenant compte de leur anisotropie naturelle. Les propriétés élastiques, la résistance à la traction et la ténacité sont mesurées après saturation, pour plusieurs humidités relatives en phase d’humidification et de séchage, et après plusieurs cycles de variations d’humidité relative. Un couplage de mesure des dilatations et des vitesses d’ondes acoustiques a permis un suivi précis de l’endommagement au fil des cycles. L’influence des minéraux argileux dans les processus d’altération a pu être vérifiée. Ces phases ont été identifiées comme des facteurs essentiels de la dégradation, car elles constituent des plans de fragilité favorisant l’endommagement et la fissuration du matériau, et ce d’autant plus qu’elles se dilatent lors des sollicitations naturelles.Flaking is a deterioration pattern widely observed on monumental stone heritage. This pattern is due to the initiation and propagation of a crack subparallel to the surface of the stone exposed to natural weathering. This research aims to better understand the alteration mechanisms and to determine the parameters involved in such crack initiation within monumental stones. A fracture mechanics approach is proposed and the influence of clay minerals on this detachment alteration is discussed. The research is carried out on three stones, a molasse, a sandstone and a kersantite, showing scaling effect when submitted to natural conditions. Their hydromechanical behaviour is studied with respect to their natural anisotropy. Elastic properties, tensile strength and toughness are measured after saturation, at various RH values during humidification and drying, and after several cycles. Wave propagation combined with dilatation measurements is used to follow damage during the RH variations cycles. The influence of clay minerals on alteration processes is verified. These phases are critical factors of stone degradation as they are weakness planes leading to damage and cracking, especially generated by their swelling

Influence des propriétés physico-mécaniques des minéraux argileux dans l'altération de la pierre monumentale

Flaking is a deterioration pattern widely observed on monumental stone heritage. This pattern is due to the initiation and propagation of a crack subparallel to the surface of the stone exposed to natural weathering. This research aims to better understand the alteration mechanisms and to determine the parameters involved in such crack initiation within monumental stones. A fracture mechanics approach is proposed and the influence of clay minerals on this detachment alteration is discussed. The research is carried out on three stones, a molasse, a sandstone and a kersantite, showing scaling effect when submitted to natural conditions. Their hydromechanical behaviour is studied with respect to their natural anisotropy. Elastic properties, tensile strength and toughness are measured after saturation, at various RH values during humidification and drying, and after several cycles. Wave propagation combined with dilatation measurements is used to follow damage during the RH variations cycles. The influence of clay minerals on alteration processes is verified. These phases are critical factors of stone degradation as they are weakness planes leading to damage and cracking, especially generated by their swelling.La desquamation en plaque est une forme de dégradation de la pierre monumentale fréquemment observée. Cette morphologie spécifique résulte de l’initiation et la propagation d’une fissure subparallèle à la surface, qui subit diverses sollicitations en conditions naturelles d’exposition. L’objectif de cette recherche est de mieux comprendre ces mécanismes d’altération et d’appréhender les paramètres déterminants dans l’initiation de la fissuration au sein des pierres mises en œuvre. Une approche en mécanique de la rupture a ainsi été proposée. L’influence des minéraux argileux, mis en cause dans ce phénomène, est étudiée et discutée. Trois pierres ont été sélectionnées, une molasse, un grès et une kersantite, présentant de nombreux exemples de desquamation lorsqu’elles sont exposées. Leur comportement hydromécanique est caractérisé en tenant compte de leur anisotropie naturelle. Les propriétés élastiques, la résistance à la traction et la ténacité sont mesurées après saturation, pour plusieurs humidités relatives en phase d’humidification et de séchage, et après plusieurs cycles de variations d’humidité relative. Un couplage de mesure des dilatations et des vitesses d’ondes acoustiques a permis un suivi précis de l’endommagement au fil des cycles. L’influence des minéraux argileux dans les processus d’altération a pu être vérifiée. Ces phases ont été identifiées comme des facteurs essentiels de la dégradation, car elles constituent des plans de fragilité favorisant l’endommagement et la fissuration du matériau, et ce d’autant plus qu’elles se dilatent lors des sollicitations naturelles

Moisture-induced elastic weakening and wave propagation in a clay-bearing sandstone

International audienceLong-term behaviour of a clay-bearing sandstone is investigated using elastic waves propagation coupled with strain measurements during moisture cycles. Ad-sorption and desorption are followed in a continuous way during several variations of relative humidity. Moisture adsorption is related to the solvation pressure and the measured elastic weakening during humidification is correlated with the clay minerals sensitivity

Evolution of the viscoelastic properties of painting stratigraphies:a moisture weathering and nanoindentation approach

In this investigation on the mechanical behaviour of paint films, we use a new ferrule-top nanoindentation protocol developed for cultural heritage studies to examine the impact of repeated relative humidity variations on the viscoelastic behaviour of paint films and their mechanical properties in different paint stratigraphies through the changes in their storage and loss moduli. We show that the moisture weathering impact on the micromechanics varies for each of these pigment-oil systems. Data from the nanoindentation protocol provide new insights into the evolution of the viscoelastic properties dsue to the impact of moisture weathering on paint films

Influence of Clay Minerals Nature on the Hydromechanical and Fracture Behaviour of Stones

International audienceElastic properties and fracture behaviour evolutions of two sedimentary stones are under study to estimate their influence on scaling effect of monumental stone heritage. Young moduli and toughness of Villarlod molasse and Thüringer sandstone are measured under moisture content variations with respect to their natural anisotropy. Several relative humidity levels are considered to investigate their effect upon humidification and drying. The stone property evolution is in close correlation with the identified clay phases of the matrix. It appears that clay minerals as smectite and glauconite influence the stone properties at very low moisture contents, whereas less swelling clays have impact only when high moisture contents are reached. This is in close correlation with the inherent deformation ability of these minerals. It is observed that nature and distribution of clay phases largely influence on the macroscopic behaviour under natural exposure conditions, as they modify the stone properties and represent the weakness planes of the stone

Mapping the mechanical properties of paintings via nanoindentation:a new approach for cultural heritage studies

A comprehensive understanding of the behaviour of the heterogenous layers within the paint stratigraphies in historical paintings is crucial to evaluate their long term stability. We aim to refine nanoindentation as a new tool to investigate the mechanical behaviour of historical oil paints, by adapting the probes and the protocol already used in biomechanical research on soft tissues. The depth-controlled indentation profile performed with a spherical probe provides an evaluation of the non-linear viscoelastic behaviour of the individual layers in paint at local scale. The technique is non-destructive and guarantees the integrity of the surface after indentation. The mapping of elasticity demonstrates the properties’ heterogeneity of the composite material within the paint layers, as well as between the individual layers and their interfaces

The use of ground glass in red glazes: structural 3D imaging and mechanical behaviour using optical coherence tomography and nanoindentation

In this study we investigate the impact of the addition of colourless glass particles to red glazes, as seen in many 15th-17th-century easel paintings. With the use of reconstructions, we examined the influence of the paint preparation process on the morphological and mechanical properties of the paint film. Three sets of reconstructions were made, a control without ground glass, reconstructions with coarse or fine ground glass mixed in, and reconstructions where fine ground glass was ground jointly with the pigment oil mixture. The latter gave the desired consistency and colour based on visual inspection. The dried reconstructions were non-invasively imaged using optical coherence tomography (OCT). A data-analysis pipeline was developed for both the segmentation of the OCT images and the measurement of the size and spatial distributions of the glass particles within the glaze layer. Moreover, we used a nanoindentation protocol to measure the viscoelastic properties of the dried red glaze film. The OCT results show an expected decrease in median particle size with longer grinding-time, for which the additional grinding with pigment/oil resulted in a more narrow size distribution and a homogenous spatial distribution of the glass particles. The nanoindentation results indicate that the addition of glass particles increases the elastic and viscous moduli of the red glaze layers. The homogeneous size distribution, obtained by grinding the oil, pigment, and glass together, induces higher elastic and viscous moduli. Our imaging and analyses approach, combining OCT and nanoindentation, provides a non-invasive and quantitative investigation of glass particles in (semi-) transparent paint layers, and their effect on the mechanical properties of the glaze. The results of this study contribute to a better understanding of the artists’ addition of ground glass in paint layers

Fluid Substitution and Shear Weakening in Clay‐Bearing Sandstone at Seismic Frequencies

International audienceUsing the forced oscillation method and the ultrasonic transmission method, we measure the elastic moduli of a clay-bearing Thüringen sandstone under dry and water-saturated conditions in a broad frequency band at [0.004-10, 10 6 ] Hz for different differential pressures up to 30 MPa. Under water-saturated condition, clear dispersion and attenuation for Young's modulus, Poisson's ratio, and Bulk modulus are observed at seismic frequencies, except for shear modulus. The measured dispersion and attenuation are mainly attributed to the drained/undrained transition, which considers the experimentally undrained boundary condition. Gassmann's predictions are consistent with the measured undrained bulk moduli but not with the shear moduli. Clear shear weakening is observed, and this water-softening effect is stronger at seismic frequencies than at ultrasonic frequencies where stiffening effect related to squirt flow may mask real shear weakening. The reduction in surface free energy due to chemical interaction between pore fluid and rock frame, which is not taken into account by Gassmann's theory, is the main reason for the departure from Gassmann's predictions, especially for this rock containing a large number of clay minerals

Bridging steady-state and stick-slip fracture propagation in glassy polymers

International audienceBoth an experimental and a theoretical investigation of the fracture propagation mechanisms acting at the process zone scale in glassy polymers are presented. The main aim is to establish a common modeling for different kinds of glassy polymers presenting either steady-state fracture propagation or stick-slip fracture propagation or both, depending on loading conditions and sample shape. On the experimental point of view, new insights are provided by in-situ AFM measurements of the viscoplastic strain fields acting within the micrometric process zone in a brittle epoxy resin, which highlight an extremely slow unexpected steadystate regime with finite plastic strains of about 30% around a blunt crack tip, and accompanied by propagating shear lips. On the theoretical point of view, we apply to glassy polymers some recently developed models for describing soft dissipative fracture that are pertinent with the observed finite strains. We propose a unified modeling of the fracture energy for both the steady-state and stick-slip fracture propagation based on the evaluation of the energy dissipation density at a characteristic strain rate induced in the process zone by the competition between the crack propagation velocity and the macroscopic sample loading rate