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Item type:Publication, Access status: with files , Final report of DFG project 448085183: numerical modelling of partially cemented soils in the stagnation zone(2026-05-20); ; ; This final report summarizes the results of DFG project 448085183, “Numerical Modelling of Partially Cemented Soils in the Stagnation Zone”. The project investigated the micromechanical behaviour of partially cemented sands by combining experimental testing, X-ray micro-computed tomography, image processing, and advanced numerical modelling. In the first project phase, the focus was on the linear-elastic response of cemented sand specimens. CT-based microstructures were converted into numerical models and analysed using the Finite Element Method (FEM) and the Finite Cell Method (FCM), including numerical homogenization to determine effective material properties. In the second project phase, the numerical frameworks were extended towards fracture and failure processes, including matrix cracking, grain-scale stress localization, and phase-field modelling. Additional experimental data were obtained by Digital Volume Correlation (DVC) to support the validation of the simulations. The project contributes to a better understanding of load transfer, stiffness development, and failure mechanisms in cemented granular materials and provides automated CT-based simulation workflows for future applications in geotechnical modelling.Publicationtype: Research Report70 48 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Geometry smoothing and local enrichment of the finite cell method with application to cemented granular materials(Springer, 2025); ; ; ; In recent times, immersed methods such as the finite cell method have been increasingly employed in structural mechanics to address complex-shaped problems. However, when dealing with heterogeneous microstructures, the FCM faces several challenges. Weak discontinuities occur at the interfaces between the different materials, resulting in kinks in the displacements and jumps in the strain and stress fields. Furthermore, the morphology of such composites is often described by 3D images, such as ones derived from X-ray computed tomography. These images lead to a non-smooth geometry description and thus, singularities in the stresses arise. In order to overcome these problems, several strategies are presented in this work. To capture the weak discontinuities at the material interfaces, the FCM is combined with local enrichment. Moreover, the L²-projection is extended and applied to heterogeneous microstructures, transforming the 3D images into smooth level-set functions. All of the proposed approaches are applied to numerical examples. Finally, an application of cemented granular material is investigated using three versions of the FCM and is verified against the finite element method. The results show that the proposed methods are suitable for simulating heterogeneous materials starting from CT scans.Publicationtype: Journal Article44 79 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Implementation of linear strain elements for image-based Finite Element Analysis of cemented natural sandMesoscopic approaches in material science aim to conceive multiphase materials as compositions of constituent phases, rather than homogenized media. X-ray Computed Tomography is a powerful tool that provides full field information of composites’ structure in the form of three-dimensional images. Image analysis combined with computational geometry concepts permit the transition from voxel images to adapted unstructured tetrahedral meshes, a conversion that delivers important computational benefits of image-based numerical simulations. The current article focuses on the case of cemented natural granular material of two cement saturation degrees. Actual cylindrical samples are scanned and reconstructed in the form of digital volumes. Image artefacts impede reliable particle segmentation. A novel image filter based on domain cylindrical decomposition and histogram compatibility enforcement is applied to alleviate the bias. The conversion of voxel images to quadratic tetrahedral meshes is proposed to facilitate accurate image-based numerical analysis. Localization of stresses is evident in the upgraded linear strain element configuration, but missing from that of constant strain element. The display of stress results and supplementary statistical analysis suggest that the granular skeleton sustains the load by forming intergranular stress channeling, while the cement matrix is partially activated at regions of granular contact to aid the stress flow.Publicationtype: Journal Article20 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Investigation of the load sustaining micro mechanisms of cemented sand using the mesoscale FEM approachWhile the mechanics of cemented granular material have already been approached by means of constitutive modeling, the capacity of introducing the precise internal structure of multiphase media in numerical simulations offers an alternative perspective of investigation. In specific, means of X-ray computed tomography are able to quantify the internal irregular geometry of the composite material in the form of three-dimensional images. Image analysis can be applied in order to distinguish the topology of each material phase and advanced meshing techniques are able to form a twin meshed domain. The current paper deals with the quantitative description of the morphology of small cemented sand samples, the application of developed image analysis algorithms to treat imaging artefacts (mainly due to beam hardening), the equivalent mesh generation via the image adapted meshing technique and the numerical testing using the Finite Element Method. Four samples are imaged, meshed and numerically tested in uniaxial compression; the mesoscale finite element simulations are able to display the intergranular stress transmission chains. A statistical analysis of the load carried by the granular skeleton provides evidence of the influence of the cementation degree and the compaction of the aggregates on the uniformity of the stress distribution.Publicationtype: Journal Article33 100 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the load bearing mechanisms of cemented granular material : a mesoscale FE approachThe numerical investigation of cemented granular material (CGM) is a demanding task that requires the combination of various scientific disciplines. At the mesoscale, CGM is decomposed into the constituents of particles, cement matrix and void pores. The combination of these constituents, as found in nature or in civil infrastructure projects, creates a highly heterogeneous structure, whose morphology defines the response of the CGM under mechanical loading. The composite internal structure is quantified by means of x‐ray Computed Tomography, which provides 16‐bit grayscale 3D images of the scanned microsamples. These images are decomposed into the three constituent materials to be investigated and modeled. Still, the ability of simple Cartesian grids to adequately carry information about curved geometries and provide accurate representations is questioned. In order to overcome staircase effect errors, the interfaces of the material inclusions are smoothed and re‐defined, making use of Gaussian blurred signed distance fields (SDF). These level‐set derived interfaces serve as a reference for the volume reconstruction of the image via tetrahedra. The comparison of the raw and the smoothened images to analytical geometry Standards suggests that the above reconstruction alleviates the grid's inherited uncertainities. In addition, a comparison between a non smoothed and a smoothed meshed image is performed in terms of mechanical response, utilizing the Finite Element (FE) Method. The results suggest that even a slight change in the granular contact fabric can alter the unfolding stress transmission mechanisms.Publicationtype: Journal Article41 93 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Mesoscale FEM approach on cemented sand : challenges and implementation of high order elements(2023-06); ; ; The rapid development of 3D imaging techniques, combined with image analysis, has enabled the quantification of the morphology of multiphase media. One step beyond, the ability to mesh the captured geometry allows the numerical testing of the digital twin, proven to be a valuable numerical tool in material investigation and micro-macro implementations. The scope of this study is the implementation of a finite element discretization of 3D images of cemented sand. The proposed method of the image adapted meshing technique has been implemented and linked to the FE framework of the commercial code ABAQUS Standard. The evaluation of the results demonstrates both accuracy and efficiency in capturing the causes that trigger matrix cracking and grain crushing. Furthermore, computational challenges of excessive stiffness and parasitic stresses are addressed and solved by increasing the order of the elements used. This attempt required the development of an add-on feature to the Delaunay triangulation algorithm, which enriches constant strain tetrahedral elements with nodes at the midpoints of their edges and upgrades their shape functions from linear to quadratic.Publicationtype: Conference Paper27