Papers by Author: L.J. Sluys

Abstract: This paper presents a constitutive relationship to describe the uniaxial response in statics of brick and mortar samples of Adobe, a traditional masonry whose components are made of sundried soil mixture reinforced with fibres. Only recently Adobe has been attracting scientific attention, primarily as a consequence of the dramatic failures these structures have suffered in regions prone to earthquakes. Furthermore, it possesses eco-friendly material properties which are attractive features for western countries forced to reduce the environmental impact of modern building industry. Nevertheless, the mechanical properties of Adobe are still largely neglected, especially with regards to the influence of soil mixture components. The study of the structural performance of masonry starts from the assessment of the material performance of its components. Thus, an extensive characterization campaign was performed by Delft University of Technology and the Military Engineering Laboratory of the Netherlands. Three types of bricks and one type of mortar with different mixture components proportions, were subjected to granulometry, moisture content, density tests and uniaxial compressive and three point bending tests. Predictive formulations for compressive and tensile strength and deformation values have been proposed by the authors. These relations include the dependency of mixture components and moisture content. In this paper, constitutive laws are developed for Adobe in pure compression and tension validated by experimental results. In compression, the force-displacement curves were interpolated according to several existing constitutive laws and the model originally developed by Priestley for concrete masonry elements was finally selected as best fitting. Despite the differences in terms of mechanical parameters, the analytical assessment revealed that the experimental force-displacement graphs of all the different types of bricks could be interpolated using the same model with the same calibrating values. Furthermore, the uniaxial response in tension was derived according to an inverse approach. A numerical model recently developed by the authors and calibrated with respect to the compressive and bending tests was used to simulate uniaxial tensile tests. Also in tension, a common trend among types was observed. The results of the constitutive modelling frames components of Adobe within the class of quasi brittle (geo) materials, with particular reference to concrete. This paper presents the experimental results of the tested samples and the related analytical and numerical modelling.

Abstract: In this work, a new damage model for mixed-mode fracture in the scope of the discrete crack approach is introduced. An energy-based internal damage variable is adopted. In the model, deformation-driven loading surfaces are defined and the dual loading surfaces in the traction space are derived. Under proportional loading, it is found that the constitutive relationship is symmetric. Further enrichment of the energy-based variable on the traction field is also introduced, allowing for a better approximation of: i) the limit surface defined in traction space and ii) non-proportional loading. However, in this case symmetry of the constitutive tensor is lost.

Abstract: Fracture in heterogeneous materials under dynamic loading is modelled using a multi-scale method. Computational homogenization is considered, in which the overall properties at the global-scale are obtained by solving a boundary value problem for a representative volume element (RVE) assigned to each material point of the global-scale model. In order to overcome the problems with upscaling of localized deformations, a non-standard failure zone averaging scheme is used. Discontinuous cohesive macro-cracking is modelled using the XFEM and a gradient-enhanced damage model is used to model diffuse damage at the local-scale. A continuous-discontinuous computational homogenization method is employed to obtain the traction-separation law for macro-cracks using averaged properties calculated over the damaged zone in the RVE. In the multi-scale model, a dynamic analysis is performed for the global-scale model and the local-scale model is solved as a quasi-static problem. Dispersion effects are then captured by accounting for the inertia forces at the local-scale model via a so-called dispersion tensor which depends on the heterogeneity of the RVE. Numerical examples are presented and the multi-scale model results are compared to direct numerical simulation results. Objectivity of the multi-scale scheme with respect to the RVE size is examined.

Abstract: A hierarchical multiscale procedure for softening materials is proposed in this paper. A standard multiscale model has been analysed with respect to macro-level mesh dependence and meso-level cell size dependence. In order to eliminate spurious macro-level mesh dependence and meso-level cell size dependence a coupled-volume approach has been proposed. A discussion on the various interacting length scales in the model is included.